Phoenix SU carb conversion

SU instructions I have for these reads: 'Top up with engine oil (preferably SAE20) until the level is 1/2 inch (13mm) above the top of the hollow piston rod...'

As the damper piston appears to be totally submerged through all the travel used, would overfilling make an appreciable difference??

Wonder what changed over the years ? Bell, slide etc appear to be the same- yet, as early as the early 70s (and maybe earlier for all I know) Rivera specifically states to add no oil.

Either something changed internally in the carb- and if so I'd be curious as to what since the dampner only affects the slide movement- or data changed over time to what was correct and what not..

As the damper piston appears to be totally submerged through all the travel used, would overfilling make an appreciable difference??

Yes, because the dampner acts like a brake on the slide moving in the bell. Oil, increases this resistance.

Phoenix motorcycles, the supplier of the SU conversion kit was started by Bernard Hooper who was the engineer in charge of the Norton development team that was looking into using the SU on 1976 Commandos. Their setup instructions specifically states to use SAE 20w/50 oil.
The full instructions are in the technical information sticky on the Commando page. PDF format scans

'The function of the damper is twofold. Firstly it prevents the piston from following the fluctuations in the airflow at low RPM, thus keeping the piston steady and secondly, it prevents the piston from rising in unison with the opening of the throttle. The reason for this is that air has less inertia than petrol, so when the throttle is opened rapidly, extra air will rush in but the petrol will take a little longer before it's flow rate catches up with the new air flow rate. When this happens the mixture becomes weak, By damping the piston such that it cannot move too rapidly we get in effect, an accelerator pump action, that is, when the throttle is opened rapidly, the piston is retarded. Sufficient, in fact, to cause a momentary enrichment of the mixture to give a sharp pick up...'

Perhaps the breathing on a big V-Twin doesn't benefit from such subtleties???

EstuaryBoy said:

'The function of the damper is twofold. Firstly it prevents the piston from following the fluctuations in the airflow at low RPM, thus keeping the piston steady and secondly, it prevents the piston from rising in unison with the opening of the throttle. The reason for this is that air has less inertia than petrol, so when the throttle is opened rapidly, extra air will rush in but the petrol will take a little longer before it's flow rate catches up with the new air flow rate. When this happens the mixture becomes weak, By damping the piston such that it cannot move too rapidly we get in effect, an accelerator pump action, that is, when the throttle is opened rapidly, the piston is retarded. Sufficient, in fact, to cause a momentary enrichment of the mixture to give a sharp pick up...'

Perhaps the breathing on a big V-Twin doesn't benefit from such subtleties???

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Nice burn lol. But I'll go with what Rivera says. Your explanation of the damper is spot on...... it acts like a brake

Yes, direct from the SU tuning guide, which, to quote again, was produced with: '..invaluable assistance given by the British Leyland Motor Corporation and SU Carburetter Company..'
So as close to the horse's mouth as you're likely to get..

Mind you, I see some for HDs offered with extended (larger capacity) float bowls. So again, perhaps this particular nag has different dietary requirements?

That's why I wondered what changed. But keep in mind. ... many things in shop manuals, has changed over the years and is uodated. Many factory service bulletins in years past, addressed such

trappnman said:

That's why I wondered what changed. But keep in mind. ... many things in shop manuals, has changed over the years and is uodated. Many factory service bulletins in years past, addressed such

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IMO, it's not the difference between Norton and HD engines - it's just a difference in SU tuning philosophy.

I'm running a Phoenix SU on my round-the-world 850 Commando that I've been riding through Sth & Central America for the last two years...and my nephew is running an Eliminator II on his 1200 Sporty.

From my understanding, the difference between the two approaches to HIF SU damping - which I simply call Brit (oil damped) vs Mercun (no oil damping) - is simply one of SU tuning philosophy. Both approaches seem to work.

In Des Hammill's book SU Carburettor High Performance Manual, he details his many years of experience with tuning SUs for ultimate high performance on automotive racing engines, much of that experience derived in the UK. In particular Hammill explains (for best power and acceleration) the necessity to get the piston to be as responsive ( i.e. lift) as possible to increases intake depression as possible. This approach requires balancing the use of lightest piston spring and the lightest hydraulic damping effect, with a needle choice that delivers the correct fueling matching the responsiveness of the piston. If piston responsiveness (acceleration) is the goal, a different/richer needle is required to provide the correct AFR than a slower rising piston (increased torque and economy)

If I recall correctly earlier on in SU development, there wasn't a hydraulic damper feature or even a spring. Damping adjustment was achieved by increasing or decreasing the weight of the piston until an optimal balance between piston flutter and piston responsiveness was achieved.

On The Norton, I'm using a slightly customised BBC (Phoenix's recommended) needle and the oil-damped approach so am more familiar with this philosophy...
...but I'm currently helping my nephew with his Rivera SU Eliminator on HD Sporty which is setup to run with no oil-damping and just an occasional spray of WD40 in the chamber.

On The Norton I can easily tell when I need to top up my dashpot as the engine surges slightly at low throttle settings, and when I remove the air filter I can clearly see the increased piston flutter. When I then add oil to the dashpot, that flutter is significantly reduced and the surging disappears. It'll be interesting to see how this plays out on the SU equipped Sporty without oil in the dashpot. I'll report my experiences with tuning it. My SU experience suggests that even on the HD engine low speed fuelling / steady performance at 0 to 1/4 throttle opening will be best with light oil oil in the damper...but we'll see.

Here's a link to a video (on YouTube ) showing an idling HD with a fluttering, un-damped piston... In my experience, this won't ride nicely at low throttle openings..

I remember reading a carburetor shoot out for HD shovel heads in the 70s that involved an SU. I believe it came in second in performance to a Lake Injector. The SU was very well liked and was a better choice than the Lake since it had a float bowl, whereas the Lake did not. Third place was a two throat Weber, if I remember correctly.

A recent s/h purchase for playtime

The later SU carbs have a bimetallic fitted to the jet apparently to reduce pollution. This makes the carb an absolute ba****d to set up without an exhaust gas analyser or a colour tune plug. And, the needle is spring loaded rather than centred in the jet resulting in premature needle and jet wear. That's progress I guess.
I cannot remember how I did it but I removed the bimetallic strip and used the time honoured method of lifting the slide a tad the set the mixture.
The SU is a great carb and I would think handle Ethanol well with its one big jet and a needle rubbing it clean.
Nearly forgot, if you over fill the dash pot it gets blown out of a small vent into the the inlet stream. In my view 20/50 oil is the best.

I had the Pheonix SU kit fitted to my 850 for a year or two a long while back. It was a great carb & the bike ran well & started easily. The main reason I removed it was the bike ran out of steam at 90ish mph, where as with the Amals it would just keep on going. I've often wondered if the manifold was the problem, being a compromise due to limited space.

Matchless said:

I had the Pheonix SU kit fitted to my 850 for a year or two a long while back. It was a great carb & the bike ran well & started easily. The main reason I removed it was the bike ran out of steam at 90ish mph, where as with the Amals it would just keep on going. I've often wondered if the manifold was the problem, being a compromise due to limited space.

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Did you have the ram pipe on it?? (Sorry, but so many appear on ebay without)
I'd be the first to admit it's not a carb to go racing with, but I can wind mine up to Amal speeds if you hang on to each gear a little longer. But in camera littered, speed restricted UK I don't miss three figure jaunts
That's my experience with it....

EstuaryBoy said:

A recent s/h purchase for playtime

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@EstuaryBoy how far does the tube protrude into the dashpot?

cliffa said:

@EstuaryBoy how far does the tube protrude into the dashpot?

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EstuaryBoy said:

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Thanks for posting! That's a neat implementation.

I guess a lot of rampipes remained unused as it meant changing the filter. Do you have one on your bike? I have to say though they don't look as if they do a lot for airflow, as they don't have a bellmouth. Maybe they are a tuned length?

For folks who aren't familiar...

EstuaryBoy said:

Did you have the ram pipe on it?? (Sorry, but so many appear on ebay without)
I'd be the first to admit it's not a carb to go racing with, but I can wind mine up to Amal speeds if you hang on to each gear a little longer. But in camera littered, speed restricted UK I don't miss three figure jaunts
That's my experience with it....

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EstuaryBoy said:

I used it with a K&N filter for which I machined up the end plates. The one on the carb side had a short bell on it I think. I may have adapted the original ram pipe to fit inside the K&N. When I aquired the kit it was new complete & unused, but the foam filter was starting to fall apart.

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cliffa said:

Thanks for posting! That's a neat implementation.

I guess a lot of rampipes remained unused as it meant changing the filter. Do you have one on your bike? I have to say though they don't look as if they do a lot for airflow, as they don't have a bellmouth. Maybe they are a tuned length?

For folks who aren't familiar...

View attachment 104466

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From the Bike article on the Norton 76: '..the sharp cut off of the venturi, where it normally bolts straight up to a car's air cleaner, produced a loss of power at the top end of the rev range. Consequently the venturi extends 3 1/2 inches.......'
And yes, Cliffa, I use it with a foam 'sock' filter round it

If I understand correctly, it is not actually giving ram air effect, as in a scoop facing forward to stuff high speed air into the carb throat, but to better smooth out, make more laminar, incoming air. Similar to what are more often called velocity stacks. On bikes and some cars, these typically do not face forward so cannot scoop in high speed air.
I've read elsewhere that passive ram air effect only becomes significant well above 100mph so not really worth chasing for a road bike/car.