compression test.

[ka-ja]

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.

[Bryanj]

Less than I would expect for HC piston but testers vary ideally check a good standard motor and compare

[ka-ja]

 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.

[ka-ja]

Compression test done cold using electric starter, no oil down the bores, all plugs out and carbs removed from the engine

[Bryanj]

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?

[ka-ja]

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.

[SteveD CB500K0]

What’s the compression ratio supposed to be?

400/4 is 9.2:1


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[ka-ja]

Not sure of the compression ratio, it is 35+yrs since I bought them.

[Andych]

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.

[THUNDERDOWNUNDER]

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 !

[Andych]

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.

[SteveD CB500K0]

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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[Bryanj]

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

[ka-ja]

Thanks for the ideas, will go through the basics again, the engine should run and tickover judging from replies.

[K2-K6]

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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Thought it's because compression ratio is calculated from swept piston volume compared to combustion chamber size. With the psi data having to add the compression ratio X the remaining combustion chamber volume + the "138" psi to give you a theoretical total psi reading above 170..........to which you have to subtract any mechanical sealing losses.

Making CR the figure that is used by the designer to burn the fuel as they want it to.

The psi figure is effectively the dynamic total achieved within the components you've built.  Hence not hitting target on psi illustrates something is not working as effectively as it should.
That could be from a race type cam timing and duration for which you may have a designated CR but a different psi from standard equipment  because of  valve overlap etc.

In short you'd probably need a different psi reading for a hot cam, which could also vary based on how you time it too.