Compression Test Results

[bnold5000]

Hello Everyone,
I bought this car last year. It has always run rough since I bought it. In an effort to try to get it running smoothly, I decided to start my diagnostics with some compression testing. This car has an aftermarket high compression head which I’m guessing probably increased the ratio to 6:1. These tests were performed rather crudely by cranking the car over the best I could by hand.
Here are the results:
Standard Automotive Compression Test:
1-68, 2-55, 3-52, 4-52

Differential Compression Test (like you’d do on an aircraft engine for an annual inspection. Base pressure for this test was 60 psi):
1-55, 2-49, 3-47, 4-53

What do you think about these results? Is this engine going to run smoothly with a good ignition system and carburetor? I’ve got new plugs and Anderson Timer and all coils seem to buzz right and produce a good spark at approximately the right time as I crank the engine over, but it still runs terrible.

[TXGOAT2]

Odd that one cylinder reading is so much higher than the others. Could it be loaded with carbon? All cylinders should fire with those numbers. I'd look at ignition and carburetion. Coils that are out of adjustment can buzz merrily and produce impressive sparks, yet still perform very badly. Coils that are badly out of adjustment can quickly spoil a new timer. Chafed wiring may work well when testing sparks with the engine not running, yet cause problems when the engine is running and vibrating. the same is true of poor connections in the coil box and elsewhere, including the ignition switch. A timer that is off-center may show good results at low speeds or in engine-off tests, yet perform very poorly at normal running speeds.

[Charlie B in N.J.]

The comp is OK. Plenty in fact. After rereading your post my first suggestion would be the coils. What capacitor do they have? It might be the original type and need replacing with the modern type. Secondly have they been set up on an HCCT or similar electronic tester? This is CRITICAL for good performance. Can't stress that enough. If they've never been done they need to be done. You can move forward with diagnosing from there but it's a starting point.

[George Mills]

The one is high yet within the rule-of-thumb 10% of the others?

That leaves carbeuration and/or timing and/or ignition.

Some thoughts...

On the carb, take the rough running and open the carb by 1/2 turn. It will either run better, or not... Then return it to where started and take it 1/4 turn in. It will either run faster and smoother, stall, or...no net change. It Will show you something. If it runs better 'lean', watch the manifold. If it starts to cherry red, stop and but the carb back to where it was.

The plugs could be suspect. Champion X say set to 025. Some do work there, my experience is that most don't. I always set them now to 030 and have never had issue...but have every time I try and prove that 025 'should' work.

Intake manifold leak? Very carefully spray a quick hit of carb cleaners at the gasket on the block while running...does it speed up?

Do the screwdriver test by shorting the cylinders one at a time. You may have one that just isn't getting there. Short the plug top tp block one at a time...there should be a noticeable change in RPM as each is shorted. No change? Thats a dead cylinder and troubleshoot from there?

When you say rough runner...is that on mag or batt? You say Holley G so maybe B4 'batt'? Hook up a lantern battery if you haven't a 6 volt source. It will last long enough to let you switch back and forth between source...it 'might' show a real difference, might show nothing (such as cruddy mag post, or intermittent mag)

Others might be able to suggest other things....keep telling yourself>>>>>its fuel, compression, ignition, timing. That's it

[John Codman]

I do not agree that the compression is OK. The high cylinder is more then 20% above the others. In Salinas, Kansas your compression pressure will be a little lower then those of us that live closer to sea-level, but either you have three low cylinders or one high one. If it's a 6:1 head I would vote for three low. Did the engine sit for a long period without being run? If so, it needs to be run for a bit under load. It could be that the compression will even out a bit. You mentioned doing a leakdown test as would be done on an aircraft engine; was compression being lost through the tailpipe? Ditto the carburetor; ditto the radiator; ditto the crankcase. From what you said about the radiator, my vote would be the head gasket. At least that would be my hope, anything else would be ugly. If your car really has a 6:1 head, the compression in the three cylinders that are close together are kind of low.

[TXGOAT2]

Valve clearance may be too tight on the 3 "low" cylinders.

[TXGOAT2]

I've had good results hooking an air line to each cylinder in turn with that cylinder on TDC of the power stroke. Even a slight valve leak can be heard at the carburetor or tailpipe. Have the car in gear and the brakes set when doing this to keep the engine from turning.