deadram
XS400 New Member
Hi all,
So 1979 xs400, shoe brakes, spoke wheels (they have a 79 with disc brakes, and rims instead of spokes). End of last season she lost power at top end. Used to top out at 120km/h, but on the 30 minute ride to work it was 100km/h at the start, 80km/h at the end. On the way back she dropped down to 80km/h top at start and 60km/h by the time I got home. Re-did electric, and carbs. I'm sure the problem is 100% not electric, 99.9% not carbs.
Did a compression test, got 150psi on the right (which is brand new spec, by the book), and only 60psi on left. I ran some tranny fluid through the carbs, made a lot of smoke, and rechecked compression, got her up to 90psi. I started her up, and blew an unlit torch around the gasket with no change to the rpm. Just to make sure, let a little blow into the carbs, and got a big change in rpm.
Put 120psi compressed air into the spark hole, got air out the exaughst, then out the exaughst and intake, then only intake, then I couldn't push the kicker anymore and had to stand on it to keep going. sealed the intake, no air out the exaughst, no air out the oil pan. Figured I'd do a valve adjustment on the left cylinder. Now the air goes out the exaughst, then exaughst seals at the same time the intake opens. Again, have to stand on the kicker with all my weight to push it past the compression stroke. No air out anywhere (except my quick-connect pushing air into the cylinder). Recheck compression and still get an easy 150 out the right cylinder, and an easy 90psi out the left, or if I give her, I can get 100psi out the left.
I have a feeling it's a sticky intake valve (due to an old spring), or maybe some muck in the left intake valve. I'm probably going to try some seafoam into the valve adjustment port on the left intake, and maybe some extra oil in there too afterwards.
Anyone have any other ideas/tips though?
As a re-cap, problems:
1) low power, especially at high speed
2) Left cylinder at 90psi
3) ~Both~ cylinders fire, engine will idle off left cylinder, with right plug removed
Suspected cause:
1) Not electrical (not until cylinder compression issue is solved at least)
2) Not fuel/carbs (not until cylinder compression issue is solved at least)
3) Left cylinder seems to keep enough pressure to reverse engine with 120psi at LF (10% before Left TDC)
4) Valves adjusted to spec, and don't seem to leak. (0.004 inches for left intake, 0.007 inches for exaughst)
5) Suspect "sticky" intake valve
So 1979 xs400, shoe brakes, spoke wheels (they have a 79 with disc brakes, and rims instead of spokes). End of last season she lost power at top end. Used to top out at 120km/h, but on the 30 minute ride to work it was 100km/h at the start, 80km/h at the end. On the way back she dropped down to 80km/h top at start and 60km/h by the time I got home. Re-did electric, and carbs. I'm sure the problem is 100% not electric, 99.9% not carbs.
Did a compression test, got 150psi on the right (which is brand new spec, by the book), and only 60psi on left. I ran some tranny fluid through the carbs, made a lot of smoke, and rechecked compression, got her up to 90psi. I started her up, and blew an unlit torch around the gasket with no change to the rpm. Just to make sure, let a little blow into the carbs, and got a big change in rpm.
Put 120psi compressed air into the spark hole, got air out the exaughst, then out the exaughst and intake, then only intake, then I couldn't push the kicker anymore and had to stand on it to keep going. sealed the intake, no air out the exaughst, no air out the oil pan. Figured I'd do a valve adjustment on the left cylinder. Now the air goes out the exaughst, then exaughst seals at the same time the intake opens. Again, have to stand on the kicker with all my weight to push it past the compression stroke. No air out anywhere (except my quick-connect pushing air into the cylinder). Recheck compression and still get an easy 150 out the right cylinder, and an easy 90psi out the left, or if I give her, I can get 100psi out the left.
I have a feeling it's a sticky intake valve (due to an old spring), or maybe some muck in the left intake valve. I'm probably going to try some seafoam into the valve adjustment port on the left intake, and maybe some extra oil in there too afterwards.
Anyone have any other ideas/tips though?
As a re-cap, problems:
1) low power, especially at high speed
2) Left cylinder at 90psi
3) ~Both~ cylinders fire, engine will idle off left cylinder, with right plug removed
Suspected cause:
1) Not electrical (not until cylinder compression issue is solved at least)
2) Not fuel/carbs (not until cylinder compression issue is solved at least)
3) Left cylinder seems to keep enough pressure to reverse engine with 120psi at LF (10% before Left TDC)
4) Valves adjusted to spec, and don't seem to leak. (0.004 inches for left intake, 0.007 inches for exaughst)
5) Suspect "sticky" intake valve
