Starting issue

[brian kirby]

I dont know that either one outperforms the other, once the bike is running, they are both excellent carbs. It is no doubt easier to start a big bore two stroke with a Bing, and there is no doubt the Mikuni is cheaper to maintain. People claim the Bing makes more power, and that may be true, but the difference, if any, is minimal and the far superior long term durability of the Mikuni makes it my choice.

Brian

[gooddirt]

I like the hit of the Bing not the Sewing Machine power band that MIC gives (100 and 125's )[:0]

[Big Mac]

Fought this same impossible to start gremlin this weekend on an old KTM I bought cheap because the seller gave up on kicking and a shop told him it was too old to service. Same lean condition you mention. Both the main jet and the pilot had a bit of scuzz inside.

Didn't have a bigger pilot to try, so I took a chance with reaming out the old pilot a bit with a sharpended wire. But the float height looked like it was shutting off fuel too soon. I bent the tang to set the floats so fuel was extra high...a hair past horizontal when held upside down and float arm resting on the needle. Starts today with two kicks when cold and choked, one kick no-choke when warm. Runs like a champ. Suggest you go big on the pilot and don't forget to keep fuel height plenty high in the bowl--should work better.

[brian kirby]

Float height is critical on a Mikuni, if it's too low it won't start.

Brian

[brian kirby]

Float height is critical on a Mikuni, if it's too low it won't start.

Brian

[Ed Chesnut]

I get a bit nervous reading this thread.  If the pilot jet is so lean that causes significant problems with starting (and it isn't plugged up), then it is lean enough to cause things to get extremely noisy then extremely quiet in quick succession whenever a low throttle position cruise situation is encountered with a Mikuni (and probably with a Bing or Amal).

A quick, though not completely conclusive, check for the Mikuni would involve:
1.  Identify the pilot air screw setting.  How many "turns out" is it?
2.  Check to see if stable idle speed changes when the pilot air screw is adjusted "in" 1/2 turn.  Do another check with the pilot air screw adjusted "out" 1/2 turn from the original starting position.  Note that "in" richens mixture, and "out" leans mixture.
3. If pilot air screw adjustments do nothing . . . either the pilot jet or pilot air passage (or both) are plugged.
4. If best idle is achieved between closed and 1 turn out, this is an indication that the pilot jet is too lean - get a larger pilot jet.
5. If best idle is achieved at 2 turns (or more) out, this is an indication that the pilot jet is too rich - get a one step smaller pilot jet.  (NOTE: it is not at all likely that a hard starting engine has a too rich pilot jet -- unless it is plugged.)
6.  When the proper size pilot jet is installed, best idle will be achieved with the pilot air screw somewhere between 1.0 and 2.0 turns out.

Ed
PS: I don't know Pentons very well . . . but am pretty familiar with the innards and operation of the Mikuni.
PPS:  Being lean on the pilot jet is not a good thing.  No.

Keep the rubber side down!

[Ron]

Important to note, because the pilot jet feeds fuel in front of the slide, it flows fuel at all engine speeds.
If your extremely lean or rich in the pilot jet, changing the pilot jet will also change your air/fuel mixture in needle/needle jet combo, as well as your main jet. Get the pilot jet right first.

[Ed Chesnut]

To be slightly more accurate, the pilot jet flows fuel at all throttle settings - so changing the pilot jet in a Mikuni has an effect on jetting at ALL throttle settings.  To be sure, the effect at WOT of a 1 step change in the pilot jet will be small . . . but it could be very important if Wide Open Throttle was already on the ragged edge of being too lean and then the pilot jet was reduced by a step.

Yep, get the pilot jet right . . . and then check/re-check jetting at all other throttle settings.

Ed

Keep the rubber side down!

[skiracer]

Thanks for all the help.  I hope to work on the issue next week.  Currently riding my 250 Herc in Fl.!


1976 MC 5 Original Owner
1976 Hercules 250 GS
1982 Suzuki PE 175
2002 KTM 200 EXC
1976 Penton 175 GS

[skiracer]

I just wanted to thank everyone who responded to my request.  I finally made the time to work on my Penton.  I changed the pilot jet from 25 to 30, and adjusted the air screw to about 1.75 turns out.  The bike started on the 4th kick after sitting since last October!  Music to my ears!  I really appreciate the help, thanks again.   James