backfire

[400exbiggun]

Was breaking in the ex motor the last few days. Started to break it in by a few idle/light revv periods. Then started and some short trips through the yard etc etc. Cooled it down then some more short trips in the yard. I was about 1/2 tank through and running out of powermist so i got on it a little bit. Never a WOT run, no over revving really. As i went to about 3/4 throttle and let off it backfired, then i slowed down from 2nd into 1st, let off the gas a bit more and it backfired again. I shut it off thinking it just was getting hot from the easy riding. Started up 20min later, took it out again and it started to do the same thing.

its a JE 11:1 416 piston, running powermist (forget the octane rating), TC ported head, WB all trak cam, CT full exhaust, lid was on with k&n and some other little crap like the 1'' holes in airbox etc etc. Below 1000 ft above sea level (900ft i think exactly).
I have a 155main, stock needle spot, and a 42 pilot. Should I move the clip up one? I never did a decent 3/4-wot run to really check the plug for the main, but it looks a decent brown color like it should.

[400exbiggun]

thats a kehin 155main BTW not a dynojet

[Aceman]

Your pilot circuit is lean. Follow these instructions for jetting it. Let us know how she runs afterwards.

http://www.exriders.com/vbb/showthread.php?s=&threadid=229923

[Wheelie]

Usually the pilot causes a backfire like you're describing, although, in your case, stepping up a couple sizes on the main wouldn't hurt either.

[GPracer2500]

Originally posted by Wheelie
Usually the pilot causes a backfire like you're describing, although, in your case, stepping up a couple sizes on the main wouldn't hurt either.

I agree. K155 sounds pretty small for your settup.

[400exbiggun]

correction, i have a 165 main in there:blah:

Sorry about that, i bought a few jet sizes and had a few in front of me. Its a 165 in there...not a 155.


Ill play with the pilot a little more tomorrow. But should i leave the needle alone?

God its been literally like 2 years since i had anything worth modding to ride so please excuse my lack of knowledge at this time.

[Wheelie]

Lower the needle one notch. The 165 sounds like it should be pretty close.

Personally I'd leave the pilot alone until the needle is reset.

[GPracer2500]

Ahhh. I 165 sounds better.

As far as the needle, I'd probably move the clip down one notch and test. Moving the clip down on the needle raises the needle in the carb, which would be a richer needle postition.

When evaluating needle postition changes, focus on how the engine responds when going from about 1/4 throttle to 3/4 throttle. If the engine loads up or boggs when making this transition you're too rich on the needle. If it cuts-out before picking its self up and reving out then you're too lean. If you're unsure about too rich or lean on the needle (sometimes it's hard to tell) then move the clip in the other direction and retest. That should help clarify what's going on. However, you shouldn't need anything leaner than the stock (middle) clip postition.

ETA: Wheelie beat me to it. Although, I prefer to set the pilot circuit first.

[JW450R1]

if u go to hmf web site and register they have the jetting chart all in words for u....

[400exbiggun]

alright, going for the 3rd oil change for break-in. After that ill drop the clip one and come back with the results.

[400exbiggun]

ok, results are not good.

Was out finally after using another 1/2 tank easy riding. Got on a trail, just crusing. Didnt revv high, shifted into next gear pretty soon, then it started to backfire again. It sputtered until it died like it ran out of gas. I was on a hill so i pop started it and if i let off the gas it would die. Anything less than like 1/4-1/2 throttle would die.

My buddie towed me back to his house, we got it there and it fired right up. It would idle like a dream but once you hit the gas it got all choppy and broke up then died.


Went back today and it fired right up (i never use choke). Idles perfect, once you gave it gas it broke up and got really choppy. Pulled the needle and lowered it one clip, installed and tried to start. Now it ONLY starts with the choke on, give it gas and it breaks up, then dies. Turn choke off and it dies right away, it wont even idle.

Pulled the plug and it looks good IMO. The base is a little black but the ceramic part is paper bag brown all around. I even tried it with airbox on and air box off, on reserve and on ON.

Any ideas?

[JW450R1]

i know this some strange,but the vent to your gas tank lid.try pulling it out and to see if it may be clogged or with u push down in it doesn't kink up causeing it to vapor lock.
i had a 87 250x doing the same thing.

[400exbiggun]

yea i thought about that, i even had the lid totally off the tank at one point.

Any ideas? Im trying to get it back home tonight to mess with it a little more, but im totally lost at whats wrong now.