400f cuts out up rough hills

For those who visit regularly you would know I have been on the carb jetting roundabout.

I finally found a good jetting combination But now that I have been on a few off-road rides have found the odd problem that if I hit big whooped sections or rough steep uphill the bike dies for a second and then goes again. I mean completely cuts instantly then comes back a second later.

I assume it's a fuel level problem in the bowl but is it floats or float valve causing it? on the flat ground it is flawless.


That only happened to me when one of my vent lines were clogged with dirt.

Yeah I ruled that out.

I took all vents off carb.

Same problem.

When you say "completely cuts instantly" that has me leaning towards something electrical, like a barely connected wire. This is where I would start.

I will definitely check through the wiring.

I do still think its fuel related because I can chug slowly up until a certain angle and it will stop and go and stop and go again it does give a bit of a pop as it dies while I'm on the gas.

If I go full WOT I can keep it going up a hill but it cuts out worse at mid.

Problem is that I can't tell if it is flooding or starving.

On reflection the feeling is very similar to a BOG off idle except this is at a steady throttle, but almost exact same feeling of the bike unexpectedly having no power and then going well again. I think I can replicate the problem by doing a quick wheel stand. So I guess it's still the same question....too much or not enough fuel?

So, you say you can replicate it in a wheelstand: do you mean the angle of the wheelie causes it, or the slap-down after the wheelie?

I'll bet we can figure this out...

Just as the wheel starts to come up, which I guess is when the fuel rushes to the rear of the carb due to the surge in power. But if I nail it on the flat and keep the wheel down it doesn't hesitate.

OK, so what you are saying is that when the wheel is coming up (maximum rotational mass up and over), you loose ALL power until the wheel stop rising, or starts going back down.

I would say for sure that is starvation.

I would raise your float level 5mm (or just properly set it to factory specs),and look at the main jet to see if it has the anit-surge plastic funnel.

You can get the bike running and tilt the crap out of it to replicate the gas position in the bowl....

If it does not drool gas out of the overflow when you tilt the bike over 20 degrees, then the float level is too low.....

Edited by Krannie

Thanks mate, that makes sense to me.

I'll give it a go.

This is the last issue to fix in what started out as a totally screwed up carb.

So.... I tried setting the float height lower ~ 8-9 mm and it was really bad. Bogged even on the slightest incline. Had to stop on a ride and pull the bowl off. Bent the tab waaay up on trailside, maybe to 12mm+ and it ran better, still bogged but I was able to get up most hills with a few pops and farts.

So the only thing I can think of is my float needle is dodgy. I think the little spring loaded piston on he back might be dead or it's jamming open on rough stuff.

Going to order a new float needle. I'd get the seat too except I don't think I could get the old one out. It looks like it's pressed in.

Any other ideas greatly appreciated as its getting a bit frustrating now.


If your float needle was not sealing, you would get fuel overflow from the overflow tube, or you would drown-out the main jet.

Yes, that seem to be the case.

When the bike is stationary it seals correctly and no problems.

But when on rough or steep ground (up or down) I think you are spot on about the main getting swamped. I can JUST keep the bike running if I go FULL THROTTLE which must be just enough to keep the fuel level down a bit but if I back off it bogs then goes then bogs then goes.

I have ordered a new float needle, so HOPEFULLY it will end my frustration.

In other breaking news, I have been running a #45 pilot with NO MAJ as an experiment and it has worked brilliantly to solve my Hot starting issues. I have just received a #40 pilot so I will put my #75 PAJ back in and hopefully get better response from the fuel screw as I'm only at 1/2 turn out right now.

It's getting so close to being a good bike I can taste it.

I'm still baffled about how much I have leaned it out to make it run well......

now at:

155 main

40 pilot

1st clip OBEKP needle

Could the float needle have been causing my extremely rich conditions all along ?? Because that means my jetting will need doing again.... :rant::cry:

I guess I'll find out when I get the new Float needle.

Sadly, yes. If the fuel level in the carb isn't set properly, via either a wrong setting or a faulty needle & seat, then your jetting will have to change to compensate. Case in point, if the fuel level is low, then it is harder for the fuel to be "drawn up" thru the jets orifices, which makes the jetting act leaner. And if the fuel level is high, then it's easier, so the jetting acts richer.

On the bright side of things, just look at how much knowledge you've gained from all of this jetting run around!!! :thumbsup:

Yes, it has been a big learning experience.

I can now strip out and rebuild a carby in about 10 mins.

It's always the simplest things that trip you up.... in hindsight I should have replaced the float needle at the start, because it did initially leak but then seemmed ok after I fiddled a bit.

Big thanks to William, Krannie and Maniac for expert guidance through the dark arts of carb tuning.

Hopefully by next week my troubles will just be my riding skill instead of the bike :thumbsup:

Edited by Thommo_400f

Well, riding skill is always being fine tuned!!! Or gross adjustments in my case!!!

I have tried everything I can think of to get this bike to stop bogging. New float needle as well.

If I nail it from standstill it pulls hard all the way. No off idle bog at all.

If I chug in 2nd and pop a low speed wheelie it bogs/coughs at the top which causes wheel to come down and then all is good again. Same if I hill climb. Coughs on steep rough stuff. It does cough on really steep down as well but less noticeable since I'm not on the gas.

I have tried the float at 9mm and it wouldn't even go up a smooth hill.

I've got it at about 12mm now and it helps but is still bad.

The one thing I am not sure about is how deep the seat should sit in the carb body. Should the hex nut section of the seat be flush with the edge of yhe orifice? Wondering if the seat could possibly be pushed too deep?

I've ordered up a needle & seat for my WR250f, which is basically the same carb, so when the parts come in next week sometime I will tear into my carb and will let you know what I find out about the seat depth.


My manual doesn't really give much help about how to remove/install. So no idea if it is sitting correctly.

You can probably guess I am grasping at straws!

Is there any way fuel can interrupt the PAJ or MAJ when sloshing about?

Had a thought....

From memory there is a retaining screw for my seat so I doubt it could be the wrong height....:/

Are there any air passages that lead down into the bowl that could be getting partially blocked by fuel splashing?

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