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Amal Carb Tuning

Submitted by john_shorter on Sat, 29/07/2017
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Nortons - General

Most manuals recommend starting by checking the "Full throttle" performance of the main jet, then going back to slow running. Surely, if the needle position is too low, it will not be clear of it?s jet, and, therefore the main jet will not be getting unrestricted fuel flow. Would this not give the symptoms of weak mixture, at full throttle?

Submitted by barry_carson on Sat, 29/07/2017

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i suppose thats why they say start with the full throttle opening deal with that first then work your way down.

Submitted by john_holmes on Sat, 29/07/2017

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The needle will only interfere with the main jet at the needles lowest position if its the wrong needle and too long. So check you have the right needle before you tune the main jet.

Submitted by john_shorter on Sat, 29/07/2017

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Just overhauled the carb (with an Amal kit). Tickover perfect, acceleration good, but top end performance poor. the bike is a 54 88. Recommended main jet 240, but I always used a 250, back in the 50?s, for fast riding. I am wondering if an even larger jet is needed, for 95 octane unleaded. The bike pulls well, up to about 75 m.p.h., but will only go faster downhill. Normally, this is acceptable, but, sometimes a bit more grunt would make overtaking easier.

Submitted by robert_tuck on Sat, 29/07/2017

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My 99 is probably still capable of 100 mph and the 88 would do an indicated 90 in third and climb slowly to an indicated 100. It would cruise at 80+ on the M1. your 88 should do 80 + even with the std jetting. A compression test and timing check would be the first things I would do.

Submitted by george_phillips on Sat, 29/07/2017

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To answer your initial query, John, if the needle is too low the symptom will be lack of top end performance as the engine will not be getting sufficient fuel for flat out speed. You could say it's a weak mixture but the end result is the same. Of course once your on low revs/tick-over the main jet is out of the circuit and you're on to the pilot jet. George

Submitted by john_shorter on Sat, 29/07/2017

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No problems with ignition, recent head overhaul, New guides fitting, a valves 're-ground. Thorspark ignition spot on. I doubt if an , iron head, 88 would ever hit 100 m.p.h. ( the best I saw was 95, lying on the tank, down hill!). Before the maximum speed limit was introduced, of course. George is not quite correct, on the operating sequence of an Amal carb, but, he could be on track. I don't see how the main jet can get full fuel flow, until the needle has completely cleared it's jet Anyway, the next step will be to raise the needle a couple of notches, and see what happens!

Submitted by david_evans on Sat, 29/07/2017

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If the Needle completely cleared the needle jet then there is a good chance that it would catch the jet edge on the way down and then stop the throttle from being closed. I don't think the needle is designed to clear the jet completely. try measuring the tip of the needle, work out the area, then measure the bore of the needle jet, work out the area of that. take the small number from the large to give you cross section of the needle jet. then work out the cross sectional area of the main jet. See what you get.

Submitted by david_cooper on Sat, 29/07/2017

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When the slide is almost fully down the needle nearly blocks the hole - so small changes in height make a big difference to the clear passage. But when the slide is fully up, the tip doesn't block much of the hole, so small height changes (moving the clip a few notches) doesn't make much difference.

Submitted by george_phillips on Sat, 29/07/2017

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I have a little tuning guide produced by Amal which involves a turning disc with symptoms on one side and actions on the other. It shows that from 1/4 to 3/4 slide opening the needle position is in charge whilst from 3/4 to fully open the main jet size is the important factor. John's original assertion is correct. If the needle is too low the result will be a weak mixture at full throttle. This can be checked by closing the air lever and noting the result. If the revs pick up the needle is too low. If they don't then look elsewhere.

Submitted by george_phillips on Tue, 01/08/2017

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I should check the thread even more often. You beat me to the punch Les. Yes - that's the one. A bit simplistic but it does the job. G

Submitted by george_phillips on Tue, 01/08/2017

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I should check the thread even more often. You beat me to the punch Les. Yes - that's the one. A bit simplistic but it does the job. G

Submitted by michael_sullivan on Tue, 01/08/2017

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Previously les_howard wrote:

This one?

Les

Thanks for that Les. I am going to print it out and mount it on some thin hardboard for future use.

Mike

Submitted by Xanna_jeannett… on Tue, 01/08/2017

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Previously John Shorter wrote:

Just overhauled the carb (with an Amal kit). Tickover perfect, acceleration good, but top end performance poor. the bike is a 54 88. Recommended main jet 240, but I always used a 250, back in the 50?s, for fast riding. I am wondering if an even larger jet is needed, for 95 octane unleaded. The bike pulls well, up to about 75 m.p.h., but will only go faster downhill. Normally, this is acceptable, but, sometimes a bit more grunt would make overtaking easier.

hello you have the wrong carburettor fitted you should have a 1 inch 276 AK /1TK main jet is a 170 side 6.3.1/2 pos 2 needle107 so you must have a 376 which came out the year after, have fun in the sun yours anna j

 


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