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Plug vacuum on carb 3 when syncing?

H

hannibal

Forum Mentor
I've finally gotten my valves adjusted on my 77 GS750 and I'm hoping to sync the carbs one night this week. I'm using a temp gasoline tank made from a plastic bottle. Just wondering if I need to block off the vacuum hose from carb 3 (that normally runs to the petcock) when I'm syncing carbs.

Thanks!
 
Absolutely, or the sync will be waaay off

Just leave the hose on the carb and plug the open end with a bolt or golf tee or something
 
I just use some vacuum caps that I got from the auto parts store years ago. the only thing that sucks is trying to pull that sucker off when the motor is hot without pliers. otherwise I feel better using that than a golf t or screw
 
60ratrod said:
I just use some vacuum caps that I got from the auto parts store years ago. the only thing that sucks is trying to pull that sucker off when the motor is hot without pliers. otherwise I feel better using that than a golf t or screw
Click to expand...

Nothing wrong with that and a bit more positively sealed, but the hose is already right there, needing to be plugged. And you don't have to slip it back on the carb when you're done
 
Big T said:
Absolutely, or the sync will be waaay off

Just leave the hose on the carb and plug the open end with a bolt or golf tee or something
Click to expand...

Yup yup. Get this, one time I took my bike in for a carb sync, an hour later they bring it back out saying they couldn't sync the carbs. I asked them did you plug off the vacuum on #2 carb....response was "huh?" never went back there. That was one of a couple times my bike has ever been to a shop. Me do now lol.
 
I like the golf tee because it won't leave marks on the hose like a screw might and I think I can get a good seal. Plus I have tees.
 
Just want to report on my progress. I started the bike for the first time in almost 4 months. Man that felt good! But after 5 minutes of running on choke, I still couldn't get it to idle off choke. If I kept the idle screw turned in a lot, the idle would race up to 4000rpm when I turned of the choke. If I adjusted the idle screw lower, it would die. I've read enough to know that a climbing idle is usually a sign of an air leak so I started to think maybe my intake boots were worse than they looked (I replaced the O rings, but did not replace the boots). I turned off the bike and began to install my rebuild rear caliper and MC.

It took me almost 2 hours to install the brakes and bleed them (left a piece of wood holding the pedal down), then I went back to try to get it to idle correctly. I was using my plastic bottle gasoline tank, and I had my golf tee in the #3 vacuum hose. Then I remembered I had loosened the vacuum ports in anticipation of syncing the carbs. Duh, that would be the air leak that caused the high idle. I tightened the ports and after sitting for 2 hours, it started immediately without choke and held a nice idle. Then it started to rain so I closed it all up and called it a day. Hoping I can sync the carbs tomorrow.

Two things I noticed: my tach (which worked 4 months ago and on the very first start today) was no longer working. After starting the bike, the tach would sit at zero for 30 seconds then come alive and read correctly (I think). The first time this happened, a little tap on the gauge brought it to life, but after that, no amount of tapping the gauge or wiggling the tach cable would get it to work. It would just wake up on its own after 20-30 seconds.

The second thing that confused me was that the exhaust pipes on cylinders 1 and 2 were cold. On my first start with vacuum ports open and a horrible idle, the pipes were hot. After installing the brakes and restarting, the pipes stayed cold but it idled great. I pulled the plug wire off the cylinder 1 spark plug, and the idle speed and sound didn't change. The exhaust on 3 and 4 were hot but not 1 and 2. Not sure what to make of this but I'll play around with it tomorrow. I'm just thrilled to see the bike run after cleaning carbs, adjusting valves, and changing points.
 
1st guess is the plug wires aren't on correctly

left coil 1-4
right coil 2-3
 
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