Balancing twin carbs

[scotto75]

I used the brass nipple to replace the external overflow tube on the bottom of the bowl, not the internal part.

Yes, you’re right about the metal strip that connects the choke on both carbs.

I hooked the carbs up tonight and measured float level by screwing in some clear tube into the drain hole, and pouring petrol into a funnel that ran to some fuel hose. I have set the float heights at 26mm, as recommended on the Honda Twins forum, but the level came out at 6mm below the gasket line. My understanding is that I should be aiming for 3-4mm under the gasket, so I could adjust the float height to get the level in the tubes higher by 2-3mm?

TBH thought my carbs were running a bit rich. The jets are all the right size, but wasn’t convinced about these Keyster rebuild kits, or the press in jets with o’rings and a spring that holds them down when the bowl is screwed back on. The engine idles nice but bogs down the second you open up the throttle a fraction.

BTW, it’s not gold. it was Maroon but someone painted it black at some point. I have a gold tank on a CB350.
____________________
Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V

[bikenut]

The engine idles nice but bogs down the second you open up the throttle a fraction.

You sure the carbs are assembled properly as per cmsnl diagram and the diaphragms are a ok?

Correct jets ( as they are new ) all clean etc..

TBH thought my carbs were running a bit rich.


set a bit rich at idle, increasing fuel height would richen the mix but that can be adjusted out....you say the engine dies with even a touch of throttle, so the dash pistons dont rise......

Engine dies with a touch of throttle, even when mainly choked by your hands forming adjustable air intakes ? This would require a second person using the throttle......try this, what happens??
____________________
nuts about bikes

[scotto75]

Found a couple issues with the carbs, which i’ve now resolved but it’s still not smooth.

I used the clear tube method and reduced the float height to 24mm and with both carbs having equal level of fuel coming 4mm under the gasket.

The clear tube test meant I found the float needle was getting stuck in the valve on one carb so it wasn’t get any fuel.

I have both carbs sync’d and with the mixture adjusted but the pressure coming out the back is not equal. The jets are clean and new but not genuine. The pilot jet is #35 which is correct, but I have to have the screw all the way in virtually, to avoid it being too rich, and when I open the throttle I can see in the Colortune that it goes bright orange so wonder if the needle jet is too rich?
____________________
Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V

[scotto75]

I’m not convinced by the quality of the Keyster rebuild kits. When I got the bike I instantly stripped the carbs, ran them a couple of times through an ultrasonic cleaner and rebuilt them with the jets and tubes from the kits.

There are three orifices, a pilot jet #35 with a rubber bung, the needle tube and a secondary jet #70, and the main jet #100 and tube.

As an example of a difference which I find a little suspect, the original needle tube needed to be tapped gently out of its orifice. The replacements Keyster part doesn’t stay in the orifice. It is held in by the jet, which has an o-ring and is held in by a spring clamp, which is held down when the float bowl is screwed down. When the clamp and jet are removed, the tube falls out. This is the same for the main jet tube too.

As a comparison I will go back to the original parts removed from the carbs when I rebuilt them. They were put through the ultrasonic cleaner so should be clear, but
____________________
Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V

[bikenut]

test ok that didnt work back in a bit that didnt work either.....
____________________
nuts about bikes

[bikenut]

ried to post cmsnl exploded views of the carbs which didnt work.

You sure the carbs are all clean, even the air bleed holes that are tiny, as well as any ports etc..

Maybe clean old jets and refit for now, poor fit of "new" parts is not good.

can you post a link to cmsnl carbs for your bike, the view i have looked at is the one nearest to the top of cmsnl list.......

Main jet emulsion tube usually fitted from above and is usually a slight press fit. The old oe one may have had to be persuaded to come out as it had been in there a long time and had probably "gummed" a little.
____________________
nuts about bikes

[scotto75]

Thanks for persevering with me bikenut. I appreciate your help greatly. The carb is here on CMS

https://www.cmsnl.com/honda-cb250k3_model50719/partslist/0050.html#.W-aHSaSnzYU

I have replaced the jets and tubes with the original ones that came out of the carbs when I put them through the ultrasonic cleaner. The tubes fit a lot better than the Keyster ones. The o-rings on the primary and secondary jets were crap so swapped with the rebuild ones. After some adjustments of the mixture with the Colortune and sync with the Synchrometer (which is my new favourite tool BTW!) I still end up with a strong idle that stalls when the throttle is opened just slightly.

I have read the Haynes and Honda carb manuals and my understanding is that the primary main jet supplies fuel between the idle circuit and the needle jet as the throttle butterfly opens. It’s supposed to smooth the transition between the two.

The idle jet gets fuel from a hole in the main jet and the mixture screw controls how much aerated fuel is allowed through. Screwing in reduces the amount making it leaner, etc.

The first cause is blocked jets restricting air or fuel flow. I have probably assumed that all passages were cleaned when put through the ultrasonic cleaner, but I will have to go through and double check.

Next is air leaks. I’m not having any issues elsewhere with rev hunting or hanging high revs. But maybe it could be coming through the throttle bushes when the throttle opens? The Colortune shows bright orange when the needle jet should be in control making me think it is rich and not lean.

I’ve tried both Keyster and original needles with the same result, but could it be when the needle lifts it gets too much fuel?

Time to get the carbs off again!
____________________
Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V

[Hong Kong Phooey]

[scotto75]

[bikenut]

Main jet emulsion tube usually fitted from above should read mainly fitted from below....sorry
____________________
nuts about bikes

[bikenut]

The main jet needles, the ones attached to the dash pot pistons are not uniformly tapered but "stepped", well su one where at least.

If it has a flat spot when opening the throttle the system usually richens the mixture to avoid this, an acceleration pump ( squirts raw fuel into intake with quick throttle openings, virtually non on slow throttle openings ) are fitted to carbs, but not these or even su's, the needle being tapered.
____________________
nuts about bikes

[bikenut]

With the engine not under "load" some misleading results can be seen, the engine should be under load for "proper" tuning, like on a dyno, but that time can be expensive.

How high have the dash pot pistons risen when colour is orange, they rise and then creep down ?
____________________
nuts about bikes

[talkToTheHat]

I've got a twin pressure gauge carb sync thing somewhere, don't use it anymore as I no longer have a twin with a balance screw that vibrates loose. like half of this https://youtu.be/t0qf11BOx6o If it helps It's available for postage cost and a beer
____________________
Bandit. does. everything.

[scotto75]

[scotto75]

[bikenut]

dash pot pistons, with the rubber diaphragm attached......the slot in the emulsion tube, aligns with a brass pin in the carb body near the bottom from memory ?

Not mentioned so far, you did not "wind up" or "unwind" the dash pot piston springs during assembly did you, and the piston slid in and out of the vac space, with the vac space cover falling off the piston rod at the same rate over say 3 to 5 seconds with the vac holes bunged ala su ?
____________________
nuts about bikes

[scotto75]

I stripped the carbs back again today and blasted cleaner through every hole and checked that everything moved as should and was aligned properly. I redid the float levels and replaced o-rings on the main jets. I also bought a pair of new intake rubbers, which was good as it turns out there was a hairline crack along one. This could have been the main culprit I suppose.

Anyway, after realising that the left points terminal was grounding on the inside of the points cover, I got it fired up and both cylinders and it runs great. Idles well and accelerates with no flat spots.

I’ll go for a long ride tomorrow and adjust the idle mixture and rebalance again.

Thanks bikenut for the support and advice!
____________________
Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V

[bikenut]

I know I bang on about the red neon plug caps, but that's for a reason.....I would take a pair with me on a ride along with the bespoke adapters if the bike had misfiring issues/rev issues, as sometimes ign problems that match fuel problem symptoms can be iiminated via the neons, surging at particular revs particularly with cdi systems.

Anyways you have found some more issues that you have illiminated ( how dos you speel this word ) and here's hoping for a problem free joyous ride, nice weather for it as well !!

Be careful out there. :karma:
____________________
nuts about bikes

[scotto75]