Need expert advice

[Glenyzracer25]

I'm working an on 05 400ex, it cuts out about 1/2 throttle and backfires. I will list what I have done. I need somebody that's knows a 400ex

-New plug.
-Cleaned carb.
-swapped cdi's,coil, stator,rectifier.
-motor is fresh
-motor is in time
-valves are in spec
-checked for air leaks around intake

It starts and idles perfect, just cuts out at 1/2 and above.

I have even swapped another carb off a 400ex and both carbs are fine on the other 400.
The leaner the jetting the better it runs.

Motor is 86mm wisceo, hotcam, full sparks, uni air filter.

I need new ideas to go on.

The only idea I have left is checking the flywheel key and that's next on my list.

[rinconrider]

whats your current jetting?

a main jet of about a 158 or 160 would be my starting point as a main jet, needle on the 3rd notch, and 3.5 turns out on the f/a screw with a 38 pilot or a 2.5 with a 42 pilot.

[CJM]

Yes, what is the jetting currently at?

I would also venture to say its possible it might be slightly out of time. Either the cam gear is off a tooth or the woodruff (key on the flywheel) is causing an issue. You have already eliminated anything electrical as well, so it can be your stator or cdi either.

But its more jetting Im willing to bet. Your bike should be running a 155-160 main roughly depending on your elevation and such.

[Glenyzracer25]

I'm 99% sure it's not fuel related. We have tried 2 carbs on this bike. Have tried a stock jet, 150, and a 162. 40 pilot. 1 3/4 all the way to 3 turns on the a/f screw. It gets worse the richer it gets.
We even put the original carb on another 400ex with just a slip on and it ran fine, telling me its not fuel or carb. Jetting can cause a small miss or cut out, even at 5 sizes to big on the main won't cause a miss at anything over 1/2 throttle.

We are gonna pull the flywheel and check the key next. Flywheel puller is on the way.

I'm just looking for something other than jetting. I need something wild that has caused a 400ex to cut out. I feel like I have covered just about every idea possible.

[CJM]

Ok, I just wanted to be sure-so many like this are jetting issues.

Its surely a timing issue I think.
1. Verify the timing chain is TIGHT, you shouldnt be able to move it.
2. Check the cam, is it in with the lobes down if your flywheel is on the "T" mark?
3. Verify again the valves are in spec of course, just to be sure.
4. Cam gear should have the 2 marks line up with the head and the other mark pointing straight up.
5. Check timing chain tensioner, does it spring back or it it loose? It should spring back.

Other ideas:
1. Check the coil and wire, verify they are good and your getting good spark.
2. Check compression, ya never know.
3. Blown headgasket

[Glenyzracer25]

Originally posted by CJM
Ok, I just wanted to be sure-so many like this are jetting issues.

Its surely a timing issue I think.
1. Verify the timing chain is TIGHT, you shouldnt be able to move it.
2. Check the cam, is it in with the lobes down if your flywheel is on the "T" mark?
3. Verify again the valves are in spec of course, just to be sure.
4. Cam gear should have the 2 marks line up with the head and the other mark pointing straight up.
5. Check timing chain tensioner, does it spring back or it it loose? It should spring back.

Other ideas:
1. Check the coil and wire, verify they are good and your getting good spark.
2. Check compression, ya never know.
3. Blown headgasket

Timing chain is good.
Timing is correct by flywheel and cam gear.
Valves were checked and rechecked.
Bike has 1 hour on top end.
Bike cut out before top end rebuild and still does today.
Coil has been swapped from another running bike.

I've looked at the harness but don't see any obvious wear spots.

[CJM]

Then Id look like you are towards the flywheel. Seems quite odd that everything checks out but still bogs.

[Glenyzracer25]

Trust me, I have went through this thing until I'm blue in the face, It almost sounds like the reverse rev limiter.

I was hoping somebody had a similar experience.

[wolfe 21]

Had a buddies warrior that was doing the same thing basically. He had a bad ground to the frame and it wasn't allowing him to get a continuous spark at the motor.
Also, does it rev any better with the clutch pulled in. Had a neutral safety switch give me all kinds of fits, til I pulled in the clutch and bike reved fine. Switch had the wires crossed and was activating rev limiter.
2 things to check that are outside of whats been discussed so far. Hopefully you get it sorted out.


James.

[powerbomb400]

wolfe 21 has a point. Check the the neutral and reverse safety switches.

[maddmaxxx]

-had the same symtoms...took me 3 dam days to figure out


-my petcock screen was rotted out...screwing up my gas flow

-maybe try fuel flow test without the carb on...into a jar

-it drove me nuts...hope thats the problem guy