1972 Jackpiner Mikuni/Fuel Problem need Help

[Rob Thatcher]

Hi,
I finally go my bike all back together.  Looks good and will start right up. But it never starts and runs cleanly.  

Fresh 93 octane with Golden Spectro Synthetic @ 44:1.  New spark plug.

Kind of chugs like a diesel when I give it gas.

However, it won't rev much over an idle and stalls out.  

Almost like fuel starvation.  I can see fuel in the lines, but one petcock doesn't seem to flow well.  

It's not running long enough to do a plug check.  I have a 30mm Mikuni (new) that was allegedly baseline jetted for the bike.  I am looking for the page that has my specs on it.

I know the timing is correct (Thanks to Chicago Jerry).  

Just over three weeks to Vintage Days and I want to ride it prior to paying for my entry fees.  

Any and all help greatly appreciated.
Thanks
Rob

[Dale Fisher]

First thing I would check out of the ordinary is the exhaust.  Make sure it is not plugged up or restricted [?]

Dale Fisher
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74 Berkshire 100
70 Six-Days 125
73 Jackpiner 175
And some silly other bikes...

[penrac]

This does sound like a plugged exhaust issue, but could be ignition related, after double checking jetting, it is possible the ignition system is losing spark when your engine has a "loaded combustion chamber full of fuel".Also do not overlook simply having a bad spark plug or poor wire connection or grounding of engine/ignition system.[?]

[tofriedel]

Rob,

I had a bike acting the same way as yours.  Fuel was in the fuel line but it wasen't getting to the carb fast enough to keep it running.  Further inspection of the petcoks revealed that they were almost totally clogged.  

I was able to clean one up, but I had to replace the other.  Problem solved.

Tony

[Rob Thatcher]

Thanks for all the replies.  I do have spark and it has no problem off the start.  I will check the petcocks tonight or tomorrow, as I am leaning toward not enough fuel.  If it had enough fuel, it would just foul or run, not attempt to run.  I will let you know.

[Steve Minor]

Do you have the float adjusted to the proper height?

Steve Minor
Wilmington, NC

[Lew Mayer]

I have seen bits of varnish from the tank partially block the petcocks and had to be blown out.

Lew Mayer

[Rob Thatcher]

Timing is set correctly, pipe is not clogged, plenty of fuel flowing.  Does anyone have baseline jetting for a Mikuni 30mm?  

I know I can search for it on the forum and it comes back with someone saying search for it instead of real numbers and real answers.

My NEW Mikuni has a 240 main and a 40 pilot (seems pretty large to me).  I have a spare 35 and 37.5 pilot and pretty much every size main.

I have not messed with the airscrew, needle is in the middle.

[Tom Murphy]

I bought a new 30mm Mik, baseline was 270 main and 35 pilot. Setup is supposedly sea level at 70 degree f.  I stepped the main down two and it is really close on my 125. Tom

[tomale]

that is a curious problem, you are going about it right though, check everything. Here is something to look at.. no one has talked about the air jets, they can be found in one of the holes inside the venturi. on my bigger bikes, it was not allowing enough air to atomize into the needle jet circuit.  you might try running the bike with the choke on too.. If it changes how it runs, then that may help find what is going on.. don't forget to play with the air mixture screw too. try it at several different settings and see how it runs..

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
74'250 hare scrambler (project)
74' 1/2 440 maico
78' 440 maico
72' cr125 Husky (project)
93' RMx 250 suzuki

[marsattacks]

Maybe check gas  cap too?  I'm stupid on this stuff but it could be a problem source so worth part of the full-on itinerary.

Mars Attacks!

[Rob Thatcher]

Checked the gas cap, changed the jets, changed petcocks, everything is tight and it still doesn't want to start.  I will be at Mid Ohio on Friday, any thoughts or help to figure out the problem would be appreciated.

[chicagojerry]

rob...   call my cell.   440 213 5146
i'm laid off this week. bring it out to me and we'll get it going.
chi jer

[Rob Thatcher]

To all following this thread,
Chicago Jerry and I finally figured out that the pipe, even though cleaned out, is still restricted.  Based upon his assessment, he feels the crank bearing are suspect and that I shouldn't ride it at Mid Ohio.  So, the engine is back out of the frame and on the bench.  I am racing the motocross on Saturday courtesy of Chicago Jerry racing in the Super Senior C on his 250 with my Jackpiner tank.  See you all there and will let everyone know when the bike is back to 100%