Sherco 250 crankshaft seals/'hunting" - Resolved

[Grasbo]

Thanks to those who replied with great suggestions. After looking at all suggested items I decided to throw the bike on the trailer and shoot it across to the folks at Powerhouse Pakenham Vic. I described the issues and took a look at it for me. The bike runs absolutely perfect now. Starts first kick and idles beautifully. The final outcome was that the air/fuel mix screw was in too far (out 1 1/2 turns - factory setting). The air /fuel screw is now out at 4 1/4 turns and held in place with a dob of glue so it doesn't fall out. I still believe that the previous work I did contributed to the final outcome......Gotta give myself and those below some credit.
Thanks again,
Graham


Folks,
I've been having some "hunting" issues with an '07 Sherco 250 that i bought. So far I have cleaned all fuel tank lines and carby several times, set the float level to 19mm, replaced the crank seal on the flywheel side, and retarded the timing. The bike is running much better since the first ride, but still hunts. I am now looking at replacing the clutch side crankhaft seal. This is where I am now confused as the exploded parts diagram shows the same seal on both sides of the crankshaft on the outside of the bearings and after removing the clutch, there is no seal, just the bearing. The depression where one would expect the seal to sit is a larger diameter (about 1mm larger) and the depth too shallow for the seal to sit. Am I crazy (dont answer that) or is the exploded diagram wrong and the seal on the other side of the bearing?

Your help would be much appreciated. Otherwise my next step is to find a good mechanic to find the issue for me.
Cheers,
Graham Johnston
Trials newby Oakleigh MC

[David Lahey]

Maybe your parts diagram is for an earlier model motor. On some modern trials two-strokes, the drive side main bearing is lubricated with gearbox oil ( the crank seal is inboard of the bearing).
Anyway, unless you are losing gearbox oil into the crankcase, I would not be changing the drive side main seal. Oil loss from the gearbox should be easily detected by having a smoky exhaust and the oil level dropping with use.
I would be looking elsewhere for an air ingress site unless you have the oil loss symptoms.
Other sites are:
cylinder base gasket
Reed cage gasket joints
Carby boot to reed cage (cracks)
Clamp joint on front side of carby to boot
Crankcase centre gasket
If you can't see anything wrong with these sites (visually), or can't see any tell-tale traces of oily dust at the leak sites (from the air-fuel mixture), you can spray the sites with WD40 at the time the engine is hunting, and if the hunting stops, then you have found the leak site

another thing to think about is the possibility that there might be something partly blocking the pilot system somewhere, that your cleaning technique might not be cleaning out. I've seen this a few times - the passages are tiny, especially where they emerge into the venturi. You should test them for clear flow while you have the carby apart, ideally using a liquid so you can see the flow pattern.

[Grasbo]

Thank you David for your prompt reply.
I thought the same thing about the parts list but it seems to be the same across all models. I'm sure you're right that the seal is on the other side of the bearing.
Here's a handy site for Sherco folk: http://www.shercousa.com/content2.cfm?action=9
The bike is puffing a bit of smoke, but since I'm historically a 4 stroker I'm not too sure if it is normal or not. I would assume it would be more prevalent at higher revs. Will need to look closer at that.
As for visual and CRC testing, yep I've done that without success. the gaskets look dry all the way around.Intake manifold and associated bit look ok too.
The carby has been apart a few times now with all components stripped off, blown out with air compressor and all jets, holes poked with different diameter fishing line and blown out again for good measure. cleaned tank, lines and fuel filters and have inline fuel filter. replaced orings on jet screws.
As you can see apart from that one seal, I'm clutching at straws.
Cheers,
Graham

When you're on top of the world, remember it turns over every 24 hours.

[Freestyle pilot]

I have just bought a 2008 Sherco 250 with the same problem. Once its hot, it hunts.
Haven't done seals yet, but have checked everything else. Timing is 3mm BTDC, and I assume thats right. Bike also tend to Ping, so Im suspecting the 98 fuel I bought is not 98.Anyone got some answers? I cant find intake leak with WD40 or Start you bastard.

[paulm]

One thing worth checking is that the fuel tank breather isn't blocked
Paul Mac

[kimbo]

Hi Grasbo, Freestyle

I had the same issue with my 2010 250 Access. It was a air leak at the reed block. Initially difficult to find.

Try a new gasket and clean all surfaces really well.

Cheers

Kim

[Freestyle pilot]

Found my problem. I had just bought the bike, and it ran fine on the test (still cold). I had drained fuel, and done gearbox oil, but hadn't checked the radiator, as its such a pain to do on a Sherco (this is my first Trail bike). MX bike are easy, and checked very regularly.
Decided to check the coolant level, as the fan seemed erratic, and found there was very little coolant in it. Topped it up, then found a 3mm hole worn in the top tank, so fluid pissing out. Repaired all this, re-tuned the bike, and it runs sweet as a nut. The problem was it was overheating and getting pre-ignition. The fan erratic, as there was so little coolant, the thermostat wasn't working.
Anyhow, the bike survived, and just proves you must check EVERYTHING.

[David Lahey]

If the fuel screw needs to be so far out, then while it may now run OK, having the fuel screw out so far means that there is an underlying cause that has not been fixed. If that underlying cause is air ingress, there will be unfiltered air entering the engine. Hopefully the underlying cause is something else, like a problem with the carby idle circuit.

[Mark K]

David, it was Grasbo (the original poster) who had the problem with the mixture screw.

Freestyle Pilot joined the conversation later, and never mentioned his mixture screw, therefore I'd be confident he has it resolved, unlike Grasbo who never told us any further results.

If I'd been chasing a problem that elusive for that long I think the bike would be at the bottom of a cliff somewhere.

EDIT: OOps Sorry, I thought you were replying to Freestyle Pilot who had posted last night but after writing the above I now see that Grasbo at some stage edited his original post, and yes from what little bit I know 4+ turns is somewhat excessive.
One day I might actually look properly and even think before I make a comment ..... maybe.