These are bits of wisdom I have gleaned from posts on the GS Resources Forums. The notes in [brackets] are mine. -BassCliff

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On this page:

Bench Sync Your CV Carbs
Morgan Colortune/Carbtune Tips
Overflowing Carbs
Removing Float Pins
Mikuni Motorcycle Carb Theory 101

Carb component ranges-throttle openings

Bench Sync Your CV Carbs
By Mr. Steve (Woodin)

To bench sync the carbs, you do need to have the rack assembled, but not on the bike. Open the master idle speed adjuster until carb #3 has a gap in the throttle plate that is large enough to put something in. That 'something' needs to be small, like a paper clip. Adjust the idle speed adjuster so there is just a light drag on the paper clip, much like you do when checking the valves. You will see adjuster mechanisms between the carbs, so loosen the locknut, adjust the screw so you have the same amount of drag on carb #2, then carb #1. After doing those two (in that order), do carb #4. Now adjust the master idle screw out to close the throttles back down. I like to close them completely, then back in about a turn or two. When you start the bike, be ready to hit the kill switch if the idle speed is still set too high and the engine races when it starts. Back the idle speed screw out some more and start the engine again. When you have the engine warmed up and are ready to do the vacuum sync, use the same order of adjustment.

Notes from other members:

From Mr. KiwiGS

  1. Assemble carbs [in the rack like they are] ready to go on the bike.

  2. Set air screws 1 1/2 turns from bottom [from lightly seated].

  3. Determine master carb [#3]. The one that the throttle cable connects to right?

  4. Put a small drill bit 1.5mm under the butterfly of the master carb.

  5. Adjust the main idle screw until the butterfly just touches the drill bit.

  6. Adjust the sync screws until all of the butterflys are the same.

  7. Check the master carb again.

  8. Reset the idle screw.

Additional comments from:

Mr. chef1366: I use a flexible feeler gauge [instead of a drill bit]. #3 is the master carb.

Mr. Steve: Start at 2 full turns out and use a smaller gauge. I have had a couple sets of carbs that would not open to 1.5 mm, so I use a straightened-out paper clip. The larger ones I have available at home are 1.05 mm in diameter, the 'normal' size clips are 0.88 mm. Just be sure that there are no burrs on the end of the clip that will scratch the throat of the carb.  Also, don't forget to re-set the idle speed before starting the bike. With the throttle open far enough to do the bench sync, revs will tend to skyrocket when started.

Ms. SqDancerLynn1: The only thing I would add is to make sure the #3 throttle blade is completely closed with the idle screw backed out. You never know what was done in the past.

Mr. Steve (again): Ms. Lynn makes a good point. What I tend to do when finished is to back off the idle screw until there is a gap in the adjuster, then shine a light through all the carb throats to verify that they all are closed. Then do a visual sync as I turn the idle adjuster until it touches, then about one turn more.

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Morgon Colortune/Carbtune Tips - originally posted by Mr. LarryD with notes from Mr. bwringer.

For vacuum syncing your carbs on the bike, please see Mr. bwringer's excellent Syncronize Carburetors guide using the Morgan Carb Tune.

Think you might have worn jets and needles? Click below for pictures and information.

Overflowing Carbs
by Mr. bwringer  
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Let me point out the obvious: If a carb (or carbs) are overflowing when the bike is not running, you have TWO problems. First, your petcock should prevent ANY flow; second, the needle valve should allow flow only up to a fixed height in the bowl (ie, not overflow). Excess fuel has to make its way past both of these parts.

(A third possibility on old carbs is for fuel to flow past the inlet seat O-ring. [n/a for VM]. That doesn't apply to newly rebuilt carbs, since the owner has just installed brand new. Right??)

I'm not 100% sure why some needle valves leak and others don't, but I'm starting to lean with Keith Kraus' suggestion, which is to use only OEM valves and seats. Simply because it works. The question why is a separate matter. (I have a theory, but no guinea pig to test it on at present).

For the petcock, you'll hear a number of suggestions. I think that the vacuum operated petcock should be kept operational, for a few safety reasons.

Several of the folks here are convinced that petcock rebuild kits are useless, and your only real option is to buy a new OEM petcock.

For now, for people who are 100% sick of carb overflow & the ensuing problems, the consensus then is to go with OEM petcock, seats & needles. it's expensive, but it seems to be the only certain way to correct this BS once and for all.

(I would add: no doubt there are people whose overflow problems began when they installed carb rebuild kit(s). The machining on the parts is often abhorrent and inexcusably amateurish, and the f&^%#ing O-rings don't fit correctly. The consensus for a while now has been to buy gaskets IF you need them, valve seats & needles IF you need them, and an O-ring kit.)

The petcock does not, in any way, REGULATE the rate of flow. It should be either on or off. If you have overflow on a running bike (and it's not inhaling fuel through the petcock diaphragm via the petcock vacuum line) then there's a problem with the inlet valve.


1) the float is no longer buoyant enough or is badly adjusted, or

2)the valve is bad, or is being held open by some foreign matter.

And finally is the standard BWRINGER's lecture:

One of the secrets of the GS850 engine and carburetion is its outstanding low-speed manners. This, in turn, allows you to much more easily exploit the corners of its handling envelope, which leads to more giddy fun than you've ever had with your clothes on. As most racers eventually find out, smooth is fast.

If you had a freer-flowing aftermarket exhaust, a good set of K&N pods, a dynamometer, exhaust gas analyzers, a degree in chemical engineering, no neighbors to bother, a few weeks off work, a large box of Keihin jets, and several fifths of good tequila so you can sleep at night, you might be able to extract a few more horsepower from the 850 engine at certain RPM. And if you're really good, you might be able to get the low end half as smooth as it was stock. Maybe.

With the stock exhaust, forget it -- all you're going to do is screw up everything else quite badly to get maybe 5 more horsepower at 7,500 rpm or something like that. These things have a certain balance, and more intake flow must be balanced by more exhaust flow.

Additional Petcock Notes: (by Mr. tkent02)

There are two paths for fuel to get through the petcock into the crankcase or onto the floor:

One path is a holed diaphragm. Fuel flows down the vacuum line into the #2 carburetor (#3 for VM carbs), and into the intake port. The petcock will still function normally as far as shutting off the flow of fuel when the engine is off, and unless the hole is very big, it will still turn on the flow of fuel when the engine is running. To test for this, check for fuel in the vacuum line. You should be able to suck on the vacuum hose and not get any fuel, none whatsoever, if the diaphragm is good. If you see or taste fuel the diaphragm is leaking.

The second path is by the rubber O-ring in the vacuum valve being damaged, or it's just too old and not sealing properly. In this case the fuel cannot be shut off completely. It just leaks all the time. When it is on the bike, with a full tank, and with fuel in the line to the carburetor, there is a vertical drop of over a foot. This creates quite a syphon effect trying to pull fuel though the leaky O-ring. If the petcock can't seal perfectly, fuel will flow slowly the entire time the bike is shut off until the tank is emptied into the crankcase. If you take the hose off to look, you lose the vacuum of the syphon. It is possible that it won't leak while you are watching it. A good test is to run a fuel line from the petcock down a few feet to a suitable container, go to prime to fill the hose, move it back to ON, mark the fuel level in the container, and go away for a while. If you come back later, or tomorrow and there is more fuel in the container than there was previously, you have a slow leak from the petcock. It takes only a few drops a minute to put a gallon or so into your oil over time. It's very hard to detect a slow leak just by looking at it.

The VM carbs on 1979 and earlier GS bikes have overflow tubes under the float bowls which should just dump any overflowing fuel onto the ground under the bike. These can clog up or be routed wrong. But if they work properly it should prevent fuel from going into the crankcase. The CV carbs on the 1980 and later GSes have no such overflow hoses, this fuel is going into the engine, period.

If you want a decent, reliable motorcycle, just skip all the testing and buy a new petcock. The old one did its job for thirty years. Get a new one and forget about it for another thirty.

Removing Float Pins - by Mr. Nessism   Back to top of page

Due to an interference fit between the float pivot pin and the support post, the post can snap off when driving out the pin. The interference fit is only on one side of the pin, next to the nail head detail. You need to support the float post before driving out the pin. Hopefully the photos are self explanatory.

Removing float pin

Support the float pin with a small socket or similar.

Using a small socket for support.

Use a punch to carefully remove the float pin.

Carefully punch out the float pin.

Mikuni Motorcycle Carb Theory 101

Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. Read this webpage for explanations that even I can understand.

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