carb question

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There was a Gold Star BSA to which somebody fitted a bigger carb and ran with the step in the port. They claimed it gave better power. But if you run with the needle jet half a thou of an inch larger than it needs to be, your motor will be slower. If the carb is jetted right, you probably need to feed the throttle on more slowly when you are accelerating. I would not run with the step in the port, but what metal you remove from the head, cannot be put back on. The big mistake many people make, is they believe bigger ports deliver more power. If you had 34mm ports in a Commando engine, you would probably need to rev it to 8000 RPM to get decent power. Rev limit on any Commando engine is about 7000 RPM, and that is with a rebalanced crank.
 
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Fast Eddie said:
On my old Nourish race bike I found that it produced slightly higher peak power with posh mk2 smoothbore Amals, but actually, it gave a better overall power curve with normal none smoothbore Amals. I have no idea why other than the none smoothbores caused some turbulence or swirl that was accidentally beneficial.
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When you changed the carbs on your bike, did you re-jet from scratch - or did you just fit the same jets as you were using previously ? A simple thing such as fitting slower taper needles can make a big difference. What you are playing with is the vacuum. The taper on the needles compensates for loss of vacuum. When you change to smooth bores, you might change the vacuum characteristic. If you use petrol as fuel, the jetting becomes very critical. That is the reason most race bikes are faster when they are run on methanol. It hides-up the tuning errors.
 
I use 34mm Mk 2 Amals on 30mm intake ports. I have tapered the ports 2mm per side , over about the first 10mm of the ports. But I would not do it to a road bike, using petrol.
 
I suspect modern bikes get much more power from similar motors to the Commando. The main reason is probably because they have fuel injection and computerised engine management systems. The new Royal Enfield twins might be better than any 1960s similar British twin. The only reservation I would have, is they might wear-out quicker. British metallurgy was not all bad.
 
acotrel said:
I suspect modern bikes get much more power from similar motors to the Commando. The main reason is probably because they have fuel injection and computerised engine management systems. The new Royal Enfield twins might be better than any 1960s similar British twin. The only reservation I would have, is they might wear-out quicker. British metallurgy was not all bad.
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Do you seriously believe that a new 650 Royal Enfield is gonna wear out faster than a 60s Brit twin ???
 
In the 70s I picked up a knackered 69 Mk2 Mini Cooper that was fitted with a factory Stage 2 head. I followed the consensus of the Mini tuners, cam, etc. The motor was flat a real nett loss in power. Against all the advice of the gurus I got a pair of 1.5" HD40 SUs from a scrap yard re-needled them on advice of SU and the bloody Mini flew. Did the same to my mates' 1100cc Mini with the same result.
The mini 998 has a longish stroke and having an iron crank I limited the revs to 7000rpm. Sometimes a motor might need bigger carbs.
 
In the FWIW department: I fitted the new 32MM Combat spec'd carbs on the stock 750 Mk1 manifolds. 30MM at the inlet and 28.5MM at the head. The only change to jetting was to move the needles to the center groove from the top as per the stock 930's I took off. I made a slight radius at the inlet end of the manifold but there is still a bit of a small step there. I was reluctant to do more in case I want to go back to stock carbs. Bike runs fine, exactly the same except it idles SMOOTHLY now :). Something the clapped out originals have not done in a long time
 
MikeG said:
In the FWIW department: I fitted the new 32MM Combat spec'd carbs on the stock 750 Mk1 manifolds. 30MM at the inlet and 28.5MM at the head. The only change to jetting was to move the needles to the center groove from the top as per the stock 930's I took off. I made a slight radius at the inlet end of the manifold but there is still a bit of a small step there. I was reluctant to do more in case I want to go back to stock carbs. Bike runs fine, exactly the same except it idles SMOOTHLY now :). Something the clapped out originals have not done in a long time
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carb question
 
MexicoMike said:
FWIW, historically, the best set up for a Norton Commando has proven to be a pair of Amal Concentric carburetors. You would have thought that Norton would have fitted those at the factory!

Oh...wait...they did!

Put 'em back! ;)
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I used to say that until I fitted the tm40 ;)
 
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