carb questions (amal)

trappnman

trappnman

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Jun 4, 2013
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What I have: A 72 combat that I've rewired with all new harnesses and gone through mechanically to where it is, with 1 exception, a reliable bike. Pazon ignition, all new coils etc. Battery new and load tested. Running stock and I assume original amals that I've thoroughly cleaned (multiple times) and did the rebuild kits on both. Timing seems spot on. New air cleaner with tight seals so no intake leaks.

What is happening: Start 1st kick whether sitting for a couple weeks, or on a hot restart. Idles perfectly, with no fluctuation in rpms. With new plugs (tried ngk 7s last try) is runs great for a few miles, then starts an almost imperceptible hesitation at steady throttle- not constantly, but here and there. No hesitation at all in acceleration. After a few more miles it gets worse (plug is carbon fouling)- yet from a dead stop- you can go through the gears flawlessly with no misses or sputters- but come down to a steady or about 1/4 throttle and it misses again. You can hear right cly cutting in and out. In the 3 years (summers) I've been working on bike, the right has been a troublespot, albeit we put 1500 miles or so on it last year before the problem occurred. Put in a new plug, and the cycle reoccurs.

What I think: Since the problem comes on after a few miles, and disappears under full throttle (and carbon fouled plug) that its a condition of running too rich. But leaning up the air/fuel mixture screw, seems to have to effect on the fouling, although leaning WAY (3,4,5 turns) increases rpms. I'm running the needle setting on the middle on both carbs. My question is this (and Gregs advice about "when Im done messing with those amals, get some Premiers)" ringing in my ears- is it worth trying to rejet, change the needle setting (and isn't this a pilot jet problem) and trying to tune my way out or am I at that point that its just a worn out carb, and I can't tune that out?

Or am I missing something obvious? On my ironhead with a SU carb, a similar problem was traced to a o ring on the enrichener shaft- but the amal really doesn't have such. Is there something else that could cause this?

This last piece of the puxxle is driving me nuts......
 
trappnman said:
What I have: A 72 combat that I've rewired with all new harnesses and gone through mechanically to where it is, with 1 exception, a reliable bike. Pazon ignition, all new coils etc. Battery new and load tested. Running stock and I assume original amals that I've thoroughly cleaned (multiple times) and did the rebuild kits on both. Timing seems spot on. New air cleaner with tight seals so no intake leaks.

What is happening: Start 1st kick whether sitting for a couple weeks, or on a hot restart. Idles perfectly, with no fluctuation in rpms. With new plugs (tried ngk 7s last try) is runs great for a few miles, then starts an almost imperceptible hesitation at steady throttle- not constantly, but here and there. No hesitation at all in acceleration. After a few more miles it gets worse (plug is carbon fouling)- yet from a dead stop- you can go through the gears flawlessly with no misses or sputters- but come down to a steady or about 1/4 throttle and it misses again. You can hear right cly cutting in and out. In the 3 years (summers) I've been working on bike, the right has been a troublespot, albeit we put 1500 miles or so on it last year before the problem occurred. Put in a new plug, and the cycle reoccurs.

What I think: Since the problem comes on after a few miles, and disappears under full throttle (and carbon fouled plug) that its a condition of running too rich. But leaning up the air/fuel mixture screw, seems to have to effect on the fouling, although leaning WAY (3,4,5 turns) increases rpms. I'm running the needle setting on the middle on both carbs. My question is this (and Gregs advice about "when Im done messing with those amals, get some Premiers)" ringing in my ears- is it worth trying to rejet, change the needle setting (and isn't this a pilot jet problem) and trying to tune my way out or am I at that point that its just a worn out carb, and I can't tune that out?

Or am I missing something obvious? On my ironhead with a SU carb, a similar problem was traced to a o ring on the enrichener shaft- but the amal really doesn't have such. Is there something else that could cause this?

This last piece of the puxxle is driving me nuts......
Click to expand...
Test, don't guess.


"gone through mechanically "

Details.
WHAT was done, and by who?
 
I was having a similar issue with my G15- would surge and hesitate a tiny bit under steady throttle and foul the left plug when run gently. If I ran it hard with a lot of throttle it did fine and the plugs stayed a nice tan.

In my case the float level was too high and inconsistent. I’ve installed stay up floats and Vinton tipped needles and it’s now behaving properly. If you are still running the original plastic floats and needles you may have the same fault.
 
Should not be pilot circuit related as at steady cruise you have throttle open more than 1/8 turn, needle jet is primary supply from there to 3/4 full open I believe.
How is compression on warm engine? Is one plug fouling more than other?
Could be intermittant electric fault that rears up at certain riding/vibration conditions. Have you cleaned kill switch contacts? Ign switch new or original? Earthing connections clean? Cracked or shorting bullet connectors?
 
the left plug is perfect.

Could be the float. Can;t remember, but the rebuild kit might have came with new floats, certainly a new float needle, although it could be the seat.

Its not a mechancial issue. What do you suggest I test? spark? coils? gas flow? Timing? Intake? Tappets? Battery? Rest assured, I have tested all of that. Clean the carbs, rebuild them, add new petcocks and filters? Done. Multiple times in fact (deep clean). all connections are new, and seem under the push to see and continuity where apt ok.

While I can't rule out electrical- I'm 99% convinced (after wrenching on vintage bikes as my only rides for almost 50 years) that its carb related. Until proven otherwise, that will be my assumption based by tests and experience.

my question was basically is there something in the carb I might have overlooked (perhaps the float as mentioned), or is it new carb time. And would changing the jetting help? after decades with same jetting, seems unlikely unless worn out to excesses levels
 
make absolutely sure there is no air leak at the flange. This can open up with heat. The phenolics are NOT gaskets. I sandwich the phenolic between to regular gaskets.
 
Mine is a '72 Combat also. When it does this, it's time for new needle jets. Every other time it gets new needles too. A 106 needle jet lasts me about 5000 miles before it starts blubbering at constant speed as you describe. It really needs a 105.5 because a 105 is too lean. Needle jets and needles are really critical for smooth running at constant speed in second gear through town.
Russ
 
batrider said:
Mine is a '72 Combat also. When it does this, it's time for new needle jets. Every other time it gets new needles too. A 106 needle jet lasts me about 5000 miles before it starts blubbering at constant speed as you describe. It really needs a 105.5 because a 105 is too lean. Needle jets and needles are really critical for smooth running at constant speed in second gear through town.
Russ
Click to expand...
that makes sense to me. I'll order new jets. It does have new needles. I do have 1 gasket at intake in addition to the phenolics
 
Are the chokes fully open?
Choke cable is counterintuitive. Pull the choke to open, push to close.
 
Another thing you might want to try is go down a size on the main jet, but you need to be careful and not go too lean.
 
trappnman said:
the left plug is perfect.

Could be the float. Can;t remember, but the rebuild kit might have came with new floats, certainly a new float needle, although it could be the seat.

Its not a mechancial issue. What do you suggest I test? spark? coils? gas flow? Timing? Intake? Tappets? Battery? Rest assured, I have tested all of that. Clean the carbs, rebuild them, add new petcocks and filters? Done. Multiple times in fact (deep clean). all connections are new, and seem under the push to see and continuity where apt ok.

While I can't rule out electrical- I'm 99% convinced (after wrenching on vintage bikes as my only rides for almost 50 years) that its carb related. Until proven otherwise, that will be my assumption based by tests and experience.

my question was basically is there something in the carb I might have overlooked (perhaps the float as mentioned), or is it new carb time. And would changing the jetting help? after decades with same jetting, seems unlikely unless worn out to excesses levels
Click to expand...
Sorry to have offended you, clearly you have it well under control. 🏁


"Its not a mechancial issue. What do you suggest I test? spark? coils? gas flow? Timing? Intake? Tappets? Battery? Rest assured, I have tested all of that. Clean the carbs, rebuild them, add new petcocks and filters? Done. Multiple times in fact (deep clean). all connections are new, and seem under the push to see and continuity where apt ok. "
 
Last edited:
ordered the new .106 jets- I think batrider might have nailed it. I'll report back when installed. thanks for all being a sounding board and giving me things to ponder on.
 

trappnman said:
1) My question is this (and Gregs advice about "when Im done messing with those amals, get some Premiers)" ringing in my ears- is it worth trying to rejet, change the needle setting (and isn't this a pilot jet problem) and trying to tune my way out or am I at that point that its just a worn out carb, and I can't tune that out?
Click to expand...
1) YES! Listen to Greg
FWIW it sounds more like a needle jet issue to me but what do I know?
 
MikeG said:
1) YES! Listen to Greg
FWIW it sounds more like a needle jet issue to me but what do I know?
Click to expand...
new jets ordered, and if they and more fiddling don't cure it, I will be changing out carbs over winter.
 
Have you tried switching the plug leads from one coil to the other to see if the problem follows to the other cylinder?
If it does, this could be a coil that is defective when it gets warmed up.
 
thought about it,, but I haven't. I wasn't sure if I could with the Pazon ignition.
 
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