Hello everyone,

I have a Fourtrax 250 that I have recently purchased. When I rev it up, it starts missing and gas vapor blows right back out of the carb into the air cleaner box. It has a brand new battery, good gas, good sparkplug. I just cleaned and put a carb kit into it, adjusted the mixture screw to 1 1/2 turns out, and the float seems to be working correectly. The machine will fire right up, idle, and run smooth up to about half throttle, then loads up, misses, and blows vapor backwards out of the carb. Any help would be appreciated.

 

<div class="FTQUOTE"><begin quote>Originally posted by: lbnrose

I just cleaned and put a carb kit into it, adjusted the mixture screw to 1 1/2 turns out, and the float seems to be working correectly. </end quote></div>

Did it start acting up after you installed the carb kit?

 

No, it did it before the new gas, battery, plug, and carb kit. I changed those things trying to fix the problem. I have rode it like that to give the kids slow short rides, and one time for 15 minutes or so it seemed to clear up, but then it went back to loading up, backfiring, and missing.

Thanks for the reply.

 

Is it possible that the petcock is plugged up and not quite letting enough gas through? I don't know man. Sound like the typical plugged carb but, if you rebuilt it then it's beyond me.

 

My daughters 90cc two stroke was kinda doing that it turned out the rings were bad and wouldnt suck the gas in it would blow back out in tube.

 

I agree with Koopa. Most people neglect the petcock. I has a filter on it as well and get overlooked easy. Give it a good cleaning and see if that helps.

 

Definately check the petcock, I had a XR100 that did the same thing. I cleaned the petcock and it has ran perfect ever since.

 

Thanks for all the advice guys. I did check the petcock when I drained the tank, which seems to be in good shape even though it is steel, and the screen was clean. I blew it out with air, and checked the rubber oring in the petcock, which I have heard will cause a problem. It seems that either it is getting too much gas (which with the tank, petcock, lines, and carb all gone through should not be the case) or the gas is not being burned as fast as it is sent in. The bad rings problem may have something to do with it. Would bad rings still run good at low RPMs? Can anybody explain to me the timng advance and components? Could it be the spark is breaking down at high RPMs or not advancing like it should. I had an old battery in it for a short time and maybe it strained some electrical component. Is there something that may be needed at higher RPMs or under load that does not affect the lower RPMs. I really appreciate all the ideas.

 

I'm far from a professional on this but, it really sounds like it just not getting enough gas. You can run a compression test to check if the rings are bad.

I'm short on time so I'll let someone else chime in on the timing.

 

Well I have removed and recleaned out the petcock and screen, tested it with air and it seems to be working fine. I don't have a compression tester, but the cylinder seems to have great compression with the plug removed and the engine does not smoke at all. A local mechanic seems to think it is either the coil or the stator plate. I pulled the coil and used it on another machine to test and the coil is fine. That leaves the stator plate, which I had avoided until now because I didn't want to start ruining gaskets until I had to. Any other advice or comments concerning this issue or the electrical/timing systems would be great. Anyone know a place online to find the wiring scems for this machine?

I will continue to update as things develop. Thanks again.