Single Amal 932Carb main jet and needle position.

N

Nortoncommandoneil

Joined
Jun 9, 2004
Messages
67
Country flag
Hi, my 850 commando mk2a is fitted with a single amal 932 carb and always seems to run on the lean side. ( plug colour light grey )
I took the carb off today and found it is fitted with a 260 main jet and the needle clip was in the middle position.
The question is do I leave the 260 jet in and raise the needle 1 notch or fit a larger main jet.
Thanks.
 
If it’s lean at full throttle, the main jet may be too small. You have to be sure enough fuel is getting into the carburettor though.

Try the richer needle notch. If the engine lurches horribly around mid throttle and soots up the plugs, change back to the middle notch.
 
Nortoncommandoneil said:
Hi, my 850 commando mk2a is fitted with a single amal 932 carb and always seems to run on the lean side. ( plug colour light grey )
I took the carb off today and found it is fitted with a 260 main jet and the needle clip was in the middle position.
The question is do I leave the 260 jet in and raise the needle 1 notch or fit a larger main jet.
Thanks.
Click to expand...
All IMHO:

Unless you're running a lot at or above 3/4 throttle, the main jet has nothing to do with it. If you think it is lean between 1/4 and 3/4 throttle and your fuel level is correct in the bowl, move the clip down one notch. Never change more than one thing at a time.

If running E5 or E10 gas, then look inside the plug at the center insulator. If black halfway up, do nothing and disregard the plug color. If more then you're rich, if less black, then your lean. Of course, you have to test at a certain throttle opening. In normal day-to-day riding you can have some things wrong without it showing.

It's important to understand that the carb is like 4 in one: Pilot, Cutaway, Needle/Needle Jet, Main Jet - they interact very little except at the transition from one to the other.
 
Unleaded pump gas burns better than gas did 40 years ago. If your motor runs well, I would not worry about the grey color. If it pinks and/or coughs and falls on its face when the throttle is quickly opened near WOT, then it may be lean if the timing is good.

Below is a chart that shows where the parts in a carburetor work in relation to throttle opening. Not an Amal but close if you can read between the lines. There is some overlap in fuel delivery even with an old Amal. (I am guessing that I.M.S. in the chart is idle mixture screw.)

Single Amal 932Carb main jet and needle position.
 
A single carb should not need different jetting than stock dual carbs, at least in theory. On dual carb setup, each carb fuels/flows only one cylinder. On single setup, the carb is only fueling/flowing air to one cylinder at a time. Yes there is added perturbations from the flow needing to keep changing left or right side of the intake manifold. At full throttle there is presumably more restrictions to max flow than dual.
My MK2 850 came setup with single 932 and I ran that for a few seasons. Did seem to be on lean side, pipes bluing etc. But that was also during my steep learning curve of how to set up the carb and dealing with pilot circuit issues etc.
Now running dual setup and have needles at the higher position/clips in lowest notches (richer). Bike is at first over bore (20 thou) in case that makes any difference. This gives best response to quick throttle input, no stumble. Plugs are tan/grey, so might still be considered lean according to old fuel type readings. But she starts first kick 9/10 times, good power and no popping on decel, 'cept for a few pops when still cold in the mornings.
 
I'm about to upgrade my engine from 750 to 850 cc (820 actually) and I wonder if my 32 mm single carb Amal MK1 will be enough or if I should switch to a 34 mm.
I'm looking for torque and power in the 3000 to 5000 rpm range and ease of use and tuning, rather than top end power.
BTW I noticed that Burlen are now offering the Concentric 1,5, external diffrence being the choke lever on the carb (as on a Mikuni or Amal MK II).
Any feedback on it?
Thanks,
Laurent
 
In the chart, what is the "slow air jet"? If that is the air correction jet it isn't supposed to be felt until the main jet comes into play.
 
Nortoncommandoneil said:
Hi, my 850 commando mk2a is fitted with a single amal 932 carb and always seems to run on the lean side. ( plug colour light grey )
I took the carb off today and found it is fitted with a 260 main jet and the needle clip was in the middle position.
The question is do I leave the 260 jet in and raise the needle 1 notch or fit a larger main jet.
Thanks.
Click to expand...
Your current carb will be fine. I would stay away from the MK1.5 carbs, getting parts can be difficult and the carbs are not even listed for sale in the dealer price list. They were originally Spanish-made for Spanish bikes.
 
marshg246 said:
Your current carb will be fine. I would stay away from the MK1.5 carbs, getting parts can be difficult and the carbs are not even listed for sale in the dealer price list. They were originally Spanish-made for Spanish bikes.
Click to expand...


My friend just bought a Yamaha XS 650 with Amal MK2's fitted on it. One was made in England the other was Spain. So it appears both the MK1.5 and 2 were both made in Spain for a while.
The bike also has a Boyer igniton, Dunstall taillight and manufactured in Canada. Guess PO wanted to make a Brit bike out of it.
 
Triton Thrasher said:
If it’s lean at full throttle, the main jet may be too small. You have to be sure enough fuel is getting into the carburettor though.

Try the richer needle notch. If the engine lurches horribly around mid throttle and soots up the plugs, change back to the middle notch.
Click to expand...
I would go in the opposite direction. If you lower the needle one notch, the motor should develop a miss when you ride the bike. then raise the needle one. How often do you ride your bike using full throttle ? - that is when the main jets work, and the only time plug colour matters. When you look down inside the plug, there should be a black ring of carbon on the porcelain where it meets the metal. If you tune using a lower number plug, you will tend to remove the black ring due to the higher heat range, so you will stay safer.
 
Deets55 said:
My friend just bought a Yamaha XS 650 with Amal MK2's fitted on it. One was made in England the other was Spain. So it appears both the MK1.5 and 2 were both made in Spain for a while.
The bike also has a Boyer igniton, Dunstall taillight and manufactured in Canada. Guess PO wanted to make a Brit bike out of it.
Click to expand...
Or, was struggling trying to get neglected old CV carbs to work again.
 
Nortoncommandoneil said:
Hi, my 850 commando mk2a is fitted with a single amal 932 carb and always seems to run on the lean side. ( plug colour light grey )
I took the carb off today and found it is fitted with a 260 main jet and the needle clip was in the middle position.
The question is do I leave the 260 jet in and raise the needle 1 notch or fit a larger main jet.
Thanks.
Click to expand...
I'm with Schwany, if you are just judging lean running from plug colour, it probably isn't too lean at all.

Plug reading is not good with unleaded fuels and you should expect a light grey, not that old chocolate brown we used to consider correct.

If it pulls cleanly up to your chosen rpm limit and doesn't make annoying noises on the overun, leave well alone.

I doubt you are on the main jet long enough for it to affect the plug colouration anyway. The way to confirm if the main jet is too lean is to hold it at full throttle in a high gear and when settled, slowly reduce the throttle setting towards 3/4 throttle, if it accelerates, it is too lean, go up one jet size and try again. If that is OK, it's all down to the needle and needle jet.
 
SteveA said:
I'm with Schwany, if you are just judging lean running from plug colour, it probably isn't too lean at all.

Plug reading is not good with unleaded fuels and you should expect a light grey, not that old chocolate brown we used to consider correct.

If it pulls cleanly up to your chosen rpm limit and doesn't make annoying noises on the overun, leave well alone.

I doubt you are on the main jet long enough for it to affect the plug colouration anyway. The way to confirm if the main jet is too lean is to hold it at full throttle in a high gear and when settled, slowly reduce the throttle setting towards 3/4 throttle, if it accelerates, it is too lean, go up one jet size and try again. If that is OK, it's all down to the needle and needle jet.
Click to expand...
The truth is here.
 
laurentdom said:
I'm about to upgrade my engine from 750 to 850 cc (820 actually) and I wonder if my 32 mm single carb Amal MK1 will be enough or if I should switch to a 34 mm.
I'm looking for torque and power in the 3000 to 5000 rpm range and ease of use and tuning, rather than top end power.
BTW I noticed that Burlen are now offering the Concentric 1,5, external diffrence being the choke lever on the carb (as on a Mikuni or Amal MK II).
Any feedback on it?
Thanks,
Laurent
Click to expand...
Go for a single 34mm Mikuni, first it will start much better, and there are good base set-ups around to get you going.

Also, no tickling, and you can choose a cable operated fuel enricher (choke) so you can do all of the start up without putting your hand under the tank or getting fuel on your gloves.
 
seattle##gs said:
In the chart, what is the "slow air jet"? If that is the air correction jet it isn't supposed to be felt until the main jet comes into play.
Click to expand...
That is a Mikuni chart, the air correction jet is the 'main air jet' noted up by the main jet. And of course you are correct that the effect of the air correction jet is felt mainly on the main jet, specifically at higher rpm, where too much air causes full throttle lean out. Amal carbs don't have an air correction jet to change.

'slow air jet' is as mentioned the slow running air screw setting. Again, whilst the effect is similar between Amal and Mikuni slow running mixture screws, they aren't the same, one affects air flow, the other fuel flow.....screw one in....and the other one out.....go on, ask me if I can remember which is which on a casual Saturday afternoon!
 
SteveA said:
Go for a single 34mm Mikuni, first it will start much better, and there are good base set-ups around to get you going.

Also, no tickling, and you can choose a cable operated fuel enricher (choke) so you can do all of the start up without putting your hand under the tank or getting fuel on your gloves.
Click to expand...
I disagree about the starting better with a mikuni
That's not been my experience
The best my Norton has started has been with twin MK1 amals
A single MK1 was ok but worked better when the choke was used aswel as the tickler for starting from cold
 
baz said:
I disagree about the starting better with a mikuni
That's not been my experience
The best my Norton has started has been with twin MK1 amals
A single MK1 was ok but worked better when the choke was used aswel as the tickler for starting from cold
Click to expand...
If the Mikuni doesn't start better, you probably have the wrong set up somewhere.

Fitted one to my nightmare starter AJS Single, now it starts first kick, hot or cold. But don't be tempted to twist the throttle open at all.
 
SteveA said:
If the Mikuni doesn't start better, you probably have the wrong set up somewhere.

Fitted one to my nightmare starter AJS Single, now it starts first kick, hot or cold. But don't be tempted to twist the throttle open at all.
Click to expand...
Yep I could have the wrong setup
I can only go by what I have tried on my own commando
I've had a single 32mm concentric a single CV 32 mikuni a single VM 34 and a VM 36 and my final single setup is a TM 40
None of these setups have started better than twin concentrics so far
My b50 would generally start first kick ,that was running a 32mm concentric
You darent open the throttle whilst kicking that bike over !
 
You must log in or register to reply here.

Similar threads

Britstuff
MK II 850 Commando 1974 Amal Carb choice.
Replies
13
Views
504
acotrel
acotrel
ILLF8ED
‘72 combat main jets
2 3
Replies
41
Views
1,597
acotrel
acotrel
D
Can you give me advice about tuning/jets etc on my single Mikuni VM34
2 3
Replies
47
Views
1,274
DennisMo
D
Bonzo
I think I'm running a bit rich...
2
Replies
24
Views
1,135
acotrel
acotrel
W
Amal premier jetting for 4000' elevation?
Replies
4
Views
312
Wireman
W
Back
Top