Jetting Fun

[Kurlon]

So, I hobbled up a bashfest engine out of spare parts after my TB160 setup blew it's cookies all over the place.

Lifan 120 with a 57mm Tak XR100 Superhead piston, TB cylinder, nice thin copper head gasket, stock untouched head with a TB 300a cam and to top it off, a Mikuni VM26 carb as it just didn't like my PE28. The other end is a 1st generation Ultima CRF70 pipe. In other words, a moderate compression 140cc import with restricted breathing in the head.

I'm working on dialing in the jetting and I've gotten down to a 17.5 pilot / 145 main, and it's still reading fat at idle. My 15 (smallest available for the VM26) just arrived so I'll test that tonight. Am I over-carbing the poor thing, or is it possible for this hodge-podge of parts to have that strong a signal at idle? Seems odd to be at this end of the range when I'm typically seeing suggestions for mid 20s pilots when others play with this carb.

Terry, is there a sling delete clutch cover setup that bolts up, no cutting of the crank arm required?

 

[The_GDFP]

Sounds close. Screw "readings", what's it feel like? Throttle response and not loading the plug in the paddock are all idle is good for I know you use the other end of the throttle cable most of the time, any way. PE28 and clean up that head a tad. You'll like it better, I'm betting.

 

[sean01]

145 main may be a tad fat, drop it a bit and it may clean the bottom up..

 

[Kurlon]

Heh, when I say 'readings' I'm going entirely by feel so far, 3.5 turns out on the air bleed and it still starts stone cold in 50 degree weather without the choke. Idle returns instantly without a dip or burble when snapped, just off idle transition range is slightly spotty but I suspect that's main/needle. Been trying to get the pilot sorted before chasing main then needle but I think you're right Sean, 145 seems fat even though I'm able to get full revs with fat popping on top so far.

 

[sean01]

yeah your idea is good and its what is usually advised but problem is most of these carbs act differently when large mainjets are fitted..

id drop the mainjet size, then tune the bottom and mid then tune the main again..

 

[Kurlon]

Dropped to a 120 main today, still wants a 15 (or smaller) pilot, and now it is clearly lean on top. WFO it dies, roll off a touch and it sputters, roll of a little more and it screams to redline.

Pretty sad to see how much HP I've lost from my prior motor, 17/30 gearing and this motor does NOT want to pull 2nd, old build would rip through before you could find the throttle stop... I'll just have to dream of a new machine while I dlal this dog in.