MKIIA 850 carb needle: bean can vs peashooter

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Re: 850 carb needle: balance pipes

I went over ALL the posts twice and never saw anyone mention the reason I have long espoused.
BALANCE pipes on ALL oem factory 850........and it seems to be in the book.... ALL 850 have the cut spray tube and 4 groove needle
So I surmise the air box and mufflers and mutes are secondary tuning considerations.
 
Re: 850 carb needle: balance pipes

Rowley's 1/4 mile and the Magic Tube .

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(Edited)
A new acceleration test was introduced for USA and from memory the test was 40 mph snap open throttle in top gear, this was fine without the mutes fitted, but with the mutes ( small disk fitted to the end of the exhaust silencer), this caused a[...??????...] the muted condition, ..............................the final cure was to install a cut away spray tube, this allowed a much smaller main jet to be used, this in turn stopped the eight stroking when trying to pass the USA acceleration test, I think 40 to 60 mph in a certain time.

It is not difficult to see how this worked, FOR THE MUTED CONDITION

Most times with the mutes fitted we would snap at the 1/4 mile point at 30 and 40 mph and be doing the same speed, 30 or 40 mph at the timing lights, after 1/4 mile, of course, you could feather the throttle back and judicially pick up speed and improve, but the idea was that a person when accelerating passed a long vehicle could not be expected to intervene and should expect a solid reliable acceleration.

You may not be aware that AMAL did not set up the Carburettors for the trade, BSA, TRIUMPH, NORTON but development worked closely with Amal, and the responsibility for final sign off and payment was down to the Test and Development department of each of the firms, as Deputy group test rider under Bob Mann's we carried such responsibility........

One final thing that would vindicate the modification was the fact that we had over 150 bikes held up and not allowed in California test Authority, and after the modification, they and all subsequent Nortons passed straight in.
I am not sure if Alan Lines? of Amal is still alive but he would no doubt give ....chapter and verse on how and why the cutaway spray tube was beneficial to the MUTED CONDITION.

Regards
Bob Rowley
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Letter 2

" I remember being seconded by the Chairman to work direct at AMALS in Witton, Birmingham, working with Alan Lines? the testing was done on a short route around Witton cemetery ( outside not in ) the 830 cc had poor top end flow, the ports being fully machine as a cost cutting, earlier head having the port wasted down around mid point from carbs to valve, this was done manually and sometimes not very consistently at that, depended on the dedication of the man doing the fettling.

We had to initially raise to quite a large main jet out of all proportion to the increase in cc, now my memory is grey but the MJ increase from the 750 cc was around 40% in JET size terms.

The mutes was added as the legislators got wise to the industry tricks our shall we say ( allowed work around's ) like the heavier test riders conducting the noise test, filling the fuel tank to the brim, using Interpol 5 1/2 gallon fuel tanks all to slow the 2nd and 3rd gear noise test, snap open and snap shut at a line on the ground.

Even raising the 2 nd gear to get a better figure on the noise.

Things got so bad the bike would not accelerate at all and of course the Noise problem was cured no acceleration no noise ( joke ), this is why I believe the acceleration test was a requirement certainly for the USA West Coast.

Alan an I had a carburetor test stand at Amals ( basically a vacuum pump ) and we played around with trying to keep the fuel mixture high at the top end, but without richening the mid range, and sometimes totally 8 stroking the motor when we forced a WOT snap test, at 30 and 40 mph in top gear.

The fully cut away spray tube enabled a smaller MJ for example only say a drop of 40 to 50 but being an improved signal, so at the lower RPM Snaps the velocity was not that great and the fuel did not bog down the motor.

From memory we had a modified NJ counter bored and a 90 deg to jet axis small drilling to keep the fuel build up down to stop some hesitation.

Every night I had to report to Mr Poore? personally and you can imagine the pressure that we was under, with whole shipments of bikes held up.

We had the breakthrough and the run around the outer wall of Witton cemetery proved that it was worth a visit to MIRA to carry out some speed test and snap test.

I rang the works and arranged for Dave Rawlins and Dick Mills to meet us, after finalising the MJ and plug readings, I let David and Dick ride for confirmation as we did this in case I was willing it to work, David produced one of the fastest standing quarter mile runs for a std bike that afternoon even with the mutes fitted.
But as expected we gained a bit of noise back, but we sort of ignored that.

You can post the items so long as you please tidy up the spelling errors and adjust to make more readable. "

Regards
Bob
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Noted, dynodave, thanks – I've avoided the balanced pipes for purely practical reasons. The bike came without them, for one thing. But I don't doubt that they were fitted for a reason.

L.A.B., should I take it from what Mr Rowley wrote that the sectioned spray tubes are a hangover from the use of mutes? Or simply that a larger main jet remains a good idea? I've never seen a Commando with mutes (although that's not saying much), and I have a beautiful pair of muteless Viking repro peashooters returning to me from rechroming as I write ...
 
NortonMKIIA850 said:
L.A.B., should I take it from what Mr Rowley wrote that the sectioned spray tubes are a hangover from the use of mutes?
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Yes, that does seem to be the case. :wink:

NortonMKIIA850 said:
Or simply that a larger main jet remains a good idea?
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Regardless of the stepped spray tubes and 4 ID needles fitted to all 850s, the Mk1A/Mk2A models (and Mk3) had smaller (230/220) main jets than the Mk1/Mk2 models (260), so an increase in main jet size may well be necessary if black caps on an 'A' have been replaced with peashooters.



NortonMKIIA850 said:
I've never seen a Commando with mutes
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....And you're not likely to in the future! :) :wink: (Although the Mk2 peashooters had their outlets reduced to 1 1/8" but probably not many of those still in use now, either)

Edit: All 850s had '3 1/2' slides.
 
Fascinating stuff, thank you!

Now I'm wondering if Mr Rowley had anything to say about balanced pipes ...
 
NortonMKIIA850 said:
.......... I've never seen a Commando with mutes (although that's not saying much)
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I have one like new muted pea shooter on the shelf. Saw a real nice pair on ebay, a while ago , the seller wanted a lot of money, I him emailed about the mutes, didn't see to care. They do exist :?
 
One thing I never do is rip the throttle open and expect the motor to respond. I always feed the throttle on in one smooth firm motion. When I ride the bike, I am always thinking a very long way ahead.
 
Each to their own – I sometimes crack the throttle open. On the (mostly) congested roads where I live, if I see a chance to do a quick overtake then I'll often take it, and want the motor to give a lively response. That's on roads where the limit is 60 mph or lower though, and the motor's perfectly capable at those speeds. But I look a long way ahead too. That and developing what I'd call defensive attennae are probably why I'm still here today, after riding who knows how many thousands of miles on bikes on public roads since I was 17. That's not meant to be a brag by the way, just that I have to feel wide awake on a bike, and the more you ride the better your sense of what goes on – the stunts other road users pull, the blind hazards, condition and features of the road, things like that. That's what I mean by antennae. They're the best protection, near or ahead, better still than good brakes and tyres, although you need confidence in those too. On the other hand, I would agree that a smoother hand is more sustainable mechanically. I just want my bike to be capable of doing what it's supposed to. Cheers.
 
I would like more info about using Mikuni needles in an amal carb. Which needles? The 6DH3 seems to be the most popular, I use the 6F9, there are a couple of others. does the amal needle clip fit the Mikuni? Is it a straight drop in?
 
I use two 34 Mk2 Amals with methanol fuel. Mikuni number 6 needles drop straight in and I think that below 30mm in Amals, you would have to use number 5s. I didn't notice any difference in the clips Currently I am using 6Ds, - 6Fs are richer. But because of the methanol, I use 0.116 inch needle jets where you would probably be using 0.107 or 0.107 inch for petrol. My motor is on standard compression as you would use for petrol - if it was on 12 to 1 comp. , I would probably need 0.120 inch needle jets. The Mikuni needles are the same diameter as the Amal needles, in the region above the shoulder. However the tapers are different. To get a commando going really quickly on petrol would take a lot of fiddling and it would become much more sensitive to weather changes. I think most guys would not know if their bike was that bit more sluggish when accelerating up through the gears. In any case most would not have close ratio gearboxes anyway. On public roads, there is no need to go there.
There is one problem and it is unlikely to encounter it, if you are using petrol. It is usual to have the main jets slightly over-rich. If you are doing plug-chops and increasing the main jet size does not make the plug reading richer, you are metering off the tip of the needle. The answer is to recess the needle jets and meter a bit lower down, so the ends of the needles remain trapped and you don't risk hanging the slides up and fly off the road.

I don't know whether you noticed about that video I posted - round one of the Barry Sheene Memorial Trophy was run in the rain last year. There would have been a jetting difference on the faster bikes between the two rounds. I think some of the guys are still using GP carbs - it is not so easy.

https://www.youtube.com/watch?v=QP8WtUdYheA
 
I have a pair of MK2 amals from a 1979 Triumph. They were absolute pigs and responded to NOTHING until I completely blocked off all passages leading to and from the enriching circuit. All of a sudden it would idle and respond to the air screw. I took it for a short ride and finally, after several months of trying any and all recommended jetting, these carbs actually started to run lean. Further experimentation stopped there but I did make a tickler that fits in the old plunger hole.
What is the opinion of MK2 carbs if they are running right? Are they as good as Mikunis? Are they worth going through a lot of trouble?

I have been working on a Norton 750 with the full Steve Maney stuff, JS rods and pistons, and the JS stage one came. Stock head. The bike is running very well on a pair of concentrics but I feel it should have more zip when accelerating from 1500 to 3500. I have tried going to 107 needle jets with adverse effects, then raising the needle one notch, still no gain. It is running a pair of 3.5 slides, I am loathe to change them because it took so much effort to get it running this well. A stock commando with a stock cam delivers better power at low revs than this one. Does the JS rods and piston combo with the stage one cam weaken the low end till about 4500? this bike is very streetable and a very acceptable idle.
 
seattle##gs said:
I have a pair of MK2 amals from a 1979 Triumph. They were absolute pigs and responded to NOTHING until I completely blocked off all passages leading to and from the enriching circuit. All of a sudden it would idle and respond to the air screw. I took it for a short ride and finally, after several months of trying any and all recommended jetting, these carbs actually started to run lean. Further experimentation stopped there but I did make a tickler that fits in the old plunger hole.
What is the opinion of MK2 carbs if they are running right? Are they as good as Mikunis? Are they worth going through a lot of trouble?

I have been working on a Norton 750 with the full Steve Maney stuff, JS rods and pistons, and the JS stage one came. Stock head. The bike is running very well on a pair of concentrics but I feel it should have more zip when accelerating from 1500 to 3500. I have tried going to 107 needle jets with adverse effects, then raising the needle one notch, still no gain. It is running a pair of 3.5 slides, I am loathe to change them because it took so much effort to get it running this well. A stock commando with a stock cam delivers better power at low revs than this one. Does the JS rods and piston combo with the stage one cam weaken the low end till about 4500? this bike is very streetable and a very acceptable idle.
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I know this goes against the grain but when I was jetting a 990cc Triple, I assumed to raise the needles . The bike ran pretty good , but like yours didn't have the low down zip I thought it should have. Eventually I dropped the needles (to weakest position) and it was like a different bike, instant low down power and no bogging . It may be that the shite that passes as fuel these days is messing up carburation more and more. Might be worth a try.
sam
 
When the motor is sluggish through being slightly over-rich, you often don't know you have a problem until you lean-off the jetting and the bike goes much quicker. In the condition where the carburation is set up so the motor delivers it's best, it becomes much more sensitive to fuel changes and changes in the weather. So sometimes it is better to live with the slightly over-rich condition. When you buy a new bike, it is probably set up to handle the worst-case scenario.
 
Well here's a thing, for an update – it turns out my carbs already have 260 main jets! It also turns out that my air filter has completely disintegrated – I have the MKIII-style black box, the filter being oiled foam on a metal frame. I remember washing and re-oiling that filter not long before the bike went into storage many years ago, so hadn't bothered to check it until now. Silly me, maybe. Anyway, I have a new filter and foam seal on their way, I expect that might improve things, as I've effectively been running without an air filter. We shall see ... Incidentally, I did a compression test today, and got 169psi in the right cylinder, 168 in the left, so apparently no harm done. I'd guess that's excellent in fact, given the (I believe) 8.5:1 compression ratio. Curious but irrelevant fact – I notice from my records that, if I'd waited two days to do the compression test, it would've been 15 years to the day since I last did one on this bike ... :lol:
 
Ok, latest news, and maybe as far as I want to take this, unless someone else knows better – I installed a replacement air filter and foam seal from Andover Norton today, all oiled up nicely. There was quite a lot of crud to get out of the plastic airbox first, almost entirely rotted air filter foam, but with some grass seeds mixed in for good measure. All clean and nicely buttoned up now. I've just been for a really nice blast on the thing, and I'd say it's working as well as ever, but no more. This is with 260 main jets, unbalanced exhausts and Viking peashooter silencers - there's a misnomer if ever there was one, it does sound fantastic! I might see what happens if I raise the needles one notch, and then try retarding the Boyer a tad to see how the advance curve matches up at higher revs, but with those jets, a good air filter and (it seems to me) superb compression, I'm not sure there's much more I'm prepared to do. The motor was rebuilt only a few thousand miles ago (but many years) by Norman White, and he would most certainly have said if I ought to have a new or different camshaft. And naturally I've checked all the obvious stuff. In the end, it's a snorting Norton, and I love it! :p

p.s. If dynodave's watching, I heard tell over at the NOC forum that Andover Norton are testing a set of balanced pipes specifically for the MKIIA, if so and the planets align that is something that I'd certainly try one day!
 
The fact that per the Norton service manual, the factory dropped the jetting size from 260 down to 230 for the same engine/carb when the bean cans/airbox was introduced shows that either the mufflers or the airbox (or both) that were installed on the MkIII were restrictive compared to the earlier 850.

Balanced pipes add power and reduce noise. This was determined a long time ago which is why nearly all production systems that use dual exhaust use either an "H" or "X" crossover. The X has proven a bit more beneficial and there are plenty of objective DYNO tests around showing it. The Dunstall performance manual from back in the day recommended that as well. A year or so later, Norton incorporated it. So it works. Whether it's worth the trouble associated with the common cracking of the Norton parts, is a different issue. I installed a NOS balanced set on mine a couple of years ago, replacing the single pipes that had previously replaced the original balanced set. It has done fine so far here in Mexico on very bumpy cobblestone streets. Here's hoping it continues to do so!

FWIW, some years ago we picked up 15 HP on the DYNO on a Mopar 340 with headers/duals simply by installing a crossover and making an appropriate jet change in the carb to take advantage of the increased efficiency.
 
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