Is 200psi too high

jay760

XS650 Guru
Messages
1,131
Reaction score
520
Points
113
Location
Somerset
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.
 
20210429_164200.jpg
20210429_164029.jpg
 
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.
 
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:
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.
 
If you do encounter pinging try backing off on the max advance on the ignition timing.
 
No pinging, was surprised there was such a difference going from 1mm to 2mm over valves, everything else is as its always been
 
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?
 
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.
 
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.

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,
 
Bigger pistons pushing into the original space will raise the CR an amount.
More CR needs better octane to be safe or ignition improver.
 
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:
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).
 
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.
 
Back
Top