Author Topic: 350 carb tuning issues revisited  (Read 157 times)

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Offline disc valve

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350 carb tuning issues revisited
« on: April 06, 2021, 03:01:53 PM »
Hi group
Trying to get my rebuilt 350 running something like.but my issues are worryingly similar to Brians of a few years ago.it wont pull away cleanly splutters and bogs.again wont run smooth on light loads and small throttle openings.ive got no exhaust pipe banging and popping though.once it gets onto mains it pulls strongly and cleanly.running 25 pilot 140 main and 2nd groove on needle.
Observations after reading Brians post

If engines are being strangled by std air filter wouldnt it effect larger throttle openings aswell/more.

If pilot circuit cant be proved because of plugged passageways might it be possible to try and remove the plugs/balls to clear/check the circuit and reseal

It would be nice to have a pair of proven carbs for members to try.

By comparrison ive just recommissioned a 175 dt thats been stood a while.sorted a few bits and bobs sonic cleaned the carbs and away it went with no running problems.good fun buzzing up and down only problem its stuck in 4 speed rotary.

Offline disc valve

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Re: 350 carb tuning issues revisited
« Reply #1 on: April 08, 2021, 12:05:10 PM »
Uk mikuni spares supplier motocarb can supply pilot jets in 3 different designs
Original 8 hole drillings,6 hole drilling and no holes.wonder if trying the other designs might help

Offline BRT-GTR

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Re: 350 carb tuning issues revisited
« Reply #2 on: April 10, 2021, 07:53:57 AM »
       Disc Valve, 
      Having spent months playing with the carbs and trying every variation in the book, I came to the conclusion the light load running issue was probably not due to the carbs !!  Most likely would be a small leak in the crankcase .
      A couple of scumbags intervened in the process, stole the bike from my garage, thrashed it round for a few hours and then set fire to it. She is repairable but I haven't yet tested the crankcases due to lockdown.

  A few comments :-
         Stick to the 8 hole pilot jets. BS spent a lot of time choosing them.
        Make sure your choke plungers are seating when choke is off, IE, slack in the cable at the bars. Leaking chokes upset the slow/light running.
          The 140 main jets were for running-in only, for normal use install 130 main jets. They won't solve the issue but she will run better when sorted.
          I was getting desparate towards the end of the process IE suspecting the air filter etc.  The exhaust popping and banging occurred when I was playing with some of the more extreme settings.
         Keep a record of all the changes you make to the  carbs and note the results. Although frustrating, I learned a hell of lot about carb setup and tuning.
         Good luck and keep us informed on progress.                                                      Brian.
Unspoiled by progress.
I'm so glad I grew up in the 60s & 70s. I did so much stupid stuff and there's no record of it.............Anywhere !!

Offline disc valve

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Re: 350 carb tuning issues revisited
« Reply #3 on: Today at 10:51:51 AM »
Thanks for your reply Brian
Very sorry to here that your bike was stolen i hope you were adequately insured

Can you get new choke plungers?

 I thought the pilot cicuit got its air bleed from a seperate feed.but looking at the carb i cant see it.it looks like the pilot gets its air bleed from the main jet body but i dont think this is right.there are 2 small holes into the venturi one above the pilot jet and an angled one would this be a pilot bypass which aids transition away from idling.
Leak down tests can be a little confusing on a 2 stroke.dont let it lead down the wrong path.

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Re: 350 carb tuning issues revisited
« Reply #4 on: Today at 11:37:10 AM »
Are your choke plungers bad or are you just concerned that the rubber on the bottom has hardened and no longer seals? If it's the rubber, I can send you 2 new rubber discs to swap out.
Confucius say... "Better to have Bridgestone than Kidneystone"

 


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