Jetting?

[shaneMember]

Erm…last time I rode (more than a few weeks ago!) up at Clarence I noticed that the bike was ‘missing’ a bit. Very noticable in fifth gear (yes, it only has bloody five gears!!!) and probably about 70-90 kays an hour. Acceleration was good as usual, just cruising along open trails the thing had a noticable miss.

Instead of tearing it down and checking the valve clearances yesterday I took it out around some secret tracks only to find the miss doesn’t appear at a lower altitude.

Question: this sounds suspiciously like a jetting problem, but before I start p1ssfarting about with jets and needles, does anyone think trying a higher octane fuel might work? I’m a tight bastard and the poor thing usually runs on normal fuel, not the good stuff!

Having said that I did notice the slide on the Dellorto is looking a little worn in places…maybe it’s that?

Don’t ask what size jets are in it at the moment…my memory has failed!

[PaulMember]

Yes it sounds like jetting, and if the instruction book says to use 95 or higher oct fuel then do that. 98 is even better.
4 strokes will ‘ping’ if the oct is too low, and that sounds pretty bad.
Is the air filter clean?
What bike model is it?

If the throttle slider looks worn that may be part of the problem but that would cause it to run too lean (and OK at altitude).
First get the filter clean then check for leaks in the intake path between engine and air filter and also check the jets are clean too.
An inline fuel filter is a good idea too, even the cheaper ones.

[Anonymous]

Normal fuel is 91RON, premium is 95RON and Shell Vortex/BP Ultimate and similar are 98RON. A low grade fuel could be the cause. I would at least be running the 95 in your bike Shane, it will be designed for at least that.

Trying a beter grade fuel is easy and worth a shot fisrt before you start to play with jets and the like.

[shaneMember]

Thanks lads.

I was thinking out loud, because I’ve noticed the ‘miss’ up in that part of the country before and a new plug generally seemed to be the fix. It had a new plug. The air filter is cleaned after every ride and that day was a quiet learners ride so it involved very little dust.

I’m going to try some of that new fangled premium fuel and see if that doesn’t solve the problem…

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