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TOPIC: '79 KZ650SR Compression Low

'79 KZ650SR Compression Low 16 Feb 2020 18:17 #819467

  • kgkomm
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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!

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Last edit: by kgkomm.

'79 KZ650SR Compression Low 16 Feb 2020 18:58 #819468

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

The engine will need some miles before the rings seat.

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'79 KZ650SR Compression Low 17 Feb 2020 06:09 #819481

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My first thought is to check the gauge. My second is to check timing and valve clearance.
I have a couple of Z1B's restored, an '80 KZ1000LTD restored, a 1981 KZ550 restored and a 2008 KLR 650 for off road fun. My wife has a 2019 Suzuki DR 650 for on and off road.
The following user(s) said Thank You: 650ed

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'79 KZ650SR Compression Low 17 Feb 2020 07:25 #819485

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I'm surprised the engine ran with 50-70 compression. If that is really an accurate number it is awfully low. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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'79 KZ650SR Compression Low 17 Feb 2020 11:39 #819497

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Maybe he forgot to pull full throttle on the carbs.

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'79 KZ650SR Compression Low 17 Feb 2020 11:43 #819498

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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.

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'79 KZ650SR Compression Low 17 Feb 2020 12:38 #819501

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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.

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