Odd compression...

Hi all, hope I can get your thoughts on this. I have a 1989 RRC 3.5 Efi (flapper). Recently rebuilt the engine and she runs rough from cold (lumpy, low RPM), once warm she's much smoother but still not as smooth as I'd like, not perfect.

I did a compression test, dry test with engine warm and all plugs removed and throttle wide open. Results as follows:

1: 155 psi 2: 162 psi
3: 130 psi 4: 140 psi
5: 155 psi 6: 160 psi
7: 165 psi 8: 155 psi

I wet tested the low cylinders (3 & 4), no change at all, so rings ok? Should be, they're new... Also odd that it's the 2nd cylinder back on both banks, not two next to each other.

Also, put a vacuum gauge on once she'd warmed up and was ticking over comfortably and it was sitting consistently at 15 in.hg, don't know if this helps point to anything? Shouldn't the vacuum be around 20-22 in.hg?

Any thoughts much appreciated.

Cheers

David

how much of a rebuild,new pistons ,rebore,new cam and tappets etc

Thanks for the reply. New cam and tappets, pushrods, rocker assemblies and valve springs. The valves themselves were good so didn't replace them or their guides. Also had the heads skimmed. New rings, new bearings including cam bearings. Had the block cleaned and the bores checked and honed at an engine place, they also fitted the cam bearings and checked the crank. All was good and went together well. Elring composite head gaskets fitted and clamped down with ARP studs, lifter preload checked and good. Injectors also refurbished.

One thing that did occur on initial startup was I had the throttle butterfly wide open (had it in the wrong place when I put it back together). She ran but INCREDIBLY lean. After idling for about 10 minutes the exhaust headers started to glow cherry red so I shut her down and that's when I found the throttle butterfly issue and rectified it. Could that burst of extreme heat have damaged the exhaust valves? Particularly on cylinders 3 and 4?

Do a wet test, squirt of oil in to each plug hole just before you test that cylinder. If pressure goes up rings if it stays the same valves.

presuming all valves open fully,you might need to let it run in since it wasnt rebored for the rings to settle, maybe look at other reason for lumpy idle

Thanks for the super fast replies (this forum is lightning!).

Thanks Wammers, I wet tested the two low reading cylinders and the reading didn't change.

Thanks also jamesmartin, I'll just drive it then? Re-check the compression at a later date? What sort of timescale/how many miles do you think for it to run in?