Jetting Restored 94 Wombat

[AKwombat]

After working on restoring this 94 Wombat all winter, I am not too far from being ready to start it up. I am interested in what you guys have to say about the initial jetting.

It has a stock 03 top end, including cylinder, head, and reed valve assy. Since it has a 94 bottom end, I installed a Super Combat piston. The carb is a new Combat Wombat 28mm with stock jetting. These parts were all purchased new from Terry.

The pipe is a repaired/upgraded 94 pipe. I replaced the headpipe, cut off the flamethrower, extended the converging cone down to 1-1/8", and installed a FMF Turbinecore spark arrestor.

The air filter is stock 94. I am at sea level, about 50F.

The new CW carb has:
main 190
needle jet 04
needle 5EJ13
pilot 25
slide 2.5

The new carb came with one jet richer and one jet leaner for the main, needle jet, and pilot.

I know the addition of the reed valve requires richer jetting. The stock 03 Wombat jetting on the 26MM carb looks to be a bit leaner, at least on the main and needle, than the stock CW carb.


I'm planning to run Yamalube R at 28:1 for break-in, and I understand the jetting procedure starting at idle, and working my way up.

What do you guys think about starting with the stock CW jetting? Should I richen it up to be more conservative? I can order a few more jets.

I would like to get it dialed in and possibly bring it to Hodaka Days.

Thanks,
Chris

[Bullfrog]

The stock 26mm carb from the Wombat (03) had "leaner" jets than what is listed for the CW 28mm carb partly because the air velocity going through the 26mm carb is higher than thru the 28mm unit. The higher velocity "draws" fuel through the jets more effectively, so "leaner" jets are capable of delivering the proper amount of fuel for the engine. This air flow velocity "thing" is why it is dangerous to compare jetting specs from different size carbs.

Your CW 28mm carb is going to need somewhere around a 240 main jet and (if I recall correctly) about a 40 pilot jet. At any rate, there are some Mikuni carb tuning rules which should not be ignored:

1) Follow the published procedures . . . which . . .
2) . . . describe tuning the pilot jet first. So do that FIRST. Yes. Remember, the proper pilot jet will make starting and idling work well AND, the pilot jet delivers fuel at ALL throttle settings. So, if you skip pilot jet tuning and go straight to main jet and needle tuning . . . when you finally get around to tuning the pilot jet, you'll change your full throttle (main jet) tuning too - and you'll have to re-do your main jet and needle tuning. Did I mention, tune the pilot circuit FIRST?
3) ALWAYS sneak up on the proper main jet from the rich side, never from the lean side. By starting with a RICH main jet, the engine is protected from lean condition seizures as you tune. If you start with a lean main jet, you could easily seize the engine before you find your way to a rich enough jet.

If it were me, I'd put start with the 40 pilot jet and a 250 or 260 main (yer gonna need a few jets to play with). Time spent working leaner through too many rich main jets is well spent compared to not starting out rich enough. Yes.

Ed

[Bullfrog]

PS: I should have mentioned - your reeded 03 cylinder and 28mm CW carb - is exceedingly similar to the situation of having a stock Combat Wombat and adding reed valve. And yes, that requires richer jetting than the CW had. Long ago, I ran a reeded CW with the standard 28mm carb . . . and that is where the memory of 240main and 40 pilot comes from. The pipe you are running may not (actually, . . . won't) reach the same level of performance as the CW pipe, so my old jetting specs may not be right for your machine, but they should be close. Only tuning will tell. (and I'd still start with a 250 or 260 main)
Ed
PPS: I wasn't dissing your pipe. It'll run pretty well, but to quote Harry Taylor's answer to my question about what would be a good all-around pipe for a Combat Wombat -- he said, "Well kid, the Combat Wombat pipe IS a pretty good pipe." Yes.

[Bullfrog]

. . . annnnnnd, you are likely to need a richer slide - 2.0?
Ed

[AKwombat]

Ed
Your recommendations are much appreciated. I have the Mikuni procedure, but still would have been all over the map without the info you provided. I will order some additional jets and a 2.0 slide. Their are no pipes available, so I tried to do the best I could with my original pipe. I will be just be trail riding my Wombat, so hopefully that pipe will work okay.
Thanks again,
Chris