Need Help-Low Compression in One Cylinder

Jack Zupan · #1 (**permalink**) 10-22-2001, 06:40 PM

I have got a problem I can't diagnose yet. I have a 351W with 2V Cleveland heads in my Cobra. I put an MSD setup in the car a couple months ago and noticed a burble at idle from the left sidepipe. I think the MSD made it more noticeable but that could be my imagination. Except for the idle the car seems to run fine. The burble is not at every stroke but only every so often, maybe every three to four cycles. I pulled the plugs on the left side the of the engine bank and found the number 5 plug to be black and the insulator also had a black carbon coating. I guess there must be oil burning in that cylinder. The other three plugs looked like I just put them in yesterday. I did a compression check and only had about 60 pounds in that cylinder. I measured 185 lbs last time I did a compression check which was probably last year. Number six had 172 lbs in it. I pulled the valve cover and can find nothing wrong there. Springs are intact and pushrods look straight. The valves move up and down when they are supposed to. There are still some other possibilities but I don't know where to look next. So I ask some of you engine gurus out there what would you look at next. I don't want to pull the head unless I have to but that looks like the next step to me. Any ideas from you guys?

Thanks,

Jack Z

Richard Hudgins · #2 (**permalink**) 10-22-2001, 07:34 PM

Hi Jack,

First thing to do is a leak down test. This will let you know where the problem is. Valves or rings.

The next thing would be to put a borescope in and look at the top of the piston for problems.

Sorry to hear of the problem, but you are going to have take it apart now matter what.

At least after doing the leakdown test, you will know whether you have to just pull the head or the motor.

427sharpe · #3 (**permalink**) 10-22-2001, 11:40 PM

Sorry to say, it sounds as if you have lost ring seal. TRy to squirt a dash of oil in the cylinder than showed lo compression, spin the motor over 2-3 times and recheck--is your compression back? If so, how long does it last? If you are very lucky, than that cylinder is running too rich and merely lost ringseal (the oil barrier that seals the rings to the cylinder). If you are like me, you lost a ring and will have to pull that piston and put on new rings. SORRY.

Usually a burnt valve will be a regular, hard miss but it's easier to check valves from the bottom half, after you remove the piston. If you dont have a borescope...good luck, and I hope it turns out simple and quick!

B L & sons · #4 (**permalink**) 10-23-2001, 08:07 AM

Just a thought,could it be a valve guide that is trying to stick?
best of luck

Jack Zupan · #5 (**permalink**) 02-01-2002, 04:48 AM

I am updating this post because I would like some opinions on options. I pulled the heads from my engine and saw nothing that jumped out at me as being a problem. The head gaskets looked OK and the heads looked OK except that the bad cylinder looked oily. The only thing I noticed was in a certain light it looked like not all of the valves were at the same depth. Turns out this is correct. The machine shop has a list about $800 long of repairs that need to be done. One intake seat was shot, no inserts in the exhaust seats, guides are shot, wrong retainers, valves are multigroove, etc. Sounds like the guy who built the engine just slapped the heads on with no work. The funny thing is they looked good from the top, 7/16" studs, pushrod guides, Crane springs, CompCams roller rockers.

Anyway here's the question(s):Since I'm in for some big money anyway I have an opportunity to make some changes. I've already decided to pull the engine and re-cam it. With the heads I can go aftermarket aluminum but that would require new pistons, new intake, new headers and possibly new sidepipes. Probably too much money and a lot of hassles. I have a set of 4V heads I could install but that would require the same as above and without increasing displacement would probably not be a good choice. Finally I could also go with a set of Aussie heads. These have the advantage of the closed combustion chamber with the smaller 2V ports and should bolt right up. Compression ratio would have to be checked of course. There is a company that sells reworked Aussie heads for $1000 which may not be too bad if they use good quality valves and the machine work is good. I don't know how to tell that unfortunately. Has anyone out there had experience with PowerHeads? I might have found them from this forum but I'm not sure. Finally I could have my machine shop rework the heads I have. If I go that way are there any ideas on what one should do to the heads besides the required repairs to get more horsepower?

As always I look forward to what you all have to say.

Regards,

Jack Z.

Bob Putnam · #6 (**permalink**) 02-01-2002, 05:24 AM

Jack,

You don't mention anything about the bottom end. With the heads off, now would be a good time to check the condition of the pistons, rings and bore - especially since you found oil in the cylinder. Even if a valve guide was extremely bad, a bottom-end check now couldn't hurt.