750L compresion stats, wtf is happening ?

[richyrich]

compression numbers measured cold with opem throttle,1-4 l=r

after new piston rings, fitted a year ago 120, 130, 135, 120,   with oil in bore 160,150,155, 160

trouble starting all year after instalation, zero mies covered

recently 130, 125, 50, 130 and valve clearances:   exhaust  25/25  25/25  30/30 25/25

                                                                                intake     15/30  15/20  20/20  20/20

lapped in exxhaust valves, couldnt shift intake valves with my old tool,

now 100, 50, 70, 100  with oil in bore 100, 90, 100, 100, valve clerances have not changed

 

can anybody suggest why ive lost compression ?

i got number three back presumably cos  the exhaust valve seats better, but ive lost number two.  i dont understand why all four are down,.  I wonder if the head gasket and head have seated properly.  ive reused the head gasket and this is the second time its been off.  it is torqued down correctly.  it shouldnt be blowing by with no plugs in and just the compression tester. i am stuck. id really apreciate any ideas

 

[Captain Chaos]

try another compression tester?

[colinworth79]

I would remove the carbs , then do a compression test using one that screws into the plug hole not a push on one . Think they should be roughly the same . If its low on a cylinder it can only be valves or you have broke a ring putting it back together. If its low on say 1 and 2 I would say head gasket. Not really good practice to re-use a head gasket especially multi layer steel one but i have once in an emergency and didn't have a problem .

 

[richyrich]

thanks, yeh i wondered about the compression gauge before on something else, it is a screw in one and the carbs are off. The readings are pretty consistent accross all four tho so i dont see the gauge not working. 

interesting about brocken rings, but that would be apparant straight away wouldnt it ? in a single cylinder.   ive gota measured thirty psi missing on all four. that had been there on diff tests months apart.

and yeh bad form on the head gasket but the engines not under any load cranking over on the starter with the plugs out, i dont wanna believe its blowing.  But hey the pressure gotta be going somewhere.  Tommorro im gonna retorque it.  Ive got faith in the valves and the piston rings, the cylinders had a bore when they went in. 

The head gasket is the prime suspect.  Its the inner two that lost compression, one each before and after the head came off.  Maybe that is the pair and theyre blowing into the cam chain tunnel. fuckety fuck fuck.

as the sod isnt actually running would i see evidence of this blow by on the gasket i wonder.  I really dont want to take the fucker out again.

[jameskat]

I take it you have checked valve timing and clearances after refitting the head.

[richyrich]

thanks, yeah the clearances havent changed at all, which im a bit miffed about, i figured number three exhaust valves would tighten up with amount of flippin twig rubbing lapping i did on them to make a nice seat.

The timing marks and number of chainlinks are all good im pretty sure, obv willl double check again tommorrow.  Can they be 180 out ?

[jameskat]

As long as notches in the cams face each other when the number 1 on the exhaust camshaft faces the cover gasket surface then all is good, every other crank rotation will put the marks out by 180 degrees.

[richyrich]

yeh exactly, do the valves start hitting the pistons if its 180 out tho, i think they do, mus be right

[richyrich]

Thankyou jameskat, colinworth and captn chaos.  The litany of confusion goes like this:  The Timing was in fact one tooth out ! fixed that, got some compresion back.  Then the valve clearances did look different to before the valve lapping, so swapped some shims about keeping notes, veery interesting.  torqued the cam caps and great; closed a loose one and opened a tight one; but hang on, valves ive not touched have tightened up s what the hell is going on.

Id let the cam chain tensioner extend then screwed it in, this tightens clearances in the middle. Probly shouldnt admit to that but dammit that was a long hour.  Reset the tensioner and presto all valves in spec and compresion of 135, 150, 140, 135.  I think thats healthy enough for a cold test.  Tommorrow it might even start...

[Crass]

Valve shims in a 750L motor - surely not?

[richyrich]

uhuh, thats how they do

possibly not for much longer in this one after i torch it and chuck it off a cliff