Author Topic: 350 carb tuning issues revisited  (Read 4662 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.
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Offline disc valve

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Re: 350 carb tuning issues revisited
« Reply #3 on: April 14, 2021, 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.

Offline moonpup

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Re: 350 carb tuning issues revisited
« Reply #4 on: April 14, 2021, 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"

Offline BRT-GTR

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Re: 350 carb tuning issues revisited
« Reply #5 on: April 15, 2021, 09:52:34 AM »
       The pilot circuit gets its air bleed from the 4mm hole at the bottom centre of the carb venturi inlet. (This hole also supplies air to the correction jet in front of the main needle jet). The air takes a left turn via the pilot air screw and then runs back into the carb body through the drilling that is blanked (just left of the 4mm inlet hole). There is then a further blanked drilling on a slant (main carb body above the float chamber) that takes the air to the pilot jet.

  Also note that current new pilot jets are ISO threaded 5mm x 0.7mm pitch whereas the original cabs and jets are JIS threaded 5mm x 0.75mm pitch  . Easily sorted, run new pilots through a good quality 5mm x 0.75 die, readily available on the internet.
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Re: 350 carb tuning issues revisited
« Reply #6 on: April 18, 2021, 12:05:31 PM »
Hi moonpup
Just trying to eliminate any potential problems.thanks for your kind offer.my email is larkins334@btinternet.com .i can forward my details
Cheers david

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Re: 350 carb tuning issues revisited
« Reply #7 on: April 19, 2021, 12:10:26 PM »
So youve just sonic cleaned the carb bodies,given them a good blow out with comp air.then normally you'd test by squirting carb cleaner to make sure alls clear.easy on the choke circuit,needle jet,fuel feed,pilot fuel but do you have to hope the pilot air is clear?have i missed anything.cheers

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Re: 350 carb tuning issues revisited
« Reply #8 on: April 20, 2021, 09:45:23 AM »
Hi moonpup
When you posted your excellent photos of sectioned carb bodies on the redundant blanked hole and pilot air screw drilling did you go further and cut away to reveal the pilot air bleed drillings

Offline moonpup

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Re: 350 carb tuning issues revisited
« Reply #9 on: April 20, 2021, 12:09:18 PM »
Hi moonpup
When you posted your excellent photos of sectioned carb bodies on the redundant blanked hole and pilot air screw drilling did you go further and cut away to reveal the pilot air bleed drillings

Yes and here is what I found...... First I cut away the "housing" that the pilot jet screwed into, so that I could get closer to where all the action took place.  :D

In the first pic, you'll see that it is drilled out towards the center of the carb body and comes out inside where the throttle body rests.

The second pic shows where that drilling exits. (the left circle in the pic) The right circle shows a smaller passage drilled in the half tunnel that connects to the passage in the left circle.

The third pic shows a paper clip inserted into a short passage that angles down to a longer passage that then runs out to the front of the carb and exits out the first hole in the pilot air screw housing, as seen in the last pic.

It appears that only compressed air or chemicals would be able to clear out this passage should it be blocked.

PS..... check your emails.

« Last Edit: April 20, 2021, 12:13:37 PM by moonpup »
Confucius say... "Better to have Bridgestone than Kidneystone"

Offline disc valve

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Re: 350 carb tuning issues revisited
« Reply #10 on: April 21, 2021, 10:46:18 AM »
Finally worked out the path of air to the pilot jet thanks to both Brian and Mike.if Mikes still got the old carb body to prove the pilot air circuits clear you could have a go at removing the pressed in ball/plug then spray carb cleaner through to prove its clear then maybe reseal it by tapping a suitable grubscrew in.what do you think?

Offline hardy

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Re: 350 carb tuning issues revisited
« Reply #11 on: April 21, 2021, 07:28:02 PM »
I have done this before with RD400 carbs, to remove the ball, drill a small hole and thread in a screw. Using a pair of plyers you can work out the ball.

When tapping the thread, only go in a few threads and use loctite to ensure it doesn’t come out.

Hope this helps...

Offline disc valve

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Re: 350 carb tuning issues revisited
« Reply #12 on: April 22, 2021, 05:04:53 AM »
Hardy
Wondering what the balls are made of.ie how easy/difficult are they to drill and what made you drill them out in the 1st place what symtems did the bike have.was the circuit blocked if so how were you able to clear it.did it work

Offline hardy

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Re: 350 carb tuning issues revisited
« Reply #13 on: May 11, 2021, 07:15:14 AM »
Hey Disc valve,

I removed the balls and original air jets and replaced the air jets with a larger jet. This was done to convert the carb from a bleed needle jet to a primary needle jet. A common mod for Rd350s & Rd400s.

The balls are soft so a sharp centre punch then quality drill bit will easily work.

I have been contemplating removing the balls on other carbs I rebuild in future just to assist the cleaning….?

Offline BRT-GTR

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Re: 350 carb tuning issues revisited
« Reply #14 on: May 11, 2021, 07:58:48 AM »
      Hardy
                 Which air jets did you replace ?
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 !!

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Re: 350 carb tuning issues revisited
« Reply #15 on: May 11, 2021, 05:39:25 PM »
On the yamaha carbs, there is a pressed in 0.7mm air jet that feeds the needle jet, I removed this, threaded and installed a larger jet.




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Re: 350 carb tuning issues revisited
« Reply #16 on: May 12, 2021, 06:57:08 AM »
Hardy
Cheers.if my problems persist ill try and remove the ball brg and prove the pilot cicuits clear

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Re: 350 carb tuning issues revisited
« Reply #17 on: May 15, 2021, 05:49:33 AM »
No worries.

Make sure you post up your results. We can all learn from them

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Re: 350 carb tuning issues revisited
« Reply #18 on: August 24, 2021, 04:36:06 AM »
Delayed update on carb cleaning/checking.i removed the 3mm dia brass ball from the blanked passage way to the pilot jet by lightly centre popping the ball then drilled a hole through and worked the ball out with a picker.i then drilled and tapped out to M4 i put grease on the tap to keep the swarf under control.didn't go too deep as its near the pilot airscrew.reverse blew out with air then checked with carb cleaner that all was clear the 2 holes into the venturi and passageways to the pilot jet.all good so now everything checked and clear good to go.havnt tried them on the bike yet but hope to soon.cheers David

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Re: 350 carb tuning issues revisited
« Reply #19 on: August 24, 2021, 04:37:13 AM »
Havn't mastered photo posting yet otherwse would have

 


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