trx440 bigbore-carb problems

[tbroady2002]

So I resently bought this 00 trx400ex w/ the 440 bigbore kit and I was having throttle cable problems. When trying to buy a new one I was having trouble because the carburator is an edlebrock. Id be happy except during the research I found out its not the carb for the 440 kit but the one for the 450r. What I'm wondering is, is this going to be a big problem or can I jet this carb for this motor set up.

thanks for the help
Tom

[Here 2 help]

A carb is just a mixer. It mixes fuel with air. Properly jetted it will work fine, but........

An overly large carb will enhance high RPM's at the cost of low end grunt. The bore of the carb should not be any bigger than the port it is bolted against. If the intake port in the head is enlarged and a larger valve installed and a larger exhaust valve installed and the exhaust port enlarged and a larger than stock exhaust pipe installed and a less restrictive air filter is installed and the inlet to the air box is enlarged, then you will get an appreciable boost in power. Bolting a big carb on a stock head is a waste of money, but it gives the owner a mind boost.

[tbroady2002]

the carb has a 40mm bore. does any one know if that is larger than the intake bore on a stock 400. and if thats ok how about jetting this thing. 440 bigbore, fmf slip on, open air box w/ k&n filter. i ride at sea levelish and 3000 to 4500 ft.
thx

[Here 2 help]

The stock carb was 38mm. 2mm larger will not have a major effect. However, the 440 bore will keep the velocity of air through the venturi approximately the same as the 450 had. Yes, it can be jetted to work on your engine. Start with the idle jetting, then jet for mid-range and finally jet for top end. Happy jetting

[tbroady2002]

so this may sound like a stupid question but how do i tell the jets that are in there are wrong without my motor blowing up first?

[Here 2 help]

You have to run the engine and ride the machine or run it on a Dyno. (This is like Carburetors 101 in a condensed version)

The first step is idle: Start the motor and let it warm to operating temperature.
Does it start easily? Does it idle smoothly when warm and the choke is off? If it won't idle without choke, try turning the fuel screw out. If it idles, but acts like the choke is on, turn the fuel screw in. If you have to turn the fuel screw out more than 4 turns - go up one size on the pilot jet. If you turn the fuel screw in until it just touches it's seat - go down one size on the pilot jet. Either way - try again after the changes are made.

Once you get the idle right - go for mid range. Ride the machine at a steady speed in third or fourth gear at about 1/2 throttle. If it surges like it wants to go faster but can't, the slide needle needs to be raised. If it blubbers like the choke is on, the slide needle needs to be lowered. (This is where you find out if the needle is adjustable. If it is GREAT! If not, you will either have to shim it to raise it or change the needle for a wider taper in lieu of lowering it)

Finally, after the idle and mid range are right, you get to jet for top end (main jet). Get the machine out where you can run it up to full throttle. As you go through the gears, listen for gas-knock (ping) which indicates a lean mixture. If you get any ping, jet up 2 sizes and try again. Continue increasing the main jet size (by 2 sizes) till you can run up to top speed without ping, then continue to jet up until at the very top end of engine rpm it falls on it's face and blubbers like the choke is on. Jet back down 2 sizes and you should be good.

Last step - run it where you normally ride checking that it drops to idle and idles cleanly after acceleration. It accelerates cleanly with no bog and no spark knock. It should start easily after running and stopping the engine.

Easy way - take it to a shop with a Dyno - pay them $500 to do it for you without having to take the carb off and put it back on 20 times.

Hope I didn't make any grammatical errors.

[tbroady2002]

awesome! thanks. And I guess since Im so cheap i better get to work.