Author Topic: compression test.  (Read 237 times)

Offline ka-ja

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compression test.
« on: May 22, 2020, 04:33:33 PM »
Hi,
    Having carburation trouble with the old 460 Yoshy, are compression readings on a cold engine I to 4 cyls readings show  as 150,140,150,150 any good.
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Offline Bryanj

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Re: compression test.
« Reply #1 on: May 22, 2020, 04:37:13 PM »
Less than I would expect for HC piston but testers vary ideally check a good standard motor and compare

Offline ka-ja

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Re: compression test.
« Reply #2 on: May 22, 2020, 04:49:08 PM »
 thanks Bryanj, sorry but no standard motor to check against, the big bore is 6,000 miles and 35 years old, and new rings seem unobtainable, unless cruzin image rings will go on Yoshi pistons.
« Last Edit: May 22, 2020, 07:07:52 PM by ka-ja »
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Offline Oddjob

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Re: compression test.
« Reply #3 on: May 22, 2020, 06:04:00 PM »
Sorry not wishing to offend you but did you remember to open the throttle wide open as you spun the motor over? Easily forgotten.
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Offline ka-ja

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Re: compression test.
« Reply #4 on: May 22, 2020, 07:06:24 PM »
Compression test done cold using electric starter, no oil down the bores, all plugs out and carbs removed from the engine
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Offline Bryanj

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Re: compression test.
« Reply #5 on: May 22, 2020, 07:18:34 PM »
It does depend on the tester for example it it has a long lead from head to gauge it increases the "closed" volume by a significant amount on a small bore, thats why i suggested testing on another engine for comparison. Rings shouldnt be worn out yet but stranger things have happened.
Was it just the yoshi bore or did it get a cam and other stuff?

Offline ka-ja

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Re: compression test.
« Reply #6 on: May 22, 2020, 07:24:58 PM »
Balanced crank, rods, gas flowed head (26mm inlet tracts) Yoshi camshaft and HD cam chain. I have now found a damaged gasket on one of the carbs, could be the carburation problem, but the original question still stands, is the compression good enough.Test lead is about 320 mm long.
« Last Edit: May 22, 2020, 07:28:15 PM by ka-ja »
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Offline SteveD CB500K0

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Re: compression test.
« Reply #7 on: May 22, 2020, 08:27:01 PM »
What’s the compression ratio supposed to be?

400/4 is 9.2:1


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Offline ka-ja

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Re: compression test.
« Reply #8 on: May 22, 2020, 08:51:30 PM »
Not sure of the compression ratio, it is 35+yrs since I bought them.
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Offline Andych

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Re: compression test.
« Reply #9 on: May 22, 2020, 10:38:13 PM »
Leak-down test might be the better option. It will give you better diagnostics on any issues..
I would think that those pressures wouldnt be too bad considering the length of hose from the fitting to the gauge.
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Offline THUNDERDOWNUNDER

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Re: compression test.
« Reply #10 on: May 23, 2020, 01:53:41 AM »
When I checked my compression not  long back it wasn’t in the 150s  you got me worried now ! And my 400/4 is just standard . No fancy bits Agh !

Offline Andych

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Re: compression test.
« Reply #11 on: May 23, 2020, 02:31:37 AM »
The manual says it should be 170.7 psi with 9.4:1 Compression ratio. I couldnt find the upper and lower limits in the quick look I had.
I am sure it will be in there.
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Offline SteveD CB500K0

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Re: compression test.
« Reply #12 on: May 23, 2020, 06:33:57 AM »
One for the scientists:

If compression ratio is 9.4:1 and sea level air pressure is 14.7psi

Why do they specify 170psi?

14.7x9.4 = 138


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Offline Bryanj

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Re: compression test.
« Reply #13 on: May 23, 2020, 06:57:26 AM »
If anybody remembers the C50L, it was a C50 restricted to 29mph to make it a moped, single saddle etc. I had one in the workshop, bout 30,000 and would not tickover. Compression on that was 115 which i considered ok but Honda said 140 so took it apart and found burnt exhaust valve due to the restriction method being a washer welded into the exhaust pipe at the head. New valve comp about 145 and tickover ok

Offline ka-ja

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Re: compression test.
« Reply #14 on: May 23, 2020, 10:06:24 AM »
Thanks for the ideas, will go through the basics again, the engine should run and tickover judging from replies.
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