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1988f150

No full throttle

5 posts in this topic

Hey guys, purchased a 1998 yz400f about 2 weeks ago and ran fine for the first week but now when accelerating i cant give it full throttle or the engine will fall on its face. It has a big bore kit, titanium valves and h/d valve springs, and dsp header and pipe. It also pops on deceleration quiet a bit. Just wondering what some of the problems could be before i tear into the carb.

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Consider cleaning and re-oiling the air filter to begin with. It could be dirty enough to choke off the engine under heavy loads.

You may also have water in the fuel, or a partly obstructed main jet.

Decel backfire is almost always the result of either an air leak in the exhaust (at the head or the mid pipe) or a lean idle circuit:

http://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

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Consider cleaning and re-oiling the air filter to begin with. It could be dirty enough to choke off the engine under heavy loads.

You may also have water in the fuel, or a partly obstructed main jet.

Decel backfire is almost always the result of either an air leak in the exhaust (at the head or the mid pipe) or a lean idle circuit:

http://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

I vote for a mixture of filter and main jet too.

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i have the same problem i just got a 98 yz400f bored out to 426cc with full white brothers pipe. my bike will idle fine but when i try reving it i have to give it full acceleration. i draind the gas dryed out my oil filter flushed the carb witch is an edelbroc carb. i got the bike dec 28 2010 and it up about mid january 2011. im very interested in any more info i should know about getting it to run properly again

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Ok so it has been a long time since I posted this thread but I do still have the bike so he we go. My problem was the "valve float" in the carburetor. It is a flat piece of metal with a rubber o-ring that sits on the slide in the carb. They can be kinda costly for no bigger than it is but you gotta have it intact. Since you have he edelbrock carb I can't really help. I would say you need to check you jetting and or buy a jet kit for it and play around with it. My guess is it is still at factory settings and since u have the big bore kit and pipe it needs to be re-jetted.

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