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Message started by Justinmkantor on 07/25/09 at 18:03:43

Title: Jetting question
Post by Justinmkantor on 07/25/09 at 18:03:43

So I was running Lancer's carb with the needle setting at the middle and a 180 and a dyna muffler. Was running good but I wanted to open its throat a little and put on a Jardine and ran real rough on a short ride to work. I live a mile away and just wanted to see how it did on a short ride. Then I upped a jet to 185 and got about a mile and it started to quit on me just before I got to the highway (thank God). Was able to sputter/push it back home and I went down to 175 because I thought it was running to rich. Still cuts out and then after its out after a couple of cranks turns over. Any words of encouragement?

Title: Re: Jetting question
Post by LANCER on 07/25/09 at 19:51:43

0C3335322F282B2D2728322934460 wrote:

Strange symptoms. You did not say what you did with the pilot screw settings, if anything. Changing the muffler from a dyna to a Jardine is significant and will affect all three throttle ranges.
There could be something else going on as well. The strange symptoms could have coincidentally been caused be something getting into the carb and creating a clog. Stranger things have happened.
I would suggest going back to the original settings for pilot, needle and main jet and see how it runs there. It should run for you if the carb is working normally, and if it won't run at all then there is a clog somewhere; most likely in the pilot system.
Going too rich on the main jet will just cause it to bog down in the 3/4 to wide open throttle setting.
After going back to original settings, richen the pilot screw by 1/2 turn and test run the bike. If it runs for you then adjust for best setting for highest rpm, then back it down to normal idle speed with the idle screw.
Then check the midrange run quality with needle in normal setting. If it is too lean then drop the needle clip one slot to richen the mix and test run.

Once the low and midrange is worked out then go to the main jet. With engine totally warmed up, test wide open throttle running, and go up in main jet size until the engine bogs down ....jet too large ... drop down to the previous size and your good to go.

If the testing of each of the throttle ranges does not act normally, then you should assume that something got inside the carb and is blocking fuel flow and is screwing up the normal function.

You did remember that the pilot adjusting screw on a VM carb works opposite of the stock carb, right ? It turns IN for rich and OUT for lean. ;)

Title: Re: Jetting question
Post by Justinmkantor on 07/26/09 at 10:43:01

I have not touched the pilot screw, yet because it ran fine at idle, but that was at medium temp not at full temp. I will go back to original jet and see if I can adjust the screw when fully warm. I will see what happens and adjust accordingly. Did not think the Jardine would have that much an effect but I guess it does. Thanks for the tips. Must remember the basics.

Title: Re: Jetting question
Post by DrunkenDwarf on 07/26/09 at 16:12:33

3E333C3137206065520 wrote:

After going back to original settings, richen the pilot screw by 1/2 turn and test run the bike. If it runs for you then adjust for best setting for highest rpm, then back it down to normal idle speed with the idle screw.
Then check the midrange run quality with needle in normal setting. If it is too lean then drop the needle clip one slot to richen the mix and test run.

Once the low and midrange is worked out then go to the main jet. With engine totally warmed up, test wide open throttle running, and go up in main jet size until the engine bogs down ....jet too large ... drop down to the previous size and your good to go.

Not to be the uppity newbie...ok, yeah, that's me.

This is in the opposite order from most of the advice linked to in the tech section. Is there a particular reason?

For example, usually the main jet is dialed in first. I think this is because a different sized main jet can have an affect on the mid throttle mixture. Or am I crazy?

The advice for choosing a main jet has been go down in main jet sizes until too small, then go up one. I'm guessing this would be the difference between jetting for fuel economy and jetting for torque?

-D. Dwarf