1970 Roadster carbs

Hi,
I have a 1970 roadster which I have had a long time but only now getting around to looking at. I have a question on the Carbs,
As received (and it sort of ran when I bought it) the Right hand carb is Amal stamped "RB 930 46" and the left hand is Amal stamped "930 L301"

I assume these are not an original pair.

Can someone let me know
1/. What the original equipment would be
2/. What the difference between original and what I have is.

Looking at the bike I don't see anything unusual about it for a commando except it has a 2 into 1 into 2 exhaust fitted

Many thanks

Roger

Your exhaust could be Dunstall 2 into one into 2, as for the Amals as long as they run well and they both 930 carbs as stampings all differ from Amal carbs 930s for the 750s and 932 for the Combat 750s and 850s Commandos, the Amals could have been replaced as the older Amals the slide wear out pretty quick as they are made from pot metal.

The right hand carb could be original although I have no idea why it is stamped with a 'B'. The left hand carb would be L930/47.

930/301 is a generic replacement LH carb.

According to my Amal list 930/46 and /47 had the following specifications.

Main jet 180
Needle jet .106 (According to Amal) .107 for 1970 according to the workshop manual.
Slide 3
Needle 622/124 (normal 4 stroke needle with 2 ID rings)
Spray tube 622/074 (flat topped).

(These may be different to suit the exhaust that's fitted)

It's possible your '1970' model could have had 930/30 and /31 carbs originally in which case the main jets would have been 220.

VArowing said:

Hi,
I have a 1970 roadster which I have had a long time but only now getting around to looking at. I have a question on the Carbs,
As received (and it sort of ran when I bought it) the Right hand carb is Amal stamped "RB 930 46" and the left hand is Amal stamped "930 L301"

I assume these are not an original pair.

Can someone let me know
1/. What the original equipment would be
2/. What the difference between original and what I have is.

Looking at the bike I don't see anything unusual about it for a commando except it has a 2 into 1 into 2 exhaust fitted

Many thanks

Roger

Click to expand...

It's best to state the seral number rather than year as what appears to be "1970" can be 1969-1971. If actually a 1970, then as @L.A.B. said, R930/46 & L930/47.

The only difference in the 30mm (930) carbs for 750 Commandos is the main jet. They would be 180 or 220 originally. With E10 gas, 230 might be called for. With your exhaust, there might be a different requirement. The only real way to know is to see how it runs at 3/4 or higher throttle in 4th gear (better have a track or forgiving police).

If you are looking to replace them with a set of Premier carbs, the 46/47 pair is AMAL part PACK 135. If I were replacing them I would probably install PACK 134 which are the same carbs but with 30/31 sub-numbers and 220 main jets. If upgrading, I would specify #19 pilot jets - they come with #17 which work but #19 start easier and idle better.

Thank you all for the information. The bike is a matching numbers 138339

So, built between May and Sep 1970, guessing in late June or in July. Indeed, considered a 1970.

VArowing said:

Thank you all for the information. The bike is a matching numbers 138339

Click to expand...

As a fellow '70 model year owner, you should be aware that early bikes like yours have a few differences than later models, so when you ask questions always mention your bike is a '70 and most of the advice you get will reflect the correct parts or configuration for your model. Some late model parts can be substituted directly as an upgrade, and some are a more complex issue...

Early bike parts that are different than late model:

both side and center stand
Fork yokes
drum front brake
engine cradle
swingarm
timed disc breather crankcase
head steady
Carb size
oil tank
No oil filter
different wiring schematic with an inline ammeter
no cush rear wheel
..... probably a few more that I can't think of off the top of my head...

Sidecovers and seat

I believe on original carbs for Commando the manifold mount flanges had a flat ground at the inboard "ear" to prevent interference.

Tornado said:

I believe on original carbs for Commando the manifold mount flanges had a flat ground at the inboard "ear" to prevent interference.

Click to expand...

Yes, they do.

That's one problem with the 300/301 carbs. They are generic bodies and you ordered them jetted however you needed. For 750 Commandos that is fine except the flanges interfering with each other, so filing is needed. 850 Commando carbs have special needles and spray bars so 932 300/301 are never correct for them.

At least 620, 622, 624, 626, 928, 930, 932 concentric carbs had/have 300/301 versions available.

The 626 300/301 carbs are not correct for Tridents as they have a special spray bar (not the same as 850 Commando) among other things.

Tornado said:

I believe on original carbs for Commando the manifold mount flanges had a flat ground at the inboard "ear" to prevent interference.

Click to expand...

Yes, also the unused inside throttle stop screw lugs must be filed/ground.

marshg246 said:

850 Commando carbs have special needles and spray bars so 932 300/301 are never correct for them.

Click to expand...

Yet my 850 Mk3 runs fine with 932/300 and 301 carbs. The 850 stepped spray tubes and 4 ID-ring needles were only there to solve a problem that a rider wouldn't encounter under normal riding conditions.

If I am jetting carburetors, I usually start with the recommended jets, then lean-off. The recommendations are probably always slightly too rich. However, my main jets are always too rich. If you are going to burn a valve or a piston, it usually happens at full throttle. If you are rich in the midrange, the bike just accelerates slower - so I lean-off the midrange, almost until the motor begins to miss. When full throttle is used, the motor gets a reprieve. With a lean midrange, full throttle is used less often, you get more acceleration. With a race bike, feeding the throttle on should give you the fastest acceleration. Some guys get out of corners then whack the throttle open -that is slower. Feed it on from at least the middle of the corners, if not earlier.
The size of main jets does not affect midrange jetting, unless they are too small. On most race tracks, I am usually a long way down the straights before I reach full throttle.

L.A.B. said:

Yes, also the unused inside throttle stop screw lugs must be filed/ground.



Yet my 850 Mk3 runs fine with 932/300 and 301 carbs. The 850 stepped spray tubes and 4 ID-ring needles were only there to solve a problem that a rider wouldn't encounter under normal riding conditions.

Click to expand...

True, but I didn't say wouldn't work, I said not correct.

marshg246 said:

True, but I didn't say wouldn't work, I said not correct.

Click to expand...

Precisely why I mentioned it.