Synchronizing Carbs

[vur]

I suspect my carb for #4 cylinder (far right) is out of sync with the rest. #4 cylinder runs cooler than the other three cylinders at idle and the spark plug is carboned up (black) with a white deposit right at the tip of the electrode. Compression check when hot is marginally above the service limit of 100 psi (#1 - 114, #2 - 111, #3 - 103, #4 - 107).

Additional information - the bike idles fine (though slightly erratic when using the choke when cold), revs without stumbling, but does have a slight surge at cruising speed.

I do not have a carb balancer, but given #4 cylinder seems to be the biggest offender I thought I would just adjust the throttle valve adjustment screw on #4 slightly to open the throttle a little and reassess the exhaust temps / plug condition.

Does anyone know how sensitive the adjustment screw for the throttle valve is? i.e. should I make a 1/2 turn or 1 turn adjustment? FYI it is a '84 GS750EF with the CV carbs.

I realize that #1 and #4 throttle valves are set with a slightly higher vacuum reading at 1750 rpm per my Suzuki service manual (and that the normal sync order is #2, #1 than #4)

 

[Nessism]

Get a proper sync gauge and do the job right. I suggest going though the carbs first and adjusting the valves as well. The carb O-rings get old and hard, including the O-rings on the intake boots, so it's a good idea to reset the clock on these parts so you can be sure they are not contributing to the running problems.

 

[Macmatic]

I suspect my carb for #4 cylinder (far right) is out of sync with the rest. #4 cylinder runs cooler than the other three cylinders at idle and the spark plug is carboned up (black) with a white deposit right at the tip of the electrode. Compression check when hot is marginally above the service limit of 100 psi (#1 - 114, #2 - 111, #3 - 103, #4 - 107).

Additional information - the bike idles fine (though slightly erratic when using the choke when cold), revs without stumbling, but does have a slight surge at cruising speed.

I do not have a carb balancer, but given #4 cylinder seems to be the biggest offender I thought I would just adjust the throttle valve adjustment screw on #4 slightly to open the throttle a little and reassess the exhaust temps / plug condition.

Does anyone know how sensitive the adjustment screw for the throttle valve is? i.e. should I make a 1/2 turn or 1 turn adjustment? FYI it is a '84 GS750EF with the CV carbs.

I realize that #1 and #4 throttle valves are set with a slightly higher vacuum reading at 1750 rpm per my Suzuki service manual (and that the normal sync order is #2, #1 than #4)

1/2 a turn or a whole turn is probably going to throw you totally out of whack! The cost of a Morgan Carbtune II is well worth it the first time you use it. I'm not sure what it works out to in $C right now but I think it was around $85 USD last time I looked.

My bike is mostly '84 Canadian GS750EF also, wanna sell those ugly ol fairings?

Seriously now, you wouldn't happen to have a spare 1/4 turn fair fastener would you? Can seem to get one in the US.

/\/\ac

 

[vur]

Carb boots are good and valve clearances are in spec (all at set at 0.005" when cold).

Yes, I agree, a proper sync tool would be ideal. The bike for the most part runs flawlessly except for a slight surge when cruising. If I had not inspected the spark plugs I would not have suspected anything significant...

Thanks for your input.

 

[vur]

Sorry Mac,

I like my fairings. An no, I do not have any spare fasteners.

VUR

 

[trippivot]

perfect sync might not get the exhaust temperatures the same. (2 HG per cylinder + or - is a acceptable tolerance window) I suggest using the mixture screws as far as possible to get a good plug color. if that is not possible thhheennnnn there is another advanced tunning alternative

I do not like to have the carbs adjusted differently but sometimes it is necessary :ie:
needles at different clip levels.
different pilot jet sizes.

to achieve the exact tune you are searching for realize that different cylinders will have different requirements especially if you are looking to get E.G.T. exactly the same. we have multi cylinders and multi carbs each are individual to it's own set.

if that #4 dosen't come around with adjustments only-- a smaller pilot jet on that cylinder alone might be your trick. just short of total engine,carb rebuild - all new rubber . new floats etc..... these are 30 YO bikes and parts.

like I said I do not like this but if you have 1 problem cylinder and all 4 are tuned the same. you have tried everything. Logic states then that 1 cylinder will need special attention. to hell what everyone thinks.

tunning is a black art sometimes. keep it secret .

 

[Steve]

.. valve clearances are in spec (all at set at 0.005" when cold).

If your valves are set at 0.005", then they are NOT in spec.

This is why you have to pay special attention to measurements, especially when dealing with valve clearances. Specified clearances are 0.03-0.08mm. In "inch" measurements, that is real close to 0.0012-0.003". Notice that both sets have a "3" in them, but one is the MAXIMUM, the other is the MINIMUM. If your valves are actually set at 0.005" as you say, they are too far open. However, if they are actually at 0.05mm, they are happily mid-range.

.

 

[vur]

Specified clearances are 0.03-0.08mm. In "inch" measurements, that is real close to 0.0012-0.003". Notice that both sets have a "3" in them, but one is the MAXIMUM, the other is the MINIMUM. If your valves are actually set at 0.005" as you say, they are too far open.
.

My Suzuki service manual (GS750E/ES) specifically specifies "Valve clearance (when cold) 0.09 to 0.13 mm", which converts to 0.0035 to 0.005 inches. I think I am OK... No???

 

[Nessism]

My Suzuki service manual (GS750E/ES) specifically specifies "Valve clearance (when cold) 0.09 to 0.13 mm", which converts to 0.0035 to 0.005 inches. I think I am OK... No???

Yes, you are fine. Steve's brain must be stuck in 2V mode, not 4V.

Regarding the surging issue, it's common with those 3rd generation 700/750's. Try shimming the needles some...after you vacuum sync the carbs if the problem doesn't go away.