850 Commando, Dunstall Exhaust, Sifton "A" Grind Cam

Hi
I am looking for advice on what Amal Premium Carb to buy for a 1973 Mk2 Roadster that has 932 R31 and L32 carbs.
Bike currenty idles fine but runs very rough when throttle is opened up.
Its a project bike I brought last year that hadn't run for decades.

Bike has Dunstall 2 to 1 to 2 exhaust and cam has a Sifton "A" grind.

Am thinking due to engine mods, I should go to Amal Premier PACK 136.
ie 32mm 260 main jet.

Rather then the PACK 137 direct replacement for my current carbs, that have 230 main jets and looks to be for a Mk 2A.

Bit of a newby on carbs, and as new carbs are expensive, I want to make sure I am buying the right kit.

Bike looked to have done high kms, so dont think rebuild will be worth while.

What kind of aircleaner do you plan to use?

I have a 74 Mk2 and use the hamcan cleaner and the32 mm 260 main jet carb pack. So I would recommend that.

But just wondering if it might be worth trying new needles and needle jets before going all the way to new carbs. The needle jets certainly wear.

Main jets are pretty much irrelevant unless you intend to fit megaphone exhausts. The jets you need to concentrate on are the needles and needle jets. it is very rare to have main jets which are too lean. If you progressively lower the needles by raising the clips on both one notch at a time together, you should be able to induce a miss. When you get that, raise the needles together one notch. When you ride the bike changing up and down through the gears, you should never get a miss. If you cannot induce a miss by lowering the needles, the mid-range jetting is too rich. A different exhaust system often requires a change to jetting, but it is usually only one notch on the needles. The whole combustion thing depends on keeping the variables constant - ignition advance, compression ratio, fuel mix and exhaust system. The fuel mix is usually the adjustable. If the motor is sluggish - indicates rich mixture , If the motor misses - indicates too lean. Too lean can do damage.

johnm said:

But just wondering if it might be worth trying new needles and needle jets before going all the way to new carbs. The needle jets certainly wear.

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The intake pulses from the engine especially on carbs feeding single cylinders push and pull on the needles rapidly wearing out the needle jets which is 1/4 to 3/4 throttle openings and then run rich. So new needles and jets should restore good running. Document the main jets, slide and needle jet sizes plus the slide cutaway once you have it running well on your current carbs and then that should be a good starting point for a 32mm replacement. I would keep the head at 30mm and use the 32mm tapered to 30mm manifolds to match the 850 MK2 for tractability.

Thanks for the replies.
Will look at adjusting/replacing needles and needle jets.
I'll try raising needles one notch in the first instance, as bike with half throttle was feeling like it was running out of fuel (missing ??).
Bike has original ham can filter.
Sifton racing cam paperwork I have with receipts from back in 1977 recommends to have 10 1/2 to 1 cylinder compression and suggests trying 40 degrees of spark lead instead of the usual norton 28 degrees that I have it currently statically set at. Think 40 degrees is for racing at high revs. Will test with timing light once I get the carbs sorted so bike is running in a reasonable fashion.
I haven't had the head off, but engine compression does feel very high.
Looking at using bike as a daily runner, not going to race it.

40 degree advance with a Norton head is extraordinary. I would very very very much doubt that's a good idea.

Increasing compression usually results in needing less advance. Don't know your advance curve but I'd expect about 28 degree above around 3500 rpm would be about right.

Raising the needle will as you say make it richer in the low and midrange. But leaness is not usually the issue with worn Amals. Usually old Concentrics are too rich and the plugs are sooty and black.

Your in NZ. What gas are you using? I use pump 95 but my CR is around 9.5. Maybe try 98?

As John says don't go past Norton's recommended 31 degrees. I have 3 engines over 10.5 compression & now run them all at 28 degrees after advice from Comstock as I destroyed my 960 running avgas & 4 star 50/50 at 31 degrees. I didn't know I needed rocket fuel! Steve Maney rebuilt it with 10.5 compression & also suggested 28 degrees. The big girl blew again after 10 years use due to my timing slipping at Pembrey. Hopefully it will be back on track in June.

kommando said:

The intake pulses from the engine especially on carbs feeding single cylinders push and pull on the needles rapidly wearing out the needle jets which is 1/4 to 3/4 throttle openings and then run rich. So new needles and jets should restore good running. Document the main jets, slide and needle jet sizes plus the slide cutaway once you have it running well on your current carbs and then that should be a good starting point for a 32mm replacement. I would keep the head at 30mm and use the 32mm tapered to 30mm manifolds to match the 850 MK2 for tractability.

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A 1973 850 should have a RH4 head with 32mm intake ports. The RH number is stamped above the right exhaust valve cover. It's dubious the manifolds would improve a RH4 head. JS Motorsport sell intake and exhaust port enhancements, but carbs need sorting before bothering with performance tuning. Dunstall advised richer jetting for his silencers, I have no data about the Blair designed 2-1-2 exhaust. 260 may be a good starting point.
I think I'd inspect the carbs internals to have confidence there isn't any corrosion or crud. Amals were well known to suffer from float bowl corrosion if fuel was left in them for long periods of time.

johnm said:

But just wondering if it might be worth trying new needles and needle jets before going all the way to new carbs. The needle jets certainly wear.

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Couldn't agree more.
I re-fitted the original carbs to my last resto, and it was pretty much unrideable above 2,500 rpm, just wouldn't pull cleanly at all.
Replacing the needles & needle jets transformed it - a very cheap and effective fix.
It got me thinking that a lot of Mk1 Concentrics have been condemned as worn out when they just needed a few choice parts replacing.

Ian Rosie said:

932 R31 and L32 carbs.

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They're 850 Mk1A carbs (230 main jets), although they could possibly have been fitted to early Mk2A but not Mk2 as they should be 932/35 and 932/36 (260 main jets).

I'd check and adjust the valve lashes(if you haven't already) prior to any carb tuning

I am probably a lot older than you guys. The first thing we always did when we bought an old bike, is replace the needles and needle jets. The compression ratio, ignition advance, fuel type and jetting form a balance. The adjustment is usually in the jetting. Getting the jetting right is not rocket science . Fit the largest recommended main jets, and lower the needles until you get the miss, then raise them one notch. If you have a road bike, the exhaust system should not lean the jetting enough to cause damage, unless it has open megaphones.