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Discussion Starter · #1 ·
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
 

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If its anything like my f1, then when i took the fairings off my bike i had this issue.

Simple fix;
Without fairing your air box and carbs are seeing different air pressure so this is why you arent getting power in the rev range...The faster you go the bigger the difference in pressure it is.

Take the atmospheric vacum tube from the carbs (in the f1 it has 2, the f2 i believe has 1) Replace this/these tubes with a longer one and run that into the bottom of the airbox (drill a little hole one the underside)

Now both the airbox and carbs will be using the same air and seeing the same pressure.

This is very common for the f1's and f2's and i believe they call it "the ram air effect"

I have images as a reference on my build thread if you need them.
 

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Discussion Starter · #4 ·
Thanks thats great advice, problem is I've already don that. Based on other advice I've gotten I'm going to try 142's, putting 38 mains back on, put one washer below the needle and see if that does anything
 

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Thanks thats great advice, problem is I've already don that. Based on other advice I've gotten I'm going to try 142's, putting 38 mains back on, put one washer below the needle and see if that does anything
I suggest you change only one thing at a time... or suffer an increased risk of confusion.

The 145 to 142 "main Jet" change will lean 3/4 to WOT.

The 40 to 38 "Pilot jet" will lean your idle.

Raising the needle will richen the midrange.

It is all about throttle position and not rpm. Mark your throttle with masking tape so you know where you are.








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