30 mm vs 32 mm Carbs

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NickZ

NickZ

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The 1973 parts book page for "Carburettors and Carburettor Pieces" (page 11) shows both 30 mm and 32 mm carbs for the 750. Why 2 choices and what are the trade-offs between them? I have a 73 750 engine that I need carbs for and would like to understand this choice. I has has a RH4 head.
Thanks in advance for any help.

Correction: head is RH6.
 
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Often I find out that there is some bit of information I have neglected to process, but as far as I know the 32mm were used only on the Combat heads with 32mm ports (750cc engines).

Measure your ports. They might be 28.5mm. Regardless you will want to make sure you have proper manifolds that fit both ends. The head intake diameter and the carb diameter. If they measure 28.5 on the head, you are not going to find 32mm manifolds unless you modify or make them. As to whether or not there is a good reason to put 32mm carbs on that head is beyond my technical knowledge.
 
NickZ said:
The 1973 parts book page for "Carburettors and Carburettor Pieces" (page 11) shows both 30 mm and 32 mm carbs for the 750. Why 2 choices
Click to expand...

Not really two choices only that the 30mm carbs were used with the "060988 Low compression '30mm' (28.5mm) RH1 cylinder head" (see parts book Group 2).

rvich said:
Often I find out that there is some bit of information I have neglected to process, but as far as I know the 32mm were used only on the Combat heads with 32mm ports (750cc engines).
Click to expand...

32mm carbs continued to be fitted to the later (post-Combat) 750 models with '32mm' RH5 (low comp.) and RH6 (high comp. but not as high as the Combat) cylinder heads so not only Combats.
 
My 850 HiRider came with 3 Amal carbs, two 32mm (one fitted to bike on a 2n1 manifold; other in pieces in parts bin), one 30mm body and assorted components. Not sure that was ever used on the bike. I'll never use it. If you want it let me know...just cover shipping.
 
Okay, I measured the intake port on the head and it is 32mm, consistent with what you guys are saying. So it looks like I will be getting 32mm carbs.

Tornado, thanks for the offer, but I don't think that 30mm will be useful for me either.

I appreciate all the info and thanks to all for answering my questions.
 
Both RH5 and RH6 heads (1973) had 32 mm intake ports and 932 carburetors, as did the 1972 Combat head. Only pre-1972 750 Commando had 930's.
 
Maybe someone can explain the significance of the 26 / 27 designation after the 932 model number. I could not find an explanation on the Burlen website. Is it specifying the jetting setup, etc? If so, I don't understand why is it stamped into the carb since you can readily change out these items? I have access to AMAL 932/300 & 932/301 carbs, which I believe are their generic 32mm Mk1 concentric offering, but I am thinking that if I configure it with the same jets, throttle valve & needle set up as called for in the reference for the /26, /27 carbs, I will have pretty much the same thing. No?
 
NickZ said:
Maybe someone can explain the significance of the 26 / 27 designation after the 932 model number. I could not find an explanation on the Burlen website. Is it specifying the jetting setup, etc? If so, I don't understand why is it stamped into the carb since you can readily change out these items?
Click to expand...

It was the original factory build specification so it does indicate jetting, slide, needle, etc..

NickZ said:
I have access to AMAL 932/300 & 932/301 carbs, which I believe are their generic 32mm Mk1 concentric offering, but I am thinking that if I configure it with the same jets, throttle valve & needle set up as called for in the reference for the /26, /27 carbs, I will have pretty much the same thing. No?
Click to expand...


Yes, so the original 932/26 & 27 specification was:
230 main jet
.106 needle jet
'3' slide
622/124 needle (std. 4-stroke)
622/074 spray tube (std. flat top)

Generic 300/301s will need metal removing from the inner blank throttle stop screw bosses and mounting flanges.
 
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L.A.B. said:
Yes, so the original 932/26 & 27 specification was:
230 main jet
.106 needle jet
'3' slide
622/124 needle (std. 4-stroke)
622/074 spray tube (std. flat top)

Generic 300/301s will need metal removing from the inner blank throttle stop screw bosses and mounting flanges.
Click to expand...
Perfect. Exactly the info I needed. Thank you!
 
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