Carb conundrum

[GoldenFailure]

Hi everyone. I just bought a 1955 ish Golden Flash, barely running and in a bit of a state. My main problem is the carb, its an old Mk 1 Concentric, when it should be a pre mono 276 or mono 376 I think? Anyway, I cant afford to change it just yet, and was wondering if anyone could advise me how to set it up?

At the moment -

626 Amal Mk1 Concenetric
25 pilot
106 needle jet and needle with four identifying grooves (both borrowed from a Mk1 932)
Needle in richest groove (1st from taper)
210 main jet (naughtily drilled out from 1mm to 1.2mm)
3.5 Slide

On one mile test runs followed by plug checks

Starts first kick, idles a bit rough

30mph runs lumpy, plugs black and sooty
40mph runs better, plugs still sooty
45mph feels good, plugs starting to burn white
50mph - 60mph still pulls well, but plugs are scary white
runs out of guts at 70mph

Ive tinkered with the pilot screw to no avail, and put the original, shorter needle and needle jet in, but with these its near impossible to start and will only run with air slide down, so put 932 needle/jet back in.

I would really appreciate any advice, I cant afford to buy endless jets and slides, or a new carb, and although Ive rebuilt a couple of engines, setting up carbs still confuses me.

[trevinoz]

Are you mixing up very early concentric components with later ones?
The first concentric had a short needle and a short jet holder.
The modified form is a jet holder which places the main jet deeper into the bowl.
The first ones had a pilot jet screwed into the bottom of the body but was changed to a drilling in the pilot passage.
It seems as though you might have the passage pilot which is blocked.
Tkae out the air sdrew and poke a fine wire through the drilling.
A bit difficult to get at but when it is clear you will be able to squirt some fluid through the pilot and see it come out through the bottom of the body.
If you have a swinging arm bike, you should have a 1-1/16" monobloc and if plunger, 276.
 Trev.

[GoldenFailure]

Hi Trev, thanks for your response. Yes, I'm mixing parts, only way I could get it working. I read about the different needle and holder, but bizarrely the holders were the same length (I had an old 932 Ive been cannibalizing) Ive blown all the passages and jets out with compressed air, but I'll try unscrewing and poking it with wire, but surely that would only explain the richness (and would it? wouldn't the pilot being blocked make it leaner?) and not the later lean ness? The pilot is a screw in affair, and the bike is a plunger, do the engines differ? (in a carburation requirement sense)

[jfligg]

Hello Have you measured the float hieght?  Jeff

[Triton Thrasher]

Do the Amal procedure as is found in manuals.

Forget mph and concentrate on throttle openings.

Idle, full throttle (main jet), 2/3 throttle (needle position), 1/4 thottle(slide cutaway), idle again. Use a new needle and needle jet.

Get each circuit too rich then correct.

[iansoady]

A 626 would be a bit on the small side (just over 1" bore) so it'll be difficult to get everything right. The 32mm would have been closer I would have thought.

Symptoms sound as though it's rich on needle / cutaway then weakening out. I'd echo what Triton Thrasher suggests to try to eliminate the problem. Unfortunately with carbs it does tend to be a process of substitution to get things right.

[ebsbury]

The original carb is 1 1/16" so 26mm is a little small. If the carburetters is too small it will be likely to run out of steam at the top end but should run well at lower speeds.

What is stamped on the side of the carb. There are lots of old 26mm Triple carbs around that cause rich running on other bikes. Ther's an article here that can help sort out what bits and pieces are in the carb: http://www.amalcarb.co.uk/TechnicalDetail.aspx?id=11

You can download the Amal Tuning leaflet from the same site.

[Triton Thrasher]

What is stamped on the side of the carb. There are lots of old 26mm Triple carbs around that cause rich running on other bikes. Ther's an article here that can help sort out what bits and pieces are in the carb: http://www.amalcarb.co.uk/TechnicalDetail.aspx?id=11

You can download the Amal Tuning leaflet from the same site.

The Triumph and BSA Triple carbs I've seen were 27mm, which is pretty exactly inch and a sixteenth. 28mm is the commonest metric replacement used for that.

If inch and a sixteenth is correct, then 30mm is probably too big for best performance and flexibility and 32mm is really too big.

[GoldenFailure]

Thanks for all your responses.

Theres nothing stamped on the side apart from 626, I know what you mean, the old 932 has additional numbers, but this doesn't.

Thanks for the link to the amal carb technical page, but thats what I was using to describe the parts in my original post.

Ive removed the pilot jet and air screw, cleaned and replaced and its made no change.

I understand that if I replace needles and jets its unlikely to be perfect first time, but its so crazily wrong right now I'm not sure where to begin.

Does anyone know (or know where I can find out) what the correct jets, needle and slide should be used on the 376 and whether they should be proportionately increased or decreased to create similar fueling in the 626?

With my exceptionally limited finances I'm not sure where to start, and I'm kicking myself for swapping 2 working bikes for a bloody great paperweight! 

[ebsbury]

An 1 /16" Monobloc on a Gold Flash of that era came stock with a 240 Main Jet, 25 Pilot Jet, 106 Needle Jet, and a 3 1/2 cutaway. As a general rule to convert from Monobloc to Concentric reduce the size of the Main Jet by 20% and leave the other settings unchanged.

Presumable the pilot bush has been removed as you have a 25 Pilot jet fitted.

The needle you are using with the four inscribed rings is a lean needle usually found on later 850 Commandos.

[iansoady]

The needle you are using with the four inscribed rings is a lean needle usually found on later 850 Commandos.

The Commando uses a 900 series carb whereas the 26mm will be a 600. ISTR that the needles are different between the 2 ranges (but my memory isn't what it was).

[GoldenFailure]

Hi all.

I just replaced the needle with the correct one, and it was a different length and taper. But its had no effect. Even on the richest needle groove position, with the recommended 106 needle jet, 230 ish main and with the idle screw correctly adjusted, the bikes running far to rich at low throttle openings and far to lean on wider throttle openings.

Starting to get disheartened, help?

[Triton Thrasher]

If it's weak at full throttle, even with a grossly oversized main jet, I'd suspect a fuel flow restriction, somewhere between the tank breather and the main jet.  What's the biggest jet you've tried?

Richness at small throttle openings is probably a worn needle jet (am I right in assuming you're not using a new one yet?), or throttle slide cutaway too small.

[wilko]

Do you have a choke?

[GoldenFailure]

Yep, have an air slide, down when starting, up when running.

You are right, guilty of cheapness and just bought the needle, figured that if it got a little closer, I'd cough up the cash for other bits.

Surely a worn needle jet wouldn't affect very small throttle openings?

I hadn't thought of fuel flow problems, theres plenty coming out of the tap, (with pipe disconnected) the pipes clear so there no blockage, BUT, I did mess about with the brass float needle seat as the carb was overflowing dramatically when tickled, so I drifted the seat up to lower the float bowl cut off. I tried to do it the bare minimum, but maybe I've lowed the fuel level too much. Would that cause such dramatic lean running?

And, theres two fuel taps, but one isn't connected to the carb, does an a10 need both taps open for full throttle running? surely not?

If it is a fuel flow problem, wouldn't it die after a mile or so, only to regain power after letting off the throttle for a few seconds?

Again, thanks for all the advice.

[muskrat]

 G'day GF,
              might sound silly at this late stage but does it have a filter in the hose banjo at the float bowl?
 A low float height will affect all but idle. Only need 1 tap to  run. The left one should be reserve.
Cheers

[GoldenFailure]

Hi Muskrat

It does have a filter in the hose banjo, but I have cleaned it with carb cleaner and blasted it with an air compressor, is that what you meant, that it might be blocked up?

Would a low float height make such a huge difference? Ill strip it tomorrow (for what feels like the millionth time) and measure it.

[muskrat]

 Might have to do a bit of plug chopping. Get some whiteout and mark the twist grip and throttle housing, now twist the grip to full and mark it. now mark 1/2 way between two marks and do the same again. You now have 4 marks. !st sector is slide 2nd the needle jet 3rd is clip and 4th is main.
 Get it up to temp (at least 5 min running) and find a gentle hill. Hold it in 3rd or 4th at the desired mark for at least 200 yards then pull clutch and kill(stop) motor. Check the colour of the plugs, deep down the porcelain. Tan good, black rich, white lean.
 Another way is to hold on a mark and close the throttle a tad, it should slow. If not it's too rich.
 Then I saw you have a choke. At each mark put the choke on to see if it gets better or worse. Better your lean, worse your rich.
 I don't enjoy setting up new carbs but once done right it's worth it.
Cheers

[Triton Thrasher]

The mixture is governed by the parallel portion of the needle in the needle jet at around 1/4 throttle, maybe less throttle with a small carb on a big engine. Needle jets wear quickly and used ones are not advisable for re-use.

Setting float level certainly is not done in relation to the tickler. I expect this forum has lots of info on float levels. There are various recommendations.

Another thing that can cause intransigent richness at low rpm is use of a sporty camshaft, perhaps with big valves. I don't know if you've got that in your bike.

Has anyone mentioned the danger of piston seizure or holing with a weak mixture at large throttle openings?  That's why we make sure it's too rich (so rich that it runs badly), then work our way down from there. I expect you live far away from me, up here on the north coast of Scotland, but I have a pretty big collection of main jets after nearly 40 years of arsing about with old bikes. If you tried my spare 350 jet and it was still weak at full throttle, then we could be pretty sure it's not the jet.  There must be similar losers to me,  with piles of jets somewhere in your area.

[Triton Thrasher]

Are you happy that the ignition timing is correct at full advance?