Smoothbores

Hi

I would just like to ask the racers on here about their views on smoothbores on Commandos.
My Maney stage 3 tuned 750 runs really well on 34mm smoothbores. I cant see any advantage on running 38mm as recommended.
Thoughts?
The Big bore 960 that Dave Watson/gary Thwaites ran, used standard 34mm Mk2s. I nicked the carbs off the 750 and the big motor runs really well on them. However I've just picked up some 36mm smoothbores & think this would be a logical progression. But, is it really?

Thanks Chris

I have always run Mikunis so I can't comment on smoothbores but I can comment on the size issue. The first version of my Norton 750 racebike had a Maney bottom end, 3mm oversize inlet valves, head porting, etc. and a 2s cam; it had 32mm Mikunis and a 2 into 1, 3/8" ID exhaust; before we worked on it, we put it on the dyno and it pulled 52 hp. We then put 38mm Mikunis, 1 3/4" ID, 2 into 2 pipes, a 7s cam, added a little bit of compression via a head shave but no other motor work; back on dyno it now pulled just under 68hp (I have that dyno sheet here). Not much point in speculating about it, you put the bike on the dyno, see where you are and try going bigger; the bike will like it or it won't, it depends what else you have, pipe size etc. Kenny runs smoothbores and he has always run smaller carbs than me, maybe they work different than the Mikunis. The big difference I noticed when going big with the carbs was a big 6000 to 7000 rpm rush. Sometimes although you can just move the hp curve around and not make more necessarily but Kenny will be more help to you than me.

It seems that big carbs alone can stifle the midrange a bit, and not always add anything to the top, but when combined with good (not necessarily large) porting, cams, higher compression as in Dougs second motor,they can work well. My experience in making these changes is limited to bikes other than the Commando, but I think some general principles apply.
Doug, how did the lower end and midrange of the two graphs compare?

Glen

If the rubber mount fits straight on , theres one way to find out .

Here is that old dyno sheet

Thanks for posting that.
So some power was lost down low and a whole lot was gained up high.

Glen

Essentially 70 hp 750 on tap! That is very significant figure to me to ponder thanks.
If the finish line is being crowded by others likely would not want to drop rev's below the 55-6000 range. On street if ya in the lower rpm range likely not wanting or needing to go fast, so so what, its just a snick down to tap into cam breathing through bigger holes and away ya go go. No one doing standing starts, starts below their power band rpms. Low down grunt does make small rear steering shifts on loose stuff handy while going slow.

Its swings and roundabouts. Fitting a larger bore carb(s) will drastically slow your gas speed through the inlet ports at low throttle opening-just look at the graph at below 4,500 rpm.
Where you gain at the top end you lose at the bottom end. Large bore carbies are really only suitable for racing use, where you have the throttle nailed back to the stop all the time.

Hi
Thanks all for your replys.
Doug thanks for the dyno reading. The 750 revs out to 6,600, It pulls nicely on the 34's so I will leave it be. However I will put them on the 960 & see what we get.

Chris