RD350 31K COMPRESSION

[gagzy]

Hi Guys

Bike been difficult to start for a long time and doesn't feel like much compression on the kick start so thought I would do a compression test.
Warmed the bike up, removed both plugs and fitted the compression tester, kicked over until it stopped rising- had about 55 psi on both cylinders. Guess this is well low as most threads seem to be pointing to around 115psi. anyone think it could be anything else before I do an engine strip? bike never been rebored, done about 10k miles and running original pistons and rings. No oil leaks, head bolts tight etc. Do you think its just wear? just a little strange as really close readings.
I bought the bike a few years ago from a guy that had started to restore it, he said he had taken the barrels off and all looked good so he just fitted new gaskets and put it back together, beginning to think there may have not been any visible damage but it still might have been worn in there!
Any thoughts appreciated. Probably time to do a strip anyway.

[jessy03]

55 psi is pretty low, are you sure you have the right compression tester?

Does it rattle - piston slap?

[gagzy]

Nov 26, 2023 17:42:12 GMT 1 jessy03 said:

55 psi is pretty low, are you sure you have the right compression tester?

Does it rattle - piston slap?

Hi Difficult to say, there is a rattle coming form somewhere which does not stop when clutch is pulled in so possibly.
Checked compression tester on a TDR 250 i just finished a full rebuild on and that was well into the 100's.

[jessy03]

Sounds like a strip and rebuild then 👍👍

[gagzy]

Your prob right, should have done it when finishing the resto really!
Thanks for advise

[midlifecrisisrd]

That is very low

On a genuine 10k motor that's been treated well I'd expect just a ring change but tend to fit new pistons too

But...

With it being that low unless it's had a seizure I'd suspect the rings are gummed up and not expanding against the bore

That top end needs to come off and get checked

Steve

[steve63]

I've never done it myself but apparently if you put a couple of drops oil into each cylinder and do the test again and the results change quite a lot then it points to bore and rings etc. being the issue. My guess is a ring or more are stuck in. Odd that it's both and they are very similar though. Time to get the socket set out.

[gagzy]

Thanks for all the responses guys- Started the strip already not going to bother trying to diagnose any further as a strip will take all the guess work out of it. As I haven't checked anything since owning going for coolant and oil drain and whilst there might as well check the clutch and basket etc. too.
Hopefully it is just gummed rings etc. At minimum do a top end rebuild whilst its all apart. Hoping crank is good though! soon find out.

[freddie19]

Had this same problem on my LC2 a couple of years back and when I stripped it down both pistons had the rings gummed up in the grooves.

Fitted new pistons and rings and I still haven't managed to get the rings unstuck on one of the old pistons! 😁

[vectisitch]

A cheap compression tester i had showed 60 psi on my 31k. I borrowed a mates one and it showed 125psi. The ones you get on Amazon are crap

[abar121]

Did you open the throttle when testing the compression?

I've never seen one that low that runs!

[JOHN-DYNOSTAR]

please check your squish clearance before you remove the barrels.
if it has chinese barrels or cornflake base gaskets.

[gagzy]

Hi Guys,
Have stripped engine out, took off barrels, both seem in really good condition, bike only done around 10k from new so on original everything.
Bottom end looks like it has been getting nicely lubricated, Bores don't appear to be worn with no marks or damage at all. Head no damage what so ever. Pistons unmarked but totally smooth skirts, rings not damaged. No evidence of up/down wear to crank. Possibly might just have got a bit of ring and piston skirt wear?
Was thinking whilst its apart might as well do a top end rebuild- new small ends, pistons and rings etc. but keep to standard size.
Even more confused about loss of compression now but might as well do a refresh on the top end and work my way back - What's your thoughts guys?

[muttsnuts]

measure the bores, piston skirt to bore clearance and the ring gaps, only then will you know if they are worn, just looking at them won't tell you anything about wear, be sure to check the bores mid of the ports as that is where they wear the most