Bean Cans vs. Peashooters on '77 MKIII

Hello forum members,

as I am new to the forum let me shortly introduce myself. My name is Thomas, I'm a 57 years old mechanical engineer from Germany, married and I have three daughters - and some motorcycles. One of them being a recently acquired 1977 Commando MKIII Electric Start with 7.500 original miles on the clock.

For reasons of sound, performance and looks I want to replace the bean can mufflers to roadster type (look-through) peashooters. At the same time the air-filter box will be replaced by a K&N filter unit, fitting to the stock Amal Carbs. There is no connecting tube between the two exhausts.

My question is: do I have to adjust the carbs in terms of jets and needle position, to get the whole ensemble running smoothly? Do you have any other suggestions or comments to these exchanges?

Thank you in advance for any advice.

Thomas

Welcome!!!
Or is it Wilkommen?
850 may be a little lean w/ pea shootters due to 107 needles and jets.
If Amal 32mm all else should be ok.
Try the peashooters and see how it responds.
If a liitle lean on the plugs trying closing down the air screw.
If your some where between 3/4 and a half turn open I would change the needle and jet as a pair to 106.
The main jet should be fine as I thing if my failing memory serves me a peashooter Norton ran a 220 main and the 850cc
ran a 260 main.
Hope this helps.
Alsways good to ask first, start cheap and work up to expensive.
By the way you will lovethe sound :wink:
Marshal

Per the Norton workshop data, your bike should have originally had 230 main jets; the earlier (peashooter equipped) 850s had 260s. All other parts in the carbs were the same so IN THEORY all you have to do is install 260 main jets and possibly move the needle from the top grove to the middle groove and you should be OK. I say "in theory" because there is no telling what some prior owner may have done to the carbs...

850 Mk III's (1975s) imported to the US had 220 main jets, 106 needle jets and cast-in-place pilot jets. When I went through mine I raised the CR to near 9.8:1, installed a Web Cams 12a and had the exhaust ports relieved. I started with 260s (in Amal 932s) and have worked down to 220s. I installed chromed brass slides with #3 cutaways, and will be going back to the "factory" speced #3.5s next time I do a valve adjustment; I have yet to see any signs of running lean and 'am getting low to mid 40s for MPG.

I'd like to hear from anyone that has a different recipe or deeper thoughts on the subject, I keep thinking that I did somethimng wrong with the 220s running so well...

RS

MexicoMike said:

Per the Norton workshop data, your bike should have originally had 230 main jets; the earlier (peashooter equipped) 850s had 260s. All other parts in the carbs were the same

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The MkIA, IIA and MkIII model carbs would have had the 4 ID ring needles and stepped spray tubes originally, to match the black airbox and annular discharge blackcap/bean can silencers/mufflers. Changing the exhausts and airbox from the original A spec. could require the needles to be changed to the 2 ID ring "4-stroke" needles and flat topped spray tubes.

Well I'm really confused...

The Norton workshop Manual for the 73 750/850, under "carburetor 850" shows a 260 main and needle in top position.
The Norton workshop manual for the MkIII shows a 230 main and needle also in the top position. All other carb parts are the same per the workshop manual. So they WEREN"T as stated? I thought the stepped spray tubes were in the early 850's - my 73 had them and had the "old" air filter and peashooters...

MexicoMike said:

Well I'm really confused...

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Well, you wouldn't be the only one, as I was also under the impression that all 850s had the stepped spray tubes at one time, however other members hold a different view, and the topic has resulted in a fair amount of previous discussion, a search on spray tubes yielding 4 pages of results.

search.php?keywords=spray+tubes&terms=all&author=&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

But whichever type an owner finds in his 850's carbs I believe it's important the 4 ID ring needles are used with the stepped spray tubes and the 2 ID needles with the flat topped ones.

Many thanks to all of you for your valuable comments. After dismantling of the carbs I will know more about their "inner quality" and I will go on step by step with changing the needle and/or its position and the jets. I will keep you informed abaout the results.
Additionally, another question came to my mind: which role has the connecting tube between the exhaust tubes? Does it have an influence on the carb adjustments? My "bean-can-machine", originating from the UK and not the US, does not have this connecting tube and I am not planning to use it with the peashooters.
Best regards
Thomas

norton-racer said:

Additionally, another question came to my mind: which role has the connecting tube between the exhaust tubes? Does it have an influence on the carb adjustments? My "bean-can-machine", originating from the UK and not the US, does not have this connecting tube and I am not planning to use it with the peashooters.

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All 850s are supposed to have had the cross tube exhaust system-and its purpose was to reduce exhaust noise (which it does seem to do), so any MkIII exhaust system certainly should have had it originally, but it doesn't seem to have any effect on carb settings if it's there or not.

FWIW, crossover tubes on V8 car engine exhaust increase power as well as reduce noise - a win/win deal all the way around. I have read that they did the same on the Commando though the crossover tube connections routinely cracked and are therefore commonly replaced with the 750 exhaust.

I looked into putting the oem 850 exhaust headers on my 850 Commando (PO installed 750s) but they are difficult to come by. The two into one into two exhaust would be excellent IMO and they are available but they require the centerstand be removed and they reduce ground clearance to the point of being unworkable here (Mexico).

MexicoMike said:

FWIW, crossover tubes on V8 car engine exhaust increase power as well as reduce noise - a win/win deal all the way around. I have read that they did the same on the Commando

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Yes supposedly, and again, this has all been discussed at some length in the past (use the search?) and the general consensus of opinion (along with ludwig's valve) is that the tube seems have no noticeable effect on power output, and in any case most owners prefer to fit the unbalanced pipes.