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Is 200psi too high

Discussion in 'The Garage' started by jay760, Apr 29, 2021.

  1. jay760

    jay760 XS650 Guru

    Was struggling last weekend with plugs and a fueling problem, I thought I had an engine problem so just done a compression test, found I have around 200psi on both cylinders, is this too high for standard fuel? or is it something else.
    LanzSilva likes this.
  2. Downeaster

    Downeaster Everything in XS Top Contributor

    Well, that's certainly higher than average, but I don't think it's dangerously so. Is the engine stock? Has it been recently rebuilt (Rings, pistons, valve job?) Pay close attention to engine sounds, especially at lower RPM under load. Looking for any indication of pre-ignition (knocking, pinging).

    If you're not getting pre-ignition, I don't think it'll do any harm. You might want to run premium fuel to be sure.
    timbeck, grizld1 and gggGary like this.
  3. jay760

    jay760 XS650 Guru

    750 engine, je pistons, 2mm over valves and unknown cam, ran the cam 2 years ago when I changed the valves I also changed the cam, now put it back to see how the valves would effect this cam, think we can safely say not good.

    Pulling strongly in first and second then sounded like it was just sucking air in 3rd, also had a plug die on start up
    Last edited: Apr 29, 2021
  4. grizld1

    grizld1 Grumpy old man Top Contributor

    If it's any help at all, I ran a 700 cc. XS650 engine with Venolia pistons for years at 180 psi on premium pump gas (91 to 93 octane) with no ill effects. As DE wrote, if there's no detonation you're probably OK. You might want to cc. the motor out to calculate static CR, and if it's north of 10:1, run the numbers for raising the deck with a thicker base gasket and retime the cam.
  5. Team Junk

    Team Junk XS650 Guru XS650.com Supporter Top Contributor

    If you do encounter pinging try backing off on the max advance on the ignition timing.
  6. jay760

    jay760 XS650 Guru

    No pinging, was surprised there was such a difference going from 1mm to 2mm over valves, everything else is as its always been
    Team Junk likes this.
  7. Team Junk

    Team Junk XS650 Guru XS650.com Supporter Top Contributor

    Maybe a jetting issue ?
  8. jay760

    jay760 XS650 Guru

    Probably, bit odd it's only noticeable in higher gears
  9. kshansen

    kshansen XS650 Guru Top Contributor

    Okay just to play somewhat of the devils' advocate here but do you have another gauge to compare to this one? Any chance the gauge got dropped or bumped somehow?
    gggGary likes this.
  10. hovel

    hovel ol' stupid XS650.com Supporter

    What did the dead plug look like ? I suspect fuel flow for whatever the reason. Higher gears is where the engine is working the hardest and needs more fuel.
  11. jay760

    jay760 XS650 Guru

    The gauge is brand new straight out of the box.
  12. jay760

    jay760 XS650 Guru

    The plugs looked right if anything a tiny bit rich, with this cam third gear is where you can normally notice a defined step in revs,
    Team Junk likes this.
  13. Rasputin

    Rasputin XS650 Addict XS650.com Supporter Top Contributor

    Bigger pistons pushing into the original space will raise the CR an amount.
    More CR needs better octane to be safe or ignition improver.
    lakeview likes this.
  14. XSLeo

    XSLeo XS650 Guru Top Contributor

    zWhen I added Mike"s 750 kit to my 75 I tested the compression on the engine stand, stock cam, no carbs, with E-start, full of assembly lube I got 195 PSI each side.
    It runs good on 91 gas, even better on 93 when I can find it.
  15. gggGary

    gggGary When a whole one is just too much. XS650.com Supporter Top Contributor

    Madness also an old school high compression cammed 750 with some (unknown amount of) miles on it when I took possession. Started out in the low 190's with a trusted gauge that shows healthy stock 650s at 150 to 155. It's dropped mebby 10 pounds in the 10,000 miles I've ridden it, no ethanol premium pump gas.
    Last edited: May 1, 2021 at 1:22 PM
  16. jay760

    jay760 XS650 Guru

    Think I have found the problem, not sure how to resolve it other than spacing the plugs you can just about see it in the picture, piston has been hitting the plug 20210502_170009.jpg
    grizld1 likes this.
  17. Team Junk

    Team Junk XS650 Guru XS650.com Supporter Top Contributor

    They sell thicker plug washers to degree the plug. Get a thick one perhaps.
    Paul Sutton likes this.
  18. grizld1

    grizld1 Grumpy old man Top Contributor

    Ouch. I know it's a PITA to pull that slug and remove the head, but it might be a good idea to lay in some modeling clay and check piston-valve clearance and check squish clearance on both axes with .040" solder (if the solder crushes much at all, bad things are waiting to happen).
    GLJ, jay760, Team Junk and 1 other person like this.
  19. jay760

    jay760 XS650 Guru

    Don't think I would be quite so worried if it was touching on both sides, the fact its just the one is a bit of a worry.

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