I have an 03 Vinson, stock that has started backfiring after
a continuous high speed run.............. backfires when
you let off the gas, coming to a stop after staying on the throttle
for a period of time. I have stock jetting with no mods. Anyone else
had this problem? What did u do to remedy the situation?? I have heard
it is extra fuel in the headpipe, .. ive heard its jetting. Any help or ideas
would be much appreciated!!!!

Thanks.... Red 03 Vinson Manual

 

If it's happening when you let off the throttle, it's occuring during the transition from the needle jet circuit to the idle circuit in the carb. A lean condition in the transition causes the backfire. I'd turn the idle mixture screw out (counterclockwise) a quarter to a half a turn and see if that fixes the problem.

Hope this helps.
Mark

 

Thanks Mark, I will give it a shot.

 

vinsonman, i know this is a old post but would like to know if it worked for you, im having the same symtoms. also id like to know if you have to remove carb to adjust idle, and is this screw located on the very bottom of carb?

 

This is from the mikuni website
"Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires."
The mixture screw is on the bottom side of carb, very near the intake runner to the head.#17 in the picturecarb
The idle speed adjust is #26. There may be a plastic or aluminum piece in the way of the screw, # 40. It will need to come out to adjust the mixture. Hope this helps.

 

Hondabuster,thanks buddy, it does help.

 

dont worry about it mine dose it every once and awhile and it sounds bad ***.

 

you may need to have a valve adjustment..... that is part of your first service if you do it through the dealer.Check into it!!!!!

 

yeah im do for the service but dont have really have the much confidence in my local dealer. small things about the way they do business makes me think twice about taking it there. i will probably go there anyways though.