MPG- Twin Premiers vs single 34 Mikuni

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Whether your motorcycle has an SU (vacuum slide) or an Amal/Mikuni (cabled slide) they all get the same vacuum signal from the engine. At sea level the vacuum signal pulls in air that is more dense than is available at, say 5,000', given that other atmospheric conditions like temperature and humidity are similar. Motorcycles with intake and exhaust plumbing and matched cams that enhances cylinder packing will be more noticeably effected by changes in altitude.

So, you select your jetting package for the altitude that you spend the most time riding in. The higher you operate at the less fuel you can burn optimally; the vacuum signal will pull the same amount of fuel at seal leave as it will at 15.000' out of the fuel issuing orifices.

You install smaller jets for higher altitudes, you are still looking for the same optimum A/F ratio as at seal level, but when get to 10,000' your engine will produce less power, still running great.

Some carburetors have more "bandwidth" than others, some have greater adjustability, some are cheap, some are expensive. I spent some time on the NOC website and read about Mikunis, Amals and SU carbs. The big Mikuni complaint was that it had too many jetting combinations. The Amal's were heralded and cursed for being primitive. SU's were difficult to install without hitting the backbone.

Be the vacuum signal...

Best.
 
[QUOTE="RoadScholar, post: 455521, member: 1139

Cheesehead wanted to know which gets better gas mileage for a cross US trip, single Mikuni or twin Amal Premiers.
 
Not sure on original question , but I get better (slightly) with my single 34VM compared to running two mik 32VM carbs , same bike same conditions etc. ... no numbers ,just seemed I was filling tank more often .....
 
"Cheesehead wanted to know which gets better gas mileage for a cross US trip, single Mikuni or twin Amal Premiers."

Well, back to an original answer...it depends on how you ride the bike. If you USE the additional power that the dual Amals can produce, it will use more gas. IF you don't, there is not enough difference to make any difference. As I said, when directly compared, a Norton running a single Mik and mine running twin Amals, we're talking approximately .2 gal more fuel use by the twin Amals in 140 miles. That includes the fact that we were both accelerating at WOT in some areas of the twisties. If we had worked hard it trying to exactly match each other's throttle usage, the difference would have been less.
 
[QUOTE="RoadScholar, post: 455521, member: 1139

Cheesehead wanted to know which gets better gas mileage for a cross US trip, single Mikuni or twin Amal Premiers.

With the Amals and the Mikuni tuned for optimum mixing, and all other variables the same for both the MPG should be very close, and as I mentioned earlier in this thread, the intake plumbing could give a slight edge to the Amals.

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“So the SU's don't compensate for altitude (at least to some degree) on their own?”

Sorry guys, unless you artificially manipulate the pressure on the float bowl vents, the aforementioned carbs do not compensate. Not even CV.
 
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Sorry guys, unless you artificially manipulate the pressure on the float bowl vents, the aforementioned carbs do not compensate. Not even CV.
I have to agree with this , we are talking about lack of oxygen or air that lets the engine to maintain that 16:1 air/gas/fuel ratio.
 
Ski-Doo has been doing it a while, before DFI.
I had 4 sleds with. Worked great.

MPG- Twin Premiers vs single 34 Mikuni


the article was slightly inaccurate. DPM could only LEAN out the mixture. Jetting was set for -4F at sea level.
I squeaked one at -25F, replaced a piston in the hotel parking lot that night.
At +35F when other engines were slobbering rich, they’d run crisp & clean.
 
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CV carbs alleviate the “lean bog” when a conventional carb is suddenly opened. It feeds the engine as much throttle as it will handle. Also makes the “pumper carb” unnecessary, as the accel pump just covers the lean bog.

CV carb was never intended to compensate for altitude or temp.
 
I have to agree with this , we are talking about lack of oxygen or air that lets the engine to maintain that 16:1 air/gas/fuel ratio.
Air density, lower barometric pressure equals fewer oxygen molecules per volume of air.
 
Yep I understand how the vacuum carburettor works
What I meant was there are people that understand carburetion better than me
The best set up I have ever had on my 750 is a 40mm TM mikuni flatslide
I have no idea why it performs so well
I'm thinking a flatslide mixes/atomises the fuel better than a round slide carb?
Or maybe it's the pumper helping here?

The flat slide Mik, was hailed as a performance UPGRADE, way back when first introduced.
The upgrade, actually was to their own bottom line. It was far easier/cheaper/faster to manufacture than the round slide VM.
JMWO:cool::p:D
 
I don't understand the comment that Mikunis have too many jetting combinations. My feeling is that Amals do not have enough. With Mikunis, there is a much greater range of needles available which have different tapers. My bike is jetted for maximum power right across the whole range of throttle openings, which means I have to feed the throttle on, as you have to with a two-stroke. If you want to whack the throttle open, you need a richer taper needle in the carb or the motor will gasp. With MIkunis, the choice of needles is about double what you get with Amals. If you are running even slightly rich on the needle jet, especially when using petrol - power and fuel economy will suffer. But if you jet for maximum power, then your power characteristic probably becomes much more weather-dependent.
Because I use methanol fuel, my jetting is half as critical as it is with petrol. Even so, I use 6D Mikuni needles in my 34mm MK 2 Amal carbs to suit the relatively low compression ratio - they are the leanest available for that size carb. With my bike, a half a thou of an inch oversize in the needle jets makes the motor noticeably more sluggish. With petrol, it would be hopeless. You can jet a petrol motor to go as fast as it will using methanol, but you need a lot of patience.
I think you probably need to compare your Commando with a Japanese bike of the same era. Every model had a different carb set-up which was about getting more power or better throttle response. Most of the differences were in the needles and needle jets. Main jets don't matter much as long as they are slightly rich.
 
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