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Low compression both cylinders

Discussion in 'The Garage' started by Triggerton III, Mar 26, 2019.

  1. Triggerton III

    Triggerton III XS650 Member

    thank you.
  2. Triggerton III

    Triggerton III XS650 Member

  3. 5twins

    5twins XS650 Guru Top Contributor

    You can't do anything about the cam rolling back when you tighten the tensioner. That's just what's going to naturally happen. That's why you want to check the alignment with the tensioner applied, to make sure it's still OK.
  4. Triggerton III

    Triggerton III XS650 Member

    my bad,
    Jim likes this.
  5. Triggerton III

    Triggerton III XS650 Member

    Hello again. I have manged to get the timing corrected. still only 90 psi. :(
    MaxPete likes this.
  6. 5twins

    5twins XS650 Guru Top Contributor

    Did you take the topend apart again to correct the cam timing (move timing chain on sprocket)? If so, did you inspect the ring install while in there to insure they were correct (top (marked) side of top 2 rings facing up)?

    It would help if you told us a little bit more about exactly what your "rebuild" consisted of. Did you replace the rings and if so, did you hone the cylinders? If you did hone the cylinders, what type of hone did you use? Or, did you have the cylinders bored and install new oversize pistons and rings? Did you install a new cam chain and/or front cam chain guide?
    MaxPete, TwoManyXS1Bs and gggGary like this.
  7. Triggerton III

    Triggerton III XS650 Member

    Hi 5Twins
    I will take a step back. I bought the bike as a non runner. poor compression. This my first foray into bike repairs. it is a standard 1979 engine. my idea is to practice on this engine, get it running correctly. once I have achieved this. I will start working on the original 1971 engine that came with the bike.
    Now to answer your questions.
    No I didn't inspect the ring install. that is my next job.
    I used a glaze buster on the cylinders. I don't know what its called. the 3 pronged type. i did struggle to maintain a 45 degree cross hatch, I used engine oil as the lubricant and thoroughly cleaned afterwards.
    using original pistons and standard rings.
    new cam chain and guide.

    I am taking engine apart again today.
  8. delagem

    delagem XS650 Addict

    Are you certain your compression gauge is accurate? I have 4 compression gauges; 3 read very low compared to my good SnapOn gauge. I have compared a few friends gauges, too. Almost all of them read low, sometimes by as much as 30psi.

    Sometimes it's down to where the schrader valve in the gauge is. It should be at the end of the hose, where it screws into the head. Cheaper gauges tend to put the valve at the gauge itself; this adds the volume of the hose to the head cc's, throwing off the test.

    More than one gauge I discovered had a bad schrader valve, which will also cause very low readings.
    MaxPete and gggGary like this.
  9. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    if pressure drops on the gauge, remove and lube the schrader valves.
    MaxPete likes this.
  10. 5twins

    5twins XS650 Guru Top Contributor

    Sounds like you used a stone type hone, the kind with 3 spring-loaded arms that have long stones on them. You have to be careful with those as they can enlarge the bore if run too long. Honing is a quick operation. You only hone for about 20 seconds or so. It's best to use a ball type or "bottle brush" hone. Flex-hone is a common one .....


    These just break the graze and are less prone to removing material and enlarging the bore. I think you better also measure your ring end gaps now. If you enlarged the bore, they may be too big. You said standard rings but were they new ones or the originals? If originals, the end gaps may have been too big to begin with.
    MaxPete and gggGary like this.
  11. Triggerton III

    Triggerton III XS650 Member

    I am not sure that the gauge is accurate. the reason I tested is becuse the bike wouldnt start. the Schrader valve is at the gauge end.

    I fitted new piston rings and i did measure the end gap. within tolerance, as specified in technicians manual.

    I havent started stripping engine yet as I had an invite to ride the new Norton V4SS. I thought it rude not to :)
    MaxPete, TwoManyXS1Bs and delagem like this.
  12. delagem

    delagem XS650 Addict

    Right, but I thought you found the underlying issue on why the bike wouldn't start when you found your cam timing off a tooth? But then because compression didn't come up above 90, you pulled the engine again?

    I'm not saying the gauge reading low is the cause of the bike not starting/running properly. But once you seem to have solved that issue (the cam chain), did you try to run the bike again?

    I didn't see any replies between "still only 90 psi" and "I'm taking the engine apart again today".
  13. Triggerton III

    Triggerton III XS650 Member

    I tried to run the engine after the cam issue no luck, tested compression readings are 90 psi. Guage doesn't lose the pressure reading. I haven't taken the engine out yet.
  14. Jim

    Jim Beyond the edge, is the unknown. Here be Dragons Top Contributor XS650.com Supporter

    Being as how your schrader valve is in the wrong place, you could be considerably higher than 90. Perhaps even up to 110-120 which would be more than enough to run the engine. With new rings not seated yet, you might even be in the ballpark. If it were me, I'd give a serious effort toward makin' it run before I pulled the motor again.
    delagem likes this.
  15. XSLeo

    XSLeo XS650 Guru Top Contributor

    Perhaps your carbs picked up some crud from poor gas or crud in tank.
    Sometimes the issues from ignition and carbs overlap.
    A might step back and go through the steps of a major tune up. This will get any of the mechanical issues in line as well as ignition/
    #1 Adjust cam Chain
    #2 adjust valves
    #3 If still running points set gap and timing. If running a Pamco set to center of adjustment.
    This gets most of the issues straight. If it still won't run. then pull carbs and clean them. I assume you have heard about the carb guide. Use it for the cleaning. Pay particular attention to the idle circuits. Very tiny ports and passages that plug easily.
    I think I'm going to have to do that with my 81.
    Got the Pamco in and timed up a few days ago. Now hard to start with crappy idle. If I keep it running long enough to run with out the choke it revs rough and is very slow to return to idle.
    gggGary likes this.
  16. Triggerton III

    Triggerton III XS650 Member

    thank you chaps, I will try those today. the pressure gauge manufacturer is Sealey. if that helps
    delagem likes this.
  17. delagem

    delagem XS650 Addict

    Just a thought I had when I first read your post. One time I bought a project bike with low compression. Turned out the PO had adjusted the valves at the wrong TDC (exh vs compression) and this caused the readings to be very low. Doubtful it's your problem but something to consider.
    Triggerton III likes this.
  18. MaxPete

    MaxPete Life with Lucille...I suggest, she decides. Top Contributor XS650.com Supporter

    Oh yes - and you are quite right on that score Triggerton III. Being rude to people who offer you rides on their expensive motorcycles is a very bad thing to do and would definitely earn you a finger-wagging from......
  19. Triggerton III

    Triggerton III XS650 Member

    Well it just wouldn't be British MaxPete.

    Attached Files:

    TwoManyXS1Bs and MaxPete like this.

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