What do you think it was

Prior to my TPS experimentation, Sunday afternoon was out doing some moto, just as Im getting ready for some pics, my bike starts falling on its face, but only on places that are carrying g-loads, like jump faces, backsides, and only on the bottom end of the range. Up in higher RPM, seemed ok. So I ran around the course a couple of times on the rev limiter, and the problem stopped completely.

So my question: Ya think it was just a tiny piece of dirt in the carb? Its wierd, its done it before, but not consistantly as this time.

I can tell ya this for sure, its scary as hell when your running at a 80+ foot jump and the thing sputters on the face. Im fourty, I dont need that kind of excitement. I can read the headline now: Local man has heart attack mid air during motocross, lands fine, dies on ground.

Maybe water in the carb or partially blocked vent tube that cleared from the vibration.


Just think... if you have a heart attack mid air and somehow survive the ordeal....


A. You can sue the track for not having a more gently angled slope to the face of the jump so as not to get a much lift.

B. Sue Yamaha for a sputtering carb (apparently this works)

C. Sue _______ (insert any thing you want related to dirt bikes)

So see... your not in such a bad situation...

given that you live.

It was some kind of blockage in the fuel delivery system. It sounds like when you revved it up it loosened whatever was blocking your fuel and now your good to go, no heart attacks even.

I had a blockage once in me fuel system

I revved up :)

Took a seat :D

And Walla

Two Courtesy flushes later, no heart attack

Shawn, have you ever checked out the air boot adaptor on the carb? Work with me here, if you look at where the air boot slips onto the carb adaptor, you will notice that the adaptor has two Allen head bolts that ONLY put pressure on the bushings. The problem with this setup is that as time goes on and you do your routine maintenance, the action of removing and refitting the air boot tends to put side torque on that adaptor and the bushings begin to lose integrity. Once this happens, the Allen head bolts don't hold the adaptor firmly against the carb intake and you can develop a small to large air leak at this juncture. An easy fix is to simply fit a proper sized washer under the Allen head bolts that overlap onto the adaptor itself. Even if the closest size you can locate is a little too large, just grind down a flat edge so that this flat edge fits against the adaptor body. The aim here is to make sure that the Allen head bolts snug the adaptor against the carb and prevent an air leak. I would invite ALL 400/426/450 owners to check this juncture. With a strong flashlight, you should be able to see how the Allen head bolts ONLY snug down the bushings. As long as the bushing retains its integrity, you won't know the difference. Just a tiny slip of the bushing and you will be chasing a gremlin forever. This happened to me on my '00 and I only discovered by chance when I was reinstalling my airboot after the third carb dismantling/cleaning. My symptoms were intermittent throttle response problems along with high intermittent idle and a series of loud exhaust pops upon decel. I could tune out the popping but then the throttle response would go nuts. It drove me nuts for a while!

I'm not saying that this is your problem, but I would still recommend that you, and everyone else, install these two little washers as insurance. I can't begin to describe the difference this simple mod made in how my bike performs now. As a simple test, simply grasp the air boot where it is clamped to the adaptor and wiggle it a little while watching to see if the adaptor itself appears to move. Watch for a gap between the adaptor and the carb. I could actually see my adaptor slide just a little over the left side Allen bolt. Sorry for the rambling, but I wanted to be clear.

[ July 24, 2002: Message edited by: Boit ]

my guess is something a miss in the float bowel

Boit, I understand what your saying and it makes sense. Since the only time it happened was on G-outs or High g-force situtations, I can see how if the problem was as you described, the wieght of the carb pulling on it could/would/did create quite the leak. I havent checked that juction since I bought the bike (can you say neglect, come on, sure ya can)so I wouldnt be surprized if your right.

Thump you could be right too, I found that my VP gas can was rusty inside, and I have a feeling that something may have gotten through. But I know the filters in the tank are clean, because I just had to replace the tank. So now Im not too sure. Dog-goneit. I hate trying to de-bug whilst experimenting. Like fire and water. Guess I'll have to disassemble and clean, so I can have a clean test bed for the TPS deal. Shoot :)


Just another datapoint... my bike was having similar symptoms. Whenever I'd try to take a sandy whoop section at the local track, it'd bog out about 1/2 through and almost put me on my lid. I called a local tuner I respect and explained the problem. I'd first thought I had a vent tube or something plugged, or it needed yet more vents, or the float needed adjusting.. etc. He said "It's your jetting" I disagreed but agreed to try some suggestions he had, after all, I called him. Yesterday, I went out to the track. It was doing the same friggin' thing, only worse. Now it was doing it on the faces of the jumps. (Full shorts time) I decided to bump the mainjet up one size. Problem solved. I didn't really think it would work, but I can't argue with the results. The weather has been hot for here (95F) and I think it just leaned it out enough to cause a few problems.



I've been hearing more about this problem. Couldn't a guy take some carb cleaner and squirt a real quick shot at the boot adapter while the engine is running and listen for any change in idle? That's how I found out I didn't tighten the airbox boot on the back of my carb.

Something to think about

Shawn Mc,

My bike did this exact same thing. Like you, I never found out what caused it. I did clean the carb the other day and it was dirty...meaning a small peice of dirt could have got in the fuel circuits and finally passed through.

Does anyone here use a filter between the tank and carb...if so what is a good one to use?

My friends 426 does this sometimes. He changes the sparkplug.

merfman wrote: "I decided to bump the mainjet up one size. Problem solved. I didn't really think it would work, but I can't argue with the results. The weather has been hot for here (95F) and I think it just leaned it out enough to cause a few problems."

That is strange because in most cases the heat will cause your bike to run rich, no lean. Remember, hot air is less dense than cold air. Possibly what you did was mask the real problem by over compensating with more fuel, just a thought??

Yea, good point. The bottom line is that it ran great after richening the main. I've still got some investigating to do but, right now, it's good to go.


Merfman, Im wondering if there wasnt a piece of crap in your carb that was pluggin the main and it came out when you changed the main jet? Im almost certain, the more I think about it, that was the problem with mine.

Im going to ride it again here in an hour or so, then its going to be time for a maint routine, I'll clean my carb too this time, see whats inside. Its probably fulla dirt. Shame on me in advance.

float level? needle and seat worn?

Might have been... I'll be going back out next Thursday so I might just put the other jet back in and see if the problem has vanished. I still don't buy the jetting issue as it only happens on whoops and high G loads. If I put the jet back in and it still runs fine, that's probably the answer.


Kirtwell. That's a good point and it will work fine as long as you don't shoot too much in that area because the vapors can sucked into the normal intake avenue and make the engine stumble. I discovered this with a lawnmower. I had just started the engine after changing the oil and shot a little carb cleaner at the oil filler spout which is on the opposite side from the carb and air box. Just that little squirt made the engine nearly stall. It's just something to keep in mind when you do that. Maybe starting fluid would work well? I've never tried it. You could even shoot a little into a shop rag and kinda hold it next to a suspected leak and see what happens? Thoughts?

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