Carb. Jetting using an A/F gage

[XR250]

I need to adjust the jetting on a new build for a 125cc Honda Navi. Started as 110cc and added a cam, pipe and carb and bore. I just installed an A/F gage on it.
If for example, it says I am at 11:1 at WOT, and am running a 110 Main jet, should I try a 11/12.5 x 110 = 98 (say 100) main jet assuming I want 12.5:1 as my target mixture?
Also, is 12.5 a good number for WOT? What should I be shooting for a low and mid throttle?


Thanks

 

[BrianPetersen]

Jet nozzle diameters are, if anything, based on diameter, not flow area, and there area factors other than merely the flow area at work. On top of that, changing the fuel jet doesn't change the air-bleed jet, nor the orifice where the fuel circuit leads into the main throat (which is carrying emulsion).

For that air-cooled engine, low to mid 12 air/fuel at full load sounds about right. Idle can be as lean as it will take. Needle as lean as it will take. I doubt if it'll be happy lean of stoich, and even that's a stretch (it isn't a very sophisticated engine). I'd try about a 102 or 105 main jet and see what happens.

Having ridden a Navi, does part-throttle even matter? A big-bore (how big?) would probably just make it less underpowered ... When I rode one, all acceleration was at wide open throttle, and all straight-line riding was at top speed.

I put 70 000 km on a CBR125, mostly at wide-open throttle, before the big-end rod bearing let go. Still have it; needs an engine. Shrapnel from the shattered bearing did some collateral damage.

 

[XR250]

Thanks Brian.

"Having ridden a Navi, does part-throttle even matter? A big-bore (how big?) would probably just make it less underpowered ... When I rode one, all acceleration was at wide open throttle, and all straight-line riding was at top speed."

Ha. I have 2 Navi's. Even with the jetting off , the modified one (125cc) is night and day faster. Hits the rev limiter at 57 mph pretty quickly. So part throttle is a thing.
Honestly, the only reason I am doing this is to kick my buddie's ass on his Grom and to have a fun winter project. I may get a rev. adjustable ECU so I can break 60 mph.
I ride my SV650 when i want some reasonable speed.