Mod Motor HELP!!

Get a KX80. No more problems.

For starters four strokes do not need that high of octane even with a high compression set up like yours. Not sure if you're using VP12 or not but that is a two stroke fuel.

I didn't spend last night in a Holiday Inn Express but I do know a former Yamaha factory mechanic.

When we modded my wife's CRF250F, we did the same thing with the piston and cam and added a falicon rod. If I remember correctly we had to go way up on the jetting. Also you may need a fuel accelerator pump (I believe that is what it is called). And I assume the cam chain tensioner has been checked/adjusted?

i recommend not using the 110 octane fuel. i had good results with renegade fuel. it was the supercross mix, meaning it was not just the mx4 i think it was the sx-mx4 race fuel really stable and its gaining popularity. gieco, star racing, and rockstar makita are running it. i would try and time your acel. pump. but only go about 1/8 turns.

You shouldn't have to deviate from a 42 pilot, 178main.

For best results send your Carb to someone who knows what they're doing.

Yep, or get an adjustable leak jet kit. ALJ

That bog from fully closed to fully open is all in your accelerator pump and leak jet. You first need to take the subframe off to remove the air filter boot so you can see the air intake side of your carb. You need to spend some time adjusting the timing of your acc. pump which can be done with the adjustment screw inside the plastic cover on the side by the throttle cable pulley. One thing to check for is that when you whack the throttle open (with the bike not running) watch the injection of fuel. You want a sharp stream that doesnt hit the back of the slide. This is where the timing comes into play. Fiddle with the adjustment screw so you can get the acc. pump to spray as soon as possible w/o hitting the slide as it comes up. Alot of people actually remove the adjustment screw and spring, drill a hole in the acc. pump lever and just safety wired the two actuating parts together so you get a direct shot as the throttle turns.

The next thing you need to do is change your leak jet. Stock yamahas come with way to rich of lean jet which allows too much fuel to return through the acc. pump diaphram. A smaller jet will restrict the amount of fuel that can re-circulate through the ass. pump diaphram, causing the injection to be a smaller, but sharper and crisper stream. I can't tell you what size leak jet to run, but i've often had to drop down anywhere from 15-30 point sizes. A good start might be around a #40 or so. The other option is to buy an aftermarket cover that has an adjustable leak jet (work very well and easier to re-jet, but not with the money in my opinion if you just have a little bit of time and patience to change jets.)

Also, you would be wise to find someone with a dyno and an exhaust sniffer and pay them for an hour's use to dial your main, pilot, and needle in. Sometimes you can chase your tail trying to jet these mod engines on the track.

When I bored and ported my 450 I wound up having to go leaner to get it right.

Change the diaphram in acc. pump, race gas is hard on the rubber.

Did this mod on my 07 and it worked like a charm. I was frustrated and at my wits end before this.

From Thumper Talk....

This is an easy job and does wonders for getting rid of the bog that many with an FCR-MX (Only, regular FCR's use the 'Taffy Mod') experience. It can be done with the carb on the bike.

I use a #78 O-Ring (7/16" O.D. X 1/4" I.D. X 3/32" thick) from the plumbing dept. of nearly any home improvement/hardware store. Cost is about $2.00 for a box of ten, share with your pals!

1) Remove the throttle wheel cover from your FCR.
2) Work the O-Ring over the timing adjustment screw.
http://i143.photobucket.com/albums/r155/WilliamLynne1/FCRO-Ring1.jpg

http://i143.photobucket.com/albums/r155/WilliamLynne1/FCRO-Ring2.jpg

3) Pull the O-Ring over the black AP cam thusly.
http://i143.photobucket.com/albums/r155/WilliamLynne1/FCRO-Ring3.jpg

4) Re-install the throttle wheel cover. Just that simple!