carb woes

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May 4, 2006
this forum is so cool. i wish i had something to offer thus far, yet i am really new to norton ownership and feel like a sponge. i have spent hours reading the archives and have learned much. so....i was running rich and fouling my right side plug. (i live at 5500') i had put 240 main jets. i stuck a 220 main jet in the right side and it quit fouling the plug. so i decided to rebuild the carbies, change the needle and needle jet with stock replacements (# from my 850 parts manual) and make sure the pilot jets were not blocked. it ran and idled fine and down the highway i went. first stop the idle crept up to 2 grand. a rev seemed to bring it down. repeated this at all stops. the idle seemed right when i first stop and then slowly crept up? it ran great at throttle, and seemed to bog abit at slow speed. (it ran just about perfect with the 2 different jets, but it just seemed odd to leave it that way.) oh yeah, i also changed the mains to 230. after the 5 mile ride i checked the plugs, and both looked sooty :cry: my 850 has 11,000 miles on it. the carbs looked pretty good inside, the usual wear from pot metal i would guess. from my reading on those great carb sites it seemed that the needle jet wears by 10,000 miles so i thought i would try to solve the problem and not mask it with smaller mains. the only other thing is the o-rings for the air screws from the re-build kit seemed too fat and not a good fit. i will change those. am i missing the obvious here? thank you all for all the great info on this site! jerome
You have read well.Sounds like you may be seeing what happens when the cables get old and misrouted. If there is too much bend were the cables enter the carb some force is transfered to the slide and it will bind from it. You may need to pull the slide out for a good polishing with Autosol. Be sure you don't have distorted carb bodies not so common on Norton's as on other users of the Amals. Sounds like you may need new cable's You can make your own and get much higher quality. Buy yourself a tech digest from INOA lots of good cable routing stuff in there.
thanks norbsa, i will look into the cables. the fact that it starts off with a lower idle and creeps up, could that still be cables? also any ideas why it may be running rich again? thanks much! jerome
What kind of ignition? Stock points or electronic?
Classic case of sloppy auto advance and one set of points out of wack.

Double check for vacuum leaks.

What are your slide sizes? Should have 3 1/2.
Probably have to lower the needle.
Verify needles are the same and correctly matched to the spray tube.

Synchronize the slides.

The rich issue has been covered, but the idle jumping up after the engine heats can be caused by air leaking past worn throttle slides. If your engine has more than 20,000 miles it's very likely the slides are worn out. If this is the case have the slides sleeved and the carb bodies bored to match. There are several sources to have this done.
thanks for the input. lets see, i have a boyer electronic ignition. the needles are clipped at the top, down as far as they go. i hope to get to winter before i re-sleeve, i do suspect wear. i changed the o-rings on the air screws and re-adjusted carbs. the high idle is gone and seems to be running okay at low speed and idle seems okay. i need to get out for a good full throttle run and see if it really running rich. every day is an adventure :eek: thanks again and all advise heeded. jerome


Make sure the timing is correct also. Maximum advance on a Commando should be 28degrees BTDC. Check it with a strobe.

Your needle position doesn't sound right. I don't run an 850, so could be wrong. Suggest you go to the manual and make all settings per factory. That usually works best for me.

Moscow High School class of '71.
Sorry David ,my Boyer instructions tell me 31 deg. before t.d.c., i run my needles in the second slot after much experimenting.
jerome said:
lets see, i have a boyer electronic ignition.

So obviously it' not the AA.

Timing is 28 for points, 31 for Boyer.

Sounds like your making progress.
thank you all again! my workshop manual says top for needle position. this weekend i will check the timing for 31 degrees. i know it is not retarded for my foot feels great. i will also syncronize the throttle cables, for i was abit lazy and did not do this. it still starts first kick, cold or warm but seems to have a vibration at about 3 grand (a shake like an improper air mixture setting? also feel it under compression while comming to a stop. norty mick, how did yours run at the top needle groove? what improvements did you find on the middle groove? it seems the elevation difference may be a factor here. will let you all know when and how i get it sorted. cheers, jerome
Hi Jerome,
She coughed, spluttered and backfired on the top groove = too lean, the middle groove certainly ran a lot smoother, after setting the air pilot and idle screw together to the best running settings she purrs like kitty cat and roars like a lion when you open the throttle. I just set the timing two weeks ago to 31 deg b.t.d.c., before i did that it to missed and popped around 3 to 4000, now you can just open it up to 5 grand and all it wants to do is keep revving :lol:
Good luck, Mike
jerome said:
it seems the elevation difference may be a factor here.

The Amal tuning manual recommends a reduction in main jet size of 5% for carburettors used constantly at altitudes between 3,000 and 6,000 ft and a 4% reduction for every additional 3,000 ft above 6,000 ft.
A BMW forum that rivals this one???

hello nortonheads, hoping this is not too inappropriate for this buddy has a 75 bmw. he is quite envious of all the incredible knowledge i am finding on this forum. (we are business partners so he gets a daily dose) he has not been able to find a good "airhead" site. anyone know of one? cheers, jerome
p.s. i am proud to say that there are close to a dozen nortons in and about my town of 3000!
I run a 1970 fastback with 80000 km on the clock I gave up playing with the twin amal carbs some years back, fitted a single amal Mk 11 34 mm and cured all my carb problems. the bike is used weekly and the carb does not go out of tune. had anyone else tried this route.
single Amal MkII

Hi Micheal,

Single 34mm Mikuni is more common, but yes the single Amal is a good choice. Single or dual carbs is a tuning decision the rider needs to make. For my enjoyment, I find the sleeved original 932 Amal MkI is the best choice. I put on less than 3,000 miles per year and like the performance of the twin carbs on my '72 750 combat engine. Idling isn't perfect, but it doesn't stall out. Anything above idle is smooth and the engine doesn't run out of steam at the top end as a single carb does.
Hi iif8ed . I read your comments. I find that up to 5000rpm there was no difference in performance and as most of my riding is done with a group this is sufficient for me. The slight lack of top end performance is more than made up for better gas consumptio, easy starting, no choking off and much smoother low speed running. The motor will pull well from 2500 revs in 4th with no problem. I am 68 years old have had this bike for 15 years and love it. I started riding british bikes in 1954, and never stopped. the bike has Boyer Ign, 3 phase 180 watt alternator, belt drive primary(dry clutch), modofied brake master cylinder, sleeved down to give higher caliper pressure, and most important, oil cooler and filter. The bottom end of the motor has never been opened.
bye for now
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