Does my RF900 engine need a rebore?

[Legotoy]

When I put my GSXR/RF motor hybrid together I suspected at the time that the cylinder bores were past it, or the rings gone, or both. I figured at the time I would run it - better to check it worked before throwing loads of money at it.

Today I got round to doing a compression test:

#1 was 95psi

#2 was 115psi

#3 was 90psi

#4 was also 90psi

So the cylinder pressures (cold) are between 17% and 21% variance of each other. When I dropped a touch of oil down the bores and re-tested the reading shot up to over 190psi. That alone would indicate the bores are shot, as opposed to valves for example.

How bad is 90psi? Rebore straight away or rag it til she blows? The motor runs but feels a bit limp... It doesn't smoke or use oil strangely though.

[skelly]

Dude.

For an accurate comp test, the engine needs to be at operating temperature.

Also an equal number of comp strokes on each cyl is needed for ballanced and repeatable results, with open throttle also.

Assuming that this is what you did, then I would say that you have a problem.

You need to get the bores measured for size,ovality, tapering, barreling and choking.

If the bores seem ok or will go ok with a corrective hone then you may be in for just a set of rings providing the pistons check out and your skirt clearance isn't excessive.

Cheers.

Skel.

 

Edited July 21, 2016 by skelly
forgot the bit about the throttle.

[Legotoy]

Thanks for the reply Skelly.

I didn't count the strokes, but did hold the starter for the same amount of time, did two tests and took the average. Test was cold since the carbs are off - but no problem with throttle being closed!

So the results will be under-reading anyway, and less than perfect, but sounds like there is a problem requiring a strip and measure at least. Bum.

It is interesting to me that the comp shot up so much with a little oil in the bores...

[Gixer1460]

The compression rings are likely shagged out - oil rings do little work so are less likely to wear and hence seal well when your shot of oil was dumped on them! They are designed to scrape oil back down not drain oil from above. Hot or cold is immaterial - you are looking at trends and as recommended pressure is around 170 -190psi (guessing) your's look terminal.

[t3rse]

With auto's, if you add a little oil to the bore and the compression numbers shoot up, then you know it's the rings and not some other problem like valves (could still be a cracked piston if one is flat).

[Legotoy]

Thanks guys for the replies, that is really useful. Shame that it confirms what I thought - rebore time! And I only just got the thing on the road too. I suppose that's what I get for using used parts.

thanks again.

 

[Sheep]

Rebore or big bore

[cregnybaa]

Did you check the valves before you built it, then check clearances.