TOPIC: '79 KZ650SR Compression Low

This weekend, I tore apart the engine in my '79 KZ650 SR because my compression was low (50-70 psi across all 4 cylinders with a slightly warm engine test). I replaced the head gasket and all the piston rings. I checked to make sure the valves weren't leaking. I put it all back together tonight and checked the compression. Essentially no change. It's still about 50-70 across the board, and I'm about ready to pull my hair out.

I tested each cylinder while cold (haven't run the bike in several weeks) with the other spark plugs in because I didn't know that it could make a difference. The carbs are off with only the intake boots attached. The exhaust is also off because I had it up on a motorcycle jack to work on it.

I'm planning on putting a little oil in each cylinder tomorrow and running the bike for a bit before I test it again because I've read that those can both make a difference. But I feel like those things adding almost 100psi is a longshot.

Is that low of a reading normal for a cold engine? I need all the help I can get. Thanks guys!

Did you adjust the valves and bottle brush hole the cylinder bores?

The engine will need some miles before the rings seat.

My first thought is to check the gauge. My second is to check timing and valve clearance.

I'm surprised the engine ran with 50-70 compression. If that is really an accurate number it is awfully low. Ed

Maybe he forgot to pull full throttle on the carbs.

From these comments, maybe my gauge isn't any good? I bought the compression tester new off of Amazon. The brand is BlueDriver I think and it had good reviews. Maybe I got a faulty one, who knows.

I've done the valve clearances and checked them. All at 0.15 mm, or within spec. I will check my timing again. I set it according to the clymer manual I have.

Engine needs to be at operating temperature, remove all 4 plugs, disconnect power to the coils and spin over for ten seconds on each cylinder with wide open throttle, you will then get a true reading, but before you do this put a few miles on it at various engine speeds to bed the rings in again.