Carburation issue

Tom P · Mar 22, 2022

I have a 72 Combat, currently with a single 34mm Amal Mk2. The bike ran great, with one problem, It would not tick over until it was warm. So I decided to strip the carb and give it a good clean in an ultra sonic bath, all jets and passages were also checked for blockages, the carb and all components are in good nick, The pilot jet did seem to have a partial obstruction but a good spray with carb cleaner shifted it.

After reassembly the bike fired up first kick and lo and behold idled perfectly when cold. However a test ride today shows the following symptoms, The bike pulls well when the throttle is opened at 1/4 to full and plug chops after sustained riding at various throttle settings between 1/4 to full show nice tan plugs, however riding in slow traffic with the throttle nearly closed, (<1/8) is choppy with the plugs fouling. I have adjusted the air screw in 1/8 increments from 1/2 in to 2-3/4 turns out, 2-1/2 turns out seems to be the best I can get, but its still not as good as it was before with the odd splutter riding through town. Could the enrichener be causing this? It is cable operated and there is slack in the cable when it is off.

(The pilot jet is in the hole in the back of the float bowl, with the blanking screw in the other orifice in the carb body.)

Any ideas, where to look next?
Thanks for any advice
Tom

concours · Mar 22, 2022

Yep, verify the enrichener seat rubber is good.
Did you remove the needle jet? And did you verify all the air holes in the needle jet were open? Did you verify with aerosol spray the air passage from the inlet horn is clear? Verify with aerosol the three idle discharge holes is the inlet throat flowed identical?
Remove the pilot jet (checked the cross drillings?) and verify flow with aerosol in all directions?

Edited to remove non-relevant Mikuni image.

Mike40M · Mar 22, 2022

All of my 8 running bikes except the fuel injected one, has lousy idling when cold. So in my experience if you have them running well when warm, they don't idle well when cold.
A pair of them has Mikunis, a race Honda is almost impossible to start without the enrichener (some incorrectly call it a choke). But it has good AFR (air fuel ratio) warm.
The Velo also needs the starting circuit (enrichener) to start cold.

marshg246 · Mar 22, 2022

Fairly sure concours first point is it. Enricher rubber bad or enricher somewhat on all the time for whatever reason.

Tom P · Mar 22, 2022

Thanks for the reply's, The diagram Concours posted is significantly different to my carb, The float is different, I have no baffle plate, the air screw and tickover screw are different and the idle jet is located in the float bowl, not the carb body.
I just had another look at the Enrichener rubber and it looks fine, I'll pull the carb again and go thru it once again.

Mart UK · Mar 22, 2022

I have a single 34 mk2 Amal on my 850. Mine doesn't match that diagram either. The diagram is available on the Burlen/Amal site. Try pushing down on the enrichener when the bike's running, to make sure it is seating fully. I am using a pilot jet 1 size down from what is recommended. IIRC, 25 is recommended and I'm on a 22. Then I'm around 1.5 turns out for smoothest tickover, good pick up and no decel popping.

I have to use the enrichener to start it when cold, even today (18 degrees C). Then straight off, or it fouls the plugs. It won't idle without throttle for a minute or two and runs rough for the first couple of miles or so, then all is good. I usually start it up, then chug half a mile up my lane at around 2,000rpm, by which time it no longer stalls with the clutch in. Tickover hot, set at 1250rpm.

marshg246 · Mar 22, 2022

Tom P said:
Thanks for the reply's, The diagram Concours posted is significantly different to my carb, The float is different, I have no baffle plate, the air screw and tickover screw are different and the idle jet is located in the float bowl, not the carb body.
I just had another look at the Enrichener rubber and it looks fine, I'll pull the carb again and go thru it once again.

Sorry to harp on it, but I had the same symptoms on a customer Bonneville except that it was dual carb and although the enricher rubbers looked fine in both carbs, they were not sealing. Once replaced, the off idle symptoms were gone and I was able to adjust the air screw to get it to idle nicely. We opted for a little slow when cold and a little fast when up to temp which meant that it idled when cold and didn't die at stop lights.

I'm assuming that you are talking about a 2900-series Amal MKII carb.

acotrel · Mar 22, 2022

I think you are correct when you say 'if the carb is jetted lean enough to get best performance, you will al;ways need to use the enrichener to start the motor, except when it is still hot''. A tan spark plug is not the sign of good jetting. If you are using a warm plug, after you have run the bike at full throttle on a long straight road, then do a plug chop. There should be about a 2mm wide black ring on the porcelain insulator right down inside the plug where the porcelain meets the metal. Plug heat ranges change the temperature of the plug which burns the carbon off the insulator. I only ever do plug chops to check my main jets are right. Normally my main jets are slightly too big, so I don't burn valves or pistons. The needle jet size and needle tapers are critical, if you are looking for performance. But with a road bike, it is normal to be jetted slightly too rich at all throttle openings. If you run the carbs lean on the needles, you should be able to tell when you are TOO LEAN - when you ride the bike the motor will cough, if you are too lean. Raise the needles one notch at a time, until the cough disappears, when you ride the bike. I usually lower the needles until I get the cough, then raise them one notch.

Triton Thrasher · Mar 22, 2022

It’s not supposed to tick over when the engine is cold.

If it did, it would be too rich when warmed up.

concours · Mar 22, 2022

Tom P said:
Thanks for the reply's, The diagram Concours posted is significantly different to my carb, The float is different, I have no baffle plate, the air screw and tickover screw are different and the idle jet is located in the float bowl, not the carb body.
I just had another look at the Enrichener rubber and it looks fine, I'll pull the carb again and go thru it once again.

Good thing I'm not flying a plane. I thought I read "mikuni".

Carry on.

baz · Mar 22, 2022

Tom P said:
I have a 72 Combat, currently with a single 34mm Amal Mk2. The bike ran great, with one problem, It would not tick over until it was warm. So I decided to strip the carb and give it a good clean in an ultra sonic bath, all jets and passages were also checked for blockages, the carb and all components are in good nick, The pilot jet did seem to have a partial obstruction but a good spray with carb cleaner shifted it.

After reassembly the bike fired up first kick and lo and behold idled perfectly when cold. However a test ride today shows the following symptoms, The bike pulls well when the throttle is opened at 1/4 to full and plug chops after sustained riding at various throttle settings between 1/4 to full show nice tan plugs, however riding in slow traffic with the throttle nearly closed, (<1/8) is choppy with the plugs fouling. I have adjusted the air screw in 1/8 increments from 1/2 in to 2-3/4 turns out, 2-1/2 turns out seems to be the best I can get, but its still not as good as it was before with the odd splutter riding through town. Could the enrichener be causing this? It is cable operated and there is slack in the cable when it is off.

(The pilot jet is in the hole in the back of the float bowl, with the blanking screw in the other orifice in the carb body.)

Any ideas, where to look next?
Thanks for any advice
Tom

Your bike should definitely not idle until it's fully warmed up

Big Phil · Mar 23, 2022

baz said:
Your bike should definitely not idle until it's fully warmed up

My 850 Mk3 with twin Amal Premier Carbs (primary belt drive so electric start removed) will not idle until it’s relatively warm which I have always thought is normal. I start it cold on full choke and I have to hold the throttle for maybe 60 seconds or so until it begins to settle down to a steady tick over whilst reducing the need for any choke. The bike runs perfectly at all speeds and I am very happy with it. My only niggle is starting can take a good amount of kicks when cold but it starts 1st or 2nd kick when warm/hot. I think it’s definitely a lack of fuel getting through when cold even though I tickle both carbs until fuel floods through the ticklers. I’ve taken to giving it 3/4 kicks with the ignition off which seems to help sometimes but it‘s still reluctant on some occasions. Once I turn the ignition on usually the first 2/3 kicks she doesn’t even fire. Then the next 1 or 2 kicks she fires and runs for a couple of seconds. I then tickle the carbs again and she starts. Like I said she runs perfectly otherwise so I’m not going to mess with her but has anyone got a starting method which might help. I’m 200lbs and can give it a good kick so doubt it’s the energy of the kick that’s a problem. I’ve tried with various amounts of choke when cold but full choke always works best. Any thoughts much appreciated as always.

cash · Mar 23, 2022

baz said:
Your bike should definitely not idle until it's fully warmed up

It should when the enricher is operated. It might eight stroke like hell and run at 1500 to 2000rpm though.

Mart UK · Mar 23, 2022

It might depend on the enrichener version. Mine isn't handlebar controlled. It's just on or off at the carb body, no inbetween. I can soot a pair of plugs beyond use within 1/2 mile, if I forget to turn the enrichener off.

Apols for the side track, but I have a set of plugs (Champion), sooted up to the point of not working (how I learned not to leave the enrichener on). Are they recoverable? Cleaner / brass wire brush? Or fit only for the bin?

baz · Mar 23, 2022

cash said:
It should when the enricher is operated. It might eight stroke like hell and run at 1500 to 2000rpm though.

Sorry I meant with the enricher turned off it should take until the engine is warm to idle

baz · Mar 23, 2022

Mart UK said:
It might depend on the enrichener version. Mine isn't handlebar controlled. It's just on or off at the carb body, no inbetween. I can soot a pair of plugs beyond use within 1/2 mile, if I forget to turn the enrichener off.

Apols for the side track, but I have a set of plugs (Champion), sooted up to the point of not working (how I learned not to leave the enrichener on). Are they recoverable? Cleaner / brass wire brush? Or fit only for the bin?

A cable controlled enricher makes quite a difference to the on off type levers
Although it shouldn't!!

Triton Thrasher · Mar 23, 2022

Mart UK said:
It might depend on the enrichener version. Mine isn't handlebar controlled. It's just on or off at the carb body, no inbetween. I can soot a pair of plugs beyond use within 1/2 mile, if I forget to turn the enrichener off.

Apols for the side track, but I have a set of plugs (Champion), sooted up to the point of not working (how I learned not to leave the enrichener on). Are they recoverable? Cleaner / brass wire brush? Or fit only for the bin?

Steel wire brush.

I’d say the cold start plungers should be closed as soon as the engine can be kept running with the throttle.

Then forget about idling, until it warms up a bit.

Tom P · Mar 23, 2022

New enrichener now on order from RGM, close inspection of the old one showed a distinct ring indented into the rubber. On a different note what are peoples thoughts on throttle slides, Alloy vs Hard anodised vs chromed brass?

baz · Mar 23, 2022

Tom P said:
New enrichener now on order from RGM, close inspection of the old one showed a distinct ring indented into the rubber. On a different note what are peoples thoughts on throttle slides, Alloy vs Hard anodised vs chromed brass?

I'm afraid with a mk 2 Amal you'll be chasing your tail eventually
I've used alloy and hard anodised
Not much between them IMO
I'd say chromed brass would be best but I haven't tried them

baz · Mar 23, 2022

Tom P said:
New enrichener now on order from RGM, close inspection of the old one showed a distinct ring indented into the rubber. On a different note what are peoples thoughts on throttle slides, Alloy vs Hard anodised vs chromed brass?

I don't think the ring is an issue
I think if there's a chunk missing you should worry

Carburation issue

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