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400ex Lack of power

21K views 67 replies 7 participants last post by  SlammedRanger 
#1 ·
Hi guys my name is Tyler and this is my first post. I just bought a 99 400ex and it was pretty much stock besides a slip-on and intake. Besides being hard to start it ran fine before I pulled it apart.

Anyway, me and a buddy (TorqueDaddy) rebuilt both our top ends with a 86mm 11:1 piston and a stage 2 hot cam. Put everything back together, and it fired right up....just lacks power throughout the whole throttle. We assumed it needed jetting so messed with that quite a bit and got rid of the slight bog it had. Now it seems to start and run fine...just has about half the power of my buddys 400ex.

While it might need further tuning with the carb/jets...it just doesn't seem like thats the issue. There is no bog anymore...just doesn't pull! Could it be clutch? valves? What do you guys think?

Thanks in advance!
 
#5 ·
Are you running a stock intake?

Does your buddy have an aftermarket intake?
 
#8 ·
Had this exact same thing happen to my quad i have now. Previous owner put car oil in it. Clutch looked like it was in a camp fire. But it was perfectly fine until i put the proper oil in it. After that it slipped so bad i couldnt get the front end off the ground in 1st gear popping the clutch even. It was just pitiful! I put an ebc dirt racer clutch with 20% stronger springs and it was back to normal.
 
#9 ·
Maybe recheck your camshaft timing. It could be off a tooth. If thats fine it might be worth your trouble to check the key in the flywheel/rotor. Take a spark plug reading after a longish WOT run... still may need to be a bit richer through the mid and top. No ignition mods were done, correct?
 
#10 ·
If the cam was off a tooth would it still idle so good? The woodruff key is good, I checked that. Im thinking it may be the clutch..

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#11 ·
If the clutch is slipping you'd probably feel (and hear RPMs come up) that, especially in 3rd gear and higher... With 11:1 piston and aftermarket camshaft made to optimize volumetric efficiency and cylinder pressures across a broad RPM range, I'd guess that your motor could idle just fine while being a tooth off. If it were mine thats the first thing I'd recheck.
 
#13 ·
If timing is a little off it will idle fine.
Lack of power could be due to timing but it would be waaaaay off and poping thru the carb. Could also hear possible piston slap.

After you put the top back on you said it had a "bog". If it was sputtering or had a backfire before you did any adjustments to the carb it's more probable that the timing is off.
 
#37 ·
The cam in that picture is 100% not in the right position if you are at TDC. The oil holes should not be pointed almost straight up like that. I bet the cam sprocket hub that goes onto splines on the cam is indexed wrong. There is 1 extra thick spline to try to make it harder to index wrong but it still can be done. But that would explain a lack of power for sure!
 
#17 ·
He has an oem original carb. I dont believe it is a carb issue. We put the oem carb off my 400, that runs great, and it was still doing the same thing (we have very similar mods and motor work)..

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#18 ·
Oem carb with keihin jets, 42 pilot and 175 main. Hasn't had any backfire or smoke just bogged/cut out until we changed the jets. I have been through the whole carb and cleaned it twice.

It could still be a jetting issue but we have gone up and down in jets and swapped carbs and it still has a seemingly unrelated lack of power...

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#19 ·
Found this on another thread..... something to check. Also was the tensioner engaged when you took the pics?

"look right behind the cam lobe nearest to the sprocket. If there is a small 4mm hole with nothing in it, that's your problem. There must be a pin and spring in it to hold the anti reverse device on the camshaft. If the pin is not there, the device is turning with the cam and your right hand exhaust valve only closes every second turn."
 
#21 ·
That's it but I just remembered you installed an aftermarket cam which typically don't use decompressors so that's not it lol
Does it rev up and won't move? Like it slipping?
I'm going back to the clutch now but it's weird timing that it would stop working after an oil change unless the old owner used a car oil.
 
#22 ·
I just looked at your photos of the timing marks on the sprocket and the flywheel and found your problem.

The camshaft is installed using the wrong mark on the flywheel... you are using the two maximum spark advance marks on the flywheel, when you should be using the Top-Dead-Center (TDC) mark.

There should be a single timing mark on the flywheel labeled "T".
Line up that "T" mark with the mark on the side cover (thru the window where you removed the plug), then install the cam sprocket on the cam so that the sprocket mark is even with the top of the head and snug up the sprocket bolts. Make sure that both cam lobes are facing DOWN when you do this.

Then go back and recheck all of your timing marks for proper alignment (they should match closely). Once you are sure you got it right, tighten up the sprocket bolts, install the chain tensioner and the rocker cover (a thin coating of hondabond or yamabond 4 on the alcohol wiped surfaces) and break it in easy for a bit...
 
#33 ·
@Carolynskiii,

You are right! I just downloaded the manual and that mark is the correct one. Thanks!

@SickNasty,

Next thing to troubleshoot is whether the decompressor is put together right, and that it is fully releasing when the motor starts. Note the order of the parts and verify the one-way clutch is good, etc:

 

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#34 ·
No need for decompressor as he has an aftermarket cam.. the spring and pin were removed.

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