Carburetor sizing

uneasy rider · Sep 17, 2024

Can I run 932 AMALs on a 1968 commando

hybridracer · Sep 17, 2024

Of course you can, but why would you want to?

baz · Sep 17, 2024

uneasy rider said:
Can I run 932 AMALs on a 1968 commando

Yep it'll run fine

acotrel · Sep 17, 2024

I run two 34mm Mk2 Amals on my 850. But it is on methanol which is more forgiving of tuning errors than petrol. It is probably important to have no internal steps.

uneasy rider · Sep 17, 2024

hybridracer said:
Of course you can, but why would you want to?

I have a good condition set of re-sleeved 832's, may need them for the '68

Ron L · Sep 17, 2024

There will be a considerable mismatch between the 32 mm carb and the 30 mm manifold.

NORTSTER1974 · Sep 17, 2024

You would have to use the intake for the 74 850 RH10 head 32mm down to 30 mm? What's the intake port size on a 68?

ashman · Sep 17, 2024

And you will have to jet it right for your motor.

ilikevelos · Sep 17, 2024

68 intake should be 30.

hybridracer · Sep 18, 2024

uneasy rider said:
I have a good condition set of re-sleeved 832's, may need them for the '68

That is a good reason. The main jets, slides and needle position may need changing but try them as they are to begin with. I tried the 32mm carbs from my combat on my stock ‘68 and didn’t notice any difference.

uneasy rider · Sep 18, 2024

So, I will get a set of manifolds from a later 750 with 932 carbs, and re-jet etc. and should be good to go. Thanks for all the suggestions.

"Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba…"

baz · Sep 18, 2024

uneasy rider said:
So, I will get a set of manifolds from a later 750 with 932 carbs, and re-jet etc. and should be good to go. Thanks for all the suggestions.

"Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba

Personally I wouldn't even bother changing the manifolds unless you are racing
I doubt there are many that could notice the difference on a road bike that the tiny little step in the port would make

maaseyracer · Sep 18, 2024

baz said:
Personally I wouldn't even bother changing the manifolds unless you are racing
I doubt there are many that could notice the difference on a road bike that the tiny little step in the port would make

Often the step is on the bottom of the port, and trying to remove the step will hurt power delivery by blending it in. There are two way I see to fit these things first would match the bottom of the port and blend the top of the head by notching the bolt holes on the manifold. The second and best solution is to make a tapered sleeve to fit in the manifold and do the transition in the manifold.

kommando · Sep 18, 2024

uneasy rider said:
So, I will get a set of manifolds from a later 750 with 932 carbs, and re-jet etc. and should be good to go.

No, as previously suggested you get the 850 RH10 manifolds that taper from 32mm at the carb end to 30mm at the head end. That way there will not be a step at the head for the incoming charge to try to negotiate.

marshg246 · Sep 18, 2024

baz said:
Personally I wouldn't even bother changing the manifolds unless you are racing
I doubt there are many that could notice the difference on a road bike that the tiny little step in the port would make

Agree and, at least for the Triumph Daytona 200 bikes they learned that smooth was not as good an answer as some turbulence in the intake track improving the fuel/air blending.

kommando said:
No, as previously suggested you get the 850 RH10 manifolds that taper from 32mm at the carb end to 30mm at the head end. That way there will not be a step at the head for the incoming charge to try to negotiate.

Of course the correct answer based on nominal sizes but for a road bike - I doubt that it will make a difference, they are hard to find if they exist at all because they are not really 30mm x 32mm or very consistent when measured. The new ones available today that I know of are nominally 32mm x 32mm and nominally 1-1/8" (28.58mm) x 1-3/16" (30.16mm). So, with 32/32 there will be a step at the head not in the airflow direction and with the other a step at the carb and probably the head and AFAIK, the originals for the "30mm" heads were really the second type available today.

I could be wrong about all this but I have measured many manifolds and have yet to find anything close to 30mm x 32mm and many have been messed with over the years so they don't fit any size specified.

acotrel · Sep 18, 2024

There was a mention on a forum some years ago, that a BSA single performed better with a step in the inlet tract. However better performance can be achieved by quite minor adjustments. Personally I would not have the step, but if the port is reshaped to remove it, there might be no easy way back to standard. A tapered manifold might be better.

possm · Sep 18, 2024

I could be wrong about all this but I have measured many manifolds and have yet to find anything close to 30mm x 32mm and many have been messed with over the years so they don't fit any size specified.

Hi Greg, I have a set of RH10 manifolds and they taper from 32-30 but the problem the poster has is the early cylinder heads have ports that are closer to 28mm and the early manifolds actually taper down as well.

marshg246 · Sep 18, 2024

possm said:
I could be wrong about all this but I have measured many manifolds and have yet to find anything close to 30mm x 32mm and many have been messed with over the years so they don't fit any size specified.

Hi Greg, I have a set of RH10 manifolds and they taper from 32-30 but the problem the poster has is the early cylinder heads have ports that are closer to 28mm and the early manifolds actually taper down as well.

How close are the ones you have to 30mm x 32mm. My MK2A 310311 whose engine has never been apart and has a RH10 head has nominally 30mm x 32mm but they are poorly matched and not very close to those dimensions. Unfortunately, they are back on the bike and I don't remember the actual sizes for sure, but I think they were around 31.5 and 29.5.

Understood. That's my point. the currently available that I know of are about 32x32 and 28x30. Putting a 32mm carb on a 28mm head probably is OK, but there is no perfect manifold for the purpose best I can tell.

Fast Eddie · Sep 18, 2024

Uneasy Rider, I’d just fit the carbs.

You are talking a 1mm step from carb to manifold.

Unless you are searching for maximum power or using a flow bench etc you won’t notice or feel any difference in performance either way.

If the step troubles your OCD, you could just file a tiny chamfer on the manifolds, but it’d be a shame to mess with them if they’re good originals. Another option might be to use a couple of those phelonic heat insultaring spacers and chamfer them.

baz · Sep 19, 2024

Fast Eddie said:
Uneasy Rider, id just fit the carbs.

You are talking a 1mm step from carb to manifold.

Unless you are searching for maximum power or using a flow bench etc you won’t notice or fell any difference in performance either way.

If the step troubles your OCD, you could just file a tiny chamfer on the manifolds, but it’d be a shame to mess with them if they’re good originals. Another option might be to use one of those phelonic heat insultaring spacers and chamfer that.

Exactly
I don't know how many people have actually fitted oversized carbs
Or are just making the assumption
But I've done this many times on many bikes for many reasons
And I'm yet to have a problem or notice a difference
My advice would be to fit them and worry about something else

Carburetor sizing

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