Author Topic: 350 carb tuning issues revisited  (Read 561 times)

0 Members and 1 Guest are viewing this topic.

Offline disc valve

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

  • Brian
  • Trade Count: (0)
  • Master Registered Bridgestone Member
  • *****
  • Posts: 920
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

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

  • Laidback Purist
  • Trade Count: (0)
  • Master Registered Bridgestone Member
  • *****
  • Posts: 2213
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

  • Brian
  • Trade Count: (0)
  • Master Registered Bridgestone Member
  • *****
  • Posts: 920
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.
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

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

Offline disc valve

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

Offline disc valve

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

  • Laidback Purist
  • Trade Count: (0)
  • Master Registered Bridgestone Member
  • *****
  • Posts: 2213
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

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 181
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

  • Trade Count: (0)
  • Full Bridgestone Member
  • ***
  • Posts: 54
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

 


Gallery


Views:25
Comments (0)
By: Usedknobby

Views:26
Comments (0)
By: Usedknobby

Views:107
Comments (0)
By: Perry L Anderson

Views:102
Comments (0)
By: Perry L Anderson

Classifieds

GTO
Date: 04/29/2021 04:31 pm
Time Left:
Looking for a TMX100 top end
Date: 03/25/2021 11:23 pm
Time Left:
TMX100 (or just 100) cylinder (jug)
Date: 02/05/2021 02:43 pm
Time Left:
Ignition keys Bridgestone
Date: 02/02/2021 02:02 pm
Time Left:

Recent Downloads added

BS200 Mk II RS & MK II SS Exclusive Parts
Rating: (None)
Filesize: 358.29KB
Date: March 07, 2021, 07:30:10 PM
1970 sales brochure
Rating: *****
Filesize: 6753.33KB
Date: July 16, 2020, 10:01:18 PM
2018 Article in Motorcycle Classics
Rating: (None)
Filesize: 3877.88KB
Date: June 29, 2020, 05:26:00 PM
350 GTR pdf version

Rating: *****
Filesize: 7366.26KB
Date: March 14, 2020, 09:41:23 AM

Powered by EzPortal