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

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gone fishing .
Hobot will take over .

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Interesting read ludwig. I can not really comment on the experiment, but from memory I can add this about silencers.
My MK3 came to me in 1975 with black caps. One of them split at the taper and started to peal itself open shortly afterword. This was before the kind of parts support we have today and not even knowing if something else was available, I got new mufflers from a dealer (Ross-sealy racing, in m Milford NH). I requested pea-shooters from an earlier 850 and the Norton items I got where not of the straight through design. About 3-4 inches in from the end they in fact had a plate blocking them up quite well but with a space to let exhaust flow get around the plate and out as well as three rows of small louvers too. AND they were as quiet as the bean cans!
Getting back to the spray tube though, Is it possible that 850's all did come with the notched type because their stock silencers were all very restrictive whether 'bean-can, or 'pea- shooters?

Rowley recalled that when testing an 850 with US silencing , snapping the throttle wide open in the higher gears at 40 mph (64km/h) , acceleration was virtually zero .
"At the end of a measured quarter-mile it would only be doing 41 mph ( 66 km/h) .."

My memory is a little dusty but this test is not be too far off for me to believe
I had a new 73 850 in 73.
The Pea Shooters had screw in mutes, 260 mains and am reasonably sure a full spray tube.
Snapping the throttle off idle had a HUGE bog.
I even brought up at the 500 mi service. Said it was normal and needed to be rolled on.
Had to develop a roll on technique.
Here's my hypothesis:
At 40 MPH in 4th with a 21 countershaft, I believe the engine is barely above idle.
Snap the throttle and hold, it goes into bog mode. The reason it doesn't die is it's forward momemtum from 40 MPH, just keeps chugging even picking up a MPH.
When a bog a hits most people roll off and reapply, at least I do.
He's snapping and holding it, not allowing a recovery.

Though I never did a test like that. I did work on it to clear it up.
The mutes fall out on their own.
Next, Full Dunstall exhaust.
Dropping the gearing helped the most as the engine is further up the range, less bog, but still there.
Not sure if a stepped tube would have helped with this. would have been nice to find out.

Memory serves me the smaller mains came with the Bean Cans and restricted air box.
Always figured it was the air box that passed less air for the smaller mains.

Would be interesting to see back to back tests with and without cut away with the correct needles of course.

Pretty soon someone will come on with mathematics and formulas worthy of Einstein, but in the meantime, I think the restrictive intake and exhaust coupled to make necessary jets of only 230 on the MK3, maybe other 850's too, but at low revs, whacking open the throttle reduced the air velocity through the carbs enough to prevent good draw through those smaller jets . Perhaps 260's would have avoided this but they were too big for WOT so a compromise needed to be found. Seems to me spray tube notched on the downstream side would add some extra vacuum at those low revs, eliminating the bog. Just attempting to think like a carbureted engine here.

Basic physics suggests that the cut away jet would have a stronger vacuum at the hole, ergo, a smaller main jet to compensate and give the same mixture, everything else being equal. JMWO :idea:

Biscuit said:

I think the restrictive intake and exhaust coupled to make necessary jets of only 230 on the MK3, maybe other 850's too,

Click to expand...

850 Mk1A and Mk2A models had already been in production for over a year, with black cap exhaust system, black plastic airbox and running 230 main jets, so if there was a problem then it must have been apparent well before the introduction of the Mk3.

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They were straight through like 750's. The mutes were simple ring inserts with a set screw.
Gave a slight pfst sound. They invariably rattled loose, I took them out. Ran a little better.
I think it shows them on the original parts fiche.
The Mark II Pea Shooter had the end molded over. Think of that molded end as removable.

ludwig said:

Was the mute in the US version removable ?

Click to expand...

According to Service Release N2/11, Feb. 1973, concerning "Achieving legal noise levels" on the 750 models, it appears to have been the dealer's responsibility to fit mutes in the US states that required them, the dealer was also instructed to change the main jets to 220 and lower the needles. Mutes and alternative jetting were apparently included in the crates.
Presumably the practice of dealers fitting mutes "as required" continued unchanged after the 850 models were introduced?

http://www.rgmnorton.co.uk/buy/muffler- ... e_1292.htm

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concours said:

Basic physics suggests that the cut away jet would have a stronger vacuum at the hole, ergo, a smaller main jet to compensate and give the same mixture, everything else being equal. JMWO :idea:

Click to expand...

We don't entertain suggestions.

ludwig said:

Altough this instruction was for 750's , could that mean that Rowley used smaller jets than 260 ? .
That would make the frase : " they arrived at a situation where the main jet size could be greatly reduced .."
even more mysterious ..

Click to expand...

Somehow I just don't think we are getting the full story? Whether this is down to Bob Rowley's recollection of events or Mick Duckworth's editing remains to be seen? :?

ludwig said:

Looks like that 'mute' is a simple ring to reduce exit diameter .

Click to expand...

According to Service Release N3/11, June 1972, the 063763 mute had a larger 1.0" outlet and replaced the previous 061984 3/4" outlet :shock: mute "In order to improve the acceleration characteristics of 1972 Commando models fitted with Combat engines, where silencing mutes have been fitted to the mufflers,...."

I hope the text below is helpful regarding jetting for mute removal (for 1971 whistlers in 1973).
It is from my 1973 "Chilton's Norton 750 & 850 Repair & Tune-up Guide - 1st Edition"
Ta.

ludwig said:

Rowley recalled that when testing an 850 with US silencing , snapping the throttle wide open in the higher gears at 40 mph (64km/h) , acceleration was virtually zero .

Click to expand...

Rather vague title here, it must be said = no mention of stepped down spray tubes for the Amals ??
Gunna be tough to find in the search function.

As on your 1st post at this, again I'd question the use of those words "US Silencing" - is that entirely refering to the peashooters with mutes.
Nice work from LAB etc if that is the case.
They were the stock silencers for the Mk1, would have been exported all over the world, all markets.... (??)

I'd comment that I had a stock Mk1 850 like that back in the late 1970s, 304xxx, with mutes and internally fluted peashooters.
The mutes had about the 1" diameter outlets, were just a thick short chunk of steel tube.
I had much dirt roads to ride on back then, and that bike could seriously spin its back wheel if you opened the throttle hard.
If you seriously tried, it could spin the back wheel in all 4 gears up to about 70 mph - had an airstrip to try this on, it was ~2000 yds long...
Avon roadrunner, rounded profile. The K81 was more triangular, liable to go sideways if you tried this.

With the mutes removed (easy to do, 1 screw, hook them out) it was considerably more rorty, although whether the acceleration was any better was diffficult to judge. Without some way of actually measuring it.
I would not have jetted it down to fit the mutes, only briefly tried them.
With the mutes installed, 850 was almost as quiet as the 1000cc BM I also had at the time.
Didn't seem to greatly affect the performance - but again I didn't time it, or test it for top speed.

Still my contention that the stepdown spray tube solved all this by the time the 850 was in production,
those Amals already had them. (from new, all of them - from what LAB posted on the earlier thread ?).
Maybe Mr Rowleys problem was in pre-production testing, with the earlier smaller version of the mutes. ??

Let us not forget either the factory hack hotrodded 850, tuned by Mark Baker and ridden by Dave Rawlins
that did 12.x sec 1/4 miles and 2 way average 143 mph at Elvington airport international speed meet in 1973.
This was a fairly stock looking Roaster bike, so someone at the factory knew how to make them go.
Even if they weren't quite as the parts book....

needing said:

I hope the text below is helpful regarding jetting for mute removal (for 1971 whistlers in 1973).
It is from my 1973 "Chilton's Norton 750 & 850 Repair & Tune-up Guide - 1st Edition"

Click to expand...

Whats on the previous page to that - some sort of size mentioned. ?
Those mutes would seem not to be the removable type - boring directly into the muffler outlet ?.

That drawing looks somewhat like the aftermarket short meggas you could buy then - with the alloy end caps.....

Ludwig, If you get in touch with Mr Rowley, it would seem to be of some importance to ask when that 40mph 1/4 mile incident took place.
Was this testing production bikes as they rolled out the factory, or testing of the prototypes, pre production. ?
Somewhat different scenarios....

Can't help thinking the customers would have noticed 40 mph 1/4 miles....

For what it's worth....
I was peripherally involved in the testing of the first "peashooter" silencer in 1970, and drew the production drawings. At that time the most arduous noise laws were for California. The mute was the only way that a silencer which suited the rest of the world could be made to pass the rules for California. Despite rumours to the contrary there were many hours spent testing. The tests were done repeatedly until results could be safely repeated. In the case of the tests I'm talking about, an old disused WW2 airfield was used, because it was flat, had no trees, bushes etc, no buildings. Reflections off of almost anything could make the results inconsistent, and therefore hard/difficult to interpret. At the time (mid 1970) the mute was the ONLY way to get a Commando to meet the Californian noise requirements without starting again on the muffler.

Sorry if I have gone away from the OP, but it seemed relevant to me.
cheers
wakeup