Just Fixed One And Then What Happens.....

[peterb]

Hi Neo,

I still reckon it is the mag side oil seal. Symptoms lead to this.

Have you replaced this?

Yes, I see that you have pressure tested the cylinder/crankcase area, but, when these seals leak they allow air to enter the crank case under vacuum, not under a positive pressure. I don't think that a pressure test would always show up a leaking crank case oil seal as we are dealing with vacuum, rather than positive pressure. I would say that a positive pressure on the inside (Spring lipped area) of an oil seal would provide a tighter seal to the shaft.

Well, that's my theory for now.

Bye, PeterB.

[neo]

Hi Peter,

Hmmm...you've got me thinking now.....very smart stuff my friend...very smart indeed :rolleyes

I'll investigate a the potential of a loose needle as well ...some merit there too.

Neither of these two theories would account for why it clears after richening the mix or why it never shows an full tank of fuel...but who knows

Cope, I've been running 98 Octane Premium unleaded for the past 2 and half year.....And Castrol TT Racing oil for the past 12 months....I love em both

Best of balance.

Neo

[neo]

Ok Gents,

I've decided I need to "Suck" on this problem

But how do I create a vacuum in the cylinder and how would I hold it there and measure it?

And how could see that air is going in through the seal?

Would this be called a "Leak Up test"

Best of balance.

Neo

[stork955]

Gday Neo, I have a Mityvac unit that I use for this stuff. You could hook up the missus' vacuum cleaner I spose, and look around for an old Inlet manifold pressure gauge like the ones that came in "tune up" kits years ago, you'd probably get one off ebay for a couple of dollars. This covers the basics. After that If I suspect a leak from a particular seal (the one under the stator particularly) I cover the area in grease and see if it stops. Have a look at -

http://cgi.ebay.com.au/RYCO-Vacuum-Gauge-i...#ht_2007wt_1166

and

http://cgi.ebay.com.au/New-in-box-52mm-Vac...#ht_1504wt_1166

You don't need mega accuracy, just a check to see if it changes.

Cheers,

Stork

[copemech]

Something like the Mityvac is the way to go. You should be able to adapt to your "plug" hole adapter.

Sealing around inlet and exhaust should actually be easier, as a small plastic plate with some mastic putty should be self sealing under vacume!

Remember you are looking for small leaks! Pulling large vacume makes things seal betteron the seals! Light suction, 2-3 in. hg. can reveal things that might otherwise be missed! Put your mouth on the hose even! Tounge against hose! Should hold!

Might need a long hose if you try this at a trial, so you can sit up in a manly way whilst sucking!

[peterb]

I would have just put a new seal in and be done with it, can't be more than a few dollars.

Bye, PeterB.

[neo]

Yes I know what you mean Peter,

It's not the money....it's the learning, the diagnosis, the discovery and the proving that make it feel like winning to me

And after all this time I don't really wont the problem to just suddenly disappear without knowing why .....with 100% confidence.

And I love working on Trials bikes.....(hate working on cars )....and I love discussing this stuff with those that know

Hope you know what I mean?

Best of balance.

Neo

PS...Thanks Stork and Cope....I'll come back to you after I've done some damage to my lungs or my wallet .....already found an Inlet pressure gauge ....only had it 25 years...still brand new!!!

Edited August 28, 2009 by Neo

[clav3]

Hi all,

I've been trying to find an answer to a similar problem.

Just on the leak down topic, my 290 wouldn't hold 5 psi and would slowly leak down in about 5 minutes, but would hold 6 psi indefinitely, tested several times.

I searched the net and found a site that suggested all new bikes should be tested, and minimum leak down pressure should be 6-8 psi. "Perfection is an engine that will hold 6 psi for 20 minutes" Quoted from said site.

Has anyone actually seen a sherco hold 2-3 psi. Mine sure won't!

Also how much pulsating pressure would actually be going on inside a running engine. Probably alot more than 6psi.

I didn't change seals but did replace piston and rings.

In less than one year the rings showed more than 0.020 inch wear. That's actual wear, 0.020 was the difference measured on the ring gap.

My bike now runs great with the Keihin. The funny thing is, before I did up the top end, it ran well with the Dellorto but not the Keihin or even a brand new Keihin.

My theory is if you're going to use a Keihin you need to keep the engine fresh.

This might be misguided, but the proof, for me, is sitting under my house waiting to be ridden this weekend.

[peterb]

Hi Neo,

It's the same for me, don't care for working on car's at all and really enjoy the bikes. I'd still put in a new seal though!

I guess you'll use an adaptor in the spark plug hole, block off exhaust and inlet and use a vacuum pump with vac pressure gauge tee'd in.

Clav3 - that's a huge amount of wear after a year, are they dykes type rings (L-section) or plain.

Bye, PeterB.

[neo]

Ok,

I'm gonna give the pump-up test another go too...But one thing that concerns me is when I set it at 2-3PSI, when I check the pressure (with a digital low pressure tyre gauge), I'll lose quite a bit of that pressure each time

But at least I can re-check for the soapy bubbles at that low pressure....I'm also going to carefully use heat around on the flywheel side of the case.

clav, are you saying that now you've done the top end you don't have the problem anymore?

Peter, yep I guess I gonna do them seals one way or the other

Best of balance.

Neo

[clav3]

I bought the rings from the Sherco importer in Aus. I assume they were standard factory. All the guys I ride with thought I was nuts doing up the top end so soon. One of them did the work for me. He is an engine reconditioner and has owned all types of bikes all his life. Very trusted. He was more shocked than anyone.

There were no other signs of engine wear, but I'd tried everything else other than seals. I was happy with the 6 psi leak down test so I thought the only other problem could be engine wear. And I was right.

Neo my bike is running so sweet! Its' been run in for about a month now and not a fault.

I've tried the Dellorto and it runs well too.

I realy believe that if your tuning at idle is on the border of just wanting to hunt for fuel, the effects of a worn engine show up when using a Keihin and don't when using a Dellorto.

Maybe that's why Dellortos' are still fitted to all standard bikes. They run well for alot longer.

Edited August 28, 2009 by clav3

[copemech]

This is somewhat interisting, and Clav could be on to something!

I can tell you for sure from my aircraft experience, rings are not rings! One can run soft(cast)rings in a hard(chrome ) bore OR hard (chrome) rings in a plain(steel) bore, yet never the two should mix or bad things happen.

The Nicasil is a hard bore similar to chrome and I might suggest the rings are indeed quite soft and may wear rather quickly. In a way I like my well broken in '07 kicking over rather gently as compared to a new bike, yet it does suggest a loss in compression.

Although I tend to dislike the Keihins for being finiky in a general sense, they may just bee that sensitive!

So, in theory, the seals are not the only thing that are in the motor that keep in pressures under positive or negative flux! The piston rings actually have to do the same thing as for the most part, themselves and the piston are what is creating the volumetric displacement to either suck mixture into the case or blow it up top to begin with, thusly, if this constant were to vary(weak rings), I suppose the overall mixture could as well, once exhaust backpressure was factored in because one could then displace the other and cause a lean condition as the partially burned mix takes advantage.

All theory,

[peterb]

Hi cope,

That's interesting, I often wondered about the plated narrow rings on the S3 pistons on Gas Gas running in a nicasil bore, if they need to be replaced more often that the CI dykes rings on earlier motors, which just keep on going and going, hmmmmmm.

Bye, PeterB.

[neo]

Neo my bike arrived in September 07 and I nearly ride as much as you, so it has a few hours on it already. When I did the engine teardown about six months ago to fix a twisted crank, new rings were installed and the piston was just within tolerence so I kept it. The problem may be piston wear, I dont know.

Hi clav....the fact that you'd changed the rings before suddenly came to me while I was riding today.....so you've been through two sets of rings in 6 months?? ....but why didn't it fix the hunting problem the first time??

Another thing I'm confused about is why would I have good (high) compression if I have worn rings??

Also, just speculating... some time back I did a post stating that my bike to needs a rev-up (clean out) each first ride of the day....Maybe....If I have a problem with a main seal and it's on the water pump side, maybe it's sucking in a small amount of gearbox oil while the bike is cooling down (at the end of my riding day)....leaving this oil in the crank case for the next time I ride.??

Peter, I'm thinking I might do what you suggest but do both seals...and what the hell.....I'll fit a new set of rings at the same time.

Best of balance.

Neo

[clav3]

Hi Neo,

I've dug up some exact dates.

New bike 27/09/07

After the engine was run in I replaced the standard head with a Cabestany head and inspected the cylinder and rings.

The factory ring gap was 0.009 inch and was re-set to 0.011.

The funny thing is, on Ryan Youngs forum, he suggests a ring gap of 0.020? Any comments are very welcome on this.

Eleven months after, 02/09/08., twisted crank, and replaced rings on rebuild. Piston tolerance was 0.0018 so I didn't replace.

At this time I replaced Boysens with standard reeds and changed idle jet from 35 to 42 and did not have a hunting problem until about nine months later.

30/06/09 I did up the top end. I'm sorry to all but I was mistaken about the amount of ring wear on my previous post.

The ring gap at 02/09/08 was set at 0.011 to 0.012 inch and on re-build at 30/06/09 ( that's 10 months after) measured 0.022 to 0.023 inch. So that's more than 0.010 inch wear in ten months.

I'd just like to add that I don't ride the whole time at 3/4 throttle, far from it.

Neo, with your having to clean out a blubbery engine when first starting, my bikes have always done the same if you don't carefully regulate how much choke you give them. Worse with Dellortos.

I hope this helps.