My 1972 Comando 750 used to have an SU. This was brilliant giving easy starting, loads of oomph and amazing fuel consumption figures.....70 to 80 mpg at high cruising speeds. However, old age and Ethanol took its toll and Burlen wanted mega bucks to revive it.
So I tried a 30mm Mk 1 Concentric.....easy starting, 60 to 65 mpg but poor get up and go when you needed to overtake.
A 32mm Mk 1 Concentric restored the bottom end grunt but drinks fuel at 45 to 50mpg. Possibly the carb settings need altering.
I checked out a few other owners with Mikuni conversions. Most were very complimentary but all mentioned the high purchase price plus the need for a choke conversion. Also several had noticed the rubber mounting starting to perish after just a few years of service.
I fancy trying out one of the new Premier Concentrics but still need advice on which choke size and what jetting?
A man of your ability ,Phi…
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Why does everybody assume…
Why does everybody assume that I know all the answers??? This is untrod territory for me.
My carb came off an 850 as well and has that strange notch in the jet block spray tube. I dropped the main jet to 240 and bolted on a K&N wire meshconefilter. Slide is 3 1/2 and needle middle position. The plug colours are good and generally the engine runs smoothly apart from the over-run. I used to always put Castrol GTX in my engines until it changed from a 15/50 to a 15 or 20/40.
I have been told that Ethanol in the petrol lowers its specific gravity and consequently causes the carb float to sit lower, hence allowing more fuel into the system. So a higher fuel level. Could this be a reason for my thirsty carb?
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i have a mikuni tm40 flats…
i have a mikuni tm40 flatslide pumper on my 750 you have to cut some of the frame gusset away to fit it but the performance is almost as good as a twin carb setup but with better bottom end and midrange and a perfect tickover the only downside is fuel consumption the best i can get is 55mpg but motorway 80mph plus and it drops to 45mpg ish cheers nick
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A man of your ability ,Phil, should have no difficulty here. I had a lot of excellent advice from John Dunn on this. However, yours being a 750 might be different in the settings.
John has a Mk3 850, same as mine with the single 32mm Mark 1 consentric. So this was easy, 260 main jet and 3 1/2 slide. Can't remember about the needle. Also used the earlier paper type air filter with the single carb front plate.
Fuel consumption went up from 45mpg to 68 mpg so the conversion from twin 32mm carbs paid for itself. Oh, you need a stay up float and to raise the fuel level to avoid running out of fuel in traffic or small throttle openings.
As for starting, the carb must have been right all along. When I changed back, after 25 years of straight 40 semi detergent oil to classic 20/50 I had first kick starting, even after standing for a week! A good battery helps with the Mk3 Boyer, I have to say.