High Compression in Cylinder 4

[Gregs672000]

Check the cam and make sure cyl #4 is good. If the duration is smaller it will increase the pressure.

I just got done illustrating this exact thing. A few days ago I did a cold compression check and had the following numbers: 215, 215, 205+ 225. I've never had that high a compression in #4 and had no answer (sealed better than the others?). I figured the lower number in 3 was due to that one having had the broken pin and maybe piston wear (?). Still ran great. Then, after a good drive which included some 7000rpm hits, on the way home it started giving me an unbalanced feeling at idle. Checked balance when I got home, checked the plugs etc and all was good....hmmm. So I decided to check valve lash even though it had only been 100 miles or so since. First one up, I check #3 intake and it is so tight I can't get a .004 guage in it! Fortunately the rocker still rocked some side to side so it had some lash, but it's tight! What the heck? Then I notice that the lock nut is loose which had allowed it to "self adjust". I progressed through the rest and come to #4 intake, and it has more than .020 lash, same situation with the lock nut loose (guess I had a brain fart, but I thought I ALWAYS double check...) only this one self adjusted the other direction. Good Lord Greg, who handed you a wrench?! So, with everyone playing right now the numbers are all back to 215 across the board. Loose added 10lbs, and tight reduced it by 10lbs.

Moral of the story, if compression numbers vary a lot then it never hurts to check lash!

[redroadster]

The header may be clogged with oil carbon,it builds a wall right at the head to exhaust opening .usually

[SMHertz]

Update:

Just wanted to say thank-you to everyone. Turns out that in a way everyone was right.

I was about to pop the head off but I figure I would try one last thing, pulled the plug and shoved a bore scope down the hole. The entire piston is covered in carbon, while I've got the scope down there I see oil dripping from the exhaust valve.

I replace the exhaust valve seal again then check the lash. Its way way out of spec, redo the valve lash and tighten everything down.

Toss in a fresh plug and take her for a test drive, she spits out one last puff of smoke and then runs clean. Once she's warmed up I go for the full Italian tune up and do a couple 6g pulls. Motor is now running great but the clutch is starting to slip up high.... On to the next issue....

Got home and dumped some Berlymans into the cylinder to help dissolve the carbon, now it time to research new clutches.

Thanks for all the advice

[Gregs672000]

Woo Hoo! Glad to hear, and a good illustration of persistence solving the problem! Thanks for posting your results.

[redroadster]

The guide inside likely would be Carboned up or worn out ( which will do that wear seals / melt them)
It's just a film lubricated
Sure it's filled on coolant ( using g a fill funnel)