Dr Big
Carburation advice - Printable Version

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Carburation advice - Foreman - 03-12-2016

Hi guys,
I have recently installed a sr43B carburators in a sr42A bike, because after cleaning its own carbs the engine run like hell, maybe cause I reused all the O-rings and float weight, etc... :oops: ...didn't have new parts, sorry.
Anyway, I had these spare carb on the shelf and decided to install them in the sr42 and...voila!...runs great, idles well. But It lacks of Oomphf untill it reaches 4K rpm, then it pulls and breaths normally I think. The bike has original air filter and exahust
So, Is there any adjustment I can perform in order to enhace acceleration at low rpm's?
Mixture screw maybe? Idle or main jets?

Thanks in advance. Thumbup

Re: Carburation advice - BigBob - 04-12-2016

I tried a set of 43 carbs on my 42 and it ran far too rich, and would hardly rev over 4000rpm.
I think it`s because the airboxes are different and there is a higher airflow on the 43.
I would suggest turning the pilot screw in half a turn and dropping the needles 1 clip.
Of course, carburation is a black art and I could be completely wrong.
Is it not possible to rebuild your 42 carbs?

Re: Carburation advice - ladder106 - 04-12-2016

Here are the carb specifications from the service manual. As Bob says, the SR43 has a larger less restrictive airbox so the jetting is different from the SR42.

Here are the specs:

[Image: SR42%20carb%20specs_zpsup8i1v7x.png]

[Image: 93%20SR43%20carb%20specs_zpsjlpyn9d1.png]

As long as I'm doing it, I'll include the 89 SR41 specs:
[Image: 89%20SR41%20carb%20specs_zpsfsxhds3u.png]

Re: Carburation advice - Foreman - 04-12-2016

Off course I can rebuild them Bigbob. I didn't do it cause I didn't have the new parts on the table at that time and I wanted the bike running asap.
Besides that, the bike runs not really bad with the 43 carb, the only thing that bothers is that annoying lack of power with high gas openings at mid-range.
BTW, I know you're keen on Suzuki OEM parts, but how about those Ebay Polish carburetor rebuild kits?

Thanks for the info ladder106!! Thumbup

Re: Carburation advice - ladder106 - 04-12-2016

Basically, swapping the pilot jets and mains from your SR42 carbs into the SR43s should get you very close. The pilot air jets are different but you should be able to compensate with idle mix screws.

Any other major variations will likely be attributed to the condition of the O-rings and grey plastic slide guides in the two different sets of carbs.

I can not stress enough that the O-rings and slide guides MUST be in good nick for these carbs to perform as designed.

Re: Carburation advice - dave#22 - 04-12-2016

Another thing to consider are the springs between the slide and the cap.
I run a 750 crank in an SR43 engine and had a similar problem with slow pickup even after rejetting, so I fitted lighter springs from something else?....and now the pickup is instantaneous. I know it's a faff exposing the carbs, but it's worth checking to see if the 42 and 43 have different spring weights.

Re: Carburation advice - O.R. - 26-01-2017

Not a problem to use SR43 carbs on SR42. Adjustment of idle mixture and changes in emulsion tubes & main jets will probibly be required.

Be aware of the fact that not only the main jets differ, but also the emulsion tubes
SR42 Emulsion tube O-5, Main jet 117,5
SR41 Emulsion tube O-4, Main jet 120
SR43 Emulsion tube O-3, Main jet 130
This means that de emulsion tubes got leaner, the main jets got richer :!:

My advice is never te reuse O-rings. You save a few but get €€€€€€€€ of misery.

You wrote that the problems occurred below 4000rpm. Asume this is not @ WOT (Wide Open Throttle)?
Take a look at this figure:[attachment=1550] In that case, main jet is not the problem. I would suspect the needle jet (emulsion tube) or the O-ring of the slide housing.

Assure float level is OK. Also check that the choke is FULLY closed after warming up.

RE: Carburation advice - liamol - 06-07-2020

Hi lads,
Can I ask if the changing of all jets including "starter jets" needs to happen.
Long story but could only get sr43 carbs for my 42 so swapped all otjer jets. Only runs with choke.
I noticed the manual states starte jet #65 for the sr42 and 47.5 for the sr43.
Carbs been ultra sonic cleaned...

RE: Carburation advice - BigBob - 07-07-2020

When you are using 43 carbs on a 42 and have changed all the jets, it`s unlikely we are going to have a magic answer.
It should run, in theory, with what you have done, even if not very well, so my guess is that something is not right at all.
You would have been better off with the 750 carbs as the jetting is virtually identical, and I ran a 42 on 41 standard carbs for years with no issues.

RE: Carburation advice - H_Lime - 07-07-2020

I'm in a similar ish scenario in so far I'm running 42 carbs on a 750. I've copied the standard jetting for the 750 needle and emulsion but air and idle jets as per 800. Its rich on the idle jet I reckon as dies on full choke and is not quite as free revving to 4k, after that she's fab, maybe dropping down to 120 mains might help more (I'm on 122.5) as slow to rev past 6k.
Tbh I ain't fookin round with changes till I have her up on the dyno and can see what the actual craic, path of least resistance for me.