Amal Carb Tuning

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

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.

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.

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

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.

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.

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.

George:

Can you post a photo of the Amal "Tuning Disc"

Thanks, Mike

This one?

Les

Attachments
amal_tuning_guide-pdf

A guy called Bushman has done a very good job of describing how to set up and live with the Amal Concentric carb'. It's here.

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

Ian

Previously les_howard wrote:

This one?

Les

Thank you.

Fritz

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

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

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

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