[img]i/expressions/face-icon-small-disgusted.gif[/img] Okay everyone, I've been having some trouble with my stock 400EX carb. I have an FMF Power Core IV-2 silencer, stock-bore 11:1 compression piston, XR400 cam, and a Dynojet kit. Right now, we've got a 175 main in it, stock pilot jet, and the Dynojet needle.

The bike runs great mid and top, but bogs and sometimes cuts out right off of idle. If you tap the throttle quickly, it bogs. If you tap it 4 or 5 times, it bogs, then cuts out and the motor dies. This happens at about 1/4 throttle, so I figure it's the needle. It seems like it's getting too much gas and bogging out at about 1/4 throttle. When the RPMs are up, and you punch the throttle, it runs fine. It only happens off of idle. We've tried everything we can think of so far, but not much luck.

Also, is the throttle valve suppose to sit on the notches or seat below it? You can see these if you take off the airboot and look inside the carb. They are on the left and right side walls below the throttle valve. I think that if it is suppose to seat on these, it could be my problem.

PLEASE HELP. I'm sure someone out there can help me out. Your help is GREATLY appreciated. I'm trying to get my quad running just right before the next MX race on the 16th of this month. Please help me out. Thanks a lot.

 

I checked out my spare 400EX carb, and the throttle valve sits on the notches. Stands to reason that it should, since the motor when at idle would at least need a small amount of air. Anyhow, when the carb is closed the valve sits on the notches leaving a small opening--about 1/4". Is your valve seating above or below these notches? If it's seating above the notches, you might have your cable adjusted too snug. Below? Something else is wrong...

As far as your bogging problem... First, when you tap the throttle even slightly, the accelerator pump shoots a small amount of fuel into the intake cavity, to reduce and/or eliminate stalling when coming off the pilot jet and air screw circuit. Repeated taps would therefore shoot even more fuel, which could cause the bogging problem you are describing. The single off-idle bog you are having problems with though, is most likely being caused by the pilot jet.

Since you've done quite a few mods to your engine, I would safely say that you definitely need to be running a larger pilot jet. Although the stock pilot jet is 38, most stock engines can easily run either a 40 or 42 pilot jet to richen them up a bit and eliminate cold running problems. With your mods I would start with at least a 42, and be prepared to go up even further. My "guess" is it needs around a 45, 48, or even a 50. You'll also need to re-adjust the air screw after you install a new pilot jet, as the pilot jet and air screw work together.

Using the same procedure as for a stock setup, first turn it all the way in until it "lightly seats" (be careful not to damage the seat), then out 2.5 turns. Warm up the engine for about 10 minutes, and then turn it "out" until the idle changes, and then "in" until the idle changes. Between these two points, find the spot where it idles the best, and doesn't hesitate when you lightly wick the throttle off of idle.

FYI, you might be able to get the jets from your local dealer, but if they don't carry them Chaparral Racing sells the jets you need for $3.85 a piece. Let me know if you need their phone number and/or part numbers.

Jeff

 

Forgot to mention...

The pilot jet and air screw circuit overlaps with the jet needle circuit, just as the jet needle circuit overlaps with the main jet circuit. If it's running fine from 1/2 on up, then I would think your problem lies in the transition from the pilot to the needle. That's why I think your problem lies in your stock pilot jet. Anyhow, check out the throttle valve, and if it's okay try bumping up the pilot and adjust the air screw.

 

Do you notice that it is hard to start? Does it run better cold or hot?It's most likely your pilot circuit. I had the same problem after I added a full system and opened up the airbox. I would agree that the 42 pilot would be a good place to start.

 

Thank you guys for replying and helping me out. It's been extremely frustrating [img]i/expressions/face-icon-small-mad.gif[/img] jetting this carburetor, and I'm ready to get my quad performing perfect for the June 16th MX race. The thing absolutely screams on mid and top, but lags in bottom. This is difficult for me, because I'm used to the typical 400EX low-end power.

NVR2L8,
I'll try to check over these things inside the carburetor and fix them. I had thought that the reason the throttle valve wasn't dropping quite enough was because of an inproperly adjusted throttle cable. I also thought that the problem was also related to either the jet needle or pilot jet or both. I'll try a richer pilot, and adjust the throttle cable and see what happens from there.

mx59,
The quad is hard to start sometimes. At times you have to pump the throttle many times to get the quad to start up. Is this because of the pilot jet? I forget right off hand whether it runs better when it warm or cold.

 

Yep, the hard starting is from the pilot being too lean. Replace it with a 42 and see how it runs. Also, make sure that your air screw is around 1-1/2 turns out.

 

Yea, jetting can be a pain! The trick is to work on one circuit at a time...

I'm pretty sure that your hard starting and bogging problem is related to your pilot jet. My 2000 400EX right out of the box was a turd to start up bone cold, and two sizes larger on the pilot cured it. You modified your engine though, and it now requires more fuel even at idle...hence the need for a larger pilot jet. Not sure why the valve isn't dropping down all the way...it's either the cable adjustment or something else is binding. Let us know what you find.

 

Well, guess what guys. You all were right. The problem was due to the pilot jet. I went to a local dealer and bought a 42 pilot jet. It helped, but didn't get rid of the bobble. A 45 pilot fixed the problem.

You all don't realize how much I appreciate your help with this problem. Thanks again for everything.

 

wcy2k400ex: Glad I could help...good luck at the races!