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Syncing my carbs

  • Thread starter Thread starter Acooljt
  • Start date Start date
A

Acooljt

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OK, got a 79 GS850G. My Haynes manual says isn't real specific on how to sync them with a dial gauge, only with the Suzuki gauge. Well I've got an engine vacuum dial gauge, but they don't specify what to set them at. It just says quote:

"Select as a datum the carb which shows the central reading."

Well what does that mean? What exactly is "the central reading"?

Also, do I plug the other 3 carbs while adjusting 1 or do I leave them all open, the Haynes manual doesn't specify on that either(if just says hook up 1 hose to each carb).

Thanks a million guys!

P.S. I'm not even sure it's the carbs that need resyncing. Here's my problem. The bike seems to pop a lot, specifically I notice it more at low RPM's(like right after I shift). My dad said he also thought he heard it missing on one cylinder but that has yet to be confirmed because I can't hear it(though I'm partially deaf so who knows). Any help?
 
jt, a lot of problems can be resolved with well tuned carbs. There are much more knowledgeable people here who will advise you, too. Can't speak for the Haynes manual, but the CLYMER's manual for my '80 GS1100E shows the procedure for syncing the carbs with a mercury sync tool. Carb #3 can't be adjusted, as it is the master. The other's are synced to #3. No specific "value" is given, just a pattern of what properly sync'ed carbs look like. It's kinda hard to describe, but carbs 2 & 3 should be equal, and carbs 1 & 4 should be equal but slightly higher than 2 & 3. Get it? I imagine if you are using gauges, the same rule would apply. Carbs 2 & 3 sould have about the same reading, and 1 & 4 should be equal & slightly higher.

Good luck!
 
I hope my info is correct for your year please double check first!

vacuum reading will vary with throttle openings don't worry to much about the level just that they are even. and balance the carbs at 1,500~2,000 rpms

something you may consider doing rather than just swapping your gauge from cylinder to cylinder while its running...

http://i32.photobucket.com/albums/d44/rustybronco/vacuumgauge005.jpg
http://i32.photobucket.com/albums/d44/rustybronco/vacuumgauge009.jpg
http://i32.photobucket.com/albums/d44/rustybronco/vacuumgauge007.jpg

the adaptors are 5mm bolts with a hole drilled through them (did mine by eye with a 1/6" drill bit no drill press was used) and then a larger hole drilled into the head (not to deep) to accept a vacuum nipple that was epoxed to it and an o-ring over the threads to seal it when you hand tighten it in your head or carb adaptors.

you open each valve (one at a time) until the vacuum gage needle just flutters, start the balancing at the number 3 cylinder remember the reading, close that valve and open number 4 valve until it just flutters and adjust number 4 to the reading of number 3 cyl, close that valve then repeat with number 2 then number one, you get the picture.

remove the fuel tank get or make a aux. fuel tank to supply fuel when running the engine and keep a fire extinguisher handy just in case
one more thing... keep two fans directed at the running engine.
 
Last edited:
Acooljt said:
Also, do I plug the other 3 carbs while adjusting 1 or do I leave them all open, the Haynes manual doesn't specify on that either(if just says hook up 1 hose to each carb).
Click to expand...
only one port is connected to your vacuum gauge at one time the others are capped .
 
Tom MLC. It's kinda hard to describe said:
I think this was required due to the flow characteristics of the stock exhaust system. AFAIK, you can have equal readings across the board if running an aftermarket 4X1.

Tony.
Click to expand...
 
Just remember, on your VM carbs, adjusting vacuum at one carb can/will effect the others. You should buy a vacuum gauge that connects to all four cylinders so you can see the levels all at the same time and your synch will be more accurate. One cylinder at a time is wrong in my opinion.
Be sure the valve clearances and ignition timing are correct first.
Bench synch as best you can if the carbs are off or if you rebuilt them/disturbed the jet needles. Make sure you have no intake leaks.
Then warm up the bike fully and place the bike on the centerstand. Then adjust your side air screws, one at a time, to achieve the highest rpm, using 1,000/1,100 rpm as a base. This assumes your pilot fuel screws (underneath) are already set correctly.
Now hook up the tool and vacuum synch. Use two larger fans.
Observe the initial levels at about 3,000 rpm. If you bench synched well, they'll be in the ballpark. Simply lower the "high" level(s) to match the lower levels. Easier that way. Don't over-tighten the adjuster screw holder nuts. 3.5 ft/lb is factory torque.
After the synch, fine tune the side air screws again for highest rpm. You may/may not notice any change.
 
If you still have a "pop", that word is hard to diagnose.
Popping, can be a lean or rich condition and/or the spark is being interrupted. Need more detail if it still does it after the synch. Plug reads help too.
 
Lots of good advice above.

Make SURE that you plug your vacuum line into carb #3 when you do the synch. I made a longer gas line out of rubber hose, have my gas tank on my work bench and put the bike on "Prime".

I have a 4 vacuum gauge set that I like. A guage for each cylinder. Its impossible to do with just one guage in my opinion (although I've never tried it).

However, on my '79 when the carbs are synched up right, my vacuum readings are right at 10 inch pounds of vacuum.

Use a couple of good fans too! don't let it get to hot!
 
Tom MLC said:
It's kinda hard to describe, but carbs 2 & 3 should be equal, and carbs 1 & 4 should be equal but slightly higher than 2 & 3. Get it? I imagine if you are using gauges, the same rule would apply. Carbs 2 & 3 sould have about the same reading, and 1 & 4 should be equal & slightly higher.
Click to expand...

Whoa, buck! This info is not correct. If the '79 850's exhaust is "stock", the VM carbs should all be adjusted to the level of Carb #3. And # 1 and 4 should be the same level as #2 & 3.

The '80 and later models had the CV carbs with the crossover chamber exhaust between #2 & 3. The crossover chamber is what causes lower vacium on carbs #2 & 3.

If you've got exhaust with no crossovers, adjust 'em all to the same level!
 
KEITH KRAUSE said:
You should buy a vacuum gauge that connects to all four cylinders so you can see the levels all at the same time and your synch will be more accurate. One cylinder at a time is wrong in my opinion.
Click to expand...
My opinion, it takes a little longer but the last one i did was at 9 inches across the board.
 
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