On full throttle up a large climb my bike has started to rattle a bit. It only seems to happen when im in second and giving the bike some.

I fear this may be expensive in the future. Any ideas what this may be? and any ideas to postpone my fate?

Thanks

Edited December 20, 200817 yr by Beta Bodger

Low Octane gas and/or Lean Jetting will cause that. Your smart to head it off as it can lead to big problems.

If your on full chat up a hill and its knocking as laser says main jet too weak, do a plug chop at the top of the hill and see if the plugs white. Low octane fuel can make bike knock as well, higher the octane less knocking(octane is a measure of anti knock) could also be the floats set wrong so the carbs lacking fuel/running out at that angle

Another possibility for lack of enough fuel under sustained load is to check for a restricted fuel vent. Easy to check for by comparing gas flow with/without cap on.

Are you sure you are anywhere near full tilt when this occurs?

Hi thanks for all the advice so far. I'm no where near full tilt and it only seems to happen if I have messed up my gears and gone up a steep hill in 2nd rather than 3rd..... I am runnning 99 Octane fuel which is sold at my local gas pump.

I checked the plug from cold the other day but appreciate I would need to do a chop to get a true reading.

Looking at the plug I thought it may have been running rich. I will try and give it a tune up at the weekend and reset the air screw.

If I then give it a plug chop and I should be able to see what I am running like.

The bike has been running strangely (jumpy with little throttle) and I though it was down to the absence of an atmospheric tube hanging off the carb. I have now put another bit back on.

Thanks all, I will post an update after I have had a go with it.

P.s I just thought the previous owner did retard the timing a little to calm it down.

Edited December 22, 200817 yr by Beta Bodger

I suppose you are running the Kiehin if this is a Caby bike?

What jets? Which needle? Whch position? Is the carb clean? Many posts in the past, these things are sensitive, and a bit of ping?

Ok....an update. The bike after much fiddling with the air screw is now running fine!

The carb was clean and appeared to be set correctly. So I reset the air screw (its a Keihin) and did a plug chop at the top of a steep climb and the plug was a nice light brown colour. Now The bike is running fine but i think that it may have been a little too light brown so i might adjust the carb to let less air in to richen the mixture. There was no oily deposits.... how white is lean on the plug?

Thanks all!

Light brown is a good colour at the top of a steep climb after full chat, expect a plug to be light brown to whiteish after a chop like that on full throttle. I'd say yours is jetted right mate

Edited December 30, 200817 yr by The Addict

Ok....an update. The bike after much fiddling with the air screw is now running fine!

The carb was clean and appeared to be set correctly. So I reset the air screw (its a Keihin) and did a plug chop at the top of a steep climb and the plug was a nice light brown colour. Now The bike is running fine but i think that it may have been a little too light brown so i might adjust the carb to let less air in to richen the mixture. There was no oily deposits.... how white is lean on the plug?

Thanks all!

BB,

The airscrew only fine tunes the off-idle circuit, so if you're worried about a lean condition on a hillclimb, you'll probably need to change the needle setting and/or the main jet (depending under what conditions the plug reading is taken).

If you're tackling a big, long hill involving sustained upper/WFO throttle, you might think about checking the static timing (I think you mentioned it had been retarded) as this sometimes causes problems under these conditions.

Cheers.

Jon

Oops, forgot to answer the question...Doh!

The color (o.k....colour) will depend on, in part, the type of fuel you are using (some race fuels will give a grey cast to the insulator when the jetting is correct) but generally you'll look for a milk chocolate tint on the insulator and don't forget to look down inside the plug cavity as that's a little more accurate.

Edited December 31, 200817 yr by JSE

And just to reiterate, even one drop of water condensation or a bit of dirt can really cause issues like this! Many times you will never spot the culprit!

I was never satisfied with the single bowl vent on the Kiehin that pointed toward the muff. Easily blocked without the ability to see it, and to close proximity to the muff to get a hose on it. So I drilled out the bowl vent tap on the Left side and installed a short downward hose as a secondary. As you see, if the bowl cannot vent to atmospheric pressure, it will throw things off considerably, specially under load!

Long story made short , they must be clean!

Just picking up on something you mentioned about the previous owner retarding the timing, If the ignition is retarded the engine will knock under load, just like low octane fuel will. Also, just to confirm, the airscrew, or mixture screw as it is sometimes called, is only effective on tickover. As soon as you open the throttle, the slide controls the air in the carb. I would suggest you put the ignition timing back to the correct factory setting.

Thanks for all the info guys. I will play with the timing and see if things improve. Although its not too bad now...

My 05 Sherco 290 used to knock a bit. Higher octane fuel helped a lot.

Retarding the timing tames them a bit. I would fully retard it and work from there. Moving only 1 - 2mm will make hardly any difference.

Those nice people at Trialsbits are the cheapest for flywheel pullers by a long way though you have to know the size you want.

Scraggydog