32 mm Amals too much or too little .

I was wondering what members thoughts were re 32 Amals on the 850 motor . I am currently building a 850 MK3 engine and going to fit the RH10 head in place of the RH4 in my opinion the 32 mm carbs are an overkill or should I say over fuel . I feel the 30 mm Amals would do the job just as well . Bear in mind it will be a stock standard head and engine and I am not looking to break any speed records past that stage . For economy and efficiency only . Welcome any thoughts thanks Norton Dave ?

They come standard with 32 mm carbies, 30mm for 750s unless combat.

Ashley

Norton Dave said:

For economy and efficiency only.

Click to expand...

10,000 miles on my '74 with the stock 32mm Amals, and the last two tanks averaged right at 49 mpg. They run just fine, except for the annoying inconsistent idle issues that comes from worn slides.

Nathan

Yes Ashley I am well aware of that , but the question was are they over carbureted with the 32 mmm concentics . Bigger is not all ways better did they put on the 32 mm just for the USA market as a selling agent or was it justified ? All the 850 models had 32 mm carbs .

If no special head work and standard cam and operating mostly below maxim torque rpm you may be surprised-pleased by the extra spunk running up to shift points with 30 mm duals but may run out of strong pull approaching the ton and may not be able to have road enough to exceed 110 in time. What little carb combo I've experienced in this range seems depend on weather ya want to have more torque below 6 grand or above. There was a flow rate post in past that ballparked our size Amals 30's worth about 100 cfm and 32s about 130 cfm. It takes about 140-150 cfm to burn 100 hp of fuel. So if ya think ya engine can make over 50 hp per jug then go with bigger carb, if not may have to go against the opinion grain to get a grin w/o getting jail time on the tickets. Would be educational on the size carbs P11's and related desert racers ran in the era.

Norton Dave said:

Yes Ashley I am well aware of that , but the question was are they over carbureted with the 32 mmm concentics . Bigger is not all ways better did they put on the 32 mm just for the USA market as a selling agent or was it justified ? All the 850 models had 32 mm carbs .

Click to expand...

They are not overcarbureted with the 32mm carb.
I doubt you would be able to feel the difference in top end power between the 30 and 32 mm carb on a standard 850. [I know I couldn't}
The 30 mm carb lasts a little longer than the 32mm carb. 30mm carbs, and manifolds on an rh10 makes a sweet running street bike. Jim

Dave,
Not sure if you are aware, but the 30mm port RH10 heads were/are used with manifolds which are tapered from 32mm to 30mm.
My 850 MK2 a always ran at its best with this set up. Very tractable at low speeds but lots of get up & go at 90+. I never tried 30mm carbs though. Apart from the 32mm MK1s I tried a SU & a single 34mm MK2.
Martyn.

The RH10 head indeed uses 32mm x 30mm manifolds; the stock 30mm manifolds seem to be actually 30mm x 28.5mm or so. I wonder whether that taper would affect optimal flow on the RH10



Tim Kraakevik
kraakevik@voyager.net