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I have 2010 Bayou 250 that came to me with no compression and smoking. Rebuilt top-end to factory specs. Has great compression and isn't smoking, burning oil anymore. Cleaned the carburetor and Rebuilt it because someone had drilled the seat hole bigger. I looked to the carb because after top-end job it cranked right up and idles smooth. It revs great up to a little over half throttle then it starts busting out or loading up is what I call it. Sounds similar to rev limiter but more of a whaaap whaaap sound. Lil puffs of black smoke and it blackened the new plug quickly. The timing is on the marks, I set intake to .006 and exhaust to .008. I replaced intake boot o-rings and checked for leaks. I checked air filter and air box. NOTHING I do seems to make it any better? It's definitely running rich and I have adjusted float several times. The new jets I put in it are by the book of what came in it. I checked muffler and exhaust for build up. If anyone can help me out, I would greatly appreciate it. Thanks.
 

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You have checked off a lot of items that would have been on my list. What about the carb needle? You did not mention if you checked for stock needle and stock clip setting for the needle. Also is it the stock needle jet? At half throttle it is all about the needle and the needle jet.
 

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Yeah, stock needle and it's non-adjustable . The other thing is in the carb kit the needle and jet needle slides in were in kit....so I replaced both. This thing has drove me nuts! I talked to a friend and he said I may need to adjust timing. (Even though it looks like its on the money)....something about timing chain stretch and the tensioner? It had 50psi and smoked like crazy when brought to me....now it has great compression and isn't smoking but has that issue at half to wide open throttle....smh
 

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This is why I posted on here.....and like you said, from what I know.....half to full throttle is on the throttle needle??? I even tried squeezing the fuel line to lean out some and it still did it.
 

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I have heard of some folks having problems with cheap carb kits having parts that don't work right. I am not saying that is the issue here, but something to consider before you try more expensive fixes.
 

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You have checked off a lot of items that would have been on my list. What about the carb needle? You did not mention if you checked for stock needle and stock clip setting for the needle. Also is it the stock needle jet? At half throttle it is all about the needle and the needle jet.
Anything else you can think of....please let me know. I have considered electronic issue but it ran better with low compression & smoking, and why would something electrical start now ya know? My friend has worked on atv's for a long time.....maybe he is right, I need to adjust timing to where it looks off to me? Either advance it a tooth or retard it a tooth? I think I will try to retard a tooth 1st. If valves are staying open as its firing, I think that's what would be going on if it was advanced a tooth too much? Running out of options.
 

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I have heard of some folks having problems with cheap carb kits having parts that don't work right. I am not saying that is the issue here, but something to consider before you try more expensive fixes.
Well, I got an Out Law kit....and the #'s on the parts were same as service manual says. Like 117.5 for main jet then there's the running the same before rebuilding carb......I think seat was drilled bigger because of the low compression, to "make" it run with low compression/intake?
 

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Discussion Starter #8
What do you think about changing timing? I figure its worth a try. I think just moving a tooth I will be safe as far as engine damage. The valves looked good and like I mentioned before, it will idle all day long and smoothly. I have a video, can we post videos on here?
 

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First I would be very careful about changing timing unless this is a clearance engine meaning valves cannot hit pistons.
I don't know if yours is a clearance engine so check that out before trying any valve timing changes.

The problem with cheap jets is quality control. The number stamped on the jet may not be accurate and the surface finish inside the jet won't be smooth like an OEM jet. This is why you are not supposed to drill out a jet. A drill will leave a rough surface. Cheap jets may be drilled instead a more precise, drilled and reamed hole. A rough surface affects flow.
 

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Discussion Starter #10
Yeah....you should see the "seat" I took out....why it was such a mess I'll never know. I'm about positive this is an anti interference engine as far as that timing deal. The Mikuni carburetor that's on it had oem jets in it and it ran basically the same way, was worse before putting carburetor kit in but that's mainly because of hole in seat being drilled. Me squeezing the fuel hose and it not making a difference kinda tells me its running rich no matter. ( if that makes any sense ). Everything I have done, barely made any difference except when I adjusted float too lean and tried running with lid off air box....they of course made run like crap.
 

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Gotcha. Thanks. I have no idea why anyone would drill out the float valve seat.

Have you checked to see if this carb was intended for this ATV? People do crazy stuff.
 

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Discussion Starter #16
I've always referred to this as needle and seat...float needle valve I think is correct name, idk? I think this was done to make it run before. I was thinking with hole drilled out it was getting too much fuel.....this and needle was ordered from Mikuni and barely made a difference. I thought for sure it was the issue....lol.
 

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Discussion Starter #17
Gotcha. Thanks. I have no idea why anyone would drill out the float valve seat.

Have you checked to see if this carb was intended for this ATV? People do crazy stuff.
I have....it is indeed the correct carb. Mikuni Vm24ss I think, I would have to look through notes.
 

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Discussion Starter #18
I'll check back later....I even checked all vent hoses, connectors that I may have bumped, etc......it's just unreal the time I have put in this. The float was set extremely rich as well when I first opened carb but again, at that time it was a very low compression engine....I guess they did that to make it run even though it was smoking....lol
 

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Discussion Starter #20
WFO-KZ , let me know if u were able to see the video and if you have heard one do like this.....any suggestions, etc. Thanks for all u have commented on.
 
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