Amal Carb size

ludwig · Sep 12, 2021

IMO, a better way would be to take the opportunity to rubber mount the carburettors.
I would never bolt a carburettor rigid to a Norton engine.

SteveA · Sep 13, 2021

Fast Eddie said:
It’s not quite as simple as that…

Partly because of the drag that is caused. Also the effective size on the carb is reduced by the needle and emulsion tube that protrudes. And flow is hampered by the machining for the slide etc.
A bigger carb will have less drag and will provide some compensation for the above.
Provided the carb is not stupidly big (big enough to not cause a proper Venturi effect and screw up the fuelling) then a slightly bigger carb is good as it ensures the port will get all it can use.
Velocity is created in the port mainly, so again, provided we’re not talking crazy carbs, a bigger carb should not negatively effect velocity.
Comnoz’ testing showed the best thing to bolt onto 30mm Norton ports is 32mm manifolds and 32mm carbs (at least).

Standard valves, 30mm ports, 32mm manifolds, 36mm carbs (or 34mms which are in use elsewhere at the moment) Works for me. And it just started with one spin of the rollers, fresh build, first start!

After 2 years of lurking.....waiting for me and my bones to heal!

It is a 750 short stroke and I think 38mm would be close to stupidly big! But I am sure someone is running them fine!

EstuaryBoy · Sep 13, 2021

SteveA said:
Standard valves, 30mm ports, 32mm manifolds, 36mm carbs (or 34mms which are in use elsewhere at the moment) Works for me. And it just started with one spin of the rollers, fresh build, first start!

After 2 years of lurking.....waiting for me and my bones to heal!

It is a 750 short stroke and I think 38mm would be close to stupidly big! But I am sure someone is running them fine!

What camshaft???

Schwany · Sep 13, 2021

ludwig said:
IMO, a better way would be to take the opportunity to rubber mount the carburettors.
I would never bolt a carburettor rigid to a Norton engine.

I feel the same. Spigot rubber mount is the only way to fly. Much easier to work with.

My carbonators are mounted via automotive fuel tank filler hose connected to my one off sawed off Commando spigot intake manifolds. My setup is unique and for the most part irrelevant to this discussion, since I don't/won't use soft metallurgy Amals with that o-ring flange seal. Life is challenging enough without adding frustration.

It is possible that with Amals 32mm carburetors into a 30mm port on a stock Norton engine in a Commando frame works best. I'm definitely not a believer in stepped intake tracts though. Makes no sense to me at all as the tuner of my own toys. Actually sounds like interweb tuner mythology that won't die.

I read the Comnoz posts about ports, and messing up heads. Oddly I came away thinking he kind of got tired of doing the testing, and didn't have a final conclusion he was that happy with. Maybe I was inventing content and reading between the lines though.

SteveA · Sep 13, 2021

Schwany said:
I feel the same. Spigot rubber mount is the only way to fly. Much easier to work with.

My carbonators are mounted via automotive fuel tank filler hose connected to my one off sawed off Commando spigot intake manifolds. My setup is unique and for the most part irrelevant to this discussion, since I don't/won't use soft metallurgy Amals with that o-ring flange seal. Life is challenging enough without adding frustration.

It is possible that with Amals 32mm carburetors into a 30mm port on a stock Norton engine in a Commando frame works best. I'm definitely not a believer in stepped intake tracts though. Makes no sense to me at all as the tuner of my own toys. Actually sounds like interweb tuner mythology that won't die.

I read the Comnoz posts about ports, and messing up heads. Oddly I came away thinking he kind of got tired of doing the testing, and didn't have a final conclusion he was that happy with. Maybe I was inventing content and reading between the lines though.

Did you see the flow bench tests?

Most of us thought those pretty conclusive!

SteveA · Sep 13, 2021

EstuaryBoy said:
What camshaft???

I originally built this engine with a genuine PW3, which is what I think is best suited to it and the exhaust, I have tried other profiles closer to '2S with a bit of sauce' which make good power.

Any road race cam would work with the head and carbs. There is going to be a bit of rider preference associated with power delivery. I really like how a PW3 works on a race track.

I think if you trace any good 'road race' cam back, you are going to find 2S or PW3 in the baseline. Other cams tend to road use (standard cam or slightly tweaked standard cam) which might work better with 32/33mm or wild flat track/drag race profiles that work with 38mm and over carbs and will just make life harder on a road race bike and should never be fitted to a road bike.

Some would not go bigger than 32 or 33 with this set up, and again, it'll work, I can't tell them they are wrong.

Schwany · Sep 13, 2021

SteveA said:
Did you see the flow bench tests?

Most of us thought those pretty conclusive!

I saw them. Other than not much of it applies to my situation, how do you kick start and ride a flow bench?

In my limited shade tree tuner experience a lot more than flow bench numbers goes into making a motor run better. And as you mention later in another post, how an engine is perceived to perform depends a lot on the rider's preference. I'm happy with my Norton motor right now, which probably means it'll blow up any minute.

Fast Eddie · Sep 13, 2021

The flow bench testing of a 32mm manifold mating to a 30mm port was actually conclusive.

It would defy physics for it to manifest itself as a negative change to performance !!

Somewhere in that thread is my post where I showed before and after flow bench results PLUS before and after dyno results, of the same head on the same bike with NO other changes except Comnoz flow work. So the correlation between the two aspects of flow testing and effect on RWHP are quite clear to see.

Note, measuring flow alone would not be totally helpful as we need to understand the balance of flow with velocity. Comnoz measured both.

alans · Sep 13, 2021

L.A.B. said:
RH5 was the 32mm inlet, standard compression post-Combat head often referred to as 'Low' compression as there was also the 32mm inlet 'High' compression (but not as high as the Combat) RH6 head.

thanks Lab for clarifying that. Al

Schwany · Sep 14, 2021

Fast Eddie said:
The flow bench testing of a 32mm manifold mating to a 30mm port was actually conclusive.

It would defy physics for it to manifest itself as a negative change to performance !!

Somewhere in that thread is my post where I showed before and after flow bench results PLUS before and after dyno results, of the same head on the same bike with NO other changes except Comnoz flow work. So the correlation between the two aspects of flow testing and effect on RWHP are quite clear to see.

Note, measuring flow alone would not be totally helpful as we need to understand the balance of flow with velocity. Comnoz measured both.

I believe it all.

My point is as usual a merely a selfish one. I personally would not leave a step up anywhere in the intake path. I certainly haven't suffered from any negative performance port matching and cleaning up intakes on cars or bikes. That said I don't have a stock head on my Norton. I do have a stock Commando head on the shelf I tried on the same Norton motor with the same carburetion without doing any small port match work. It was very underwhelming in real world use. It didn't get along with my cam as well as the ported head. I know not apples to apples, but that's what my experience was trying out a smaller port head. All the charts in the world wouldn't change my mind about port matching. I'm a hard head stuck in my old guy ways. Ignore my babble as appropriate.

Amal Carb size

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