Amal Carb Jetting

[Duccout]

So did my Mototrans Strada 250, all the same except that the main jet is a 180, probably due to the more restricted silencer.

Colin

[Duccout]

Wonders will never cease, I seem to have got the bike running decently with the Amal! It is a bit rich, so I will try it with the needle down a notch, but for a 45 yr-old carb, it is a bit of a miracle.

I persevered because I suspect that Mikunis may not like running with a downdraught? I don't believe that I have ever seen a Japanese bike equipped with Mikunis that did not have the carbs mounted horizontally, and I believe that my problem with weakness low-down may be due to the downdraught angle lowering the fuel level.


Colin

[Dave354]

Hopefully this will help the carb rebuild!

[Duccout]

Very good!

[George]

Wonders will never cease, I seem to have got the bike running decently with the Amal! It is a bit rich, so I will try it with the needle down a notch, but for a 45 yr-old carb, it is a bit of a miracle.

I persevered because I suspect that Mikunis may not like running with a downdraught? I don't believe that I have ever seen a Japanese bike equipped with Mikunis that did not have the carbs mounted horizontally, and I believe that my problem with weakness low-down may be due to the downdraught angle lowering the fuel level.


Colin

Good news Colin. Never give up trying. I am a fan of Amal. Never had problems.

[Duccout]

Hi George,

I am a big fan of Mikunis, they are beautifully made and simple to work on, and infinitely adjustable, BUT this infinite adjustability is a drawback when trying to set up a carb that is non-standard. The big Japanese companies can afford to set their bikes up perfectly, but a private owner just keeps chasing their tail, and it works out expensive. Amals, on the other hand, have a big advantage for us, in that Mototrans have done all the work for us!

If my old carb ends up being too worn, I can simply buy a brand new one with identical jetting from Burlen, which is what I should have done in the first place!

Cheers,

Colin

[Mike]

Hello - wondering if someone can help me through some carb tuning (I feel a bit out of my depth whenever there are no fuel injectors involved!)... I had my wee 250 Monza running reasonably well last year on 140 main (on a Amal 626 concentric with very few miles on it).

It sat doing nothing over winter and wouldn't start a few weeks back, so I stripped and cleaned the carb. It starts and idles just fine, but it is clearly running quite rich. I have a Gunson gastester device, and it's showing about 8% at idle.

I can easily find myself a smaller main jet, but I think I'd take it down to 110 or thereabouts (from where it is at the moment 140) to get the CO percentage down to a reasonable level, and reading through this thread, I'm surprised that others are talking about 180-210 on their jets - should I even be having a "rich" problem at a 140 jet?

Thanks,
Mike

[Duccout]

Hi Mike,

Where is it running rich though? The main jet only affects the engine when the throttle is 3/4 open, below that it is the needle and needle jet. If you have not done so, try dropping the needle down a notch and see if that makes any difference, plus (just checking) you do have the choke slide open all the way (the lever should be fully tightened).


Good luck,

Colin

[blethermaskite]

Mike, I have run 600 series mk1 concentrics on many things, currently a pair of 626's on my pre war racing car, as an educated guess I would expect to be starting with a 150/160/170 main jet depending on your exhaust system (the more restricted the smaller the main jet), with a 140 main I would be checking out your needle and needle jet size's and slide cutaway, I would be going for a std needle, 106 needle jet and a 3 or 3.5 slide. This is all a bit of a starting point guess, however if it was running ok before???? the first thing I would look at is the float and float needle....its probably a good idea to replace them both before you start messing with everything else. good luck, George

[ranton_rambler]

Mike - you say it sat all winter. Have you put some fresh fuel in? And presumably you are using E5? And the basics of timing, valve clearances are OK?

My experience with Amal carbs is limited, but presumably they have a pilot jet with some sort of fine adjustment screw for the idle mixture.

I know it's not a Dellorto, but the tuning guide for those carbs is quite helpful in explaining which components influence which part of the operating range. It's easy to find on-line e.g. https://www.vortex-rok.com/gestione/all ... /88_40.pdf

Ian