300ex jetting/backfiring...

[GMC ZR2]

I have a 2001 300ex with a pro-circuit T4 pipe, 350x air boot, Uni-Filter and Powroll airbox lid. Pretty much ride at sea level. When I first jetted it I ran a 128 and it was backfiring on decel...the fuel screw was out 3 turns or so.

Went to the dealership to buy a #40 pilot jet and they informed me I should be running a 130 main. So I bought both. I installed them today and went for a ride. I still have backfiring like crazy on decel and it runs great when wide open. The fuel screw was
out 2 1/2 turns and then I adjusted to 2 3/4 turns while on the trails. The needle has not been touched, but I do not think that would effect the backfiring.

Still backfiring.....

Anyone have any ideas what is going on or what I can do? Should I run a 42 pilot???

At this point I am ready to buy another quad...

[Sjorge450R]

its your needle.Mess around with that, I believe you have to move the clip down.

[bwamos]

Originally posted by GMC ZR2
I have a 2001 300ex with a pro-circuit T4 pipe, 350x air boot, Uni-Filter and Powroll airbox lid. Pretty much ride at sea level. When I first jetted it I ran a 128 and it was backfiring on decel...the fuel screw was out 3 turns or so.

Went to the dealership to buy a #40 pilot jet and they informed me I should be running a 130 main. So I bought both. I installed them today and went for a ride. I still have backfiring like crazy on decel and it runs great when wide open. The fuel screw was
out 2 1/2 turns and then I adjusted to 2 3/4 turns while on the trails. The needle has not been touched, but I do not think that would effect the backfiring.

Still backfiring.....

Anyone have any ideas what is going on or what I can do? Should I run a 42 pilot???

At this point I am ready to buy another quad...

Actually.. with whay you've got (if your below 3000ft) I'd go with a 132 Main, Needle up 1 slot (clip down to slot 4, 1 being the top slot), air screw at 3 turns out. Stock pilot jet is fine.

You should be very close here. Fine tune from that point. Let us know how it works. Post fix feedback helps us all fine tune our knowledgebase.

[GMC ZR2]

So if someone was to ummm....strip the screws in bowl, how would one go about fixing that. We wont name any names.... :rolleyes: :rolleyes:


I will try a 132 main, but it runs mint at WOT, just when I left off its pops like crazy. Will the needle really make a difference?



The screw on the bottom of the carb is the fuel mixture (how many turns out 3?


Whats the screw on the side behind the choke lever? Should that been seated completely or backed out a little?


Thanks for all the help

[Sjorge450R]

just use a pair of vise grips to remove the stripped screws. As for the screw, probaly 2 1/2 -3 turns out. And yes the adjusting the needle will make a difference.

[panteramatt]

popping on decel is a lean condition usually caused by the needle, richen her up first, leave the fuel screw and 1.5 turns out and after riding it, THEN adjust the screw. One thing at a time to get her perfect.

[ohsobad_chevy]

Originally posted by panteramatt
popping on decel is a lean condition usually caused by the needle, richen her up first, leave the fuel screw and 1.5 turns out and after riding it, THEN adjust the screw. One thing at a time to get her perfect.

Agree, popping is normally a sign of being too lean. Richen that bad boy up. It took me several tries to get my jetting perfect, but when you do, you will know!

[GMC ZR2]

Thanks for the advice I will give it another shot this weekend. I think I have the info I need know thanks to you guys.

So whats the screw on the side of the carb behind the choke lever. Should it be screwed in or backed off.

[Sjorge450R]

Originally posted by GMC ZR2
Thanks for the advice I will give it another shot this weekend. I think I have the info I need know thanks to you guys.

So whats the screw on the side of the carb behind the choke lever. Should it be screwed in or backed off.

i think you are talking about the screw that acually holds the choke on... So it should be in there good.

[ohsobad_chevy]

GMC, Don't take this the wrong way, but you should really invest in a service manual (not the owners manual), I am slowly learning the ins and outs of 4 strokes as well. Service manuals are definately worth their weight in gold, no doubt. :D

[GPracer2500]

Backfiring on decel with a closed throttle is an indication of a lean pilot circuit. To confirm your pilot jet:

1) Warm up the engine to full operating temp.

2) Turn up your idle a few hundred RPM. This will make it easier to hear small changes in RPM.

3) Turn the fuel screw in until the idle starts to drop and miss.

4) Turn the fuel screw out until the idle peaks and smooths out. Go too far out and the idle may start to drop again.

The goal is to find the setting that provides the highest and smoothest idle.

If the peak RPM is reached somewhere between 1-3 turns (for a CV carb) then your pilot jet is correct. If you end up less than 1 turn out then your pilot jet is too big and you need a smaller one. If you end up more than three turns out or the fuel screw seems to make little difference as you continue turning it out than you need to go up on your pilot jet.

The above is assuming there are ZERO obstructions in the pilot circuit. It is always a good idea to thoroughly clean (w/ carb cleaner and compressed air) all passages in the carb to eliminate that as a potential problem.

I don't understand those that are saying that the needle is the issue. It could very well be that needle needs adjusting, but on a closed throttle the pilot circuit is the only circuit in play. The needle clip postion comes into play between 1/8 or 1/4 throttle (depending on where the taper of the needle starts) to 3/4 throttle.

The other screw your talking about is the idle speed screw. That is what adjusts your idle speed up and down.

[GMC ZR2]

Thanks for all the help, I havent had a chance to work on it yet. But I will use the advice when I do work on it.

I do know where the idle screw is..I will have to take a picture of the side of the carb so you can see what I am taking about this screw would be behind the yellow choke lever when the choke is off.

Again thanks for the tips..

[GMC ZR2]

Well I messed around with the quad today, got the screws in the bowl out, changed the needle. Took it for a ride and only backfired once. Not tons of popping like before. I have a 130 main jet, 4th needle clip and 2 1/2 turns out on a #40 pilot.

The only probably is the overflow at the bottom of the carb is leaking gas. I think that is from the float being stuck open, I have taken it apart twice trying to get it sit in there correctly with no luck. Also tapped the carb with mallet before starting it.

I will have to do some searching on what else i can try