Dalstdon

400ex rebuild, need help!!!

6 posts in this topic

I just bought a 2005 400ex for a grand cause it was smoking slightly(white). I got it home, stripped it to bone to clean everything, and while it was apart I just decided to do some aftermarket addons to add performance while dealing with the smoking issue! I removed carb and head and cylinder! I removed piston, removed all old gaskets and residue, and used a very fine wire wheel to polish up all surfaces! I changes o rings in valve adj caps, cylinder sleeve, carb boot. I have all other gaskets when that time comes. Polished up valves, seats, and installed new valve seals. I just ordered Namura 11.1:1 piston, stage 2 cam, White Bros CDI, Lexx slip on, K&N Filter, and here is where I gotta stop til I get help! First, I need to know what would be a good pilot jet, main jet, and needle position for me to adjust the carb, I have dyno jet kit on hand, my altitude is 550 ft, second, I need to know how or where to get Info on readjusting my valves, third, is my Cometic head gasket ok for this setup, And last, is stock cam chain and rockers ok to reuse? Would greatly appreciate the help, thanks!

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First off, welcome to Quadcrazy.

I would definately get a repair manual, you will need alot of the info in there for putting that motor back together. You will find valve adjusment procedure and clearances, torque specs, wear tolerances etc.. The valves are easy to adjust, on the end of the rocker over the valve, there is a small screw and jam nut, loosen the nut adjust the screw with the proper feeler guage between the screw and the valve, tighten the nut without moving the screw. The manual will illustrate this and tell you what the clearance is. It will also tell you what the wear tolerances are for the rocker arms and shafts, my guess is that they would still be ok, '05 is not that old. As for the timing chain, it just depends on if it is worn or stretched, it is very possible that it is still good. I don't see any reason why you could not use the Cometic gasket. Good luck with everything.

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Actually I just got done rebuilding it! Everything actually went together well despite one screw up, I tried removing the flange off of old cam to reinstall on new one and broke a lobe off! Thank god someone who was helping me today had a friend 30 minutes away with a 400ex for parts so $10 got me a NEw cam with flange on it, got it home and used a vise setup instead and got it off after playing around with it for awhile, pressed it onto new one quite easily, aligned cam gear wheel with top dead center, bolted all back up and lubes it all, than replaced head cover with new gasket! Set timing to .004 intake and .005 exhaust, buttoned rest up and it started right up! I let it around 2500 RPM for a few minutes, let it cool off, than went for a quick 10 minute ride around streets without letting it bog or revving to high, than let it cool down again! It ran beautiful! I was surprised I didn't even have to adjust the air/fuel screw after installing 42 pilot jet, 170 Dyno main jet, and needle in 3rd position from top! Im really shocked as this was my first rebuild and it came out perfect! Now just gotta break it in properly! Thanks for advice tho! Not too bad tho, bought a mint stock 2005 400ex on Tuesday dirt cheap cause it was smoking slightly, brought it home and added all these extras, it's Saturday, 4 days later, and it's totally mint and rebuilt and I'm only in it for $1550! Can't do too much better if I meant to!!! Lol!

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Damn, you move quickly. I'd buy a like new 400ex with a fresh motor for $1550 any day of the week. Good show sir.

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Gimme a day or so and I'll get some before and after pics uploaded of I figure it out, ha

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Hey does anybody know anything bout proper size jetting for a 2008 Yamaha YFZ450? My lil cousin bought it 2 years ago and with the new purchase price has dumped a total of $16000 on this thing! He races it locally so had to remove a bunch of stuff to make it race ready! He didn't do anything to the internal motor to add power, just put on a velocity air filter with no lid on box, scary fast plate for stock carb, and full FMF pipe. All the money invested is in suspension, tires, carbon fiber rims, etc! Somebody messed with his carb and he can't figure it out to get it running again! Our elevation is 550ft up to 1500ft. What pilot jet, main jet, and needle position sounds good to run! Dynojet kit on hand! Thanks guys!

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