Race Fuel Jetting???

[400 SS EX]

Ok I need only the guys with first hand experience to answer this question, because its kind of a toughie. Ok I have a 400ex with all the mods on my sig. and it's jetted alright at the moment it's a little lean especially the 40 Pilot. I have been running regular 93 octane gas, yesterday I bought some VP Racing U4 and mixed it 50/50 with premium. My question is about what size main/pilot do you think I should be at right now??? I ordered a 45 Pilot and a 170 , 172 , 175, ,main. What about the air/fuel screw did you guys have to mess with that at all. I'd like to be able to drain my system and switch out the jets with in a night after a race in case I just wanna go riding and don't wanna burn $16 a gallon gas. Any advice would be great, expecially 400ex specific. I'm doing all I can to hang with the 450's on the flat tracks:(

[GPracer2500]

What is your current jetting? Also your sig doesn't say what size piston you have. Unless someone has a nearly identical settup and runs the same tracks as you, knowing someone else's jetting specs is only going to be of marginal usefulness.

What would be helpful is if someone could share their experience on how switching to U4 affected their jetting (i.e. how many main jet sizes did they have to go up to accomidate the fuel). Hearing "I ran a 170 with U4" doesn't help much, but knowing "I went up 2 jet sizes with U4" is useful.

I'd be surprized if you had to go more than 1-3 sizes up for a 50/50 mix of U4. I'm just guessing though.

Waiting for U4 users to chime in...



On a different issue I notice you're using an aftermarket CDI and a ignition advance key. Does the CDI also advance the ignition as most do? If so I'd look into that because then you'd be running LOTS of ign advance and too much will kill power.

[400 SS EX]

Sorry I didn't put that up, I wasn't thinking. I am currently running a 40Pilot, 162 Main, 4th clip on the needle, and am on the Stock 85mm bore. The CDI brings up a good point as well. The CDI doesn't advance the timing but I do have a 6 degree timing advance key from Sparks.

[ohsobad_chevy]

Is U4 oxygneated fuel or what? Never heard of it. Would you have to change your jetting by going from 93 pump gas to 100 octane race gas that wasn't oxygenated?

[GPracer2500]

Originally posted by ohsobad_chevy
Is U4 oxygneated fuel or what? Never heard of it. Would you have to change your jetting by going from 93 pump gas to 100 octane race gas that wasn't oxygenated?

Yes, U4 is oxygenated. That's why it's more likely to need jetting changes than run-of-the-mill race fuel.

[ohsobad_chevy]

Originally posted by GPracer2500
Yes, U4 is oxygenated. That's why it's more likely to need jetting changes than run-of-the-mill race fuel.

TY sir. I am going to get a 11:1 comp piston, cam, and mild port job done. I will be running 100 octane(not oxygenated), Thats why I asked. :)

[GPracer2500]

It's always a good idea to confirm jetting specs anytime you change fuels, no matter what the fuels are. A small change may or may not be helpful depending on the fuels.

[ohsobad_chevy]

Originally posted by GPracer2500
It's always a good idea to confirm jetting specs anytime you change fuels, no matter what the fuels are. A small change may or may not be helpful depending on the fuels.

DAMN 4 strokes, They are soo picky.

[400 SS EX]

Anybody have any suggestions has to how much of a jet increase??? I have my old ones listed above and like I said I am now running U4 50/50 with premium. I switched to a 45 pilot and 165 main for now and went to races tonight and got my @$$ whooped. Last place in both heat races and in my first main the tire went flat around a corner, so I called it a night and just left! After the first practice I did switch back to the 160 main because the 165 was so shi*** but maybe it was lean and I leaned it out more. I really just need to do some good ole plug readings. But anybody with a 400 Z or EX would help me out , just an idea of how many sizes you jumped when switching to this fuel setup.