Modified Vinson Backfiring

[CPolariski]

Buddy's Vinson has a pipe and supposedly the right jet kit, but suffers from backfiring. Any ideas what is wrong and how to fix would be appreciated.

[whopper809]

Does it back fire through carbs or exhaust system. Also does it back fire at acceleration, deceleration or idle?

[CPolariski]

Backfires through the exhaust at deceleration. Needle is set at second notch from the top and 128jet.

[whopper809]

It could be a plugged or dirty pilot jet in the carbs, the air/fuel screws might not be turned out enough, exhaust could also have a leak. To find it mix up soap and water spray at all connections and watch for bubbles. I'm not fimiliar with the jetting you need (128 sounds kinda lean though), but it could be a lean condition on the main jets also, but it's a slim chance. Those are all things I would start looking at. Do you know what brand the main jets are (dynojet, mikuni, kiehin) Also what's the brand of exhaust on it and is it a full system or just a slip-on?

[CPolariski]

He thinks its an HMF silencer, and a mikuni carb. He tried a 126jet, but that didn't change anything. I'll get him to try screwing out the air/fuel screws tonight and see if that helps.

[sledneck48]

Sounds like it lean on the top end. If its an HMF they recommend a 142 main at sea level so prob 138-140 round here. Also do a plug check for the top end

[whopper809]

Sledneck48 is right HMF slip-on should have a Keihin 142 jet for 900ft elevation, so go 138-140 with Keihin jets. In mikuni jets thats close to a 120-130. and if your using dyno jets thats 128-130 jets.

Stay away from dyno jet products, there junk quality.

[QMAO]

Seems to me when I had a Vinson with HMF i had a 145 main jet.
I still have the Dynojet kit in a drawer in my garage.......for sale cheap!

[toyz]

why did you not use the dynojet kit. I have no air box lid and a modified stock exhaust. Main is a 150 and i have tried all sizes in idel jet and it still back fires terrieble on decel. I was thinking of trying the dynojet kit.

[QMAO]

I did use the dynojet kit. I removed pipe and filter and changed jets to stock as my dealer did not like the modded quad for a trade in on my King Quad.

i was going from memory, I thought I used the 145
It does sound as though your quad is still running lean.

I am not very informed on this mod, But I know there is a carb spring mod that people are doing.
I understand that what is done is remove the slide from the carb, then cut 2 or 3 rounds off of the spring on top of the slide. This was supposed to help with throttle response.

[toyz]

How much do you want for your dynojet kit. I think my problem is the stock needle is lean in the mid range.

[QMAO]

$30.00 pm sent