general (amal) carb question

joe czech said:

just askin' - running a #3 throttle slide vs, a #3.5 will richen the mixture mid-range - correct??? if so, can that be offset by moving the needle clip up by one notch? which component would have a greater effect on mixture? second question - with everything being equal, what differences would one see between a .106 vs .107 needle jet??? I'M GUESSING a .107 would also richen the mixture mid range - ????

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Most bikes will run OK if you use any of the std OEM jetting, You need to ride the bike at the RPMs you usually use and see how it goes and the spark plugs read,, then you can go from there, A bike will not run well when cold, So don't idle it to long or not at all when warming it up, and then after several miles of moderate speed/rpm driving , it should be warm enough to evaluate how it feels. Think of a car when started cold the idle is high and there is some form of enrichment with an old fashioned choke or the fuel injector and computer sensing temp of the engine. The hardest customers I used to have were the ones that were more concerned about how low the bike could idle, when I was more concerned about how they ran once fully warmed up. Until your warm enough to vaporize the fuel, the bike should not run perfect, unless your pouring lots of raw fuel in, Just like a choked out car. The idle speed is the last thing I try to set, especially when I breaking in new engines, as you should not be idling them in the first place.

My 74 has now been slightly modded and will be set up with a K&N filter instead of the plastic bin, I will have 2 separate exhausts as the balance tubes have been removed and I’ll be using peashooters]

The carbs are new 932s. They have .106 needle jet and 260 mains. Using the tall tube along with 2id ring needles in no.3 slides - I do have 3 1/2 slides to hand as well if necessary.

Do I clip the needle at the top notch, or the middle? I noticed when I removed them they were one at the top and one in the middle.

Landspeedracer said:

My 74 has now been slightly modded and will be set up with a K&N filter instead of the plastic bin, I will have 2 separate exhausts as the balance tubes have been removed and I’ll be using peashooters]

The carbs are new 932s. They have .106 needle jet and 260 mains. Using the tall tube along with 2id ring needles in no.3 slides - I do have 3 1/2 slides to hand as well if necessary.

Do I clip the needle at the top notch, or the middle? I noticed when I removed them they were one at the top and one in the middle.

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i would think you would want everything, carbs, as well as needles (w/ clips) the same. no expert here, but i would balance the carbs and read the plugs. BTW, my 74 Mk2, 3.5, 260, 106, clips - middle notch. factory, stock OE exhaust. my 2-cents.

Yes but I’m converting away from mk2a which, over here, had the restrictive mk3 style air filter and balance pipes and black cap silencers, and had 3.5 slides, 220 mains, and 106 needles.

Now it will have a less restrictive system. I only want to know if the 2 ring needles are clipped at the top or middle.

Middle position works for my bike.

I've been dealing with oil leaks. I no longer smell burning / hot oil. Now I can smell unburnt fuel! I'm happy there are no leaks, tank to carb. My plugs are greyish at the electrodes, but sooty around the rim and initial threads, so suggests I'm running rich. I have a single 34 Amal mkII concentric.

Today i asked a friend to ride behind me and he confirmed a strong smell of petrol. We were at 1/4 to 1/2 throttle, 2,500 to 3,500 revs. The only other fuelling issues I've noticed are:
- after ticking over at traffic lights for more than a couple of minutes, it can die if i open the throttle slowly;
- in 1st gear at a fixed throttle (not accel/decelerating) when I'm somewhere between 2,500 to 3,500 rpm, it can feel like it misses occasionally. Obvious when the bikes cold, but still happens when warm, but less pronounced.
Are these signs of running rich? In all other respects the bike runs great, accelerates happily through the rev range. Sits at 70mph, raring to go faster.

I'm thinking, raise the needle clip one notch, to lower the needle?

Needle jets wear out quickly, rule of thumb is 3 jets to 1 needle. A new needle jet rather than drop the needle and if you don't know the history then new needle too.

Mart UK said:

I've been dealing with oil leaks. I no longer smell burning / hot oil. Now I can smell unburnt fuel! I'm happy there are no leaks, tank to carb. My plugs are greyish at the electrodes, but sooty around the rim and initial threads, so suggests I'm running rich. I have a single 34 Amal mkII concentric.

Today i asked a friend to ride behind me and he confirmed a strong smell of petrol. We were at 1/4 to 1/2 throttle, 2,500 to 3,500 revs. The only other fuelling issues I've noticed are:
- after ticking over at traffic lights for more than a couple of minutes, it can die if i open the throttle slowly;
- in 1st gear at a fixed throttle (not accel/decelerating) when I'm somewhere between 2,500 to 3,500 rpm, it can feel like it misses occasionally. Obvious when the bikes cold, but still happens when warm, but less pronounced.
Are these signs of running rich? In all other respects the bike runs great, accelerates happily through the rev range. Sits at 70mph, raring to go faster.

I'm thinking, raise the needle clip one notch, to lower the needle?

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A problem that’s worse cold than not would indicate weak mixture … think about it, your MK2 has an enrichener to make it start / run better when cold, without it, it’s too weak. You don’t use the enrichener when warm, it would be too rich.

General Amal Carb??
Is that another statue in the bin?

kommando said:

Needle jets wear out quickly, rule of thumb is 3 jets to 1 needle. A new needle jet rather than drop the needle and if you don't know the history then new needle too.

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The carb is 8 yrs old, but only 6,000 miles.

Fast Eddie said:

A problem that’s worse cold than not would indicate weak mixture … think about it, your MK2 has an enrichener to make it start / run better when cold, without it, it’s too weak. You don’t use the enrichener when warm, it would be too rich.

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The enrichener is helping the pilot circuit and that seems to be working ok, apart from when (fully hot) feathering the throttle at traffic lights. That may be 'bogging down' and dying (lean), or it may be 'stuttering', trying to take off, but as it moves onto the needle jet, the fuel is swamping it (rich). I think I'll address that problem if it persists after the fuel smell problem is sorted. At the moment I'm hoping it's the same problem and more to do with the needle jet circuit than the pilot circuit / choke.

There's no smell of fuel at tickover. The smell of fuel and occasional stutter is at 2.5 to 3.5k revs +. I'm guessing that's approx when the pilot circuit is no longer solely in control. So, my thinking is, the problem appears to be when I move into the needle jet circuit at around 1/4 throttle. At this point, accelerating, it doesn't bog down or stutter, quite the opposite, it pulls well. So I've read it as:
- fuel smell; not lean, more likely rich
- stumbling at steady throttle just past transition to needle jet; not bogging down (lean), more like a stuttering trying to go (rich)
- soot / carbon around the rim and threads of the plugs; doesn't look like it's running lean, more like rich
- accelerating it never stumbles and is willing; so, not overly lean, but could still be overly rich.
Therefore = too rich on the needle jet circuit.
The cause may be a worn jet or needle, but I thought it worth trying to lower the needle a notch, to see if it helps.

Please pull my diagnosis apart, I won't be offended , I'd appreciate it. We're in London this weekend, so I'll be onto it on Monday.

I’m not gonna pull it apart, I think you might be right.

The only point I’d add is that it’s not actually rpm that matters, is slide position.

So a good tip is to mark your throttle at 0, 25, 50, 75, 100% positions (Tipex is ideal, it’s white so is easy to see, but is easy to remove with carb cleaner).

Then you can really see at what throttle openings your symptoms are happening. And that tells you which jets and circuits are in play.

Pull the plugs and analyze the colour. The goal is a light grey on the tips . Dark / sooty is too rich.

Mart UK said:

but only 6,000 miles.

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The jet takes a battering from the needles back and forth movement with each engine cycle. 6000 miles can start to wear it.

• Needle Jet Notes:
  Needle jets are the only Amal jet where the number refers to an actual inch dimension. The rest of the jets refer to flow in cc’s per minute.
• Gasoline carburetors, both two and four stroke, are commonly fitted with needle jets ranging from .105″ to .107″. A .106″ needle is by far the most common one used. Needle jets mesuring .1065″ have been made.
• Alcohol carburetors are commonly fitted with needle jets .120″ or larger.
• Changing one size either way from the proper needle jet has a strong influence on just off idle to mid range running.
• A slightly worn, or oversize, needle jet will cause the motorcycle to run very rich at throttle openings up to approximately 1/3rd throttle.

kommando said:

The jet takes a battering from the needles back and forth movement with each engine cycle. 6000 miles can start to wear it.

Vintage Bike Magazine » Amal – A comprehensive look

vintagebikemagazine.com

• Needle Jet Notes:
  Needle jets are the only Amal jet where the number refers to an actual inch dimension. The rest of the jets refer to flow in cc’s per minute.
• Gasoline carburetors, both two and four stroke, are commonly fitted with needle jets ranging from .105″ to .107″. A .106″ needle is by far the most common one used. Needle jets mesuring .1065″ have been made.
• Alcohol carburetors are commonly fitted with needle jets .120″ or larger.
• Changing one size either way from the proper needle jet has a strong influence on just off idle to mid range running.
• A slightly worn, or oversize, needle jet will cause the motorcycle to run very rich at throttle openings up to approximately 1/3rd throttle.

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That's a really helpful link. Thank you.

joe czech said:

just askin' - running a #3 throttle slide vs, a #3.5 will richen the mixture mid-range - correct??? if so, can that be offset by moving the needle clip up by one notch? which component would have a greater effect on mixture? second question - with everything being equal, what differences would one see between a .106 vs .107 needle jet??? I'M GUESSING a .107 would also richen the mixture mid range - ????

Click to expand...

Joe, depends what you mean by mid-range. Engine revs are more dependent on load than throttle opening position.

Mart UK said:

That's a really helpful link. Thank you.

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I've put in a new needle jet and needle, cleaned the pilot passages as best I can with a below size guitar string, carb cleaner and air pressure. Cleaned the air filter (K&N). Lowered the needle 1 notch.

It wouldn't tickover happily. The plugs were sooty again (photo). I cleaned them and checked the gap 100 miles ago. They were sooty all over the threads and electrodes. Changed the plugs, set at 0.65mm. Tickover was much happier, slight adjustment of the pilot air screw and it as good as I've had it before. I tried to lean the mixture a little, but it fluffs and dies when I gently roll on the throttle.

I've never managed to get a 1,000rpm healthy regular thump. At 1200rpm, it ticks over steady, but still with occasional misses. I have replaced the pilot jet (Amal mk2) and no difference. I also tried a different size up and down, which it didn't like at all. I think I've either a partial blockage of the pilot circuit I've not been able to clear, or the Single carb set up with a K&N doesn't suit the bike at tickover?

I got a quick few test miles in.
- At <1/4 throttle, misses still apparent in 1st gear. Not intrusive, but noticeable.
- At 1/2 throttle, picks up smoothly and romps away.
- Over 1/2 throttle, got up to 70 in second and it felt good.
- Some decel popping from 5,000 to 3,500 rpm, stops below 3500. So running a little lean?
- No smell of fuel now.

So, I'll run it like this for another 100 miles then see how the plugs look. Hopefully, no longer running rich on the needle jet, they will be ok and I wont have to change them every 2,000 miles. The bike came with Champions, not sure how old. They needed changing after a few hundred miles to cure a similar poor running problem. I switched to NGK BP7E S and have now gone back to Champion N7YC.

I'll let you know how i get on.

A colortune will identify the misses as being lean or rich if its a mixture issue, it will also show if there is no spark for an ignition problem. You cannot use a colortune underload, only on an engine running in neutral so it has its limitations but good for diagnosis at tickover and on initial throttle opening. Ultrasonic cleaning would be in order if lean mixture is identified not to do with the pilot jet but the circuit.

Sorry if already asked, but have you ruled out air getting past the rubber pad on the cold start plunger?

Triton Thrasher said:

Sorry if already asked, but have you ruled out air getting past the rubber pad on the cold start plunger?

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Good question. It was replaced with a Burlen part, just before I got the bike. I have pushed on it to check it is seating. So I think it's ok. I wonder if a previous owner had the same symptoms and tried a new choke.