32mm Amals on 71 Commando ?

G'day, I have a 71 Roadster that is currently fitted with a single Mikuni carb. I decided a while ago that I would like to try going back to twin Amal carbs as I do like the look of twin carbs as opposed to a single. Anyway, my bike is currently on the other side of the country in my fathers care and has been for sometime. I don't hear him complaining though ! So, I was scrolling through the parts section on ebay the other day, as you do, when I had a rush of blood and bid on two overhauled 32mm Amal carbs complete with velocity stacks. Jeez I thought, they would look good on my bike. So I subsequently found out before the auction even ended, my bike actually came out with 30mm carbs. Guess what I won the auction. So now, can I fit these to my bike ? I'm guessing the inlet ports on my head are only 30mm. I don't want to go to the trouble of increasing the port size. Is there manifolds that go from 32mm down to 30mm ? I guess this would defeat the purpose of having bigger carbs. Is the manifold bolt hole distance the same on my head as the heads meant for 32mm manifolds/carbs ? What's the best way to go here ? Any suggestions, or should I just on sell them and get the correct size ?

Yes yes. No Yes yes. No . :wink:

ozzie041974 said:

I'm guessing the inlet ports on my head are only 30mm.

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A '71 Commando head would normally have had 28.5mm inlets and tapered 30-28.5mm manifolds.
http://atlanticgreen.com/nhth.htm
There are 30-28.5mm, 32-30mm and 32-32mm inlet manifolds, but no 32-28.5mm manifolds as far as I know.

Would there be enough thickness in 30-28.5mm manifold to machine the carb end out to 32mm ? Would such a taper have an adverse effect on flow ? Appears this is my only option if I don't want to start machining head ports.

ozzie041974 said:

Would there be enough thickness in 30-28.5mm manifold to machine the carb end out to 32mm ?

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Probably, as it is only a small amount, a die grinder or a small file could be used to remove the 1mm step.

ozzie041974 said:

Would such a taper have an adverse effect on flow ?

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I don't know, but others here might?

Matt sent me link with a good diagram, bottom one. [url=http://commando.yolasite.com/...://commando.yolasite.com/performance-mods.php[/url]
There is a 4mm difference in taper from end to end. So it seems this wouldn't be a problem. Might give it a go. Should go like a rocket ship !

There was a quite exhaustive thread about flowing Norton heads here, and the main conclusion that I came away with after reading all the threads, is that smaller may be better. 30mm Amals tapering down to the stock manifold inlet diameter may result in better performance than opening up the diameter of the head or intake manifolds.

Read this thread here and then decide what you want to do:

head-flow-testing-t8640.html



My guess is you'll realize the best thing to do is to get the 30mm carbs, and the correct manifolds. Write to user hobot, too as I recall him referencing surprising experiences in a post, regarding superior performance from a 28mm 750 head when compared to a larger ported combat head.

Somewhere the point is reached where a carb flows enough to satisfy the demands of the engine, and I think that some of the more knowledgeable guys here have done all the work you're contemplating, and have figured out that a pair of 30mm carbs will provide all the flow and fuel the engine can use. You may actually end up going backwards in the horsepower and flow departments by enlarging things.


Review the thread about flow-testing, and I think it will give you the information you need in order to make an informed decision.

Were 30 dia ports ( at the bend ) tapered to 32 at the head / manifold face std on most Mk IIIs ? .

Hey ozzie,
I believe that 30mm carbs are more than ample for 375cc, and all the playing around associated
with trying to get the 32s to work properly will give minimal gain on a standard engine.
Usually big carbs are only beneficial with big revs, and the loss of low down will outweigh any gain
without the right head and cam etc.
I was fortunate enough to own and race a 1975 CCM (Clews Competition Machinery) here in S.A.
As a 500cc, they came standard with a 30mm Mk 2 Amal, and the hot up kit came as a bigger inlet valve, MX cam
and a 32mm carb. About 40 Clydesdales from memory.
We (the team) raced those bikes in MX, Dirt track, 1/2 mile speedway, Mile long track, Hill climb and even Road raced them at Mac Park,
and there wasn't a hot Yammy or Honda (4 or 2 stroke) that could touch them, most of which were fitted with all mannor of big Mikunis or Kiehins.
Try the 32s if you like to play around a bit, or flog them off and get a pair of 30s.
Just my 2 cents worth.
AC.

My 73 750 came with 32mm from the factory and has 30 mm heads. The manifold tapers down. These were used on late year models after a certain serial number

Was this a good idea Norton had? they tried many things to make improvements which turned out to be the wrong move,
like the main bearing saga, high compression, oil scavenging at the Combat era, IMHO the 32mm carbs on the 750 is another.

Some say (hobot) that the interference can have a positive effect on mixture flow. I would say find a 32 to 30 mm manifolds and let it fly.

Also, For these to work to the optimum then all thing should be up to snuff. Valves, guides, seat, ring. Good compression/pressure. Good flow on the exhaust. Things like that that will promote the increased venturi demand.

Rebuilt 32's isn't an encouraging note. My second suggestion would be to resell them and get a new set of 30's or a good used 34TM flatside.

Hi. Take a look at the JS Motorsport site. They have manifolds that adapt to 30mm heads. Do not know if the amals would bolt up though. Worth a call.
Mike

Hi

Run 32mm on a 750 cant see any difference?
Try it first before you spend out or cut metal.

Chris

The problem with just bolting them on, is no matter which way I do it there will be a lip around the circumference either where the carb meets the manifold if using 30-28.5mm or at the head if using 32-30mm manifold. I'm sure this lip would create great eddies of air as it enters and thus drag effecting the flow rate and hence filling efficiency of cylinder. I'm not so sure if this would be a good indicator of what it would run like with lips removed. To remove the lips on set of manifolds and taper in is no big deal. If it doesn't work then just take them off again. That Mikuni would be sitting on the bench watching with interest anyway should things turn pair shaped. It does run sweet with that on but I just much prefer the look of dual carbs.

Snakepit said:

My 73 750 came with 32mm from the factory and has 30 mm heads. The manifold tapers down. These were used on late year models after a certain serial number

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Both of my '73 750's have RH6 heads and they have 32 mm ports at the head. '72 Combat heads also had 32 mm ports.

mikie3117 said:

Hi. Take a look at the JS Motorsport site. They have manifolds that adapt to 30mm heads. Do not know if the amals would bolt up though. Worth a call.
Mike

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It's interesting that these are 32-30mm manifolds, but they say they are "acceptable" for use with 28.5mm heads. I wonder what "acceptable" means. They are nice looking carbs though. Will definitely bookmark that page and perhaps give them a ring down the track as you suggest. Thanks