Carb Jetting question

greetings,

72 interstate, with pre-combat cam.

I am running stock Amals with the K&N filter that Matt sells.

106 needle jet
220 main
3.5 cutaway
needle clip in the middle.

I recently started running 100 octane aviation (no ethanol) fuel in the bike.

It runs great, idle is perfect, acceleration is perfect, the plugs are perfect, but it will have a slight hesitation off of idle at slower speeds, like slowing down to go around a corner followed with light acceleration.

Thought I'd ask here before I start experimenting.

Thanks for all replies.

Not an expert, just a diddler - raise the needle one ring. 220 mains seem a little lean for my liking but every bike is different. I run Av fuel in my hotter bikes, runs so much cleaner it's harder to read plugs. If any sign of glazing or yellow your lean!

10drum said:

greetings,

72 interstate, with pre-combat cam.

I am running stock Amals with the K&N filter that Matt sells.

106 needle jet
220 main
3.5 cutaway
needle clip in the middle.

I recently started running 100 octane aviation (no ethanol) fuel in the bike.

It runs great, idle is perfect, acceleration is perfect, the plugs are perfect, but it will have a slight hesitation off of idle at slower speeds, like slowing down to go around a corner followed with light acceleration.

Thought I'd ask here before I start experimenting.

Thanks for all replies.

Click to expand...

The correct cutaway is 3, not 3.5 for your bike and year - it's causing you to be too lean between approximately 1/8 and 1/4 throttle. It probably was slightly little better when you were running E10 before, if you were. Everything else you mentioned is correct. That info is from AMAL - not sure what Norton says, but since you have the symptom to match, I would say AMAL is right.

Keep in mind the 100ll a gas has a LOT of TEL in it and can/will lead foul plugs with any sort of extended low speed running.

Have you retuned the tickover/idle for best idle, a weak pilot circuit will give you a hesitation.

And I thought 2.5 cutaway was correct , I too ran race gas cut with hi-test 60/40 I think

Yep, slide cutaway controls off idle pickup. Hesitation says it is a bit lean. As per recommendation try a set of 3's. The slide number is the amount of cutaway in 1/16". So a 3 would be 3/16", a 3.5 would be 7/32, a 4 would be 1/4". The more cutaway the leaner the mixture.

I use number 3 cutaway slides in every motor regardless of whether I am using petrol or methanol. The idle circuit and needle jets compensate sufficiently for any slight error in the slide cutaway. The most critical thing, once the low running is set correctlly is the shape of the needles , the internal dimension of the needle jets, and the height of the needles. If you lower the needles one notch, you should get a hesitation or a miss, then raise them one notch. Main jets do not mean much as long as they are slightly too rich. Most riding involves mid-range throttle positions. Plug readings only relate to main jets To set the mainjets, you need to ride the bike at full blast for at least one mile. On public roads, that is not so easy. Even on a small race track, it is difficult. My main jets are always too rich - it does not matter. When I use them on a long straight , my motor gets a slight reprieve. The only time you are likely to burn pistons or valves, is when you use the main jets.
Changing the exhaust system, raising the comp. ratio or advancing the ignition timing can cause the motor to run lean. Those three things work in balance with the fuel/air mixture to create the combustion conditions within the cylinder head.

The tapered part of the needle doesn't come into play until at least 1/3 throttle open, so the position of the needle clip has no effect on any throttle openings less than that such as idle and just off idle. Below 1/3 it is the straight part of the needle. With that said the diameter of the needle jet is important below 1/3 and is often worn oversize due to a steel needle in a brass jet. Needle jets should be replaced on a regular basis.

htown16 said:

The tapered part of the needle doesn't come into play until at least 1/3 throttle open, so the position of the needle clip has no effect on any throttle openings less than that such as idle and just off idle. Below 1/3 it is the straight part of the needle. With that said the diameter of the needle jet is important below 1/3 and is often worn oversize due to a steel needle in a brass jet. Needle jets should be replaced on a regular basis.

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Yes I agree. If the needle jet has been in for a decent period it may well be worn.

I beleive it was John Healy who emphasised this point.

I tried unsuccessfully to add a link but if you search google for John healy motorcycle carburetor tuning you shoukd find it. About 8 page pdf and his notes are about page 4 onwards.

I just had a cateract operation and I know this is full of typos but sorry I'm half blind at moment.

If you have a look at this thread from 2019 you’ll see why it’s a good idea to look very closely at the needles and jets. Granted, mine may have been an isolated case, but still worth checking…

Thanks for all of the replies, it started a good discussion.

The question about 3 and 3.5 cutaway makes me wonder?

The book list for the combat engine a 3, but for the 850 a 3.5?? It also list a 220 main with mute, and 230 without mute for the combat. I am not running mutes (chokes). As I stated above the combat has been taken out of this bike as per rocker lift measurements. The engine is the only part of this bike I have not rebuilt, so I'm not sure what all has been done inside??

The plugs appear to be a perfect brown with no glazing.

The idle circuit is good, I recently pulled the carbs cleaned everything because the ethanol fuel has been an issue with alll of my old Amal equipped bikes. I do tend to set the idle mixture a little toward the lean side, so I'm gonna richen that mixture first, then if that doesn't effect it, I will raise the needle to the bottom notch.

I will report what I learn later.

Thanks for the replies.

10drum said:

The book list for the combat engine a 3, but for the 850 a 3.5??

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Yes, 3.0 slides for all 750 Commandos not just the Combat and 3.5 for all 850 models.

10drum said:

I am not running mutes (chokes).

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Just for clarity, the term ‘mutes’ here is in reference to the reducing bushes added to the end of the peashooters.

‘Chokes’ is the term given to the slides that drop down to ‘choke’ the carb throat and aid cold starting.

So mutes aren’t chokes.

Fast Eddie said:

Just for clarity, the term ‘mutes’ here is in reference to the reducing bushes added to the end of the peashooters.

‘Chokes’ is the term given to the slides that drop down to ‘choke’ the carb throat and aid cold starting.

So mutes aren’t chokes.

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My bad, I forgot that Norton silencers can have mutes. I assumed "mutes" in the carb data section meant chokes. Thanks for clearing that up.

I did richen the idle mixture and it seemed to help with the hesitation a little, but it doesn't idle down as well. I'll raise the needle today, if that doesn't solve it, I'll pick up some 3.0 slides.

In my personal experience, setting Amal’s up to run at their best does not result in the nicest idle, and vice versa.

So I priorities good running and accept a slightly higher idle speed.

Raising the needle has no effect on idle or just off idle. You are in the straight part of the needle in those throttle positions. The taper is only having an effect in the mid range. If you raise the needle you will only richen the mid range and if your plugs look good now that is probably something you don't want to do.

10drum said:

I did richen the idle mixture and it seemed to help with the hesitation a little, but it doesn't idle down as well. I'll raise the needle today, if that doesn't solve it, I'll pick up some 3.0 slides.

Click to expand...

On the same time , you pick up 3 slides , and new needle jet + new needle ....

When I set the idle, I wind the screws to richen the mixture and start the bike. I then turn the screws to lean of the mixture until the motor starts to miss, then back the screws off until the motor idles smoothly. On my bike, the screws adjust the fuel, on other bikes the screws sometimes adjust the air. When I adjust the needle jets, I fit a big ones, and if I cannot induce a miss be lowering the needles, I fit the smaller ones, until I can get the miss at about half-throttle, then I raise the needles one notch. For performance, thev taper on the needles is important, but heat build-up can also happen when using petrol. I only ever use #3 cutaway slidess, but my needles have the slowest available taper. When you do this, you always have to stay on top of your tuning. For a road bike you might do better to run slightly richer - you do not usually need the extra performance, and weather dependency. Really cold weather leans-off the mixture.