Another Carb Thread - I'm Sorry

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Eric Bauer

Eric Bauer

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I apologize in advance for another carb thread. '73 850. The bike has been time recently, valves checked, etc. Bike has since I've acquired it always run a little lean on the right hand side. I have a Colortune and can get it to a good spot, but at that point the air screw is maybe a quarter turn from all of the way in. The left side is normal. When I tried a carb synch they were way off, with the right side (one with problems) having a much higher reading and you have to idle the two carbs to almost 1,800 rpm to get them to synch on the digital manometer. Now the issue is that I can get it to idle by bringing down the throttle screw (when the bike is warm after a decent ride). But then if I take it for a ride the next day it idles very high again once it warms up. Did the same thing next day and it goes back up to 1,500 rpm when hot. A little bit perplexed. Both sides are set perfectly from the color tune perspective (even though the right hand carb screw is way too far in).

I have cleaned out the right hand carb pilot jet with a #78 drill bit and have also checked for air leaks (spraying with carb fluid on both carbs and there are no smoking guns there. Any thoughts? Thinking the right hand carb might just be shot (slide or otherwise), I believe it is, if not original, very old. Any thoughts? Thank you.
 
I'd suggest to make sure both throttle needles are in the same position in their clips, and that the
main jet is tight in it's holder. Confirm both chokes are wide open as well. Good luck and don't give up.
 
The pilot mixture screw controls the air, so only needing 1/4 out of air for a correct mixture says the pilot fuel circuit is not supplying enough fuel as so little air is needed. As the engine does not come good until 1800 rpm also indicates the pilot circuit is not working.

Another Carb Thread - I'm Sorry


So if you have probed the pilot bush with a #78 drill then there must be a blockage somewhere else in the pilot fuel circuit, so use the above diagram to follow the fuel route and confirm all is clear. Other possibility is an air leak into the pilot fuel circuit, make sure the plug is not leaking air where its marked 'Gas side is plugged', a smear of silicone goo over the plug is a short term diagnostic fix.

Full info here on tuning of Concentrics

http://www.jba.bc.ca/Bushmans Carb Tuning.html
 
Have you checked for an air leak downstream of the right carb ? Cracked manifolds are not unheard of.
 
I stuck with my original carbs and must have taken apart and poked the holes at least 6 times. If there is gunge in the chamber then poke all the holes and flush with carb cleaner aerosol from each orifice while blocking all but one of the others in turn.
Bit like a Chinese finger puzzle!
Poke. Flush. Repeat!

I have found this tool to be useful. The pokers are removable from the handle and there is one nearly perfect for each hole. It is available on eBay and loads of other places. Apologies if everyone is already using these.
Amazon product ASIN B07F7Y9NMW
 
SteveC said:
I stuck with my original carbs and must have taken apart and poked the holes at least 6 times. If there is gunge in the chamber then poke all the holes and flush with carb cleaner aerosol from each orifice while blocking all but one of the others in turn.
Bit like a Chinese finger puzzle!
Poke. Flush. Repeat!

I have found this tool to be useful. The pokers are removable from the handle and there is one nearly perfect for each hole. It is available on eBay and loads of other places. Apologies if everyone is already using these.
Amazon product ASIN B07F7Y9NMW
Click to expand...

It looks good to me - I'll try it!
 
What is really perplexing to me is why I can set the air screw mixture ok (at least at idle) with the colortune and set the idle properly when hot one day, but then after a ride from cold the next day it runs right back up to 1,500 . That is bizarre to me.
 
Perhaps a bit of crud moving around in the pilot chamber?
The vacuum of idle sucking it into a hole but not blocking exactly the same each time?
I had similar in that I would set idle and after a couple of blips of the throttle the idle would vary.
After repeated prodding and flushing, whatever was in my pilot chamber must have cleared. The carbs then set up nearly perfectly at 11/2 turns!
 
SteveC said:
Perhaps a bit of crud moving around in the pilot chamber?
The vacuum of idle sucking it into a hole but not blocking exactly the same each time?
I had similar in that I would set idle and after a couple of blips of the throttle the idle would vary.
After repeated prodding and flushing, whatever was in my pilot chamber must have cleared. The carbs then set up nearly perfectly at 11/2 turns!
Click to expand...

It must be something odd like that - I am wondering if my slide is shot and when the carb body heats up and expands (at varying rates) it causes the slop amount to change causing an air leak.
 
Eric Bauer said:
And on a side note, when does one choose just to buy new carbs?
Click to expand...
When the slides are loose in the body and the body is showing signs of being worn, originals suffer badly as the slide and body are made from exactly the same material which is an engineering no no as it promotes galling. Hence the Premier slide is anodised so making it a different material from the body.
 
Eric Bauer said:
It must be something odd like that - I am wondering if my slide is shot and when the carb body heats up and expands (at varying rates) it causes the slop amount to change causing an air leak.
Click to expand...

Varying idle speed with temperature is a classic symptom of either worn slides or an intake air leak. Mine is acting up right now in that when hot the idle goes up and I've not been able to identify the cause. Gonna replace the manifolds just because it's the only thing I haven't done yet.
 
When I first got my current Commando, I did buy new Premiers.
One was perfect and the other just would not idle properly. At the time I kind of assumed that it wouldn’t be a new carb causing the problems, so went through all the usual stuff. Checking compression, changed coils, spark plugs, earthing. After a month or so I send it back to Amal for testing and repair. They found that the brass plug that seals the pilot chamber was not sealing properly and as far as I can tell they tapped it in a bit more with a hammer!
The trouble was it took them a month to do that, during which I continued fiddling with the old carbs, using info from this forum and NOC and actually got them working.
I also preferred the colour of the old carbs. I found that the pot metal of the Premiers was a little off from matching with the manifolds and engine - just my opinion.
Anyway, I sold the Premiers on ebay and replaced the slides with the newer version and replaced the pilot needles, o-rings and floats.
Runs pretty good now with a reasonable tick over.

I don’t know if there is a test to prove if the slides are leaking.
Would blocking all of the holes in the top of the carb prove it? Shoot me down if this is stupid!
 
SteveC said:
When I first got my current Commando, I did buy new Premiers.
One was perfect and the other just would not idle properly. At the time I kind of assumed that it wouldn’t be a new carb causing the problems, so went through all the usual stuff. Checking compression, changed coils, spark plugs, earthing. After a month or so I send it back to Amal for testing and repair. They found that the brass plug that seals the pilot chamber was not sealing properly and as far as I can tell they tapped it in a bit more with a hammer!
The trouble was it took them a month to do that, during which I continued fiddling with the old carbs, using info from this forum and NOC and actually got them working.
I also preferred the colour of the old carbs. I found that the pot metal of the Premiers was a little off from matching with the manifolds and engine - just my opinion.
Anyway, I sold the Premiers on ebay and replaced the slides with the newer version and replaced the pilot needles, o-rings and floats.
Runs pretty good now with a reasonable tick over.

I don’t know if there is a test to prove if the slides are leaking.
Would blocking all of the holes in the top of the carb prove it? Shoot me down if this is stupid!
Click to expand...

My T100R daytona has new basic concentric carbs on it (not premieres) and it is so much easier to tune and idles so much better than my Norton or my T150V that have the older originals still on so I'm guessing there's something there...
 
Eric,
Worn Slides would do it. Miss matched float height as well... judging from your comments on the Idle screw. 932 original Amals wear the most due to galling between the similar pot metals of the slide and the body @ less than 10k miles on the clock.
If the slides get worn then so does the needle jet from the needle rubbing against the jet due to the slide rattling back and forth in the carb body.
with the air filter removed, raise the throttles slightly (less than 1/4 throttle to get your fingers under the slide) and see how much back and forth movement you have between the slide and body. If there is quite a bit , i.e. 0.020-0.030 Inch then these need to be re-sleeved or just buy new. The Premiers are the way to go, IMHO if you want to keep it looking stock with the stock ham can air filter.
Cheers,
Tom
 
CanukNortonNut said:
Eric,
Worn Slides would do it. Miss matched float height as well... judging from your comments on the Idle screw. 932 original Amals wear the most due to galling between the similar pot metals of the slide and the body @ less than 10k miles on the clock.
If the slides get worn then so does the needle jet from the needle rubbing against the jet due to the slide rattling back and forth in the carb body.
with the air filter removed, raise the throttles slightly (less than 1/4 throttle to get your fingers under the slide) and see how much back and forth movement you have between the slide and body. If there is quite a bit , i.e. 0.020-0.030 Inch then these need to be re-sleeved or just buy new. The Premiers are the way to go, IMHO if you want to keep it looking stock with the stock ham can air filter.
Cheers,
Tom
Click to expand...

Many thanks Tom - I ordered a new set of Premieres. In the mean time I will remove the float bowl etc and give her one last go on all of these excellent recommendations and see if I can fix before the new carbs get here. If not, I should hopefully be all set anyway. The bike has around 12,000 miles on it so being worn makes sense to me.
 
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