jetting and airbox help

Randy Grein randygrein at
Wed Aug 25 17:24:15 PDT 2010

IIRC 150 is about right mains for an open airbox, but they do need to  
be staggered - try a 147 front/150 rear, and work in the classic  
tradition - get the mains sorted first, then dial in the needle. Just  
be aware that an open airbox will never run really clean - what looks  
like nice open space has real problems when it's not isolated enough.  
Probably causing weird pulses; you do know the airbox is a resonance  
chamber, right? Weird physics, and trying to outsmart the original  
engineers is hard.

I did a lot of early work with a 1st gen open airbox, and while the  
ideal jet numbers were  bit bigger than with a BMC RACE filter (use  
near stock jets for the street filter, it has a stock-sized opening  
without a trumpet) it actually makes a bit less power. Of course the  
race filter came with a jetting kit that was pretty good and included  
needles, that tells me you probably have the street air filter. If  
that's the case go back to near stock settings; try 140 front/142 rear  
to start and use the 3rd needle position. That might get you in the  
right area, but you didn't say where the needles came from.

Randy Grein, WMRRA #141

On Aug 24, 2010, at 6:17 AM, matthew patton wrote:

> I finally got the front carb to stop dripping raw fuel down the carb  
> throat. somehow replacing the float needle and it's brass fitting  
> and the float did the job. Not sure why but I'll take it.
> I've got 17.5 pilots, 150'ish mains (I think) and aftermarket  
> needles (not dynojet, name escapes me) at clip 4 from top, BMC air  
> filter and Yosh race can.
> I've been trying various things and can't seem to make any decent  
> progress. What is a good starting point?
> How much jetting difference should it make with open airbox vs BMC?  
> 1 needle clip? 2?
> Without the BMC it runs a little stutter at >1/2 throttle and 6000  
> RPM till about 7.5K and then it cleans up and seems to pull  
> reasonably strong to 9K. With the BMC in it gets massively ragged at  
> 6K and anything past 1/8 throttle and never cleans up.
> I'm guessing my BMC is clogging up the airflow (I'll clean it again)  
> and I'm jetted way too rich for the BMC but ???? for open airbox.
> Can anyone clue me in?

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