Amal idle adjustment

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Jul 8, 2007
I've searched the forums and the articles, but haven't found this issue addressed. My bike is a 73 850 Roadster, the carb internal parts (inspected and replaced as required) are in good working order, the Boyer timing is 31 by the strobe, 1/8" slack in all throttle/choke cables, the bike starts easily and will idle nicely if I hold the throttle open slightly.

My question is about exactly what controls the actual hands-off throttle-closed idle speed. Getting directly to the point, at idle should the slides be held open by the throttle screws or should the slides be held open by the throttle cables? I'm thinking it should be the throttle screws, and the throttle cables should still have some small amount of slack in them for use in synchronizing.

What I find is that if the throttle screws are backed out until only the o-ring keeps them from falling out on the ground (no threads on the screws engage with the threads in the body), that I can set an acceptable idle speed between 500-1000 rpm. This unfortunately results in no slack in the throttle cable, as I have to screw the cable adjusters in/out to regulate the idle speed plus synchronize the two carbs. I'm confident that this is not the correct approach as the throttle screws are effectively not used at all and there is no slack in the throttle cables.

Alternatively, if I screw in the throttle screws to where the screws engage the threads in the body even by a 1/2 turn (roughly 6 1/2 turns out), although this still leaves me with slack in the throttle cables for synchronizing, the idle speed is not less than 2000 rpm to 2500 rpm. When the throttle screws are turned out until they don't engage the threads in the body, the filter side of the slide (cutaway side) is open about 3/16". When the throttle screws are turned in until the threads *just* engage, this gap is about 5/16" which could explain the higher idle speed.

I'm tempted to conclude that the throttle screws are too long, except they are stock (as are the rest of the carb parts), so clearly I must be doing something wrong.

Any suggestions would be very much appreciated!

Best regards,
San Jose, California
Yes something is wrong, and your intuition is right in there. Yes the screws in the body that are at a 45 degree angle control the slides to set the idle and at that point the cables need about an 1/8 inch free play. There are a second set of screws that control the air flow feeding the idle circuit they are horizontal and are set rough at three half turns off the seat. The air screws are used to optimize the speed of the base idle. If they are not set right the height of the slides will need to be quite high to keep the motor running.
When I start from scratch building new cables I cheat by setting the slides using two 3/16 drill bits. Using the fully assembled carbs minus the air cleaners. I use the bits like feeler gages under the slides and I set the idle screws so they just slide like a feeler gage trapped by the springs force above pining the drill bit to the bottom of the carb body throat, slide on top.
3/16 gets you about a .010 air gap at the back side of the slide thats about right for a base line. At this point if you don't have some slack in the cables you need to trim just a bit from the outer casing on the cable but don't hurt the inter cable while doing so. I have had to do this a lot with pre made cables. One of the reasons I make my own.
Your not done with the bits by the way because they also make good meters to synchronize the cables when you pull on the grip the drill bits need to both move right together lifting the slides just so. The outer cable adjusting will get this done. The drill bits very quickly move as they are released by the spring pressure.
Now you get it running and made adjustments just the same on each side using your start settings as a gage to work from.
If you have not used a .016 drill bit glued in a stick on the idle jets you need to. All stock settings depend on the small hole in there being this size. To proceed without this done and the floats set is a waste of your time you will go mad and end up hating Amals.
re: Amal idle

Thanks for reading my post and your reply. I appreciate it very much. I think the float height is correct, though I've searched for Dyno Dave's tool to verify float height with an external tube. I didn't find it, but found an email address for him and have requested it. I'm not familiar with the drill bit on a stick, so please point me to that tool and I'll check it out. The air screws (mixture screws) are set for 1.5 turns out, as always.

I just came back in from the garage where I tried your drill bit procedure, which I found to be helpful. Imagine that you are installing the throttle stop screws (the 45 angle ones). When you first push one into place, the threads on the screw don't meet the threads in the body, so turning the screw nets nothing. At this very point I have 1/8" slack in the throttle cable and a 3/16" bit works nicely as a feeler gauge between the slide cutaway and the carb interior floor (air filter assembly removed). Starting the bike is easy but if I let the throttle close, it dies. If I hold the throttle slightly open then it will idle nicely between 500-1000 rpm for as long as I want. Alternatively I can take slack out of the throttle cables via the carb top adjusters and make it idle acceptably, but theoretically this is seems to be the wrong approach.

If you push in a little on the throttle screw then the threads start to bite and turning the screw makes it thread into the carb body. Stop at 1/4 to 1/2 turn, just enough for the threads to mate and take hold of each other (this is about 6 1/2 turns out). At this point I still have 1/8" slack in the throttle cable but it now takes a 5/16" bit used as a feeler gauge to close the aforementioned gap. Screwing in the throttle screws from not-threaded to just-barely-threaded, raises the slide by 1/8" (3/16" -> 5/16"). Starting the bike is still easy but it won't idle at less than 2000 rpm, which seems too high but makes sense as I've raised the slide.

I inspected both throttle stop screws under a magnifying lamp and they seem identical. My factory parts book shows no R/L handed-ness to carb parts excepting the carb bodies themselves. I'm not afraid to shorten an outer cable sheath as I've done it before, and even customized some side cutter pliers to make this job easier; I'm not sure that is the issue here. I understand that my symptoms contradict desired behavior, but to paraphrase Donald Rumsfeld, you must deal with the symptoms you have rather than the symptoms you wish to have. I could sure use another clue, please.

Best regards,
San Jose, California
Are you sure everything is Amal stock? But then again Amal did, perhaps still do get things wrong from time to time.

Why not bite the bullet and shorten the tickover screws, they don't cost a fortune.

I remember machining up my Mk2 bodies to make them into a matching pair, clearly no QC at all.


The Amal Concentric Mk1 throttle stop screws are 7/8" long overall, and it should take roughly six turns of the screw from the start of the thread before it even starts to lift the slide when the slide is in the fully bottomed position (cable completely slack) and then take another half turn to even raise the engine side* of the slide to give any throttle opening at all, at which point the head face of the screw will be approximately flush with, or just inside, the carb body screw boss.

*The actual slide height dimension on the intake (filter) side will depend on the slide cutaway (cutaway number) that is fitted?

So something does seem to be wrong?

Useful info:
Kenward, Part of solving problems like this Making assumptions it would be hard to put forth every possibility. When these carbs wear the slide gets very loose at the bottom of the travel. This condition can do things that make an even idle hard to achieve.
The first picture in the bushman's link provided by LAB shows the pilot bushing location it's a reach in behind the air screw. A # 78 drill bit (.016) does not have a long body. If you put it in a pin vice the little chuck would foul on the carb body before you reached the bushing. So a drill on a stick is called for. Take the little red plastic tube that comes on cans of brake clean that you will also need for this job. Mix up a very small batch of 5 Min epoxy and get some inside the tube at one end put the shank if the bit in that end and let it sit using some paper to shim it straight. Now you have the tool that all Amal fiddlers should start their work with.
Pull the air screws keeping the drill bit spinning it in your finger slowly approach the small bushing you will feel the tightness if no one has drilled them out. Do the work by spinning don't push at all you will be removing calcification inside the small opening. Now rinse the the system out with brake clean. I use O-ring grease on the screw seals, all four. When it's working right you will find the air screws have a sweet spot that is very sensitive if turning the air screw doesn't have a dramatic effect in the running you have more problems.
It almost seems like you have air screws in use were idle screws are called for. If you use the 3/16 drill bits under the slides and then adjust the height so that the bit just slides the idle screws should be near flush to the body of the carb.
We may have a BINGO here. The previous comment by NorBsa made me wonder if it was possible that I had swapped the mixture screw and the idle screw.

The screw in my mixture circuit is 7/8" total length, 1/2" of threads, blunt tip.

The screw in my throttle slide circuit is 1" total length, 1/4" of threads, quite noticeable long tapered tip.

The images in my manual are not specific on these dimensions, but I'm imagining that I could have these two screws reversed. Can anyone please confirm this?

Best regards,
kenward1000 said:
The screw in my mixture circuit is 7/8" total length, 1/2" of threads, blunt tip.

The screw in my throttle slide circuit is 1" total length, 1/4" of threads, quite noticeable long tapered tip.

The images in my manual are not specific on these dimensions, but I'm imagining that I could have these two screws reversed. Can anyone please confirm this?

Yes, you have had them fitted in the wrong holes.
Throttle stop screw:

Mixture (volume) screw:
BINGO: swapped throttle and mixture screws

I imagined that if I supplied enough details and clues, someone smarter than me would puzzle it out, which L.A.B. and NorBsa both did. Today I setup throttle screw(s) for 3/16" clearance with 1/8" cable play, mixture screw(s) for 1.5 turns out, 1/8" cable play in chokes (synchronized). Started easily and idled although a little slow. Raised the idle speed via the throttle screws, checking for continued throttle cable play. After a few iterations of idle speed adjust then synchronization via cable adjusters, I had a nice even idle just below 1000 rpm plus both carbs pulling vacuum at the same time when throttle is opened.

Since both screws are the same diameter/thread pitch, and are nearly the same in length, I wonder how many other mis-assembled Amal twins there are? There are five ways to do it wrong:

1. throttle & mixture swapped on R only
2. throttle & mixture swapped on L only
3. throttle & mixture swapped on both <- my case!
4. 2 throttles on R and 2 mixtures on L
5. 2 throttles on L and 2 mixtures on R

But only one way to do it right.

Next up: mixture check/adjust, re-bleed front brake, deal with drive chain rubbing in chain guard, why doesn't the rear brake work?, found two small wear-through holes in left exhaust header.

Best regards,
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