1967 Dauntless Compression Pressure

[bmorgil]

Yes you are correct The service manual focuses on how well the motor compresses during the first four strokes. If compression jumps up "quickly" it indicates good pressure. If it is slow to come up and needs all four strokes to peak, it is getting "weak". If it is varying between cylinders, something is up. On these motors this is still a good way.

This is vague to say the least! Back in the 40's and 50's however it did the job. Basically back then, if it didn't pump up quick, was hard to start and smoked all the time the rings were bad. If it did all that and didn't smoke all the time, the valves were bad. These little guys "wear their heart on their sleeve". Now in these days we do like to know the numbers. The specification for the L and F motor is 90-110 and 120-130 respectively. Bellow those bottom numbers and you would start to watch for symptoms of low compression. The top numbers are a best case, engine warm and all new. Using a leak down and/or a compression tester remains a bit of a mechanic's call and technique derived from experience. Performance drops off as compression falls. As long as it is running acceptably, some are hesitant to touch it. Most builders are going to start looking when the pressure on a warmed up engine is off by 15% on a leak down or 25% on a cranking compression test. Power is going to be off at this point. It will be slow to start. It is probable it is going to start fouling soon.