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

Sounds terribly relevant.
If Bob Rowley was involved in testing early versions of the peashooters, his recollections of that 40 mph 1/4 mile could have been well before the 850.

BTW, those early peashooters you drew, did they have the straight-through design with little triangular 'flutes' all up the walls,
do you recall, or some other design of baffling. ??

Mufflers and carbs being at the opposite ends of the power production, but rather inter-linked.....

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The build up of the silencer/muffler was, from the inside, a fluted tube with three rows of flutes, the basic tube was about the same size as the inlet tube, it may even have been the inlet tube, this was wrapped with a single wrap of perforated metal, the gap between the perforated metal, and the external steel cones of the muffler were packed with (I think it was) 19oz of fibreglass wool. The purpose of the perforated metal was purely to stop the fibreglass wool blowing out the back too soon. You could see right through from inlet to exhaust. I seem to remember asking about what happened to the wool when the whole thing was welded up with the response "don't worry about it".
A couple of years later I had a Honda 500/4, with a 4/2 exhaust and a pair of these silencers. The wool didn't last very long, maybe 10,000 miles, then it sounded fantastic. I still can vividly remember riding for the ferry after the 1973 Bol d'or at Le Mans. Going through a small town, with high building on both sides of the road, and no one about. I got separated from my mate who was about 300 or 400 metres in front. Stopped for absolutely nothing at some traffic lights, when they went to green I gave some (a lot) of beans and the noise was amazing, reverberating around. Codge on the front bike said he could hear it.
cheers
wakeup

Although it seems that Bob Rowley must have been at NV at the same time as me I don't remember him. I left in November 70. The silencer testing was done by Brian Dolman, I think. There were two Dolmans in the Development Dept, one a development fitter, the other a test rider, I think Brian was the test rider. The other brother road raced a Greeves/Triumph.
cheers
wakeup

wakeup said:

The silencer testing was done by Brian Dolman, I think. There were two Dolmans in the Development Dept, one a development fitter, the other a test rider, I think Brian was the test rider. The other brother road raced a Greeves/Triumph.

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The Oct. '11 & Feb. '12 issues of Classic Bike Guide magazine both contain articles written by Brian Dolman about the development of the annular discharge "black cap" exhaust system.
He also mentions his brother, Les, who joined the company as a general test rider.

Well blow me down.
cheers
wakeup

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The Amal records show that the specification for the first 850cc carburettors was first officially written down on 5 December 1972. The initial specification called for the conventional four stroke needle and spray tube with a 290 Main Jet, .106" Needle Jet, a No 3 Slide with the needle clip in the middle slot. On the 21st December the specification was changed to a 260 Main Jet, 3 1/2 Slide and needle position of 3. The needle became the 928/104 which had first been designed in 1967 while the spray tube became the screened type 928/107, the drawing for which is dated 20/12/72 indicating it was a new part. Screened spray tubes at the time were more commonly found on two strokes and can confer different characteristics depending on height. Thus a Dell 'Orto two stroke carburettor with a 7mm high screen will produce a richer mixture throughout for a particular Main Jet size while a 9mm height will give a weaker mixture at lower speeds and a rich mixture at higher speeds.

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Great letters from MR Rowley. After reading them, something occurred to me regarding all discussions about the benefits or no benefits of notched tubes. In past posts, plenty of people have said it made no difference one way or the other to how their bike performed,
BUT since the notched tube was evidently introduced to improve performance with very restrictive mufflers, their benefit would be lost (or neutralized) when running straight through peas...Like perhaps 90% of us are.

Good point Biscuit.

A simple test would be on a dyno with an A/F measure. Conduct a few pulls on a dyno with cut away and without cut away tubes would be very telling.

Good stuff. Learning the inner corporate workings back when.....

Carry on...................

MichaelB said:

Good stuff. Learning the inner corporate workings back when.....

Carry on...................

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+1
Thanks for the reboot on this topic Ludwig. I think the highjacked version needs to go to the Pub :roll: along with many others that end up taking a South bound train.
Cheers,
Thomas
CNN

Its a shade unfortunate that Bob didn't put some dates to some of those things he mentions.
Especially that snap 40 mph test.

(P.S. Accelerating from 40 mph in TOP gear. ??
Riders in a hurry would still be in 1st....).

Ludwig, is that mute you show the early or late version ?
We note mention someplace that Combats "should use the larger mute"
That would have been a year or more prior to any 850 kerfuffles ??

P.S. Didn't the full throttle rollon at some point subsequently become a 2nd gear test - with noise requirements.
Thats where Nortons raised 2nd gear (ratio) in the gearbox.
The slightly taller gear restricted slightly the accleration - so it passed the noise tests
??

I have read Bobs excellent message a few times now.

His conclusion seems to be that the stepped tubes are beneficial ONLY IF running with mutes in your peashooters AND accelerating in an impractical and counter intuitive way.

I would venture to suggest that neither of the above apply to the vast majority of current Commando riders!

However, there have been a few people post on here in the past saying that they wanted some way to quieten their Commando's. So knowing the mute / tube combo and history may helps those folks.

I've followed both the newer threads on this subject and read many of the links to older posts, mainly because I am interested in tuning issues and learning more about how to do it right.

With that said, I may be stupid, but this seems like a simple "breathing" problem caused by the addition of restrictive exhaust and possibly air intake.

Given that the Norton long stroke engine doesn't generate vacuum as quickly and smoothly as shorter stroke engines, this perceived "flat spot", would seem to support a lack of sufficient vacuum when the throttle is cracked open quickly. The fact that it was not overcome in 1/4 of a mile seems moot to me because if you hold the throttle in that position, it never develops any RPM to generate any additional vacuum. Given that the engine breathing is restricted in this configuration that seems to be a simple conclusion.

The carb modification (stepped tube) likely allows more fuel to enter under a lower vacuum, and thus "corrects" the problem although it seems like a bandaid to me.

I know on my 850 changing from Amal (purists please don't slam me here), to a Mikuni VM, made a difference in low rpm performance and throttle response. Changing to a Mikuni TM flat slide improved the performance even further because the TM provides fuel with less vacuum according to Mikuni. The idle is rock steady with the TM. So much so as to be noticeable. Throttle response seems quicker. Is it measurable? Steadiness of RPM and response is measurable, but it is really subjective to some extent.

I am not responding to make any claims or to change minds about which carb might be better, but make a comparison for the sake of understanding. Given that the vacuum or lack there of is possibly one reason why the stepped tube was a solution, the fact that a carb that needs less vacuum to perform seems to support this.

Fast Eddie said:

However, there have been a few people post on here in the past saying that they wanted some way to quieten their Commando's. So knowing the mute / tube combo and history may helps those folks.

Click to expand...

Indeedy, I have quietened down some very loud aftermarket mufflers on something else simply by fitting a short length of thin stainless tubing into the exhaust outlet - made it slightly smaller than the original outlet. Quietened it down quite noticeably.
Didn't check if it bogged down at low rpm giving it full throttle though, that didn't occur to me to try !!

Weird when some little thought through legislation creates problems out of nothing ??

Have 850 mk2, ham can, single pipes, JS 0 cam and lifter, new amals 260 jet AN peashooters
straight through. The bike is loud if you give it a handful. As the rpm rise it gets really loud and
high pitched.
Got a set of mutes from AN and it took a small amount of the higher pitched noise out. SMALL.
Performance may be reduced but it is equally small seems to me.
Be nice if someone did a dyno test.