Welli did a search for jetting and only found the newest topic from June. I live on the Texas Gulf Coast. Pretty much sea level. I have a problem with the plug fouling a great deal. Several locals say I am too rich (jetting, definitely niot money) and I need to lean it out. i am considering the BK mod, maybe a pipe and ignition if available. I ride moslty MX now and I am a beginner since I haven't ridden in 10 yrs. The bike is still more than I can handle, so a major mod is not necessary. I just want it to run better and stop fouling plugs. I know the starting routine to the letter and I do not race the throttle. I did have a bit of a dirty filter, but the problem persists even with a clean filter.

Secon, are pressure washers taboo? thanks :)

OK, i made some changes. I cleaned the filter for starters. The pilot screw was 1 3/4 turns out from the factory. I turned it in 1/4 turn to lean it out. I adjusted the pilot jet to the 3rd clip. Left everything else alone. Al electronics checked. I cannot really check the CDI. Given te way it runs with a new plug i do not believe the CDI to be defective.

Next, I checked the pump squirt time and found it to be about 2.75 sec. it squirts about 6-7 feet out of the carb. Could I need the BK Mod? If so, what changes are needed in jetting? Any help would be appreciated. Thanks.

This plug fouling problem is an enigma. Some 426's run just great and never foul plugs while the next identical machine will foul plugs using the same jetting. Obviously, there is something out of the ordinary going on. Some owners have found that the CDI was defective while others have discovered that the carb had internal leakages. What I'm getting at is that there is a distinct possibility that jetting is not your problem.

Well that sort of sucks. How can I determine if there are gas leaks internally? I know there is no warranty, but is it worth taking to the dealer? I am afraid they will rip me off. This based on the crappy breakin service I received for $90.

Anyway, it ran for over 20 hrs on the same plug until last weekend. Since then the avg daily temp has dropped over 10 deg. The humidity has dropped some 20%. It has fouled three plugs in three rides. I will see what the jetting does. Any other suggestions? :)

When the temperature drops, the bike will run LEANER. If your bike is fouling plugs on cooler days, and not on warmer days ( with the same jetting ), that's an interesting problem.

I'm new at this whole Thumper thing, so I can't give any insight as to why it's doing that.

I have an 01' 426 with stock jetting, and I ride at sea level or between 1500 - 2000 ft, and mine has never fouled a plug while riding.


Originally posted by bullitboy:

I adjusted the pilot jet to the 3rd clip. I checked the pump squirt time and found it to be about 2.75 sec.

>>You must mean the needle that you adjusted to the 3rd clip position.

>>The squirt at 2.75 seconds is a bit long. Mine was at 3.25-3.5 seconds. I did the BK mod and after some jetting changes the bike runs MUCH better. Generally, with the BK mod you'll need to increase the size of the pilot jet a size or two. I ride at 3000'-4000' the majority of the time so my jetting specs would be no good to you.

>>Did it run any better with the clean air filter and the fuel screw turned in???

>>A 10 degree change in temp is not a big enough change for it to run that poorly. What kind of gas are you running?? If you get a bad can of pump gas it will act like that too.

>>Just as a safe check, make sure the plugs you are now running are the correct heat range...CR8E or equal plug.


I have fouled about 10 plugs on my 01, mostly in

cold or humid wheater. I also stalled the bike

under braking in bumps.Turning the pilot screw

from full in to 4 turns out had very little effect

on rpm or running.

The tread for one of the outer screws holding the

middle portion of the lower section to the upper section of the carb was drilled and threaded in to

the float house.

After putting some epoxy in to that hole, I haven't fouled a plug.The bike also runs perfect

using BK mod. and 170 main (cold wheater).

bottom half mating to the top half

Well I got it back together and ran it up and down the street. As i expected it ran fine. It popped a little on deceleration, but I was only able to get it inot second and pin it for 50 tds or so. I ran it until it died with the fuel valve closed. I suspect all the rattling aroung in the truck during transport may be flooding the engine before I get to the track. We will see on Saturday. :)

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