Leak down test - slightly disheartened!

[ringding]

I've built LC engines up a lot of times over the years, although not in the last 10 or so. I've never previously performed a leak down test but thought I'd give it a go out of interest, not really expecting to find any issues. Boy was I wrong!


First off the joint between the carb rubbers and the reed block was leaking like a sieve on the RHS. Then I had to deal with a slight leak from my testing set up, and finally the balance tube. I was still left with a loss of 2.5psi over 5mins. Having exhausted all other options I took the flywheel off to get to the crank seal on that side, and it's the culprit.


It's a new one, came with my crank rebuild, but I had the crank done a while ago and I don't kno if it was gen yam or not.


Anyway, whilst I'm very impressed with the leak down test (and wish I'd known about them years ago), I'm a little despondent about having to strip the engine down again. I know it doesn't take long but my free time is hard to come by so I don't get much opportunity to work on the bike.

Are there any short cuts? If I flip the engine can I get to the crank seal by splitting the cases and leaving the top end in place? I can't quite picture how it'd work. Any advice?

Cheers.

[mikee]

I’d build it as is then check again after a few heat cycles , seals can take a wee while to settle
But that’s me

Mike

[dusty350]

The inlet rubbers are often found to leak, having read a few peoples experiences of leak down testing. I always use clear silicone on new Yam gaskets to try and avoid that. I had a cross over tube leak too - new Yam inlet rubbers, and I had left the clips off, not thinking it would leak, but it did, and fitting the clips solved it. Crank seal is different though. Guys have flipped the engine and removed the bottom case, so not disturbing the top end, but I have never done it myself. Cant see why it wouldn't work, although you may be able to reuse your head gasket if it's newish, and base gaskets aren't stoopid expensive like pv gaskets are, so doing it the way you know probably wouldn't take too much longer, may be easier too.

Dusty

[ypvs400]

What pressure did you use ?

[ringding]

Cheers chaps. I don't really want to run the risk of having to take it back out the frame so havtalked myself into stripping it back as normal. Just a bit frustrating.
I only used 5psi when presurrising it, thought that would be low enough.

[markhoopy]

Oct 20, 2018 20:36:46 GMT 1 ringding said:

If I flip the engine can I get to the crank seal by splitting the cases and leaving the top end in place?

Yes you can do this relatively easily to save stripping the top end.

[mikee]

Check the crank end where the seal sits for wear when it out

Mike

[stusco]

Use tie wraps around a stud to hold the crank in the top half it’s a pain but better than a total strip down I also find it leaks at the primary drive cog so I put locktite on the shaft to seal it

[ringding]

I stopped sulking, man'd up and stripped it down.

There is a polished line around the crank where the seal sits. I can't feel a groove though. The seals on this end of the crank aren't supposed to be handed are they?

I've ordered new gaskets and will start on putting it back together week after next.
Thanks for the advice.

[stusco]

5 psi is fine

[4l04ever]

Put some oil on the crank where it goes through the seals, as they may leak if assembled dry. I had a very slight leak previously, but ran engine then rechecked and it was fine. Seems like some seals may need to be run to get a good seal.

[mikee]

As rob says ^

[ringding]

Just rebuilt it and it held 5psi for 10mins. No drop at all. Also managed to improve the squish to 1.1mm. Very pleased. Lesson learnt. Always use genuine Yamaha crank seals.

[dusty350]

Worth the effort. Great news 😉

Dusty😊