jetting spec recomendation

[tboone]

Greeting
I have an 03 engine with a Cicrcle F down pipe and a 28 mm carb with the stock 98 jetting. I would like to find if anyone might have suggestions as to any jetting changes that I may need to make with this combination.

Thanks

Tom

[olddogs]

The 03 cylinder is pretty much a 97 Super Combat jug with a lower exhaust port. With a down pipe similar to the 97SC, I would start closer to a 125s fuel needs than a 100. We have raced for years with 03 reed valve cylinders on 125 Combat Wombats using the stock jetting in the 32 mm carbs right off the Super Combats. Terry at HPI sells them pre jetted for SCs and we have had excellent results with them.

If you stick with the 28, just start big and then go small when it comes to jetting. Both pilot and main jet must be dialed in together. Remember the main jet will only flow the fuel the needle and needle jet will let pass, so comparing the 98 Rats 250 main jet to the 400 main jet of the 125SC is not comparing apples to apples. The carb section in the HPI site had some jetting combo and carb recommendations. Learn all you can before you start.

[tboone]

Thanks for the info.
The 32 mm carb does sound like the best way to go and I would do that today, but finding an intake to accept that carb seems unlikely. The HPI site has them as not available at this time.

I will look for the jetting and carb recommendations you mentioned and see if I can make this 28 mm carb work.

You mentioned a #400 main jet in the SC. Is that with a #35 pilot?

Tom

[olddogs]

The 28 mm will work fine and supply enough fuel once you get it jetted, since the 03 came stock with a 26mm carb, Do some research and check what jetting the 28 mm on the 95 CW uses. One area many people neglect to consider is the needle jet. A 250 main jet may be fine with a certain needle jet but way to rich or way to lean depending on what size needle jet and needle was used. This is a common swap and maybe someone who has done it will share the work they put in to dialing this combo in.