Author Topic: 350 Carb tuning issue - how does yours run?  (Read 357 times)

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Offline BRT-GTR

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350 Carb tuning issue - how does yours run?
« on: August 17, 2019, 06:33:48 AM »
      Put a rebuilt engine in the GTO, compression 130psi +, ignition at 25 degs, good spark, jumps 9mm. Now put 200 miles on it. Carbs clean, new float valves, float height 24mm. Chokes are sealing.

    Jetted the NOS carbs to best info on site, 22.5 pilots, 130 mains, needle in 2nd slot.  Started easily but rode poorly with pipe banging on overun and maybe some detonation. Worried it was running weak put 140 mains and 25 pilots back in, needle centre slot.
     Improved power and reduced banging but despite which pilot jets and airscrew setting I use she refuses to run smoothly between 2-3000 rpm on light load, or 1/8 to 1/4 throttle. Can't decide whether it's rich or weak ( I suspect rich, get worse as engine warms but most tuning guides say weak for surging?, stuttering, snatching). Get it up to 3200 and she smooths out (coming on to needle ?).
    Throttle range points finger at carb slide cutaway, raised this from standard 1/8'' (3.2mm) to 4.0mm with little detectable improvement. May go higher.
     Modern petrol is part of the problem, totally different to 60s stuff not just the ethanol, never got pipe banging on original bike back in the day. Using Shell V power, best I can get, 99 ron, 5% ethanol.

   I am determined this engine will run smoothly at all revs if it kills me and it might  :(. Would help to get get feedback on how yours runs at light load and what jets you are using. Am I missing something obvious, retard timing, disc valves? any ideas? Thanks in advance.

    Three things I have found :-  Plug gap is super critical, set at 0.6mil but had opened to 7. Resetting to 0.6mm put a 1000rpm on the tickover, had to back idle screws off.

  They will run happily on a range of pilots from 22.5 through to 27.5 with varying airscrew settings within the acceptable range of 1 - 2 turns out. Pulls away best on the richer 27.5, almost unstallable. 22.5 may be slightly too small, open airscrews to 1 1/2 and above and she bogs on takeoff.
25 is probably the best compromise in UK climate.

   Engine runs far better with the side covers off, our engines are being strangled by the standard air filter, more so if you add foam !! That's why original foam was so thin, BS knew this. I have done a mod to the element that could increase free area by 25%. Will post if it works.

   Would appreciate any help you can give, blokes won't normally ask for help  ;D but getting desperate,                                                                                                              Brian.

      Meant to say full credit to Rocketman who picked up on the marginal air filter area way back in June 08 2009, took me 50 years  :o      https://bridgestonemotorcycleparts.com/index.php?topic=305.msg755#msg755
« Last Edit: August 24, 2019, 08:48:19 AM by BRT-GTR »
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Offline AlanJohn

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Re: 350 Carb tuning issue - how does yours run?
« Reply #1 on: August 18, 2019, 03:50:30 AM »
l had one exactly the same it really got on my nerves nothing seemed to work but l got a pair of new old stock carbs and it now runs perfectly l can't believe the difference
l even had the carbs set up by a man that l know knew what he was doing even sonically cleaned them so ld eliminated the carbs but l was wrong that was the problem there must be a chamber or something a sonic clean can't get to
New carbs Brian you know we're l'am if you need any help.
« Last Edit: August 18, 2019, 03:54:47 AM by AlanJohn »

Offline Old BS Guy

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Re: 350 Carb tuning issue - how does yours run?
« Reply #2 on: August 24, 2019, 12:07:33 AM »
I'm going to stay close to this thread, as I'm just starting to do the tune on the latest 350 GTR purchase.  It's definitely soft on the bottom and has a mid-range surge / bang at light throttle. Just starting to examine the previous owners work, and everything so far is suspect.
'65 BS Homer 50, '69 Mach II RS 200, '67 350 GTR,(2), '66 Sport 90 o.i., '65 90M Surf Rider, '68 100 Trail, '71 100 TMX ,'79 Honda CBX, '82 Yam 650 Turbo, '74 RD-350, '70 Triumph 500 Trophy, '65 Honda CL72 250 Scrambler, 67 Honda CL77 305 Scrambler.

Offline davis

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Re: 350 Carb tuning issue - how does yours run?
« Reply #3 on: August 24, 2019, 05:13:52 AM »
I have a similar situation Brian, with a similar determination to make things right: NOS carb, standard settings, zero ethanal fuel, runs rich. Operation is smooth enough, but a little low on power and 35 mpg at 3-4,000 rpm.

My thinking for the moment is that even though NOS means not used, the carbs still sat around for 50 yrs. The choke plunger seals could have hardened (?). Untested solutions:
1. Stretch or replace springs to increase seal pressure.
2. Get some rubber replacements seals from Mike Dunham. Takes some skill and patience to replace the plunger seal.
3. Try getting a new plunger from Sudco for a VM26 Mikuni   http://www.sudco.com/catalog38.html#206.

Happy hunting and let us know how things work out.

- Bob

Offline BRT-GTR

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Re: 350 Carb tuning issue - how does yours run?
« Reply #4 on: August 24, 2019, 08:44:35 AM »
        Comment
  Guys, thanks for your replies, you won't believe how much you have restored my faith in this site, was beginning to think was wasting my time. Still disappointed by lack of responses, it was a simple enough question. Is this site dying ? No wonder people are switching to facebook - instant enthusiastic replies but not as comprehensive.    It's what the modern generation wants.

     Bob, Here's the answer to your choke plunger sealing issue, maybe I should be on commission  :o     
  https://bridgestonemotorcycleparts.com/index.php?topic=4663.msg27745#msg27745
         The bike will be almost unrideable if they are leaking badly and was the first thing I did before putting the carbs on this engine. Getting the old ones out takes longer than putting the new ones in. Dig out the middle of the old seal with a fine pointed knife or anything with a sharp point. Eventually the sides pieces can be pulled out from under the peened edge with needle nose pliers. Its not that difficult, takes minutes. Make sure there is a good 1mm of slack in the cable at the bar end when refitting.
     Did see a post once where a guy had found the plungers were under size, could allow extra air into the mixture - might have another look at mine.   

      Made make a bad mistake the other day, getting late and been at it all day. Fired engine up for a test run. Idling at 3000rpm, carb cable hung up on throttle slide, wedged throttle open and flicked it back in. Put everything back and started up again to hear a rapidly rising scream from the engine !!!!!!!!!!!!! Shut key off as quick as I could, maybe 2 seconds. Yes, throttle still  wedged wide open, don't know what rpm she hit, hope it hasn't done any damage. Shouldn't have fired ? but it did
      Getting old   :'( .                     Brian.
« Last Edit: August 24, 2019, 05:57:19 PM by BRT-GTR »
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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 !!

Online moonpup

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Re: 350 Carb tuning issue - how does yours run?
« Reply #5 on: August 24, 2019, 09:07:59 AM »
You beat me to it Brian, I was just going to post a link to your previous thread.   8)

Also, just to clarify, are the carbs you're currently working with nos? I ask because in your previous thread you wrote..... "Pulled them out, prised out the tip rubbers (rock hard but NOS carbs) with a sharp point and needlenose pliers."
« Last Edit: August 24, 2019, 09:17:29 AM by moonpup »
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Offline BRT-GTR

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Re: 350 Carb tuning issue - how does yours run?
« Reply #6 on: August 24, 2019, 09:58:57 AM »
          Mike,
                    Yes, NOS, not new, came with bike, still bright and shiny with 9mm float valves. All NOS carbs may have been made by BS Tailung, Taiwan back in the 70s when their attitude to quality was questionable, now second to none.
     Found two definite faults (which I thought were the answer to my running issues) but no.
    1) Pilot bypass holes (pilot to carb venturi) were drilled as far off center as they could be and 50% blocked by the pilot tip. Made a tool and moved them over.
    2) One of carb cap retainers had been threaded on the skew (not crossthreaded) and was flipping one side of the carb top up a good 0.5mm when tightened. Changed for a better one. All tuning articles stress the importance of the carb top sealing washer but our carbs can't have one due to the indexing lug, not much we can do.                                      Brian.

             P.S. I am on Paypal, 50%   :o

   
« Last Edit: August 24, 2019, 06:01:21 PM by BRT-GTR »
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Offline BRT-GTR

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Re: 350 Carb tuning issue - how does yours run?
« Reply #7 on: August 26, 2019, 08:31:22 AM »
        Will keep this short, engine refuses to run smoothly at light load, 2-3000 rpm, starts readily and runs great above 3k. Tried every pilot jet / airscrew combination in the book. tried various carb cutout sizes.
      In desparation put new refaced choke plungers and new left carb on yesterday (noticeably more pipe bang on left side). No difference. Had slight spit back on left carb but cured that.

    Beginning to think may not be carbs but what can it be, it is not air leak on disc covers or right crank seal, all new and rechecked.  NOS crank, all new rods and bearings but lab seal had a slight tight spot when pushed to left and spun. Had crank checked by reputable specialist firm. Said they had rebuilt crank and OK but I have my doubts they took it apart. Lab seal still had tight spot.
     Could wear to lab seal cause poor running at light load.

     Any help / advice gratefully accepted, it's driving me crazy.              Brian
« Last Edit: August 26, 2019, 04:09:49 PM by BRT-GTR »
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Offline SR-175

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Re: 350 Carb tuning issue - how does yours run?
« Reply #8 on: August 26, 2019, 08:38:59 AM »
Hi Brian,

Have been reading all this but have nothing constructive to add yet. It's a tricky one to say the least !!

ray

Online moonpup

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Re: 350 Carb tuning issue - how does yours run?
« Reply #9 on: August 26, 2019, 12:06:18 PM »

All NOS carbs may have been made by BS Tailung, Taiwan back in the 70s when their attitude to quality was questionable, now second to none.
   

Brian, since I’ve never owned any nos carbs I wouldn’t know the answer to this, but weren’t all 350 carbs made by Mikuni or are you saying that the parts supply carbs may have been copies made by BS Tailung?

Do they not have this molded into the sides of them....... ?
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Offline Bridgestone Man

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Re: 350 Carb tuning issue - how does yours run?
« Reply #10 on: August 26, 2019, 12:32:00 PM »
I am with Alanjohn on this one, get a new set of carb from Richard and my GTR has ran great ever since, this seems to be the way to go.
But for you guys who want to go with used/rebuilt/cleaned carbs I do enjoy reading your posts.  popcorn

Sam Keys

Offline hardy

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Re: 350 Carb tuning issue - how does yours run?
« Reply #11 on: August 26, 2019, 08:52:14 PM »
I thought the NOS 350 carbs dried up years go?

Also been reading intently, have not had much to add.

What colour reading are you getting from your plugs?

Perhaps try a rich pilot jet and work your way leaner. I found my 350 to be happy with a 25 pilot.

Online moonpup

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Re: 350 Carb tuning issue - how does yours run?
« Reply #12 on: August 26, 2019, 08:56:23 PM »

I thought the NOS 350 carbs dried up years go?


 Me too....   :o
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Offline Old BS Guy

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Re: 350 Carb tuning issue - how does yours run?
« Reply #13 on: August 27, 2019, 12:09:21 AM »
Brian; Have you physically checked the mechanical advance mechanism for a weak or broken spring / piece? (OK, I'm being a smart ass). Question for the group, since I haven't had to do much altitude tuning with the 350's in particular; The book says 2 turns out on the idle air. I'm at about 2600 ft. Anyone have any suggestions on other than book settings? I'm at 1/3/4 right now, stock pilot, and it comes off idle pretty well, but you'd think, (normally), it might want more air. It returns to idle well. I haven't taken it out and gotten a good warm run on it yet. Might change.......
« Last Edit: August 27, 2019, 01:38:29 AM by Old BS Guy »
'65 BS Homer 50, '69 Mach II RS 200, '67 350 GTR,(2), '66 Sport 90 o.i., '65 90M Surf Rider, '68 100 Trail, '71 100 TMX ,'79 Honda CBX, '82 Yam 650 Turbo, '74 RD-350, '70 Triumph 500 Trophy, '65 Honda CL72 250 Scrambler, 67 Honda CL77 305 Scrambler.

Offline hardy

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Re: 350 Carb tuning issue - how does yours run?
« Reply #14 on: August 27, 2019, 03:00:55 AM »
When you say stock pilot, what size do you mean?

Book states #22.5, I ordered NOS from RC and received #30. When received it had #30 in it.

Just to clear up any confusion??

Offline Old BS Guy

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Re: 350 Carb tuning issue - how does yours run?
« Reply #15 on: August 27, 2019, 03:36:17 AM »
25....if memory serves.
'65 BS Homer 50, '69 Mach II RS 200, '67 350 GTR,(2), '66 Sport 90 o.i., '65 90M Surf Rider, '68 100 Trail, '71 100 TMX ,'79 Honda CBX, '82 Yam 650 Turbo, '74 RD-350, '70 Triumph 500 Trophy, '65 Honda CL72 250 Scrambler, 67 Honda CL77 305 Scrambler.

Offline BRT-GTR

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Re: 350 Carb tuning issue - how does yours run?
« Reply #16 on: August 27, 2019, 10:54:06 AM »
         The original manual states pilot jet 30, main jet 140 for run in and then change to 130, Slide No2 with needle in third groove. BS played safe and supplied them with rich settings. However, over time it was found they run better, in most cases, with the needle in second groove and a 22.5 pilot. This last figure has been amended in the onsite manual.   New carbs still come with the 30 pilot.
       Hope this clears the confusion.
« Last Edit: August 27, 2019, 01:23:04 PM by BRT-GTR »
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Offline BRT-GTR

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Re: 350 Carb tuning issue - how does yours run?
« Reply #17 on: August 28, 2019, 08:22:24 AM »
     For the sake of my sanity and maybe my marriage, I am giving up on this one. Have to assume there is a fundamental fault with the carbs or some other engine  fault causing this issue. Tried every carb variable in the book and only slight improvement, will run it as is. Will swap to a pair of old carbs as a final test.

     Wrap up loose ends,
    Mike, did underline MAY have have been made etc, specially with you in mind, Knew you would come back, really !    Did believe 9mm float valved carbs were later NOS carbs but have seen a post that said it was changed during 350 production life. Taiwan ? they do not have the same quality as original carbs, yes, noticed the JAPAN mark but could be made under license. The 350 carb configuration could be unique to our bikes (possibly also early 350 Kaw Avenger) Just a theory.
    My carbs are definitely not original fitment, look too new, no corrosion, no point in spending £300+ to change them.
    Alan, there are no hidden chambers in carbs, all drillings and passages can be proved with the exception of the pilot circuit, has two plugged passages that can't be proved without drilling out.
   OBSG, Wish I could find advance unit, might solve problem  ::) .  Pipe bang is due to poor combustion, other special conditions and maybe modern petrol, have found leaning off relevant  circuit reduces it.
   BSman,  Some of us enjoy solving problems and not merely bypassing them with the cheque book (Paypal?) even though we could afford to !!  Saves the environment, resources and those expensive new carbs for others. Who's going to be in the best position when they finally run out ?  popcorn
   Hardy, Plug chop test is really best for main jet size, can be used at other throttle settings but is difficult and unreliable, try turning 350 key off when riding  :o 
                             
                                                                                                        Brian


« Last Edit: August 28, 2019, 08:57:41 AM by BRT-GTR »
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 AlanJohn

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Re: 350 Carb tuning issue - how does yours run?
« Reply #18 on: August 29, 2019, 03:05:31 AM »
Brian your obviously correct l'm no mechanic but l had a badly running hurricane put a pair of nos carbs on it the difference was amazing and exactly the same with one of my Gto's they went from virtually unrideable to perfect l know it didn't find or solve the root cause but it was a simple fix to a problem l couldn't fix and one a good mechanic also couldn't fix even after a sonic clean
good luck with yours
Alan

Offline BRT-GTR

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Re: 350 Carb tuning issue - how does yours run?
« Reply #19 on: August 29, 2019, 05:58:09 AM »
        Hi Aan,
                At least you attempted to sort the carbs before buying new unlike the previous owner of my GTO who ditched perfectly serviceable carbs (I asked for them) and forked out for new ones (with only 3k on the clock !).  Hadn't had a good look at them before but will now try them to prove whether fault is carbs or engine.                                             Brian
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