New guy questions.

My01EX · #1 01-08-2012, 06:42 AM

Hello, new guy here. I just bought an 01 400EX and had a couple questions for you guys...

1. The previous owner had the cylinder bored .030 over. I don't know the formula for figuring out displacement so does anyone know what that would make it now? I know, stupid question sorry.

2. I noticed that when I thumb the throttle, a little stream of fuel squirts from a spot where the float bowl bolts to the carb body. I know it's not overflowing like the floats are stuck, it's not coming out of the overflow tube. It's just a little stream shooting from between the two parts. Would that just be a gasket replaced fix or is it something more serious? I think that's what's causing it to bog down when I lay into the throttle too quick, and it idles lower than I think it should. Hopefully I can just change a gasket or something or maybe something got trapped in there when they put it back together because the old owner said he had the carb worked on when he changed the exhaust. Also, given the .030 over bore, would stage 1 hot cam be worth the expense and trouble, or would stage 2? I know there's a lot of knowledgeable folks here so any help would be greatly appreciated.

Here 2 help · #2 01-08-2012, 01:38 PM

The 400EX with a stock bore of 85mm and a 70mm stroke, is actually a calculated 397.3788434631148 cc's displacement. Bored to .030 inch oversize (.75mm oversize), it becomes 404.422349088166 cc's.

The carb has an accelerator pump on the RH side of the carb. Fuel is pumped through a passage to the jet nozzle in the bell of the carb. I will presume the carb is either missing an o-ring seal or the float bowl gasket is damaged. It can cause a stumble at the beginning of acceleration if the fuel is not getting there, but it shouldn't affect the idle speed.

Changing the cam is your choice. I run bone stock, don't foul spark plugs (mine is an 04 and still has the original plug) and still trophy nearly every race I enter. BTW - my machine never fails to start and is not "finicky" to start if I stall it on the trail. Some of my buds, who have changed the air box, exhaust, cam and replaced or rejetted the carb on their machines, get PO'ed at me when I beat them because their machines have "more power" than mine. Their's are usually a little harder to start hot too. It's not the machine, it's the rider.

If you want to change the cam - a stage one improves low to mid-range power for agressive woods riding. A stage 2 is best for wide open high speed operation, like riding in the desert or motocross racing.

My01EX · #3 01-08-2012, 05:57 PM

Thank you Here 2 help! All of that was just what I was looking for! I think I'll try to get around to pulling the carb today and see if it's an o ring or gasket issue. As far as the cam goes, I think I'll leave it stock for the time being. I really don't plan to race it, my soon to be bride and I just do a lot of trails mainly so I think if I do change it, stage one is the one to go with, based on your info. Thanks again so much!

My01EX · #4 01-08-2012, 10:15 PM

Quote: Originally Posted by My01EX

Thank you Here 2 help! All of that was just what I was looking for! I think I'll try to get around to pulling the carb today and see if it's an o ring or gasket issue. As far as the cam goes, I think I'll leave it stock for the time being. I really don't plan to race it, my soon to be bride and I just do a lot of trails mainly so I think if I do change it, stage one is the one to go with, based on your info. Thanks again so much!

Ok so I pulled the carb off today and right before I did, I removed all the plastics and the gas tank and hit the throttle and sure enough, gas still squirted out of that one spot, but it seeped out other places from the float bowl gasket. Thanks again man!

hillbillystan · #5 01-20-2012, 02:22 AM

if you do a 13 to 1 piston with a stage II hot cams i wake it up youll know a defrince as long as the jeting is right

My01EX · #6 01-20-2012, 04:25 PM

I have a .020 over 11:1 piston, stock cam, aftermarket header, DG R Series pipe, ProCom Rev Box CDI, K&N filter and no air box lid. Does anyone know what jets I need to go with for all of that? I'm rebuilding my carb and I want to change the jets while I have the carb off the first time. I need to order new plastics and a new chain so I'm doing it all in a day so it can be done all at one time rather than having to take stuff apart multiple times.

Here 2 help · #7 01-20-2012, 07:53 PM

Unfortunately jetting is as individual as the changes you make and where you ride.

A good rule of thumb around here is: stock pilot jet and adjust the fuel screw as required for a good idle - no change to the slide needle position until after test riding - bump the main jet up two sizes at a time till you get the desired performance (usually bump up till too rich then back down two sizes) - after the main jet is as desired, try riding in 3rd or 4th gear at about half throttle. If it runs clean you are done - if too lean it will surge like it wants to go faster but can't - too rich and it will blubber like the choke is on. Adjust the slide needle down (move the clip up) one notch if rich or up if too lean. Repeat test riding and adjusting until you are satisfied with the performance.

My01EX · #8 01-21-2012, 12:14 AM

Thank you. Yeah, my carb kit came in today but I have to go in to work tomorrow so I won't get time to mess with it till next weekend probably. I'll go through those steps you mentioned and see what we end up with. Thanks again.