Compression Loss # 4 cylinder

[Greg Greer]

After rebuilding and adjusting the carburetors on my 66 912, I was trying to read the spark plugs. Cylinder numbers 1,3, & 2 looked minimally black. Number 4 looked brown. I kept trying to match the other mixture screws to get a brown look on all plugs. At some point, the # 4 cylinder plug started looking its metal color, no longer brown. I had only been adjusting on the 1,3, & 2 cylinder mixture screws. However, the # 4 cylinder is no longer firing. I cleaned the carburetor out, have spark, new distributor cap, adjusted points gap, set timing, and adjusted the valves. The # 4 cylinder is still not firing, and only tests at about 65 lbs. of compression. The other cylinders are around 100ish. Could I have burned a valve this quickly, during the approx. 12 hrs of run time (no high RPMS, or abusive running) after the rebuilt carburetors were reinstalled. Engine had always ran on all 4 cylinders before this. It seems like a fairly sudden and dramatic compression loss Any advice greatly appreciated.

[raygreenwood]

[Greg Greer]

Thanks for the quick reply, Ray!

Yes to the valve adjustment.

Wouldn't a head to cylinder leak affect Number 3, too?

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would not necessarily. As you know no head gaskets on this one, so valve lapping compound to seal before assembly. However usually a small head leak will pump compression, just not leak down very well. SO if the valves are in adjustment, I would do a leak down check (differential cylinder pressure test) to see if that is the problem

[Starbucket]

Recheck the valves as they tend to move during break-in and retorque the cyl. heads if the valves are good. A very loose valve measurement might mean a valve sticking in guide. Check #4 plug wire for spark at the plug with that plug removed and grounded as a lot of plugs are being made in China and are bad out of the box.. Look down the carbs for a good pump shot of gas down both bores, as a piece of grit might be blocking the accelerator pump tube and if that is good remove that idle screw and blow some air in to unblock any grit that might be blocking it. A good way to check carb linkage is to hold a straw down each carb to the butterfly and have someone slowly step on the gas, both need to start opening at the exact time. That's all I can think of to find the problem.