750 Roadster: which carburator?

Lorenzo · Nov 13, 2012

dear All,
I found out that my Commando Roadster 750 year 1973 has the 32 mm carburator instead of the 30 mm

I undestand the 32 mm were mounted on the combat engines, but at the same time I am not sure if mine is a combat (actually I think and wish it is not a combat)

one of my dearest friends and great expert on commandos told me there's no difference nor contraindication in having the 32 mm on the 750 (apart from the speed!). I believe he is right..

anyhow, I am curious: what is your experience? what are your suggestions?

comnoz · Nov 13, 2012

The later model 750s came with 32mm carbs. It didn't make much difference in performance. Jim

Hobot · Nov 13, 2012

If not desiring the cream of the crop Combat Bomb upper 6000 rpm 2S thrill, then likely will appreciate the stabler idle and finer throttle response character of smaller throat carbs. Smaller the throat the faster can snap open w/o bog.

Nortiboy · Nov 14, 2012

Lorenzo said:
dear All,
I found out that my Commando Roadster 750 year 1973 has the 32 mm carburator instead of the 30 mm

Has anyone seen a Mark V 750 (1973) with 32 mm carbs ?. All 1973 750's I have seen have 30mm.

I have original 30mm on Mk V and about 9.5:1 compression and it runs down standard 850's easily.

L.A.B. · Nov 14, 2012

Nortiboy said:
Has anyone seen a Mark V 750 (1973) with 32 mm carbs ?. All 1973 750's I have seen have 30mm.

According to the parts book, riders handbook for late 750 models and Norton Service Release N3/23 the carb size for the majority of late '72 and MkV models (with either the standard RH5 or RH6 32mm inlet port cylinder heads) was 32mm.

http://britmoto.com/manuals/Manuals/850parts.pdf pdf (p.25 & 26)

http://britmoto.com/manuals/Manuals/Riders_2.pdf (pdf.4)

Quoted from Service Release N3/23

All 750 C.C. Commando models
Worldwide
Introduction of the current 32mm carburetter version
of the standard Commando engine unit, replacing the
previous 'Combat' specification (w.e.f. engine number
211110) has necessitated the introduction of two new
cylinder heads which are listed below.

Interchangeability can be effected as shown, utilising
the available range of cylinder head gaskets, in
conjunction with the deletion of the cylinder base
gasket where indicated.

Compression Ratio (Nominal)
Using 0.030in. Cyl.Head Gasket

With Cyl.Base Gasket ......W/out Cyl. base Gasket .... Identification

060988 Standard Commando 1968/72 (30mm) 8.9:1 .... 9.2:1 .... RH1
063327 Combat 1972 (32mm) ..................... 9.7:1 .... 10:1 .... RH3
064048 Standard Commando 1972/3 (32mm) 8.9:1 .... 9.2:1 .... RH5
064097' Standard Commando 1972/3(32mm) 9.3:1 .... 9.6:1 .... RH6

J.A.W. · Nov 14, 2012

Cheers L.A.B., I enjoyed going through those factory documents, most informative. Interesting that tanks/side panels were ready painted as spares [understandable if G.R.P., but if metal...] not just in primer, classy..

Lorenzo · Nov 14, 2012

thank you All for your great suggestions and references..

.. from what you say, the 32 mm and the 30 mm are quite interchangeable..

I am wondering:
what are the contraindication in mounting a 32 mm on a later 750 (non combat) engine?
perhaps it is more difficult to set up every time?

dynodave · Nov 14, 2012

Gotta love it.
Even the service release does not match the factory parts book!!!!!
060988 RH1 is in the parts book for 73 but it is not listed on the service release as a 73 head.
FWIW for all the newbe's a RH1 is 28.5mm at the head and 30mm refers to the CARB not the port...(a 74 850 RH10 head is the only head in the NHT line up that is a 30MM port)

2. An amal concentric bored to 32 has LESS support in the body for the same sized slide used on a 30mm carb. So it may be minor but the 30mm may last longer in use due to the better support.

L.A.B. · Nov 14, 2012

Lorenzo said:
thank you All for your great suggestions and references..

.. from what you say, the 32 mm and the 30 mm are quite interchangeable..

RH1 head = 30mm carbs.
RH5 & RH6 heads = 32mm carbs.

comnoz · Nov 14, 2012

Nortiboy said:
Lorenzo said:

dear All,
I found out that my Commando Roadster 750 year 1973 has the 32 mm carburator instead of the 30 mm

Click to expand...

I have original 30mm on Mk V and about 9.5:1 compression and it runs down standard 850's easily.

It would also outrun a 750 with 32mm carbs and head if the compression ratio is the same. The small port model makes a little more stuff in the midrange where it counts.

If you want an 850 to outrun a 750 you have to make the compression ratio the same. Jim

Hobot · Nov 14, 2012

To help confuse things beyond Norton references, fastest combo I've had on a Combat was the small port head with over size 34mm single carb and no effort to blend the lip at manifold-head. It didn't have as much nice low down response below 3000ish,ie: had to open it more for similar response, as two 32's but after that it was more repsonsive and better top end. With rubber type carb to manifold gasket I didn't find the 32mm carb flanges a deforming sealing issue, but the slide bores wore to fluted surfaces till impossible to idle back until anodized slides fitted in #3 cut out to make up for the bore flute leaks and worked a treat idle to top end. Don't go too long on factory slides, get the anodized one's soon as can to keep tune as long as can.

pete.v · Nov 14, 2012

Lorenzo said:
I am wondering:
what are the contraindication in mounting a 32 mm on a later 750 (non combat) engine?
perhaps it is more difficult to set up every time?

Little to no noticable gain in performance but a big bore exhaust may compliment.
Noticable loss of economy.

If you have a Combat, detuning through a 30mm setup is not a bad thing. Although when 6000rpm is reached, the banchee will still rear its head. I know this to be true.

750 Roadster: which carburator?

Lorenzo

comnoz

Hobot

Nortiboy

L.A.B.

Moderator

J.A.W.

Lorenzo

dynodave

L.A.B.

Moderator

comnoz

Hobot

pete.v

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