86 FXR "Pressure" from carburetor

Discussion in 'FXR Models' started by katman96, Jul 10, 2010.

  1. katman96

    katman96 New Member

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    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
     
  2. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

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    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
     
  3. Hoople

    Hoople Account Removed

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    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.
     
  4. glider

    glider Veteran Member

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    My money is on the pushrods out of adjustment (loose lock nut)
     
  5. Bud White

    Bud White Well-Known Member Retired Moderators

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    mine to
     
  6. Hoople

    Hoople Account Removed

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    Me Too.

    What am I saying!!! Your both wrong. It's normal for that year bike.
    Watch and see.:p
     
  7. katman96

    katman96 New Member

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    Thanks everyone who has responded so far....

    Hoople, what would be the easiest way to check that? Look in the cylinder with the plugs out and try to figure out if its ocurring on both cylinders? I might need a little help on this..
     
  8. Jack Klarich

    Jack Klarich Expert Member

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    TDC gauge, bet you have a pushrod problem:s
     
  9. Jack Klarich

    Jack Klarich Expert Member

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    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
     
  10. Hoople

    Hoople Account Removed

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    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.