I have a recently rebuilt evo, with maybe 200 miles on it. On a recent ride, the bike stopped running. Upon trying to restart it, it started back firing through the carb. It never actually started though, just coughed and died. After trailering it home I took a closer look. I suspected timing so I opened up the nosecone and found that the timing might have changed. The screws were loose. Not having a timing light, I moved the timing plate a little in each direction. Still didnt start. Then I noticed something a little more disturbing. I took off the air cleaner cover and turned the motor over with the spark plug wires disconnected. As it turned over there is a rythmic puff of "air" coming from the carburetor. I am begining to think of a collapsed lifter? Should I pull off the pushrod tube covers and watch the lifters? I know when it was rebuilt that adjustable pushrods were installed. (for whaht thats worth) Any suggestions???? Thanks in advance
86 FXR "Pressure" from carburetor
- Thread starter katman96
- Start date
If there is a puff of air coming from the engine through the carb it would tend to indicate that an inlet valve is not closed
so with that in mind work out which cylinder is on the compression stroke when it occurs and then work from there
but perhaps the adjustable push rods arent right or a valve is sticking
Brian
so with that in mind work out which cylinder is on the compression stroke when it occurs and then work from there
but perhaps the adjustable push rods arent right or a valve is sticking
Brian
Hoople
Account Removed
It can also be the profile of the cam. On some cams the intake valve closes PAST bottom dead center. Under dynamic conditions the air has inertia so that "blow back" you are seeing does not happen. Check to see if the other cylinder does it. If it does, it supports the theory it is a profile characteristic of the cam. If the other cylinder does not do it, then it is time to check some of the reasons why.
This is the reason why you can not simply compute an engines compression ratio by comparing cylinder volume at BTC to TDC.
This is the reason why you can not simply compute an engines compression ratio by comparing cylinder volume at BTC to TDC.
mine to
J
Jack Klarich
Guest
TDC gauge, bet you have a pushrod problem:s
J
Jack Klarich
Guest
This is a long shot check your advance mechanism, if the locating pin has sheared it could cause this kind of problem, I think the big twins had this on there advance mechanisms, I know sportsters did and have seen this problem several times on sporties:s
Hoople
Account Removed
You never did say how the bike was running just before it quit. If it did run strong just before it quit my guess would be you now have an ignition problem, or a stuck pumped up lifter (if hydraulic). Knowing how it ran before it quit would tell me a lot.
Yes if an intake pushrod was too long, it would destroy the performance in BOTH cylinders. I'm thinking if a pushrod became loose, it would hammer itself shorter not longer(?). That would also leave you with 1 good cylinder and it sounds like both of your cylinders are below par. But of course just 1 intake lifter stuck in the "pumped up" position would destroy performance in BOTH cylinders also and give you that puff through the carb.
Perform a common compression check. If you build up good/equal pressure in each cylinder, I would assume both valves are fully closed at TDC of the compression stroke. That would answer some important questions.
Reason why I said it may be a profile of your cam is because back in 1988 the emissions were easier to pass than today. I am thinking what you may be seeing are the few degrees of piston travel (going up) after the piston passes through BTC on the intake stroke. Just a guess since it's a 1988 cam. I have seen exactly what you described with performance cage engine cams since the intake valve closes past BTC of the intake stroke.
But if I was where you are, I would do a compression check just to see what it was and to see if both cylinders were close to each other. Worth a shot.