Jetting

[Larry Perkins]

Brain pick here.  If you know what jetting works for a bike and you want to go 2mm bigger on the carb what size main jet, needle jet, and pilot jet would you go with in comparison to what you know works in the smaller carb?  Would you not want at least a bit more in the main for the more air?  Thanks for direction on this.

Larry P

[brian kirby]

I've wondered the same thing. A good guideline would be someone who swapped a 38mm Bing for a 36mm Bing on a 400 since they are essentially the same carb with different bore sizes.

Brian

'72 Berkshire

[john durrill]

Larry,
I would go larger on the main , pilot and a click at least on the needle.
My thinking is [?] , you have a bigger hole for the air to flow through
( more air should mean bigger jets ) , and  no change in the suction provided by the crank case ( volume is pretty much fixed ) .
 You might need a bigger jet to supply more fuel just because the draw on the jets is not as great. One engine tuner last month posted that with a 36 - 38 mm Mikuni on a heavy ported  100 cc cylinder he was using main jets in the 500 and up range.
 Did i send you enough info on the project we talked about last month?
Didnt have any luck catching up with you on the phone chuckle chuckle.
john d.

[Larry Perkins]

Thanks John.  Once again your brain comes in handy for picking.  Yes, you also got me going in the right direction on the X-15 project.  I hope to have it ready for NEXT year at Mid-Ohio.  As always, many thanks.

Larry P

[Ron]

Hi Guy's
I agree with John more air needs more fuel, however I think the larger carb is going to flow more air (more RPM's, despite crankcase volume) and I would think draw commensurate amount of fuel.
So, unless your main jet is on the border of being to lean now, I would think your going to end up pretty close to what you had.
However, you may have a weaker signal at idle due to the larger carb size (venturi effect) and may require a larger pilot and less slide cut-away.
I think I would start with the existing jetting and proceed with caution.
Ron

[john durrill]

Ron,
 Wouldn't the pipe ,ports and combustion chamber fix the RPM where the engine would run best?
 Unless the 2 mm larger carb is exactly the same , other than bore size , as the one you are replacing , wont the Signal to all jets will be different ?
 I think for me Larry , not being the sharpest tack in the box chuckle chuckle !!!!! , i would get the idle mix right. Have it the right size so you get a drop in rpm off the sweet spot as you adjust the idle mix screw in and out.
 Go bigger one or 2 sizes on the main and jet and get that right ............. unless you have a cable choke ( the enrichner type ) with a lever on the handlebars.
 Then go after the needle needle jet combo and slide cutaway . I would do this one last because the main jet does affect the needle and needle jet some. The larger the main jet increase the larger effect it would have on the needle and needle jet.
 For me Larry that would be the safest way.
  I really , really , really , really !!!!!!! don't like that feeling the engine gives me just before it seizes [B)] [xx(] .
 john d.  

   
 
 
 
 

[Ron]

Hi John,
I guess what I was thinking is a bigger carb is going to flow more air and the engine will rev higher, but the jets will feed fuel at the same ratio, unless like I said the jet size is already close to having to step up.
When I went from a 26mm to a 28mm on my Hodaka it picked up more RPM's (maybe 500 - 600 WAG).
The carb I installed had stock Super Rat jetting and I put it on a D/S motor. Jetting was almost perfect. I ended up stepping down (one size 220 to 215) on the main jet, but no other changes.
I was suggesting Larry's stock jetting is some what of a safe starting point.
Ron

[Ed Chesnut]

But Ron, you moved a properly jetted 28 from one Hodaka 100 to another Hodie 100 which used to have a 26.

The question here is changing carb sizes by 2mm - but the new carb has not been properly jetted on the same size engine (it's a grab bag) -- so what jets should be installed to get close?

I think John is on the right track.  Or even more simply put, "Find RICH immediately and work toward lean".  Applies to every tuning range - pilot jet, slide cut-away, needle jet/needle and main jet.  Working richer from where ever it happens to be can result in that nasty squeaking thing wayeeee too easily.

Ed

Keep the rubber side down!