Carb Jetting question

I'm running ethanol-free premium at 2500 ft. altitude. I've found that if I set the idle mix so the bike starts well, there is an annoying burble/missfire on the overrun as when coasting on a slight downgrade with the throttle just cracked open. If I lean out the low-speed mix the bike is hard to start and the idle speed is increased. This is true of my Matchless single, Atlas and MKIII, all running concentrics. I believe they are 3-1/2 cutaway except the Atlas which is a 3 cutaway and also has early concentrics with screw-in jets. Other than the idle issue they run well. I've cleaned the pilot jets with a wire-brush bristle and aerosol carb cleaner but I suspect there may be some obstruction yet.

Unlike the original poster, I haven't noticed an off-idle hesitation unless I lean out the idle mix further than necessary to cure the overrun problem but I suspect that both our problems are related to crud in the idle-speed mixing chamber. So that leads to the discussion about the blanking plug removal.
 
marinatlas said:
On the same time , you pick up 3 slides , and new needle jet + new needle ....
Click to expand...
Raising the needles didn't do much, the plugs are slightly richer, but still good. I'll install some 3s before I start changing jets. I don't know why this set of amals came with 3.5s? I don't remember where I bought them.

The 3.5s handled our Illinois high ethanol content 93 octane fuel just fine. It's just that if the bike sat for just a little while, it'd develop idle issues. As a daily rider it'd be fine.
 
Thomasa said:
I'm running ethanol-free premium at 2500 ft. altitude. I've found that if I set the idle mix so the bike starts well, there is an annoying burble/missfire on the overrun as when coasting on a slight downgrade with the throttle just cracked open. If I lean out the low-speed mix the bike is hard to start and the idle speed is increased. This is true of my Matchless single, Atlas and MKIII, all running concentrics. I believe they are 3-1/2 cutaway except the Atlas which is a 3 cutaway and also has early concentrics with screw-in jets. Other than the idle issue they run well. I've cleaned the pilot jets with a wire-brush bristle and aerosol carb cleaner but I suspect there may be some obstruction yet.

Unlike the original poster, I haven't noticed an off-idle hesitation unless I lean out the idle mix further than necessary to cure the overrun problem but I suspect that both our problems are related to crud in the idle-speed mixing chamber. So that leads to the discussion about the blanking plug removal.
Click to expand...
I think you did a better job describing what it's doing.

It's not so much off idle as much as when slightly opening the throttle, like when slowing down for a slow turn, then slightly cracking the throttle. It would never happen if my son was riding it, he riders every bike like he stole it. He doesn't know how to slightly open the throttle at all.
 
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...
There is some interesting info on improving the slow speed running of the pilot jet at the Victorian (Australia ) NOC site Technical info
nortonownersvic.org

Technical

Clutch Nut Torque Dominator = 70Lb/Ft Commando = 40Lb/Ft Commando is less because the circlip behind the clutch shears at 55Lb/Ft – you have been warned Lubrication guidelines 8…
nortonownersvic.org nortonownersvic.org
 
HRD said:
There is some interesting info on improving the slow speed running of the pilot jet at the Victorian (Australia ) NOC site Technical info
nortonownersvic.org

Technical

Clutch Nut Torque Dominator = 70Lb/Ft Commando = 40Lb/Ft Commando is less because the circlip behind the clutch shears at 55Lb/Ft – you have been warned Lubrication guidelines 8…
nortonownersvic.org nortonownersvic.org
Click to expand...
Interesting article.
 
Keep in mind that AV gas isnt MO gas. Which is why all the guys who worked on airplanes had cars
that needed valve jobs....hint hint. It is rather like the fact that Aeroshell 50 isn't the best oil for a
bike motor other than the fact that is was, er, inexpensive.
 
Onder said:
Keep in mind that AV gas isnt MO gas. Which is why all the guys who worked on airplanes had cars
that needed valve jobs....hint hint. It is rather like the fact that Aeroshell 50 isn't the best oil for a
bike motor other than the fact that is was, er, inexpensive.
Click to expand...
Hiny hint, back in the day, cars needed valve jobs sooner or later.

I do not have access to higher octane fuel that does not contain ethanol.

Ethanol draws in moisture, which seems to quickly gum up the carbs if not ridden regularly.

I have 5 carbureted bikes, I can't ride em all regularly, so I'm looking for a solution.

So, do you have a solution?
 
10drum said:
Hiny hint, back in the day, cars needed valve jobs sooner or later.

I do not have access to higher octane fuel that does not contain ethanol.

Ethanol draws in moisture, which seems to quickly gum up the carbs if not ridden regularly.

I have 5 carbureted bikes, I can't ride em all regularly, so I'm looking for a solution.

So, do you have a solution?
Click to expand...
Yes.

Mentioned MULTIPLE times.

Fuel stabilizer.
 
Absolutely right! We have bikes and cars with normal E10 pump gas/StaBil in the tank sitting unused for 6 months every year. They start/run as if they had fresh gas/were running yesterday. One motorcycle sat that way for over a year with the same non-issue as far as starting/running.

I have friends who add a shot of stabilizer with every tank of gas. They say it's because they never know when the bike might end up sitting for an extended period. I've never done that but it makes sense - it's always "ready to store" as far as the fuel system is concerned.
 
Onder said:
I usually just drain the carbs...
Click to expand...
Carb Jetting question



Draining the bowl is not enough.
There are places that don't drain.
 
I've tried fuel satbalizer in the past but still end up needing to pull the carbs in the spring.

In the fall I do drain the tanks and the carbs.

I'll pickup some 3 slides and report back, about the AV gas.
 
You must log in or register to reply here.
Back
Top