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sync problems!

  • Thread starter Thread starter tconroy
  • Start date Start date
So what is the recommended cm HG setting for a 77 GS750

So what is the recommended cm HG setting for a 77 GS750

I have just finished rebulding the carbs on my 77 GS750 and am ready to vacuum sync the carbs. ..... but my shop manual refers to a OEM Sukuki vacuum gage.

Question - what is the recommended cm HG setting? 12? 14?

I will be using a Carbtune Pro. Anyone else use a Carbtune Pro?

Comments/Suggestions?

Thanks

Dom
 
ddominik said:
I have just finished rebulding the carbs on my 77 GS750 and am ready to vacuum sync the carbs. ..... but my shop manual refers to a OEM Sukuki vacuum gage.

Question - what is the recommended cm HG setting? 12? 14?

I will be using a Carbtune Pro. Anyone else use a Carbtune Pro?

Comments/Suggestions?

Thanks

Dom
Click to expand...

There is no recommended vacuum setting except that they should all be equal (or inside-outside equal with CV carbs). The _amount_ of vacuum makes no difference; the _relation_ of vacuum between the cyls is the important thing. Make sense?
 
I just took the time to read through all the above posts and was amazed at all the misleading information. :shock:
The job can be intimidating enough without adding to the confusion.

Tconroy is apparently doing a carb sync on an '83 650L. \\:D/

Dave8338 says "About 4 feet of 5/16 should do the trick, make sure that the petcock is in the PRIME position..."
Length of hose is irrelevant, it only needs to be long enough to run from your temporary tank to the carbs. PRIME position is only necessary if you do not also run a vacuum line to operate the petcock. If you do NOT run a vacuum line, plug the port on carb #2.

Rustybronco - I am glad the gear lube bottle works for you, but I am concerned about an open hole that would leak if it fell. And, just how do you hang the bottle upside-down from the handlebars? My temporary tank is only slightly less dangerous. 8-[ I use a one-gallon oil container with a screw-on lid. I have drilled a hole in the lid so a fuel line is very snug going through it. By laying the jug on its side, fuel flows nicely, even without a vent. Simply turn it upright and fuel flow stops. Easy enough to drain back into the tank and store for next time.

Bradleymaynar - I have recently learned (by reading the factory manual) that carb sync is to be performed at 1750 rpm. I have always done it at idle, 1100 +/-100 rpm.

Tconroy - vacuum at 4 grand may not be enough to suck the mercury up the tubes, but be careful of sudden throttle changes. THAT is when strange things happen.

Bradleymaynar - Nice idea about leaving the adapters on and covering them with rubber caps. If that is your only bike. Some of us have more than one bike, so we have to remove them. If they fit snugly, the chance of losing them while riding should be minimal. After all, they are being sucked into place by vacuum.

Tconroy - Not sure what book you are using, but you are confusing measurement terms. The lines on your sync gauge are not cFm, they are just cm. That's centimeters. The lines are 2 cm apart. The actual value of the vacuum is not important, it might be 8 or it might be 12. What matters is that the outer two are .5 cm, that's one-half centimeter, or 1/4 the distance between the lines, higher than the inner two.

Earlfor - the vacuum vent on #3 is only on the earlier VM carbs. Tconroy is working on an '83 which should have BS carbs. Remember, carbs (and cylinders) are numbered from clutch hand to throttle hand.

Ddominik - Actual values are not important. Mercury gauges are actually more accurate than the factory gauge since there is no way for them to become uncalibrated. See my comment just above to Tconroy about values. Since you have the older VM carbs, note that your vacuum port to the petcock is on #3.


Not trying to sound "holier than thou", but there seems to be a lot of confusion out there. Just trying to keep it simple. 8-[

.
 
Steve said:
I just took the time to read through all the above posts and was amazed at all the misleading information.
The job can be intimidating enough without adding to the confusion.

Tconroy is apparently doing a carb sync on an '83 650L.

Dave8338 says "About 4 feet of 5/16 should do the trick, make sure that the petcock is in the PRIME position..."
Length of hose is irrelevant, it only needs to be long enough to run from your temporary tank to the carbs. PRIME position is only necessary if you do not also run a vacuum line to operate the petcock. If you do NOT run a vacuum line, plug the port on carb #2.

Rustybronco - I am glad the gear lube bottle works for you, but I am concerned about an open hole that would leak if it fell. And, just how do you hang the bottle upside-down from the handlebars? My temporary tank is only slightly less dangerous. 8-[ I use a one-gallon oil container with a screw-on lid. I have drilled a hole in the lid so a fuel line is very snug going through it. By laying the jug on its side, fuel flows nicely, even without a vent. Simply turn it upright and fuel flow stops. Easy enough to drain back into the tank and store for next time.

Bradleymaynar - I have recently learned (by reading the factory manual) that carb sync is to be performed at 1750 rpm. I have always done it at idle, 1100 +/-100 rpm.

Tconroy - vacuum at 4 grand may not be enough to suck the mercury up the tubes, but be careful of sudden throttle changes. THAT is when strange things happen.

Bradleymaynar - Nice idea about leaving the adapters on and covering them with rubber caps. If that is your only bike. Some of us have more than one bike, so we have to remove them. If they fit snugly, the chance of losing them while riding should be minimal. After all, they are being sucked into place by vacuum.

Tconroy - Not sure what book you are using, but you are confusing measurement terms. The lines on your sync gauge are not cFm, they are just cm. That's centimeters. The lines are 2 cm apart. The actual value of the vacuum is not important, it might be 8 or it might be 12. What matters is that the outer two are .5 cm, that's one-half centimeter, or 1/4 the distance between the lines, higher than the inner two.

Earlfor - the vacuum vent on #3 is only on the earlier VM carbs. Tconroy is working on an '83 which should have BS carbs. Remember, carbs (and cylinders) are numbered from clutch hand to throttle hand.

Ddominik - Actual values are not important. Mercury gauges are actually more accurate than the factory gauge since there is no way for them to become uncalibrated. See my comment just above to Tconroy about values. Since you have the older VM carbs, note that your vacuum port to the petcock is on #3.


Not trying to sound "holier than thou", but there seems to be a lot of confusion out there. Just trying to keep it simple. 8-[

.
Click to expand...

...............................Ah,................NO, I'll refrain. :-\"
Tim, any questions, you can PM me.
 
Steve said:
Rustybronco - I am glad the gear lube bottle works for you, but I am concerned about an open hole that would leak if it fell. And, just how do you hang the bottle upside-down from the handlebars? My temporary tank is only slightly less dangerous. 8-[ I use a one-gallon oil container with a screw-on lid. I have drilled a hole in the lid so a fuel line is very snug going through it. By laying the jug on its side, fuel flows nicely, even without a vent. Simply turn it upright and fuel flow stops. Easy enough to drain back into the tank and store for next time.
.
Click to expand...
I have made a hook that goes around the neck and attaches to the side of the bottle with wraps of duct tape and then hangs over the bars.
Like every thing in life there is danger, and what if's. I guess it's no difference than carbs leaking on a bike and a hot exhaust, a shorted wire and the loom and bike go up in smoke, what if...

your tank idea sounds good.
 
Last edited:
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