The Stovebolt Site Forums The Parking Lot Big Bolts SBC Low Compression

Hi Everyone,

I believe I have a chevy 350 (I still haven't gotten a straight answer) and the engine is in dire need of reworking. I did a compression check on the cylinders, and I got 92-110 PSI across them all. The pressure dropped as I went back from the front cylinders as well. I poured some oil into cylinder 1, and it raised the pressure by 3 PSI, so I'm guessing the rings are good, or at least passable.

My confusion starts at the valves. The lashing on them were shot, varying widely from heavy preload to a large gap. As I backed out the adjusting nuts, the pressure kept going up, until I got to 120 PSI from cylinders that gave me 95 previously (I stopped at that point, it may still go up).

Would bad valves be a serious culprit for my weak engine?
Is this a common problem to SBCs or is it an outlier scenario?
Is there any way to get it back, barring a costly valve job?

The engine ran ridiculously rich, and the plugs are pure black. The carb will be cleaned and rebuilt, but is there any similar hope for the heads?

Are the lifters hydraulic? (probably, but let's not A S S ume anything!) Start out by doing a proper valve adjustment, where each lifter is preloaded 3/4-1 turn after removing any slack in the valvetrain with the piston at top dead center. Do a search on the forum for my "companion cylinder" valve adjustment method- - - -it works on all engines, 6 or V8, solid or hydraulic lifters. It's the most accurate way I know of to establish when a cylinder is on TDC and ready for a valve adjustment.

Once you're sure all the valves are adjusted properly, run the engine up to normal operating temperature. set the RPM at about 2,000-2,500 wit the idle speed screw, and slowly dribble a Coke bottle full of hot water down the carburetor throat. The carbon buildup in the combustion chambers will be steam-cleaned away by the water as it hits the burning fuel and flash-evaporates, and there is virtually no way to do any engine damage unless you dump a huge shot of water in all at once.
Jerry

do you know the history on the engine? I would start by adjusting the valves properly and see what you have after that, a good tune-up, and cleaning your carb.

Assuming hydraulic lifters, you loosen them up, tighten to take the slop out of the pushrod, and tighten 1/2 turn. Here is a link to the full procedure and proper sequence http://www.centuryperformance.com/valve-adjustment-procedure.html

DOH, Jerry beat me to it! Good call on dribbling the water down the carb to clean the carbon out!

The engine isn't going to be running anytime soon; I tore it out to prepare for an engine relocation, and to patch some leaks. Also, I ran the nuts out 3-4 turns before I got that high on the readings, I'm not sure I can adjust it, and get the engine to agree. I figure it's hydraulic; the engine is a mid to late 70s, can't imagine it was mechanical at that point, but I shouldn't put it past, knowing my luck.

I can tell you the history on the engine. One of the previous owners didn't feel the original six was sufficient, so they took a random v8 and dropped it in, along with a "new" transmission. At least they brought the radiator along with the engine, I fear the original wouldn't have covered this thing.

Does the steam trick really deep clean it out? I was thinking of the Sea Foam approach, but if water will do it, I'll go that way.

A little water down the carb can't hurt anything, and it's certainly cheaper than seafoam. I've tried several methods, such as using a spritz pump like the one on a Windex bottle, but the simplest method is just filling a soft drink bottle with hot tap water, hold a thumb over the opening, and dribble the water into the carburetor with the engine running at cruise RPM. The engine will shudder and shake a little, and steam and maybe some carbon will belch out of the tailpipe.
Jerry