Jetting discrepencies

[rotaxrookie#9]

I purchased a r32 previously owned by Jahan Asadi, it is jetted @165 main and 60 and 60 pilots. I put MRP info into maxjet 1.7 and it states I should run 152 and clip pos 2. as I said I am currently running 165 clip 4. I really only want to get seat time this season but i dont want to load up the motor nor do i want to detinate it running to lean. also running 13 teeth and 83 teeth, RPM's at MRP 11900 at 79 miles an hour

[Garry Lobaugh]

The question please? I'm sure that when you arrive for your next practice one of the guys will give you a ball park for that day. The Max Jet is strictly a comparison. Plug color and engine sound are going to help one in the final determination as to what is the precise maximum jetting opportunity. But since you are looking for seat time, I'd error on the side of safe (a bit rich). I'm sure Bruce W. can offer a few thoughts if he happens to be logged in this weekend.

In terms of gearing, for your size Bob (upper body) my guess is you are going to want to move up at least one tooth and maybe try two. But that is just from memory of days past.

[Marc Miller]

Hi Bob-

I'm an open book when it comes to set-up. Though Tim L. tuned my engine (jetting) I know that we had a 160 jet in for last weekend's double-header, and I "believe" it was a K98 needle with clip at 2nd from the top.

Gearing - I ran 12-85 for the retro C track... and 86 the day before backwards. I would try 84-85 rear gear and try to run a little higher RPM. I never gear for the straight - I gear for the track... and if I give up a few MPH's at top end, fine by me.

Good luck!

[Bruce Woodrow]

Bob,

152 definitely sounds too lean - I was running 162/clip 2 (K98 needle) and putting in some reasonable times, so this wasn't far off. In Maxjet there is a 'slider' control that needs to be adjusted to suit your particular carb. I would enter the conditions for MRP (making sure that you use the correct combination of altitude and altitude corrected barometric pressure or just absolute barometric pressure) and move the slider until Maxjet reads somewhere in the 160 to 162 region and go from there. Also worth bearing in mind, if you use pump fuel for practice, it will probably contain about 10% ethanol and will require a jet about one size bigger than predicted for race fuel (i.e. 165 instead of 162).

By the way, do not trust the needle clip position given by Maxjet - its calculation is not based on physics, just on an interpolation between jet sizes - this is just plain wrong. The competitor software, Jet-Tech, gets this right. (I have no vested interest here as I have written, and use, my own software).

In terms of gearing, Marc's memory is playing tricks with him :-), his, and most other Masters driver's, front sprocket was a 12 tooth with rear srockets in the range from 84 to 87. However, once rear spockets get above about 85 teeth they are in danger of being damaged in any off track excursion, so it is then worth moving to an 11 tooth front sprocket to allow use of a smaller rear sprocket to give the same gear ratio (I was using 11/80 which is the equivalent of 12/87). I would recommend that you start out with 12/84 (Gildersleeve's favorite) at MRP as this will give good lap times, good straightaway speed, keeping the engine speed at a reasonable level for reliability. Also, the 12 tooth driver is friendlier for maintenance than the 11.

I hope this helps,

Bruce

[Bruce Woodrow]

Bob,

Another thought:

Make sure that you are measuring the air temperature in the shade - this will make a big difference.

Bruce

[Marc Miller]

Sorry Bob - Bruce is right - it is a 12-85 I had on. I changed it above.