Running Rough

My 1972 Commando runs great when it is cold, but as it warms up, it starts to run roughly. At a steady state, with light throttle, it will buck and fart occassionally. If I twist the throttle and start to accelerate, the issue goes away. It appears at about 1/4 throttle, in a light load situation. No hint of the problem when cold, but the more it warms up, the more evident it becomes. This is true even when the air temperature is only 55 degrees, so it is not a hot environment situation. The bike has twin Amals, very low hours.

If the bike runs well when cold, and poorly when warm, it is probably too rich at that throttle setting.

This is a great site on Amal carb tuning: http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html
It shows which adjustments affect the mixture at which percentage of throttle.

I am thinking the first step is to lean out the idle screw, and as long as it still idles, it might solve the problem.
Second step: lower the needle?
Third step: different cut-away?

I'd appreciate any thoughts.

Stephen Hill

How old are your needle jets and jet needles. As soon as my '72 starts having a problem not being smooth with slow throttle openning I replace these parts and clears up the problem. Vibration does a job on these. If you run your thumbnail along the lower portion of the jet needles and they have ridges (not smooth) they are worn out.

One thing I'd also check for hidden air leak, bowl and beyond via wd/4o, brake clearner etc sprays. Must always try new plugs just in case. Also suspect wires rubbing the insullation opens up or softens up with hot.

illf8ed said:

How old are your needle jets and jet needles. As soon as my '72 starts having a problem not being smooth with slow throttle opening I replace these parts and clears up the problem. Vibration does a job on these. If you run your thumbnail along the lower portion of the jet needles and they have ridges (not smooth) they are worn out.

Click to expand...

The stuttering with light load is the giveaway. Needles and needle jets every 5000 miles!

Since the carbs have less than 20 hours run time on them, I doubt that worn needles, and jets are an issue.
Hobot's suggestion about air leaks was my first thought But the bike runs well when cold, and the rough running starts when it is hot. Also putting the choke on when the rough running starts doesn't help, So I am thinking the problem is richness, rather than leaness.

I tried adjusting air screw, but I backed it out.. And it made no difference. I think I should have screwed it in. Screwing it in leans the mixture, correct?
I am also noticing some pinking under load, so I will look at reducing the advance, and check out the plug heat range.

Other ideas?

Stephen Hill

Screwing in (closing) the air bleed screw richens the mixture.

Presuming jets etc. are as the book & timing is crack on, my first guess would be an air leak or a condenser failing.
Check the O rings on the pilot adjusting screws & the caburettor flange faces.
Start with pilot screws about 1 turn out.

If screwing the pilot Air screw in or out don't affect the idle much then the pilot jet is clogged & or the float level is too far off.

Believe in Steve.

Dave
69S

Hmmmm. No hours on carbs. Adjusting the pilot air screw makes a big difference at idle, so the idle jet is not plugged.
I tried unscrewing the pilot air screw to about 2 1/2 turns, but it made no difference to the rough running. I don't think I should have to bring it further out.

Hobot, if an air leak is the issue, that means the mixture is lean. Wouldn't putting the choke on reduce the problem?
If it is running lean, then I guess I could try running the pilot air screw in more, to about 1 turn. I am convinced that it is rich, not lean.
Occassionally it actually pops like a backfire. Is this rich or lean? And it is definitely carb related. Soon as I load up the engine, everything clears up, with no pulsing or bucking.

I reduced the advance to get rid of the pinging. Need to see if this makes any difference.

Stephen

Hasita gotta the Balance Tube itted between the intake manifolds , ? :lol: :shock:

Outside of that , a plastic tube , to your ear , lets you listen for sucking noises , where they should & shouldnt be .
ALSO the silly carbs arnt supposed to entirely sit on the manifold hard , the ' O ' Ring is Supposed to Seal It . :lol: :shock:

Whacking the two carb bolts down hard isnt a good idea . Decent lock nuts ( Nyloc ) please .some leave say 5 thou gap at the flanges .

Id start with the idle screws out far enough to let the carbs shut entirely , & set the cables to gt both slides at the top even , W F O ish .
Fingernail at the top of the carb bore , checking theyre both just ' Spot On ' .
And sit down quitely with it and see if it starts trying to let you know things.

:mrgreen:

I thought I'd report on my efforts to get rid of the rough running at low load, partial throttle.
Because the carbs are virtually new, I decided the needles and jets were not worn.
And because the bike did not display these issue when cold and only when hot, I decided it was not a lean issue from leaking manifolds or carbs.

I could hear pre-ignition when the engine was warm and loaded up. So I retarded the timing slightly (3 degrees or so, by eye).
Result? No more pre-ignition. No more kickback on startup. Better idle. And best of all, the rough running issues at low load partial throttle are gone!

Stephen Hill

What grade gas are you putting in?

Premium. 91 Octane. In Canada. I think the octane numbers don't mean much anymore. But it is the best stuff I can get.

Stephen Hill

Ya, that blows. They kinda need 93. But if you tamed it by dialing it back some. You should be ok.

Is you head a RH4?
If so check for cracks in the intake port area.

Gosh darn Stephen my Trixie has been slowly developing exactly your symptoms which confused me thinking all the things mentioned prior, ugh and finding them a few times, but non of that is currently present so looks like I've learned again off more seasoned riders... Being a cute gal and knowing it - likely is saying - yeah but what have ya gotten me recently...

This is not an issue for 99% of my riding but today for first time naked in a city as part of the parading it was annoying not to have the beautiful Norton roll off sound and soft pitter patting of light throttle peashooter heaven, with all eyes/ears on you know who. So will let ya know real soon, right after I clean plugs and dial floats back in, if that don't do it, time to toss the race tapper needles 15,000-ish mile used ones. Its very similar to air leak of loose bowls I though for sure just an external nip up - but no not on my Norton, all the way in and back out or just foregetaboutit.

One little complication in Trixie's Amals is I finally got around to filing a 16th inch slot in spray tubes a few days ago as so easy to do but that's when I most noticed its jerky-fart back surging onset, + maybe a bit more off idle response, so will first try just dropping needles before acting like more responsible owners. WAIT A FREAKING MINUTE*

It just struck me, it definitely was more off idle low down response, just realized I almost pulled throttle only wheelies in today's blip up zoom up parading. Seemed more prone to spin in THE Gravel too, but I'm getting more comfy riding it daily = might just be me.

On this light throttle note, its one of the most pleasant things at rallies to hear/feel the soft pitter patts of even firing lolly gagging Commando all over the place and I was encouraged to let Trixie idle with puff balls hitting the lookers drinkers smokers 4 ft away from parking line. Impossible to appreciate in the Big Ole Hog Coughing Uneven Bangings. You can not tell the difference between a Hog and a Chev V8 with glass packs, oh i've tried for years, but everyone seems to imprint on unique 360' peashooters, that do carry about as far when on Throttle.

Gosh darn Stephen my Trixie has been slowly developing exactly your symptoms which confused me thinking all the things mentioned prior, ugh and finding them a few times, but non of that is currently present so looks like I've learned again off more seasoned riders... Being a cute gal and knowing it - likely is saying - yeah but what have ya gotten me recently...

This is not an issue for 99% of my riding but today for first time naked in a city as part of the parading it was annoying not to have the beautiful Norton roll off sound and soft pitter patting of light throttle peashooter heaven, with all eyes/ears on you know who. So will let ya know real soon, right after I clean plugs and dial floats back in, if that don't do it, time to toss the race tapper needles 15,000-ish mile used ones. Its very similar to air leak of loose bowls, so thought for sure just an external nip up would do it again.

One little complication in Trixie's Amals is I finally got around to filing a 16th inch slot in spray tubes a few days ago as so easy to do but that's when I most noticed its jerky-fart back surging onset, + maybe a bit more off idle response, so will first try just dropping needles before acting like more responsible owners. WAIT A FREAKING MINUTE*

It just struck me, it definitely was more off idle low down response, just realized I almost pulled throttle only wheelies in today's blip up zoom up parading. Seemed more prone to spin in THE Gravel too, but I'm getting more comfy riding it daily = might just be me. Not holding throttle as nil room too but ON/OFF in a rhythm wanting more to whiplash neck then front started to come right up. I'm wheelie shy so Must practice that some more while saving for the gear box wear.

On this light throttle note, its one of the most pleasant things at rallies to hear/feel the soft pitter patts of even firing lolly gagging Commando all over the place and I was encouraged to let Trixie idle with puff balls hitting the lookers drinkers smokers 4 ft away from parking line. Impossible to appreciate in the Big Ole Hog Coughing Uneven Bangings. You can not tell the difference between a Hog and a Chev V8 with glass packs, oh i've tried for years, but everyone seems to imprint on unique 360' peashooters, that do carry about as far when on Throttle.