Single carb on a 750

Hi All,

I'm about to install a single carb on my 750 engine, and I have got diffrent advices re: the bore of the carb. Most recommend a 34 mm, others say that sticking to the original 32 mm is OK since there is only one cylinder fed with mixture at a time (makes sense isn't it ?).

I'll use it mainly in the mid-range rpm (3000-5000) for touring rides.

Thks for your view about that. Merci beaucoup.

Laurent

Hi laurent, if you have the cash go for the Mikuni 34mm way , lot of feedback , and in your rev range it will be perfect , the amal way , what ever the size , even with the MK2 ( which are not available right now as said by Burlen, due to manufacture problem), is less reliable .................my two cents!

Yes, I agree a 34 Mikuni works great at them revs, in my experience they tend to run out of steam above that.
By the way marinatlas, do you own the beautiful BRG Roadster I saw in Dinan port a few years ago ?
I love the St. Malo/ Dinard area, and it's great to see all the vehicles exiting the port on the way to the Classic Le Mans, me and my missus usually stay in Dinan every year for a week, travelling on my purple Interstate or black Hyde 973 T160.

I have a 32mm Amal on my '69 Fastback and it seems to rev quite well.

I rebuilt the engine about 8,00miles ago and had new valve/ seats etc fitted along with a new 2S cam (should have gone for a Peter Williams cam but couldn't afford it). I used a Dremel to blend the valve seats into the inlet and outlet tracts (I don't think the firm that fitted the valve seats were bothered about gas flow!), and also blended the 2 into 1 inlet manifold to the head and the carb. The mis-matches between the components could be measured in mm!). I made sure the ports were the same cross- section using washers on the end of some wire poked down the holes. I know nothing about gas flowing so God knows if what I did was any good but I can say the bike will rev to 6,000 very rapidly and pull over a ton with me sitting bolt upright in my bulky riding jacket.

I haven't ridden any other Commando so I don't know how mine compares to others but it seems to go well and requires some rapid up-shifting when accelerating hard in the 5 - 6,500rev range. I don't red line it 'cos I'm a coward but it feels like it's capable of redlining.

I'm not sure how this will affect Ms Peels phase 3 handling on Kentucky gravel roads though, while overtaking wheeling R1s at speeds in excess of 150mph, but I'm sure we'll hear…….

"others say that sticking to the original 32 mm is OK since there is only one cylinder fed with mixture at a time (makes sense isn't it ?)"

That's not what actually happens - the reversion effect with the single carb is quite different than it is with dual carbs. If it really worked as "others" say, then a single 32mm carb bike would stay with a dual 32mm carb bike all the way through the reve range but it won't. In the low range there is little difference but up higher an 850 with the oem twin Amals will smoke an 850 with a single Mikuni. We did this experiment directly when I lived in NY. Two 850s, one with a single Mik and mine with the Amals did a bunch of roll ons in every gear. They were bout equal up to about 4000-4200 RPM. Beyond that the twin carb bike easily pulled away. To eliminate the possibility that one of the bikes was just inherently stronger than the other, we switched the carbs/manifolds between the bikes and the twin Amals again easily pulled away.

IMO, the single is, of course, easier to deal with overall and if you don't care about pushing the bike that hard, I'm sure it's a good choice. But if you want the bike to perform to it's maximum capability, the twins are clearly superior.

I had opposite confusing experience on weird Ms Peel that may or may not apply to others. I put on 34 Miki carb with standard 28 mm head instead of the CHO head, just to do the initial run in on. I didn't worry about the lip created by bigger 32 mm Y manifold and even did a sloppy rough intruding gasket at head, knowing I was going to take it easy for first 1000 miles and did. Then the day came to see if I could blow it up, OMG! Response to throttle that I had to prep for or bike would run out from under me and pull till speedo needle showing well over 130 any time I dared. Allowed Peel to embrass the sneering angry sports riders to point it became a hobby to tease em before a ride, the ones i didn't just catch up too and pass along the way. Then put on dual 32 Amals and got a bit more response below 3500 but not near the pull to red line under load the 34 gave. So in general duals seems to flow better for top end but there are exceptions to the rule that only experimenting might reveal. For fit and forget the 34 Miki is hard to beat.

Laurent
If you have a 32 in hand then use it, if not then get the 34mm - the difference between them will be minimal in most applications. As stated by most, the single carb setup runs well for most cruising/street applications but may lack the top end response ( 4.5K and above ) that a dual setup provides. I have a single 34mm setup on my 850 currently and it runs very well up to about 5K - since i want the engine to last i rarely go above that.