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

Onder said:

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.

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Hi Onder.
Your bike sounds like the ideal candidate from your description.
Would you consider rolling-throttle (then snap-open) dyno runs with and without the restrictors please?
I feel confident that forum members would contribute to the expense as 1 person = $$$$$$ or many = $ each.
Ta

Onder said:

single pipes,

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needing said:

Your bike sounds like the ideal candidate from your description.

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Single pipes though, note.
We wonder if the connected balance pipe may be somehow related ??

After all, Combats had the mutes available to them too.
Note that note somewhere warning to use the larger mutes in Combats, not the earlier smaller ones.
P.S. Anyone recall the mutes on a Combat ? Don't recall seeing this noted previously ?
Or is this post production, when later parts would have been retrofitted ...

Upon further pondering... I have a conspiracy theory to float here Ladies...

Is it possible that someone within US government deliberately devised a unique roll on test that particularly suited Harley's and particularly penalised just about everything else ... ??

Not impossible, madam.

One of the bike magazines noted that they had to adjust their magazine layout to suit the very low rpm that HDs can manage,
so the dyno chart would fit on the page.
Heavy flywheels in particular contributing to this.

Since the AMA did deliberately impose a 500 cc rule when racing against 883 H/D which lasted until the 1970s, the Regan bike tax on any foreign imports above 700cc and others I have not been informed about- it would not surprise me if this was another rule that favours the H/D company

Gone fishing .
Hobot will answer all your questions .

ludwig said:

What did I do wrong ?

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Well, pardon me for asking-but what has all this to do with the full-throttle-roll-on test conditions as conducted by Bob Rowley?

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ludwig said:

If the stepped spray tube was the breaktrough for their problem , then I would think it has a lot to do with it .


"Alan an I had a carburetor test stand at Amals ( basically a vacuum pump ) ...
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. " (quote)


If you want to set up a carburettor , wouldn't it be nice to know what the different components do ? .

Besides , it is the essence of this thread .

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My point is that you don't appear to be replicating the test conditions that demanded the changes in the first place?

All you seem to have done is measure flow at set throttle openings.

ludwig said:

Read the title .

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Why don't you go back and read what Bob Rowley is actually saying?

I run a MK2a 32mm concentric on a B44, I use a choke on cold and warm starts for reliable starts which may confirm Ludwigs findings. On the main jet with the MX cam and the port opened to 32mm but then tapered to meet the std port I run a 290 main with the std exhaust with mute and 310 without mute. Std B44 main jet is 240. So the effect found by Rowley seems to work only for very particular circumstances. No flat spots anywhere in the rev range.

I think L.A.B.'s point is that the issue was with a WOT, not fractional throttle opening.

What are the differences between the spray tubes with a WOT?

Dances with Shrapnel said:

I think L.A.B.'s point is that the issue was with a WOT, not fractional throttle opening.

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No, that wasn't my point at all.

L.A.B. said:

Dances with Shrapnel said:

I think L.A.B.'s point is that the issue was with a WOT, not fractional throttle opening.

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No, that wasn't my point at all.

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Well that was my point and apparently more than one person has missed yours, so please elaborate so we are not left guessing. :lol:

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ludwig said:

I believe that LAB's point was that I am hijacking my own thread .
Unless I overlooked it , I have not seen similar remarks from our moderator on the first thread I started about spray tubes , where the likes of Rohan an hobot are now happily and undisturbed chatting about diving Spitfires and Antonovs ..

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If that's the best reply you can come up with, then we might as well shut this thread down and go play on the highjack thread.

Gents,

Ludwig is making an effort at analytics in good faith and L.A.B. is pointing out that it is missing the mark but would be nice to get some specifics. My interpretation is it is a WOT throttle behavior we are trying to get to the bottom of, not individuals behaviors.

Apologies if I came across a bit cavalier or smug or whatever in my previous post but short of dyno pulls with exhaust gas analysis Ludwig is taking an analytical approach; I personally like that and encourage guidance.

Ludwig
Its an interesting issue you’ve raised and I appreciate your persistence with it on this forum despite the usual distractions and general off topic Hoo Hah. It was especially interesting to hear directly from someone who was there at the time – the horses’ mouth Mr Rowley.
I will offer a suggestion as to a direction or line of enquiry you might want to pursue as it might be rough guide to what was going on with Mr Rowley’s tests.
While the test was described as a snap roll on, the fact that the condition persisted for some time suggested a steady state condition, most likely a richness induced flat spot in the power curve. Two clues I have picked up on are the fact that it was effected by changes in the exhaust system and that the solution (partial? ) of the stepped spray tube was something Amal had previously introduced for 2 strokes. (It would be interesting to know for who and when the stepped spray tubes were introduced.)
Early piston port 2 strokes with expansion chambers suffered from peaky power bands due to their exhaust and intake resonances being out of phase with the engines requirements when the revs were outside of the power band. My guess is that the stepped spray tube was introduced to reduce the effect of intake reversion,( ie the double pass of the intake air drawing more fuel up the main jet and richening the mixture).
Four strokes, especially race engines with lots of valve overlap and tuned exhausts also suffer from this effect. I have witnessed it on my own Weslake twin when running it on the dyno. It had a pretty significant flat spot in the 5000 – 6000 rpm range and when running in this range you could see the a/f ratio spike rich and also observe the fogging of fuel vapor in the entrance to the bellmouths . I was able to tune it out by introducing spacers ( in ¼ inch, 6mm increments ) between the bellmouth and the carb until I’d pushed the effect far enough down the rev range that it didn’t matter. I could probably have tuned it out also by messing with the exhaust lengths but that was a lot more work and they did provide very good top end. Previously I had queried Dave Nourish (the maker of the engine) and he had suggested switching to stepped spray tubes. I did switch to stepped spray tubes (and I think it needed a different needle) but it didn’t seem to cure the flat spot and I didn’t get the chance to run a back to back experiment on the dyno. My guess is the flat spot was too severe to be significantly affected by the spray tube change.
On the other hand it is possible that for Mr Rowley’s Commando the spray tube change was just enough to reduce the flat spot sufficiently to a more acceptable level and let the bike accelerate. As others have pointed out the operating conditions were a bit unusual for normal road use. Many years ago I managed to put a significant flat spot into an otherwise standard Triumph Tiger TR7V simply by fitting a shorter aftermarket silencer with goodness knows what inside it.
My guess is that the stepped spray tube reduces the extra fuel pick up through the main jet under reverse flow conditions, relative to a flat spray tube
Hence, perhaps your experiment should measure the difference in fuel pick up between the stepped and flat spray tubes when the air is passing through the carb in the opposite direction?
That the effect Mr Rowley describes was only affected by the exhaust mute indicates to me that it was an interaction between the exhaust and intake tuning upsetting the carb operation over a limited rev range. The fact that the 850s seemed to be more affected by the mute than the 750s could be down to the 850s having the crossover pipe and hence different exhaust characteristics– I can’t remember, did all the 850s have crossovers?
As I said at the beginning I hope this is found to be useful and/or interesting. It does talk in generalities and contains no cross references and citations so will probably not satisfy the more querulous members of the forum.

Cheers Duncan

Dances with Shrapnel said:

Ludwig is making an effort at analytics in good faith and L.A.B. is pointing out that it is missing the mark but would be nice to get some specifics. My interpretation is it is a WOT throttle behavior we are trying to get to the bottom of, not individual behaviors.

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What I think may have been overlooked by some is what can happen when a slide carburettor (especially where there's one carb feeding one cylinder) has its throttle snapped wide open when the engine is revving comparatively slowly.
In that situation the airflow velocity drops suddenly, and the fuel no longer atomises and mixes with the incoming charge of air but instead the fuel turns to fine droplets so the fuel does not mix fully with the incoming air causing incomplete combustion and the engine "bogs down" (one reason why CV carbs became popular with motorcycle manufacturers is because the CV carb slide lifts according to demand and not by how fast the throttle is opened). I expect most engines subjected to those conditions would eventually pull themselves clear of it, however, for some-as yet unexplained reason, the addition of the mutes prevented the 850 engine from doing so?

The required rapid opening of the throttle from low speed and need to hold it there was a necessary part of the test and the consequent bogging down that the 850 engine seemed unable to climb out of before the introduction of the stepped spray tube is what Bob Rowley talks about, so my point is, as we know, the stepped spray tube obviously did something to alleviate or improve the situation-however it may be the overall effect the stepped spray tube has during this dynamic period that we should perhaps be looking at a little more closely, so thinking as much about the quality of the air-fuel mixture resulting from the fitting of the stepped spray tube rather than just fixating on fuel quantity as in: "richer or leaner at a particular throttle setting?

SeeleyWeslake said:

I can’t remember, did all the 850s have crossovers?

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All production 850s supposedly had the balanced exhaust system (no idea if that applied to the test 850s?).

Hi ludwig.
From your recent testing of spray nozzzles, running at 40 mph utilises only the pin holes and throttle slide cutaway for air:fuel supply. At this speed, there is not enough airflow across the flat top nozzle to raise and atomise fuel.

Fully opening the throttle slide quickly means the 'trigger' for fuel draw around 6+mm is bypassed leaving fuel 'frothing' below the nozzle lip. Meanwhile, the pin holes continue to supply fuel to sustain 40 - 41 mph.

I postulate that the sensible solution to that scenario would be to 'step' the spray nozzle a la Rowley/AMAL to allow the fuel 'froth' to spill over the lowered lip. Equally, the angled cut with the lower lip toward manifold would work. Personally, I think a 0.25 mm slit or even a judiciously drilled hole would be as effective.

The pipes and aircleaner discussion simply confounds the real issue of how AMALs respond (or not) to sudden changes in available air due to manifold pressure.
Ta.