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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!
 

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Never smoked, and have had multiple plugs in. I will check compression again but it was good after it was put together. Thanks for the reply. Anyone else have any ideas?

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Sounds like clutch to me. Stock 400ex clutch isnt good to begin with. And when you add anymore power they are really bad. My .02
 

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Are you running a stock intake?

Does your buddy have an aftermarket intake?
 

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Both of us have a stock intake with K&N filters. I used honda GN4 oil but who knows what previous owner was using.
 

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Both of us have a stock intake with K&N filters. I used honda GN4 oil but who knows what previous owner was using.
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.
 

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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?
 

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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?
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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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.
 

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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.
Me and sicknasty will check that thanks for the advice


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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.
 

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Is the carb an OEM Honda original or is it an aftermarket carb? If its not OEM you'll probably have to put the original carb back on to get it right.
 

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Discussion Starter #15
I think the timing looks good...is this how it should look?


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What about the carb... OEM original or no?
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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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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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."
 

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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."
Tensioner was engaged, and there is a small hole in the head casting near the last lobe. Isn't that for the decompression pin/spring for easier starting? Thanks for all the replies
 
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