Hi all,
It's been a long while since i've visited the site but this time i'm here hoping for some expertise. I've just recently rebuilt the top end on a new-to-me GT750M and it's got a few problems after only 100-200 miles.
The first thing I noticed is that it had a surge on deceleration, which is typical for GT750's with the CV carbs. In an effort to get rid of this I went to sync the carbs and noticed that if I synced the carbs with vacuum gauges the left cylinder would quit firing. To get the #1 cylinder to contribute equally it required the sync screw to be almost all the way in (more throttle opening). While working on the engine I noticed a bit of piston rattle from the left side as well. Now that I'm thinking about it I don't know if the rattle was present when I first put it together, but I don't think it was. I parked it for a couple weeks and just today dug it back out and started looking at the problem. Suspecting that maybe I didn't do a good enough job checking the crank seals I started with the quick test by pulling the carbs off and cranking the engine with my hand over each inlet to feel how much each of them "sucked" my hand in. the #1 cylinder again seemed to be weaker. I removed the pipe and sealed the outlet before applying ~5 PSI of air to the intake side. I don't have the correct setup for a proper leakdown test but I did my best and wasn't able to find any air leakage past the seals. I even pulled the left cover and sprayed soapy water on the seal, the cylinder base gasket, and the case seam but never found any leaks. Since I found no leaks I figured i'd do a compression test and found that the left cylinder was 20 PSI lower than the right. So that's where I stopped for the night.
It's clear to me the engine is going to have to come back apart.My guess is one of three different things:
1. The ports did not have enough chamfer and broke one of the rings.
2. Some sort of blockage in the oil passages to that cylinder.
3. A leaking crank seal that I couldn't find.
So that's where i'm at now. Is there anything else I should be looking at before tearing it down? Or something I should specifically be looking for while doing so? My main concern is that I never found a crank seal leaking, so if I take it apart and don't find a piston/ring problem, then what? I'd hate to put it back together and have the same issue. I'd also like to avoid doing the crank seals if it doesn't need it. The outer seals that I am able to see look brand new. Whats your take on it?
It's been a long while since i've visited the site but this time i'm here hoping for some expertise. I've just recently rebuilt the top end on a new-to-me GT750M and it's got a few problems after only 100-200 miles.
The first thing I noticed is that it had a surge on deceleration, which is typical for GT750's with the CV carbs. In an effort to get rid of this I went to sync the carbs and noticed that if I synced the carbs with vacuum gauges the left cylinder would quit firing. To get the #1 cylinder to contribute equally it required the sync screw to be almost all the way in (more throttle opening). While working on the engine I noticed a bit of piston rattle from the left side as well. Now that I'm thinking about it I don't know if the rattle was present when I first put it together, but I don't think it was. I parked it for a couple weeks and just today dug it back out and started looking at the problem. Suspecting that maybe I didn't do a good enough job checking the crank seals I started with the quick test by pulling the carbs off and cranking the engine with my hand over each inlet to feel how much each of them "sucked" my hand in. the #1 cylinder again seemed to be weaker. I removed the pipe and sealed the outlet before applying ~5 PSI of air to the intake side. I don't have the correct setup for a proper leakdown test but I did my best and wasn't able to find any air leakage past the seals. I even pulled the left cover and sprayed soapy water on the seal, the cylinder base gasket, and the case seam but never found any leaks. Since I found no leaks I figured i'd do a compression test and found that the left cylinder was 20 PSI lower than the right. So that's where I stopped for the night.
It's clear to me the engine is going to have to come back apart.My guess is one of three different things:
1. The ports did not have enough chamfer and broke one of the rings.
2. Some sort of blockage in the oil passages to that cylinder.
3. A leaking crank seal that I couldn't find.
So that's where i'm at now. Is there anything else I should be looking at before tearing it down? Or something I should specifically be looking for while doing so? My main concern is that I never found a crank seal leaking, so if I take it apart and don't find a piston/ring problem, then what? I'd hate to put it back together and have the same issue. I'd also like to avoid doing the crank seals if it doesn't need it. The outer seals that I am able to see look brand new. Whats your take on it?