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2008 250 Rev3 Knocking


scoobystu
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Recently got this bike and it does a fair bit of knocking especially after revving it when the engine is coming back to idle. Gets worse when it is warm. Stripped and cleaned the carb but it hasn't made any difference really. It has a 48 pilot jet in a keihin carb.

If I turn the air screw right in it goes away but it doesn't sound right and no power (strangely enough).

Looking for suggestions to try next....

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Betas are known to not tolerate low octane fuel. They will knock and ping rather badly. The other common thing with the Keihin carb is they have tiny orifaces for the bleed air circuit for the pilot so you have to blow them out with compressed air to properly clean them. If you look at the exit hole at the bottom of the carb bore just behind the slide it looks fairly large but it isn't. About 2mm down it constricts to a tiny hole that is so small that a water bubble can block it and it will stay blocked because the surface tension won't let it evaporate. I noticed this on mine while looking at it through a microscope. I couldn't figure out what the silvery sphere was blocking the hole.

My advice is to pull the carb again and go at it with compressed air. Get some Torx security bits so you can pull the center tower.

After that good gas with no water. No ethanol if possible. For me this is a twice yearly drill.

Oh and don't run wire or anything else through the jets.

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Thanks for the response. I did a partial strip down of the carb and blew it out with compressed air. I will try it again and put more emphasis on that area. It had been sitting for a long time in a garage before I got it so it is likely that there could have been condensation in the carb. There is a fair bit of instant gasket around the inlet manifold. Might be worthwhile taking it apart and sticking new gaskets on it. I am pretty sure it is carb related. Could timing be incorrect... how do I check that? 

 

The sound does not disappear when the clutch is pulled in. It's not really a mechanical knocking noise. 

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Yes, it does sound like it's leaning out.

That's something you notice more when warmed up.

Also screwing the air screw in will enrichen the mix, same goes for applying a little "choke" which tends to counter the lean mix at idle.

 

The clue might be all the instant gasket, someone might have been chasing a leak for a while before you got it and attempted various "fixes".

 

Dan's right and right and right again.

Clean carb,

Only run them on 98 or 99 octane fuel.

Try some carb or brake cleaner spray, see if you can find the leak, the engine note will stumble if it sucks the stuff in, worth replacing the instant gasket goo with the proper paper ones.

Edited by goudrons
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  • 2 weeks later...

Quick update. I changed the pilot jet as the original looked a bit dodgy. Cleaned carb again and stuck it back together. Still knocking so I gave it a spray with carb cleaner around the inlet manifold and it died. Tried it several times with the same result. Will change the gaskets which will hopefully help. If not is it likely that the inlet could be dodgy? No cracks on it but when I spray at the edge it kills the bike.

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Stripped it and the face where the crankcases meet isn't great. I trimmed a piece of the crankcase gasket that was sticking out. It is a bit better now. The inlet manifold flange face is shaped like a banana so I will get a new one - hopefully it will be flat....

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You would hope that when you get a bike, or have to buy "matching" case halves they would at least machine some of the surfaces and holes with them bolted together. Otherwise what's the point. Sounds like you're on the right trail though. Pity there's no substitute for taking it apart and having a good eyeball. Something I have to do with my front caliper. It's sticking on and I'm wobbly enough without the bike making control decisions for me.

Edited by dan williams
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You might be lucky, it looks like someone before you had found the air leak and tried, with limited success, to fix it with all the instant gasket.

 

The gasket between reed assembly and mouth/crankcase is quite a thick one and it should take up some slight unevenness where the the two halves of the crankcases come together and form the inlet mouth.

 

I seem to remember thinking the same on all of mine, all the Beta's I've owned didn't really seem that flush in that area, though the manifolds were flat!

 

Bit of a tip, if you flip the airbox to carb rubber around the other way, it makes removing and refitting the carb easier, seems the airbox end is slightly larger and a bit more pliable.

Edited by goudrons
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Cured! Well it runs a lot better now that I changed the carb inlet boot. I also took the advice from the chap at Trials and tribulations and flattened the spacer and the reed basket on a piece of glass with some 400 grit sand paper. Looking forward to getting a proper run on it now. 

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