Fouling plugs at top end

[Bruce V]

Hello Everybody,I have a 1991 KDX 250,I just bought it a couple of months ago,until last weekend I had not been any where to really hit hard through all the gears,full open,The first time down the flat track it started to bog down at the top end of first and would not clean out,by the time I got to third it was more or less just coasting.When I stopped it wouldn't even fire.The plug was trashed.The only other time I fouled a plug was when I changed oil types,and mixed it to rich. I went through two plugs the same way,flat draging.Until now it has always had no problem at top end on short burst.I'm running BelRay MC1 at 50:1(the other plug was at dealer rec. 32:1) FMF pipe and silencer,The last owner didn't say anything about any other mods other than pipe.Any sugestions appreciated!!!!

 

[Matt90GT]

They are jetted way wrong to start with.

First off make sure you have a new plug, BR9ES. once you get the jetting on, then switch to the BR9EIX NGK plugs for the best performance.

Also clean the air filter before starting and make sure the reeds are new or still in excellent shape. I would also suggest to switch out to the .26" thick headgasket from the stock .46 unit. It will give you more power and compression everywhere. You can find them for the 88-91 KX250s. There are different options for the years, so be sure to get the correct one. Also repack the silencer and install new pipe O rings. This will ensure the bike is in to shape so that the only things that you can rule on is the jetting.

now assuming you have good compression ("fresher" top end) and your KIPS valves are working (remove the governor cover. That should start moving at 6k with is about 1/4 throttle), then you are set to jet.

Now on to the jetting. Here is what I run in that bike and it will clean things up a TON. It will cost about $100 for the parts:

Pilot 38
Main 155-160
needle R1367H third clip (tune the clip position from there)
Slide #6 (stock is 5)

The that will lean everything up in the carb. the needle, jets and slide. It will help a lot and you will about double the HP from where you are at since you are so fat - at least it will feel like it.

Beyond that, run a dynoport pipe and VorceII reed cage for more power. After that port the cylinder and run 50/50 gask mix and the bike will scream for new suspension at that point!

 

[Matt90GT]

FYI I run the amsoil series 2000 @ 50:1 for premix.

 

[Matt90GT]

It should not need to be said, but you also remove the baffle from the airbox lid. You can also drill additional holes in the lid and some on the top sides for more airflow into the box.

last trick is to advance the timing all the way.