Compression tests

I did a bunch of compression tests on the Bandit this weekend, as the sparks were looking pretty weak and the bike doesn’t seem to be feeling that well in general (shockeroo).

The dry compression test went fairly well; all four cylinders were in the ~140psi range. I don’t remember the exact numbers offhand, but they were all right at the low limit of acceptable. The manual says compression should be between 140-206psi. Have I already talked about this? It seems familair. Ah well. Losing my mind already.

Anyway, since the readings were low, I figured I’d do a wet compression test. I’d never done one before, but here’s how I understand it: you put a small amount of oil (a tablespoon or so) into the sparkplug hole and then redo the compression test. The oil will seal up any gaps/holes in the piston rings, so if the new readings are a lot higher than those from the dry test, it’s probable that you have piston ring problems. If the readings are the same, then the oil didn’t make a difference and your problem is somewhere more in the top end (valves, etc).

I think I did something wrong with the wet compression test, though, because suddenly, I was getting really super high readings: mid- to high- 200 psi. That just doesn’t seem kosher. I’ve been thinking all day about what could cause the compression readings to go sky-high after adding oil.

It was suggested on ST.n that I added too much oil (entirely possible as I just eyeballed it), thus changing the compression ratio to 14:1. I haven’t wrapped my head around that explanation yet, though.

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3 Responses to Compression tests

  1. Alex says:

    Hey Carolyn,
    Another way to test, would be a mity-vac. I have one that I use to bleed my race-bikes brakes with. (Much quicker). They sell cheap add-on items that you can use to test compression with. Its much easier to tell via mity-vac if you’ve got leaky valve/valve seats, or etc.
    How many miles on that bike? I just got done rebuilding my 900SS/CR with 38K on it, and the rings we’re fine. I replaced ’em anyways, but I doubt your rings have problems. Maybe carbon build-up on the valves, so their not seating correctly? *shrug*. Take the engine apart, its fun, trust me. 🙂 (I’m serious!)

  2. carolyn says:

    It’s a 1991 with 5200 miles, meaning that it’s been sitting around for a loooong time. Many parts have rust/corrosion/weather damage (it sat around outside, uncovered).

  3. Michael A says:

    Another thing to try on compression tests is to make sure the throttle is WIDE OPEN – otherwise you can get false-low readings because of the restriction of the throttles being closed… as to why your wet readings were so high, well, I don;t have a clue! But it is consistant with my experiance in doing comprtession tests. Let it sit a while so the oil will drain down past the rings, and I think you’ll get a more accurate reading.

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