I am troubleshooting low compression on a 88' QuadRunner LT4WD.
So far:
initial compression reading was tested dry on a cold engine with throttle wide open, decompresion lever in the down/off position,using the electric starter. reading:114
Tested again wet-
Reading: 120
I then checked valve lash at top dead center of the compression stroke with the timing marks aligned. Engine was cold, and valve lash was in spec, two thousandths on intake four thousandths on the exhaust. I continued to disassemble the engine, engine still at top dead center from checking the valve lash, i verified that the cam timing marks were level with the top of the valve cover surface and cam lobes facing away from where the rockers would sit with the cover on. So I am confident the valve timing is correct.

Moving on to the cylinder head I removed the valves, long story shorter- no abnormal wear throughout the cylinder head including valve seats valve guides, valve seat width etc. I did notice a lot of carbon build up on the intake valve, exhaust was not as bad. I cleaned the valves and valve seats with solvent and the extra fine grey scotch-brite trying to be careful as possible not to remove any metal material, my mistake as i found out, even extra fine scotch brite though great for removing carbon build up, has abrasive compound in it. The green scotch brite that you might use to scrub dishes does not and i probably will use that next time.

So now with nice clean head and set of valves, albeit, with dulled edges on the seating surface(probably from polishing with the scotch brite) I sent the head to a local machine shop to have the seats cut and and valves ground and lapped in.

The cylinder itself was probably fine with minimal wear. I took a rough measurement with calipers and compared to specs in the service manual and found it to be within spec, in fact you could still see some of the cross hatch pattern in the cylinder wall. The piston measured out within spec and also had minimal wear. Being the fussy SOB that i am and having the engine apart I ordered a new top end piston and rings set. I sent the cylinder to local machine shop to be bored oversize and honed. I probably could have got away with just honing the cylinder but I wanted to start with a fresh top end for piece of mind.

So with the new oversized piston, rings, fresh cylinder bored over and honed, valves ground, valve seats cut, new valve guide seals, new base gasket, head gasket, new sealing copper washers on the cylinder head cover. Everything torqued in sequence, valve timing checked and set, valve lash checked and reset as there was more clearance with the recut valves-makes sense right. So everything is back together engine runs a little better then before, still breaking it in. I probably have just under an hour on the new top end, all with varied throttle not much idling and some spirited full throttle blips under load.

I keep checking compression as the engine is being run-in but as of now the compression reading last checked was 112-117
Service manual standard is 142-199psi
service limit is 114psi

I am running out of ideas so I thought i'd throw this one out to the interweb
and see what i get back. Any help is appreciated.

 

Did you check the machine shops work? Piston to cylinder clearance is way tighter on 4 strokes than 2 strokes. For example piston to cylinder clearance on a Polaris 330 is (.0015-0032.)which is tight. Even tighter on the Sportsman 500 (.00015-0018) .004 clearance is considered max allowed on both. Possible machine shop bored it too large. I've seen that happen before several times.

 

In my experience anything over 100psi is fine. If you were running a racing engine, maybe you would need it to be up there at 140 but on an 88 Quadrunner?

 

I did not check the machine shops work as far as piston to cylinder wall clearance. I did talk to the owner of the shop for a while and he specifically asked for that spec saying he usually gets to within a thousandth. He seemed very competent and I was pleased with the work he did on the valve seats and valves being that he doesn't advertise doing head work. Before installing the piston I did check the fit in the cylinder from what I could see and feel, its hard to get a good measurement without a set of the right t-bore gauges. Tolerance on that quad is (0.0016-.0020 In.) piston to cylinder so it is a tight fit.

All I can think of at the moment is that with the valve seats being cut and valves being ground now the valve is riding higher up in the head, not by much I'm thinking a couple thousandths maybe but if the valve springs were weak to begin with now they would have even less tension on them.

I tried a crude version of a leak down test last night up to 90 psi at TDC of the compression stroke using the fitting for my compression tester with the quick connect end hooked up to the compressor line, I used the valve on the compressor to slowly feed pressure into the engine and at fifty psi the engine wants to turn over so I hold it at TDC with a breaker bar as best as I could. Anyway it was hard to tell exactly how much pressure was leaking and where but there was an audible hiss from within the cylinder head that would get slightly louder as i rotated the crank a few degrees either way of TDC. Mind you this is without the cylinder head cover on so there is no pressure from the rockers on the valves when the engine is rotated. I need to come up with either a real leak-down tester or a better way to replicate the test and make a fixture that holds the crank right where i want it at TDC.

Anyways I digress, the low compression has me worried because in the manual 114psi is the service limit, where the standard should be somewhere around 140-199psi. Being that its a new top end I would hope to be well above the service limit for the lowest compression allowable. Yes the engine will run, but not well, it does not like quick bursts of throttle and there is a little oil burring on startup that goes away at running temp. Maybe it just needs more time to run in for the rings to mate to the cylinder wall. Thanks guys, I will be sure to keep posted of what I find, it might save someone a small headache in the future.

 

This is on a 2 stroke,but look at the 7:25 point on this video on the poor mans way of checking piston to cylinder bore clearance.