Engine power issue

[Ketchupsandwhich]

So I'm pulling my hair out here. 79 Gl1000, I can't figure out what my actual problem is. I've had the carbs off a dozen times, has all original parts in it, floats adjusted to 21mm, I bench tested with gas for leaks and there are none. Did the randakks vacuum test for the air and fuel circuits, both check out good. Spray wd40 on the carb boots when running, no change to rpm, new throttle and choke cables all seem to operate well.

Problem is upon initial startup the engine is bogging down. The number 3 carb seems to backfire through the intake, indicating a team condition from what I understand, but that's the one that gets fuel first so I don't understand why it would be lean. My main suspect and from feeling this problem on other bikes is the vacuum piston isn't operating properly. I've polished them all, seem to push easily on the bench and vacuum tested these as well and also put new caps on them so I'm pretty confident I don't have any vacuum leaks.

What I really can't figure out is that once I start driving and really give it some gas, suddenly the bike wakes up and blasts off like a rocket. I've synched the carbs as well. After it picks itself up like that it runs and idles perfectly, full range of power, it's just the initial start where it falls on its face. Also when doing a cold start at full choke it doesn't want to go above 1500 rpm, like something is stuck somewhere but I've verified all the butterfly's open as they should and together. At one point a few months back it was doing fine, start on full choke and it would jump up to 3500 rpm with full power and I could tell everything was working as it should. Now it has this weak start issue and I just don't know why.

Carbs are spotless, spark is good, has anyone had this issue before? At this point I'm suspecting maybe the fuel pump even tho it's only a year old. I've noticed bubbles in the tube coming out to the carb and they get worse as rpm goes up. My theory is maybe when I goose the throttle it's sending more fuel pressure to the bowls which allows them to operate properly. Also maybe the timing advance unit could be hanging up? I've got electric ignition on it also. I just can't understand why it would bog down then just take off suddenly, and after it does that once then it's totally fine until I shut it off for a few hours. Any ideas?

[Rambozo]

Doesn't really sound like a fuel pump issue, but that is easy to test. Just tee in a fuel pressure gauge and test drive it.

[cfairweather]

You mentioned that it backfires through the intake on #3 when it is cold. This tells me you could have a bent valve. Did you recently replace the belts or have you ever had a belt break?

[sjk4]

Have you tested the fuel pump pressure and vacuum? I had a similar problem on my '83 until one day it wouldn't start at all even tho it was running the day before. Zero fuel pressure. I replaced the pump with a new OEM unit and it solved the cold start issue. With enough fuel in the tank it will gravity feed enough to run not provide adequate pressure.

[Jonny426f]

I have a similar issue with my '82. It cold starts with a skipping cylinder (or two) but smoothes out as soon as it warms up for a few mins. If I ride it cold it will sputter and pop back through the carbs. I am pretty sure I've got a bad igniter, but time has not allowed for diagnosis. I am an auto tech by trade and have run into temperature sensitive ignition components on many vehicles, including a friend's 1986 Goldwing. You can remove the false tank and use a hair drier to warm the igniter a little before the first start up to see how it runs. If she runs well, take one igniter at a time and toss it in the freezer for 10-15mins, re-install it, and restart your bike. You can narrow down the defective unit in this manner. Good luck.

[Ketchupsandwhich]

*Solved. Timing advance unit was stuck closed. Been chasing this for 2 weeks thinking it was a carb problem. Cleaned it and reinstalled, runs like a champ.

[cfairweather]

Congratulations! Now enjoy it while the weather is nice.