[Neo]

Hi Gents,

I few weeks ago I leaned out the pilot jet a bit more and found my bike ran smother than ever. And it ran really well like that for about 4 weeks (ridden most days).

But three week ago the climate turned a bit colder and it got down to around 3 degrees Centigrade over night. I rode my bike on one of those colder mornings and was shocked to experience how quickly it revved up with such small amount of throttle.. Like one eighth of a turn would top it out in less than second!!! ...And this problem has continued since.

I gave the carb a clean and slacken off the throttle cable (lots) but no real change. And I've also noticed that sometimes it is worst than others.

Now I'm sure some MX rider out there would love this to happen to their bike but it's personally driving me nuts so I was wondering if anyone could post me their ideas of what they think might be happening?

I intend to swap the carb tomorrow with the one off of our 125 (24mm OKO) but aside from that ..... your thoughts on this folks would be appreciated?

Best of balance.
Neo

This post has been edited by Neo: 19 June 2010 - 09:54 AM

[feetupfun]

Neo you have gone too lean on the pilot jet. It was probably marginal until the weather went very cold. Cold air is denser and makes the bike run leaner. The fuel is also probably being affected by low temperatures and is harder to ignite (narrower flammability range).
What seems to you to be the motor being more responsive is just that you now have a flat spot where the pilot jet has the most effect (just off closed throttle). You are actually moving the throttle through the flat spot/dead bit and when you go far enough open with the throttle, the mixture becomes strong enough to support combustion, and the motor takes off suddenly.
To maintain a perfect state of tune with a carburettor equipped motor requires consideration of the temperature, humidity, barometric pressure and any changes in fuel type, oil type, oil ratio and fuel age.

[Neo]

Hi Feetupfun,

I could agree with what you are saying except....
1. If it is lean why doesn't the airmix improve things....even marginally?
2. I have also ridden in warmer days these past two weeks. But it been the same then too.
3. There is no flat spot at any time.... no matter what gear and what revvs I'm doing or how much load it's under.....it pulls like a train!!!

Found out yesterday that when I put the cap back on the carb (when I cleaned it) I wasn't paying attention and put the cap on backwards. That made things worse but so it's a lot better now. There's still a lot more power but I'm kinda getting used to it now.

Fuel/oil mix have remained the same too.

Best of balance.
Neo

[feetupfun]

Maybe someone else will give an opinion that you can agree with

[Neo]

Hi Feetupfun,

I think I might of expressed my thoughts badly in my last post .... what I meat to say was "I can see where your'e coming from....but"

I truly appreciate your's (and anyone's else's) help and advice and I hope my previous comment did not offend in anyway?

Best of balance.
Neo

[feetupfun]

Neo it is quite frustrating to try and help people with bike problems over the internet with such limited imfomation.

I will try and answer your questions:
Adjusting the idle mixture screw may or may not noticably change the severity of a flat spot caused by too small a pilot jet or a part-blocked pilot jet.
I'm sure I already said that it is not just the temperature of the air and the fuel that matters but also the humidity and barometric pressure.
A flat spot caused by a carburetion issue is not usually revs, load or gear related. It is usually throttle position related.

How about humouring me by either fitting the standard pilot jet or testing it out something like this:

Stick some paper masking tape to the inner end of your twistgrip and another piece to the housing. Make a mark on each so that they line up just as the slide starts to lift off the stop. Warm your bike up fully and ride your bike in 4th gear along a flat smooth surface like a road.
Hold your ground speed with the rear brake while moving the twistgrip from fully closed throttle to about 1/3 throttle, moving the throttle slowly in both directions. If you do this and still can't feel a flat spot, then there probably isn't a flat spot and we can go back to square one.

There are other possibilities for the symptoms you have described but they are very unlikely compared with the possibility of a part-blocked or undersized pilot jet.

[Neo]

feetupfun, on Jun 21 2010, 09:13 PM, said:

There are other possibilities for the symptoms you have described but they are very unlikely compared with the possibility of a part-blocked or undersized pilot jet.

Hmmm... that sounds very logical.

Ok looks like I've got a few things to do over the weekend ..... Will come back to you after the testout.

Many thanks and best of balance.
Neo

[copemech]

Other than what was mentioned, I have no clue what you have done(not being there).

Just how lean did you go on this pilot?

Did you try the other carb?