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Differences between VM & BS carbs

  • Thread starter Thread starter Anonymous
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Anonymous

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I have checked out the article on in the Garage section here about rebuilding the Mikuni 'BS' carbs and it is very informational. But as I took apart my first carb on my '79 VM carbs I see that there are quite a few differences in the details even though the basic principles of the two are the same. Is there anything that I should pay particular attention to when I'm cleaning these things? I got myself an o-ring kit from our buddy over at cycleorings.com.

Details that I've noticed so far.

Can I soak the float bowl needle valve? There is some kind of spring mechanism in there. Any rubber in there that I need to be aware of?

Do I need to fish out any o-rings from any orifices?

There was no float bowl gasket. I think this might be a bad sign to the upkeep from the previous owner.

Any advice would be appreciated.
 
I just finished rebuilding the ones on my 78 750 which are very similar to yours.

The needle valve can be soaked and/or sprayed with carb cleaner. There is no rubber or plastic inside.

The o-rings are all on parts that you remove - nothing is hidden
- drain plug
- side pilot air screw
- bottom pilot fuel screw
- main jet holder
- choke plunger assembly

There may also be o-rings on the lines between the carbs where the gas flows. On some carbs the tee and other 2 lines are rubber coated with raised o-ring-like ridges on them. Others are just metal with o-rings.

There are plastic spacers in the slide assemblies which should be removed before soaking or spraying.

I'm surprised that it wasn't leaking if there was no bowl gasket. There is a small raised ridge cast into the bowl. I suppose if the bowl were screwed on tight enough it might seal, but, like you said, it's not a good indicator of the care the bike received from the pervious owner.

There is another thread - http://www.thegsresources.com/_forum/viewtopic.php?t=19145 that has some good info.

Good luck.
 
I would not be surprised if it was leaking before, but since the bike was not running when I bought it I can't say. The carbs actually were not on the bike when I bought it. So who knows. We'll see if I can get this thing started or not.

Thanks a ton for the info. Seems like all I have to do now is clean everything and put it back together. And hopefully it will run.
 
Pauls advice is good. Let me add: keep a record of how far out the pilot fuel screws (underneath) are, you may need it later to fine tune. Do a good job of bench synching the carbs and your vacuum tool synch will go much quicker and easier. Be SURE to vacuum synch these carbs to avoid mixture problems. Replace the manifold o-rings and apply some hi-temp bearing grease to help them last and torque to 6 ft/lb. I recommend replacing the manifold bolts with Allens.
Any other questions, be sure to ask. :)
 
Are all the pilot fuel screws supposed to be adjusted to the same position? I already took the first screw out without checking the position. But I have three more to check.
 
The side pilot air screws and the bottom pilot fuel screws are originally set at the factory and tuned specifically for each cylinder. If you're not sure where they were set, a good starting point is 1 1/2 turns out from the bottom for the side pilot air screw, and 1 turn out for the bottom pilot fuel screw. That should be good enough to get it started.

Once it is started and warmed up, set the idle for 1000-1100, then slowly turn one of the side pilot air screws in and out to achieve maximum idle. Reset the idle back to 1000-1100 and adjust the next one.

Then sync the carbs.

The bottom pilot fuel screws are fine tuned by reading the plugs. Turning them out increases fuel flow and richens mixture. Turning them in decreases fuel flow and leans the mixture.
 
It's common to find the pilot fuel screws adjusted differently for each cylinder. Be sure to only seat them LIGHTLY. The sharp tip can break off in the carb body.
The side air screws are adjusted to achieve the highest rpm, then you reset the idle by using the idle adjuster knob.
 
OK so this first carb I have already taken out the one pilot fuel screw. Should I start it off at 1.5 turns out when I put it back together? And then someone said that this screw is tuned by the plug color? That could be a pain. But it shouldn't be too bad to pull off the float bowl after the carbs are back on the bike.

Thanks a ton for the advice. I've been on the road this weekend, and I can't wait to get back home to get these things done!!
 
I would initially set the pilot fuel screw (underneath) at 1 turn out, for a stock airbox. Trying to get an accurate plug read can be difficult with the pilot circuit. The carb vacuum levels can jump around at idle/closed/near closed throttle positions and influence any reads. If the performance is good (no slow warm ups or "spitting" out the carb/too lean... or temporary fouling, especially with the choke on/too rich) then you should be OK. The screws are sensitive to adjustments.
I ride the bike in 4th gear at a steady 35 mph or close for a couple of miles and chop the bike off. You have needle circuit overlap at this minimal throttle position, but it works well enough for me.
You DO NOT have to remove the bowls to access the pilot fuel screw. You just need a short screwdriver, a light, and a mirror to see well. Be sure you turn the screw the correct way. It's easy to confuse and turn it the wrong way.
 
Spray out with carb cleaner and compressed air. I don't have an air tank. Can you simply use the compressed air like you use to clean a computer key board? The computer compressed air is not really air and can leave a residue. Just want to make sure that it's OK.

And, where's the best place to get bowl and top cover gaskets?
 
I use compressed air, usually around 150 psi. I've never heard of "computer compressed air".
I buy the gaskets at my local Suzuki dealer. I'm sure you can get 'em at Bikebandit, Dennis Kirk, Parts'nmore, etc.
 
I don't think there would be a problem using cans of 'compressed air' that are made for blowing out computer innards and keyboards, but you might go through a lot of them. With 4 carbs and all those parts...

I don't know what I'd do without my compressor. I use it all the time, mostly for cleaning stuff, but also for pumping up tires, drying off parts, and the occational sand blasting. If you're going to be doing a lot of work on your bike (and/or cars) you ought to look into getting one and a few accessories. Even a small, cheap one from Sears should last a long time and you will find more and more uses for it.
 
Lots of good info given in the comments and links provided; I'd like to emphasize one thing I ran into in cleaning my 750's VM carbs. There's one passage I found to be particularly difficult to get open. Others have told me they've had no problems with theirs, but I'd dipped my carb bodies in the gallon can of carb cleaner, some twice, and still these passages were totally clogged in all 4 of my carbs. Blowing with the spray type carb cleaner, even after dipping, didn't clear it, either. The passage I'm talking about is the one near the front right corner of the float bowl. There is a tiny passage between the opening at the bottom of the float bowl and the cylinder at the edge of the float bowl. I had to use a tool made from a wire brush bristle, as shown in the carb cleaning series here at the Resources; I worked it in the 'edge cylinder'. I then followed with the spray can of carb cleaner with the plastic tube attached. When you can stick the tube in the 'edge cylinder' and see spray shoot out the opening at the bottom of the bowl, you know it's open. Careful, if it's not yet open when you spray, you'll get a strong back spray; you don't want that stuff in your eyes!
 
Sounds like you're talking about the starter tube/pipe and it's feed.
The choke won't work if this is blocked. I also always say if you spray cleaner or air into a passage, it has to exit somewhere. If it doesn't, it's not clean. Even then, it's best to do some extra cleaning to be sure the passage is 100% clear.
 
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