Confusing Jetting Problem
 

I have 92 f2 streetfighter, stock air filter, 4-1 aftermarket header, and a gutted shortened pipe (almost like a straight pipe). I have keihin 145 mains, keihin 40 pilots, dynojet needles with the clip on the top position with no washers below. It runs great from idle to 7k then cuts out and sputters from 7-9k. If I alternate between half throttle and gunning it, I can get it to 9k. From 9-15k it pulls like monster.

I don't get it, with my stock air filter, why do 145 mains yield the best pull up high, yet down low from idle to 7k it runs well with the clips on the top position. BTW the float bowl vents run to the bottom of the air box. THanks in advance for some help

thats because your not running on your main jet untill around 7500.. stock mains are 135.. i would start with them...

why would I go from 145 to 135 if my 9-15k range is great? Wouldn't that lean it out too much?

Edit: I started with dynojet 126's (Keihin 135's according to the conversion chart) and it wouldn't go past 7k at all. Then I bumped up to 140s and it would pull to about 13k then cut out. Now with the 145s it pulls real strong from 9-15k. I can't figure out if my 7-9k stumble is because it's too rich or lean. idle-7k and 9-15k are perfect

it also does the crackling popping when it winds down from 9k-7k leading me to believe its lean there. Am I right? The carb tuning guide says select the mains that yield the hardest pull from 7.5k to redline. my prob is 7-9k:confused:

Your problem area is where the main jet comes on from the pilot jet overlap. You said you were running the needles at the top clip setting. This drops the needle into the needle jet making it leaner. Could be that the transition onto the main is being magnified by this setting. Try setting the clip to the middle position and see if it lessens the bog.

I rode my f2 home on the pilot jets from 3000 to 7000 because my mains had not been tightened properly by yours truly and had fallen off into the float bowl!! Still managed a decent pace.

at first i thought with your mods 145 main sounds alittle big but if its workin for you then go with it...now that the issue is at 7-9k thats about where the transition from needle jet to main jet is happing so like gpfan said start with dropping the clip.. being your at the top of the needle theres a good chance this may fix your issue

Like gpfan1 said, you're running the leanest jet needle setting, and that will affect your transition - honestly, if you go to the middle position, you should probably try the 140 mains again, as even with a hi-flow type filter, like a K&N, 145 mains would be a touch rich.

With a full race exhaust, and clean K&N filter, I'm at 142 mains, and it runs like a raped ape.

By the way, with a gutted pipe, that could give you popping in a lot of decel ranges - even if it's shortened, if you put some packing in, it might cut that down a bit.

Also, 13,000 is slightly over-reving an F2 already, if you're running 15,000 regularly, you might run that motor down really quick... did you remove your rev limiter?

I have the needles in the leanest position because with 140 mains with the clips in anything below the second position it would get have problems in the 5-7k range (too rich I believe). Now with the clips in the top position, the 5-7k range is perfect.

If my problem area is where the main jet comes on from the pilot jet overlap, cold my 40 pilots be causing this problem?

I'm going to try 142's, putting 38 mains back on, put one washer below the needle and see if that does anything. Yea I'll be more careful with the 15k reving, I usually try to shift at like 12.5 tops. I thought the stock rev limiter was set to 15.5k?

Thanks for the help guys

Edit: what are the benefits of 40 pilots?

this may sound like it has nothing to do with your issue but is your heat shield in place?

No its not, I'm guessing the absence of the heat shield will allow it to draw in more air?