|Hitachi Carb Versions 2/3|
|Hitachi Carb Version 1|
| HITACHI CARBURETORS
(Note: the word "carburetor" is a French word. In English it refers to the fuel mixture dispenser residing near a cylinder. However, you should be aware that the meaning of this word in the native French language is quite different. In French it means: "Don't mess with me".)
The first Viragos to hit our shores in 1981 had Hitachi carbs on them and this make of carb lasted on the larger displacement (700, 750, 920, 1000, and 1100cc) models through the 1987 model year. The early carbs by Hitachi were basically good and are still running many Viragos very well today.
In discussing these carbs I am going to assume a certain amount of basic understanding on the part of the reader. If you are starting from scratch go back and read "CV Carbs". It attempts to describe what goes on inside these beasts.
There are also a couple of considerations relating to the fact that the Virago is a V-twin. One is that the butterfly valves have to rotate in opposite directions as they are opened. Hitachi solved this problem with a somewhat strange engineering approach that involves slides and a wheel. Another consideration is that due to differences in cylinder temperature (and perhaps other factors), each carb needs to be tuned slightly differently from the other to achieve optimum performance for that cylinder. As a result you will often find different needles and main jets specified for the two carburetors.
I'll now try to give you a quick history of these carbs, mainly to record how they changed over the years.
In what we will call Version 1, when you remove the float bowl, you'll see two removable jets, the main jet and the pilot jet. On removing the top and pulling out the diaphragm/slide you see a tube running across down below. This tube provides an air supply to the choke circuit and is held in place by a plug which can be removed with a 6mm Allen wrench. Once the plug is out you can coax the tube out and you will be rewarded by a look at the main and pilot air jets, which supply air to these circuits in the "premix" stage. These carbs also had two separate pilot circuit outlets into the bore. One is called the "pilot outlet". It lies in the bore on the engine side of the butterfly valve. The flow of mixture from this outlet can be regulated by means of adjusting the pliot adjustment screw. The other outlet is called the "by-pass outlet, and lies pretty much right under the butterfly valve. It provides more fuel as the butterfly opens and a greater airstream moves across it.
The Version 1 carbs were used in 750's through 1983, and 920's through '82. However, for 920's things changed in 1983 and what I will call Version 2, was introduced. This carb has different shaped float bowls from Version 1. More importantly, the removable pilot jet was replaced with a "tube jet" which is a long tube pressed into the carb body. The actual jet orifice sits in the top of this tube out of sight. No more removing or changing the pilot jet in 920, 1000, and 1100 Viragos. Also, the choke air tube was no longer removable, the 6mm plug being replaced with solid plug pressed into the carb body. No more direct access to the main and pilot air jets. Version 2's had different lips on the carb-holder ends, and require different carb holders than do Verion 1's.
The two pilot system outlets in Version 1's were sort of combined into one oblong outlet and fed through one passage. Part of this oblong outlet was positioned just engine-side of the butterfly and approximates the former discreet "pilot outlet" hole. That end of the oblong is regulated by the pilot screw. The other end of the oblong sits under the butterfly valve and serves the "by-pass" function. With the pilot screw fully seated, fuel will still come out that end of the oblong. The design of the pilot adjustment screws was also changed--presumably to accommodate the changed pilot circuit design. Version 1 pilot screws had a groove in them in which the O ring sat. The spring sat above the O ring against the upper flange of this groove. In Version 2, the groove was gone. The correct assembly of the components onto the the pilot screw now went like this: First the spring is placed on the pilot screw. Then the washer. Then the O ring. I mention this because Yamaha never showed this change in their documentation. People looking at the carb blowups in the factory manuals, and Clymer, get confused as to the proper way to place components on these Version 2 screws--particularly when they didn't pay close attention when they removed them. Why these design changes? I'm afraid the reasons and advantages are beyond me.. Anybody know a former Hitachi carb design engineer?
Version 2 was used in the 1983 920's and in the '84 and'85 700's and 1000's. The '84/'85 carbs can be recognized by the green Teflon coating applied to the slides. A port was also added on the outside of the right carb (California models) to pull fumes from the charcoal canister back into the engine for burning. The canister collects raw gas fumes from the carbs and gas tank.
With the advent of the 1100 in 1986, Version 3 of these carbs made its appearance. The main (and perhaps only) difference was the addition of a "Coasting Enricher" circuit which had the effect of richening up the pilot circuit on deceleration by cutting off half of the "premix" air supply to that circuit. The intent was to promote better burning and reduce backfiring caused by a lean misfire condition.
Part of these changes were no doubt due to mandates from the EPA, and many were made to discourage owners from tinkering with their carbs to richen them up. While we started out with our pilot screws blocked, this was easy to overcome. But with Version 2 we lost access to our pilot jet, and air jets. Of course, being red-blooded motorcycyle mechanics we are not that easily discouraged, and where there is a will there is usually a way.
While I'm at it I might mention changes in the smog equipment. Through 1985 all Viragos (750, 920, 700, and 1000 models) had a gizmo on them called a "mixture control valve". These "gulp valves" were intended to correct a very short period of richness which supposedly occurs when you shut down the throttle suddenly and droplets of fuel are sucked off the intake walls. Are they really? Beats me. Anyway, few, if any, mechanics ever understood what these things did, mainly because they could never detect any change whatsoever when they removed them. (I personally consider them a worthless piece of equipment which can only cause trouble if they start to leak air. I routinely remove them and plug any resulting open vacuum lines--would recommend that you do the same.) In '84 the Air Induction System (AIS) was introduced, which arguably has some beneficial effect on emissions. However, all those pipes running around don't look that great and did get in the way of adjusting the pilot screw on at least one of the carbs. (I remove this system also, and cap the stubs coming out of the heads with 1/2 inch copper pipe ends from Ace hardware. The gaskets are left on the stubs, and the pipe ends snug right on with a nice tight fit. I use a little JB Weld to hold things together. A little aluminum paint, and it's done)
That pretty well sums up my knowledge of Hitachi carb changes. If any of you carb lovers know of others, please come forward and we'll add them to this paper.
TUNING HITACHI CARBS
For this section we'll assume that your carbs are in good working order. We'll also assume that:
* Your compression is good. If you have any doubts, run a compression test (holding the throttle open as you do it.!)
* Your valves are in good adjustment.
* Your plugs are good and properly gapped.
* Your air cleaner is clear. If it isn't clean/replace it.
* The rest of your fuel system is working properly. (Petcocks function properly, no tank venting problems.)
* Fuel levels in the float bowls are correct (ball park). See note on setting fuel levels near the end of this paper
* Your carb vent tubes are clear. See note on vent tubes later in paper.
* The brass plugs placed over your pilot adjustment screws at the factory have been removed. If they haven't, you'll need to do this. The technique is to drill a hole in them, insert a small screw, and wrestle them out. BE CAREFUL!!. These plugs are roughly 3mm thick, and your drill can get sucked through the soft brass as it exits, and bugger the heads of the adjustment screws, which lie just underneath the plugs! Go slowly with that drill and keep it under tight control!
Equipment you'll need to tune your bike includes:
* Mercury sticks to check synchronization
* A small screw driver (you may want to make or adapt one) to turn your pilot screws.
* Wrenches and screw drivers to adjust set screws, reach the rear synch screw, etc.
* Optional equipment might include a CO meter, or possibly a Colortune Plug.
* A fan to keep your engine from overheating isn't a bad idea, either.
Tuning is done on a fully warmed up engine. I find the best procedure is to lay out the equipment you are going to use, then take your bike for a spin until it is fully warm. Back home
* place your bike on the centerstand.
* Pull any components that you need to in order to access the carbs (e.g. air cleaner pod).
* Move you gas tank up by some method (I made a little bridge that fits around the front mounting points on the tank so I can suspend it from a rafter)
* Hook up your mercury sticks to the vacuum ports in your carbholders, and you are ready to go.
Note that after a run your engine should be about the right temperature for tuning. And if you work fast, and things go right you can get most of it done without a cooling off period. But if you are going to need to idle for extended periods, then you do need to shut off and let things cool down peridically or run a fan. How hot is too hot? It's a judgement call, I'm afraid. If you can touch the engine side covers and/or cam sprocket covers for a moment without getting burned, you are probably O.K.
But what if you engine won't run well enough to take a warm-up spin?. If the engine won't run properly, or at all, you can't very well tune it. This maybe the case if you just "rebuilt" the carbs. Things can get so far out of adjustment that the cylinder(s) won't fire. If this is your problem here are some checks:
-Make sure you have fuel in the float bowls. For bikes with petcocks, set them to "prime". This should fill the float bowls. If you have a fuel pump, activate it enough times (it will pump a little each time you turn on the ignition) to fill those bowls. A fuel level check may be in order here. You can back out the drain screws to make sure fuel is there.
-Make sure your butterfly valves are a little bit open. If a butterfly valve is completely closed that cylinder won't fire on start-up, or idle.
-Set your pilot screws at around 3 turns out. See warning below!
-If you have any doubts about ignition, check your spark. You've got to have it to run.
In summary, to tune carbs, you've got to work through any problems to the point where the engine will run and idle on both cylinders.
* Synching Carbs At Idle: You will first synch the carbs at idle. This is done by means of the two screw adjusters ("throttle stop screws") on the sides of you carbs. The left one is a set screw, and the right one is a thumb wheel screw. Loosen the retaining nut on the set screw so it can be turned. Working these two screws, set you carbs to where the mercury is even in the towers with the idel at 1000 rpm on your tach. Leave the retaining nut loose for now.
* Adjusting Your Pilot Screws: These screws control the mixture in your pilot circuit, which controls off-throttle running (idle and deceleration) and running when the throttle is cracked open just a bit. The screws control fuel , not air! You turn right (in) for less fuel (leaner) and left (out) for more fuel (richer).
WARNING! Never bottom these screws hard. You can injure the seats or the screws, making good adjustments impossible. If adjustments don't seem to get results, this may be your legacy from an earlier tinkerer. Very likely your pilot circuits are plugged.
These screws came set very lean from the factory (EPA stuff) and what we want to do is set them somewhat richer to help reduce warm-up time, improve low-end running, and inhibit a tendency to backfire.
With the engine idling, turn one screw in until the engine begins to slow due to fuel starvation in that cylinder. Then back out say 1 and 1/2 turns or until the engine runs its fastest and smoothest. Then do the other side. This should get you close. If you chose, you can try doing one cylinder at a time, that is, if the engine will run on the other cylinder. Pull one plug out and be sure to ground the plug wire you pull off. Ungrounded plug wires on running engines are hard on coils. You are searching for a smooth, strong idle with each cylinder firing nicely. Once this has been achieved, you'll then want to recheck your rpms at idle and your synch and readjust if necessary. When you have decided that your idle is solid, tighten the left side retaining nut on the set screw. If you experience backfiriing, "spitting" through the carbs, or other signs of leaness, back the pilot screws out a tad more.
Note that some instructions will recommend a setting of "so many turns out". In my experience turns will vary between bikes and between carbs on the same bike. I think these screws are best set by sensing/measuring what the engine needs.
If you decide to use a Colortune Plug, just follow directions. Orange is rich, and when I was using them I would adjust until the burn showed orange and then slowly turn in to where the burn just turned blue. That, I figured would be on the rich end of "good".
If you have access to, or own a CO meter, you are looking for around 3% CO on a fully warm engine at idle. If you have aftermarket pipes that come from each cylinder you can check CO with a probe shoved into each these exhaust pipes by turn. (A local auto shop maybe able to help here and give you a reading.) But note that if you have a stock exhaust, this is not possible because both headers exit into a common collector box and the exhaust from each cylinder is mixed up with the other before flowing out who-knows-which pipe. There are ports in the headers where exhaust gasses can be read, but you'll have to make adapters to access them. The adapters should be long enough to stay cool, or your hoses will start smoking. These ports are plugged with 5mm bolts, so an adapter can be made by drilling a hole in a 5mm bolt. You can then attach a pipe in some wayto extend the adapter out.
Note that backfiring on deceleration is typically the result of too lean a pilot mixture, although air leaks can also contribute. This causes "lean misfires" and resulting back fires. As noted above, if you experience this, try backing out your pilot screws a tad more and see if this helps. You can also check for vacuum, manifold, and exhaust pipe air leaks.
* Synching Carbs At 3-4000 rpm: Once you've got your idle squared away to your satisfaction, you now need to synch your carbs at higher rpms, where you will do your riding. This synch is accomplished by turning the synch screw located between the carbs in the back. You'll most likely need to remove the rear carb breather pipe to get to the screw, and move the tank up out of the way a bit. I have a screw driver that goes around corners to turn this screw. Once you get to it, it is usually quite simple to rev the bike to 3-4000rpm, hold it there, and check your synch. Then let go of the throttle, make a correction, and try again. A couple of adjustments should get it. Note that on some carbs the linkage between them has taken some serious wear and has slop in it which maybe difficult to take all the way out. Due to play in this linkage, the throttle valve of your right carb (where the throttle cable connects) will always open just a tad before the left carb throttle. This maybe noticeable at tiny throttle openings where your right cylinder will hit harder than the left, but if you are synched properly at idle (using the set screws) and at higher rpms using the synch screw, this effect will go away as the throttle is opened further, and is not usually bothersome.
Note, however, that badly worn (loose) linkage between the carbs can cause problems here. If you have trouble synching at higher rpms and have to screw down heavily, recheck your synch at closed throttle to make sure that one of your little arms (left one usually) has not been lifted off its stop, disturbing your idle synch.
* Drilling Pilot Jets. An early article by Joe Minton recommended drilling out your Hitachi pilot jets to .5mm. In my experience this should not be done, in that I have always been able to achieve the CO% I wanted with the stock .41 jets. And a number of people have written me regretting this move, when they couldn't bring their richness down into line with the adjustment screws.
* Tuning the run circuit. The above procedure covers the basic tuning needed with a good running, stock Hitachi equipped Virago. If you install aftermarket pipes, at a minimum, you will want to go through the above procedure (aftermarket pipes tend to lean out the pilot circuits) but you may also need to get into "rejetting". This may involve changing the position of the needles-usually raising them to provide more fuel sooner, and possibly main jet changes to dial in the top end. Stock needles can be shimmed, thereby raising them and richening the mixture you get as the slide rises. Main jets in 2 step increments from 110 to 134 are available from K&L supply. Dynojet will sell you a kit which includes adjustable needles, main jets, and instructions. In my experience these give good results and the recommended settings are O.K. However, you may have to pull your carbs more than once to get things just right and some knowledge of speed tuning techniques, and/or help from a truly knowledgeable tuner will certainly be of benefit. If you have a shop install aftermarket pipes, make sure that they can handle the tuning chores as well. I've had many e-mails from owners who had pipes put on, only to have the bike run badly. And the dealers said, "It runs good for us", because they really didn't know how to tune it. Dynojet also sells a wide range of main jets at very resoanble prices.
Trick: Pulling carbs on and off can be a pain. Hitachi float bowls can be removed with the carbs in the bike, but it is a hassle. To make it easier, the first time you have them out, replace the screws holding on the float bowls with Allen head screws (avaialable from your local bolt store or K&L Supply). This will make it easier. Once the bowls are off you have access to the main jets, the pilot jets (on earlier Hitachis) and you have the ability to change fuel levels by tweaking the floats. What you don't have is access to the needles.
Water in float bowls: If the bike has sat a while, or has been tipped over, it is possible that water can wind up in the bottom of the float bowls. This condition raises hell with smooth running, and since the water is heavier than fuel it will just sit there and not go away. If you suspect this condition, just drain your float bowls to get rid of any water and crud that maybe in there.
RESTORING HITACHI CARBS
REMOVAL, DISASSEMBLY AND CHECKING
Hitachi carbs are growing old, and with normal use, neglect, etc. they can start to give problems. Many of these problems can be corrected, but a few are tougher due to unavailability of parts.
Before you can work on your carbs you will have to remove them from the bike. Without going into every detail, here's generally how to do it:
1. Remove/disconnect all the obvious stuff: air boots, air cleaner pods in later models, throttle cable, choke cable vacuum lines, fuel lines, etc. Pay attention to how everything connects. Take some digital pics if you want, to help you go back together correctly.
2. Now for the crux move: with a nice thin 10mm open end wrench, loosen the inner (left upper) retaining bolt on the front carbholder.
3. Loosen the clamps on the carb holders and use your strength to wrestle the carbs out of the carbholders. Now you can reach the inner (right upper) retaining bolt on the rear carbholder. Loosen it. Completely remove the long Phillips head bolts that tighten the carbholder clamps. Spread the clamp ends out of the way. Now it is easy to remove the lower carb holder retaining bolts. Once these bolts are out, slide the carbholders down slightly and remove them. They have O-rings in them, so be careful. The carbs will then come out on the left side. Drain the float bowls.
4. Note the presence (!) and condition of the O-rings in the carbholders, and make a note to replace any bad ones. Check the carbholders for excessive cracking or damage, and replace them if they are bad.
5. With the carbs out, deal with one carb at a time, so you don't get parts mixed up. If you have any doubts about how things go together, take some digital pics along the way to refer to later. To get into either carb you have to remove the upper bracket that holds them together. Note where the fuel hose clips mount on the top of the carbs. I draw a picture of these. While you are at it, remove the linkage between the choke plungers. Note how the springs set up and try not to lose the tiny "E" clips which can get away from you if you aren't careful. Now you can disassemble one of the carbs.
6. Unscrew the remaining screws in the top and remove it. Note how the spring seats in the middle of the cover. Pull out the diaphragm-slide assembly. Check the diaphragm for tears and pinholes. Check the slide for play and the needle for excessive wear. Remove the choke air tube if possible (6mm Allen plug) and expose the pilot and main air jets. Spray some silicon on the rubber diaphragm and don't leave it about. If you quit for lunch (or a nooner? No!) remount the diaphragm in its place with the top screwed down on it. If left around dry it has a tendency to shrink, making it difficult to reinstall.
7. Remove the choke plunger assembly and check the rubber tip for excessive wear.
8. Remove the float bowl and floats. Remove the drain screw from the float bowl (sometimes a problem).
9. Remove the float valve assembly (both pieces) and check as described above.
10. Remove the pilot circuit adjustment screw. If the brass plug is still there (unlikely), it must be removed first. As noted above, be VERY CAREFUL
11. Remove the main jet and then the needle jet (referred to by Yamaha as the "main nozzle") which will drop into the carburetor bore if you hold the carb upside down. Remove the pilot jet if possible.
12. Now study your carb and the diagram linked to this article and locate/learn every passage in it. Choke passages--air jet passages, fuel passages, etc. I don't have access to a "soaker", so my method of cleaning involves a combination of carb cleaner, and air. I work those passages until I'm satisfied that they are dead clear. Where a circuit has more than two openings, like the pilot circuit which has four, I'll close two with my fingers while pushing cleaning out one path at a time one path at a time. Look for passages everywhere. There is one in the float bowl that frequently clogs.
13. I modify the cleaner straws in two ways. First I file down the ends to a sharp point which will seat in small openings like the choke tube. Second I make a collar to go around them, so I can get a seal on bigger orifices like the tube that the main jet screws into. As an example of how this works, to push cleaner through the main (run) curcuit air jet--with the main jet and needle jet out--I hold my finger over the oulet in the bore, and squirt with my special straw (which seals) into the main jet tube from the bottom. The only open orifice left in the circuit is the main air jet, so I amable to push cleaner and air through that jet, even though I can't actually get to it with a straw.
To summarize, you need to understand your passages and see see carb cleaner coming out of every one.. Where there are several outlets to a circuit you need to close all outlets but one so that you can push cleaner out each outlet in turn. In carbs where you can't see the air jets, you can still see that cleaner coming out.
The tube pilot jet in Version 2 carbs is a special problem. It can get clogged way up in there at the top. Go to your friendly music store and get yourself a guitar string. A high E or a B string might work but it has to be smaller than .41 mm. (I measure them with a digital caliper). Cut it a couple of inches from the ball end. You can then use this to prob through any crud build-up in the top of the tube. Keep working the probe, and cleaner, and air, until jet orifice feels smooth and completely clear.
14. Some people will take their carbs to the shop to have them "soaked" in carb cleaner, and get good results. The manual says "no caustic solutions." Note that the carbs need to be stripped down as far as possible with all rubber pieces out when doing this. Also note that you have two rubber seals where the butterfly valve turns in the carb body which you won't be removing. Make sure your shop understands this and assure yourself that they know what they are doing and won't be hurting these seals. In general you don't want to leave carbs in the soaker to long.
I've been doing some research on cleaning solutions, but as yet haven't come up with a short recommnedation. there are some out there that claim to be gentle and will not harm rubber parts, etc. However, there is a line of reasonaing that says "The gentler they are, the less they will clean." Anyone who has found a proven solution here is more than welcome to contribute.
I've mentioned some of the checks that are needed as I've gone along. I'll try for a list here that will cover (hopefully) almost all of them.
-Check your diaphragms as noted. If you find tears or pinholing, your slides are not going to lift properly. You maybe able to do a temporary repair with super glue, or some other flexible, fuel resistant glue. Unfortunately, these diaphragm/slide assemblies have been discontinued. But recently, there have delveloped at least two sources for them that would be worth investingating (not cheap). Try:
-Check your choke plunger tips, and make sure those little air dams are clean and working. Occasionally these plungers will leak fuel causing the carb to run rich. These plungers are also discontinued. A quick test for a leaky choke plunger can be attempted by gaining access to the top of the plunger and pressing it down with a screwdriver when the engine is idling. If the idle changes, you've learned something.
-Check your needles and needle jets (nozzles) for wear. Look for "ovalling" in the needle jets. If there is wear, you are not going to be able to meter fuel correctly and will run on the rich side. Both needles and needle jets can be checked with a dial or digital caliper for ovality. Dynojet does supply new needles in their kits. I know of no source for the needle jets accept good ones in a used carb. You might talk to Factory (on the web).
-Check jet and needle numbers against the specs to make sure they are right, and in the correct carb. Sloppy mechanics get them reversed sometimes.
-Check your float valves for wear and replace them if necessary. Leaky float valves can flood your carbs and make your bike unridable. Check that the little strainer cups in the float valve bodies are clear.
-Check floats for any signs of damage or cracks.
-Look for excessive wear in the linkage between the carbs.
-Look for causes of any problems that prompted you to remove the carbs in the first place.
As a general suggestion on parts. Don't be afraid to get on the net and search hard with google or otherwise. And don't be afraid to call carb rebuilders and see what they have. See list at end of this article.
Reassembly goes in the reverse order of what you've done. Work slowly. Be careful not to over-tighten jets and small parts. Use small tools and get things "tight enough but no tighter". Use WD 40 on moving parts if you want, but not on diaphragms.. Once you've got the carb together push the slide up with your finger to make sure the spring is seated correctly and the slide goes up and down normally.
Note on setting fuel levels., Before you put the upper bracket and choke linkage back in place, I would recommend that you now deal with any fuel level problems you found earlier. Most manuals will cover this, recommending a 6mm clear fuel line, and filling your float bowls from the tank by running your engine. The problem with doing it that way (carbs in the bike) is that removing and installing carbs is a time consuming pain. But remember the "Trick" noted above. By installing allen head screws you can quite easily remove the float bowls with the carbs in the bike.
In any event, I much prefer to do this procedure on the bench. I made a fixture that allows me to put my carbs in a vice and position them in the same attitude they will be in on the bike. I then put fuel to them from a fuel pump or remote fuel supply (depending on what feeds the carbs on that bike) and do my checking and adjusting on the bench. I measure from as close to the middle of the float bowl as I can get (idle screws are in the way a little) and from the bottom of the "meniscus", the curve of the fuel in the measuring tube. You should get your levels close to spec, but I wouldn't sweat a millimeter this way or that. Higher levels mean richer running, lower levels mean leaner running. Watch for signs of "creeping" fuel levels that might indicate that the float valves are no quite sealing properly, or that the floats are taking on fuel and going "heavy". Not common, but does happen.
Sometimes during this process a perfectly good float valve may stick and cause flooding. Don't panic. Just start over and tap the carb to seat the valve as you add fuel. Speaking of fuel, it can go BOOM. Exercise proper safety precautions. If you have to adjust float levels, you'll need to empty the float bowls each time so be prepared to handle that fuel.
When you reinstall the choke linkage, check to see that both plungers are being raised roughly evenly, and bend the tangs to achieve this if necessary. There is some poor metallurgy in these brackets which bend way too easily. A touch of grease on the tangs eases matters (for a while, at least).
When mounting the carbs in the bike, remember to tighten that inner back carbholder bolt before popping the carbs back into the carb holders. With the carbs in place, you won't be able to reach it. Spray some silicon on the carb lips and carbholder lips.. I first seat the back carb, and then coax the front one in, often using a hammer handle or wood equivalent as a lever. Sometime it goes better with the front crab first. Some mechanics recommend against this practice (broken cooling fins) but I find that serious persuasion is often needed to seat those carbs, and have never had a problem. Some go in easier than others.
Note on venting tubes: Those venting tubes coming out of the carbs near the top are important. They provide air to the main and pilot air jets, and maintain open air pressure throughout the carb body (except above the diaphragm). These tubes have little filter pods on them which contain two screens with a thin layer of foam sandwiched in between them. In many of them by now that foam has turned to dust. When you pull off your diaphragms and slides upon disassembly, look for dirt/dust coming in from those tubes. If there is some, you may want to deal with those filters, that is, provide the venting system with some fresh foam (UNI has some). I'll leave the details of this project up to you. Otherwise, just blow the filters out well with air and keep going. The main requirement is that they breath freely and are not shedding particles into your carbs..
The other important point about vent tubes is that they need to exit into quiet air to where wind and other sudden changes in air pressure don't find their way in to the air jets. Older Viragos provide for this with outlets behind the left side cover, and newer ones with outlets in the pods.
CONCLUSION, OPTIONS, AND SOURCES
Well that pretty well covers my store of knowledge about Hitachi carbs. I don't know it all so check out other good sources of info. Many older Viragos are running around very happily with these carbs still in place. Since I'm in the problem solving business, I sometimes loose perspective on how good these oldsters can be.
But let's just assume you run into some serious Hitachi carb problems to the point where you decide they are shot and finished.. You do have some options:
First you can go through the rebuilding procedure and if you can find parts that you may need (diaphragms, needle jets, or choke plungers, linkage, etc.) you may come out O.K. If you need discontinued parts, a diligent search throughout the salvage world may find some, or check the sources below Try to always get a used carb for parts that is the same one that is on your bike. Don't try 920 parts in a 750. Some of the differences may be obvious, but some, like diaphragm springs, may not be. Also beware of the "carb rebuild kits" that are offered for Hitachis. They have some usable stuff in them--float valves, drain screws, gaskets, and mayber pilot screws--but they don't have the hard to find parts noted above--the ones you may really need..
Or find a set of good used carbs.
Or e-mail Mike Belluomo for help at mbelluomo@BGProd.com. Mike specializes in rebuilding Hitachi carbs and maybe able to revive yours with a thorough cleaning, and some good used parts, etc. Mike also has a website http://www.geocities.com/mike33div/My_Bike_Page.html
Or get in touch with motorcyclecarbs.com. These folks know Hitachi carbs really well and have developed a source for new diaphragm/slide assemblies. Not cheap, but it is a new option that I wasn't aware of.
Or convert to a set of the Mikunis used from 1988 on. 750 carbs should work well straight across in older 750's, and later 1100 carbs might work without much rejetting on 920's (never tried it) and on 1000's. Some rejetting may be required to get top performance. If you have the early Version 1 750 or 920 Hitachis, you'll need new carb holders since the lips changed around 1983. Mikunis will bolt right up, since the dimensions of Virago engine did not change, but there will be some tinkering and adapting with throttle cables, venting tubes, etc. to get it all to work.
Or see about adapting some aftermarket carbs like Mikuni TM series 40mm pumpers (accelerator pumps). Not a cheap or easy alternative! Mainly for the project/performance minded.
Or consider a KJS single carb conversion. (Check them out at kjsmotorcycleworks.com) They take your old carbs and send you back one that they have reworked, plus their single carb manifold. I must add that reviews on these kits are mixed. Many people like them, but a few have had problems..
So there are some alternatives, and all may not be lost if your carbs go South on you.
Many parts are available, and the situation is actually getting better. K&L Supply offers Hitachi style main jets, float valve assemblies, and the "rebuild kits" that I mentioned earlier. K&L does not sell direct, so you need to see a dealer for this stuff. But motorcyclecarbs.com maybe the best place to go for hard to find parts and of course you can order over the phone, or on the web. Also, as noted, Dynojet offers jet kits which contain needles and main jets.
Finally, I don't know everything. Corrections and comments are welcome and will be incorporated in this paper if they have merit.