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MPG- Twin Premiers vs single 34 Mikuni

Discussion in 'Norton Commando Motorcycles (Classic)' started by Cheesehead Commando, Jan 15, 2020 at 8:17 PM.

  1. Cheesehead Commando

    Cheesehead Commando

    Joined:
    Dec 2, 2008
    Hi all-

    Was having a conversation with a friend regarding expected miles per gallon for my ‘75 MKIII Interstate running Premier's vs 34mm Mikuni, and we can’t seem to agree. I’m planning a long trip with the bike and am mostly concerned with reliability, ease of maintenance, and MPGs in wide open western U.S. I just bought a set of Premiers for my ‘74 Roadster, but giving serious thought to a single Mikuni for this bike and trip as I suspect the MPG difference in particular on the highway is significant. Any thoughts or experience with this issue is appreciated.

    Thanks!
    Tom
     
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  2. illf8ed

    illf8ed VIP MEMBER

    Joined:
    Dec 30, 2003
    Never had a single Mikuni on my combat roadster long enough to check mileage. I’m getting 47mpg at 70mph with sleeved Amal MkI twin carbs. Hope you’re not going on this trip in the winter, it’s cold out there.
     
  3. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    I have a friend with a single mikuni Norton that I ride with. Both his bike and mine have fastback tanks. In normal cruising we both end up stopping for gas at around the same point - typically around 140 miles. We both fill to the top when starting out and mine usually requires around .2 gal more to fill when we stop at approx 140 miles.

    Obviously it will depend on how you ride. The twins CAN use a lot more fuel if you are heavy on the throttle because they can make more power. When we do our occasional drag races, mine will pull away from his as soon as we get into 2nd gear even though mine has a one tooth higher ratio. But if you are just cruising, my experience is there is not much difference in fuel usage. That assumes, of course, that the carbs are properly jetted, air filter is clean, etc.

    Ultimately, the distance you need to go between gas stations is the issue. Frankly, I'm happy to pull into a station and refuel every 100-120 miles or thereabouts just to get off for a couple of minutes. With your interstate tank on my bike, I would be MENTALLY comfortable with up to 170 miles between gas stops; my butt wouldn't.
     
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  4. Cheesehead Commando

    Cheesehead Commando

    Joined:
    Dec 2, 2008
    Thanks guys, good comments!

    Agreed on the range thing, dictated more by my bladder than fuel tank. So 120-140 is about my max. However, it’s nice to have the option out in the middle of nowhere to stop again 30 miles down the road if the fuel is of higher quality. I ran in to that issue a few times a couple years ago while in WY/MT/SD on my ST1100. Doing a long trip on the Norton will fulfill a long time desire. Last one was 1982.

    Interesting to note that mileage didn’t vary much between Commando’s at highway speeds (70-75) between either twin Premiers or single Mikuni. Makes me start to rethink my approach..

    This is a long term rebuild project, trip anticipated for summer of ‘21. But busy buying parts now.

    Thanks again!
    Tom
     
  5. Cheesehead Commando

    Cheesehead Commando

    Joined:
    Dec 2, 2008
    I should have added... I’m planning on using 22 tooth counter sprocket, if that makes much difference.
     
  6. ashman

    ashman

    Joined:
    Jul 11, 2010
    It amazes me every time this subject comes up, Norton Commando's were design for twin carbs they were classed as a high performance motorcycles in their time, if you want to go single carb all good but really its good to know with twin carbs when you need to open it up you have the fuel to do so, once the twin Premiers are set up they are very reliable and a simple carb to work on, if you want to save on fuel they work just as good at low revs and if you are going 22 tooth sprocket well even be better cruising, I am not a fan of single carbs on a big Norton engine, but there are also folks who just love single Mikuni carb's and if your bike is turned right the twin carbs should get good mileage if you keep them on a constant cruising rev.

    Ashley
     
  7. maylar

    maylar VIP MEMBER

    Joined:
    May 13, 2007
    Here's an interesting note - when my 850 was new with Amals and the crossover head pipes I got 60 mpg on the highway at 65 mph. After switching to 750 type pipes I lost 5 mpg. That's what it's been now for 40+ years, around 55 mpg.
     
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  8. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    Really? That's quite a difference!
     
  9. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    The crossover head pipes are a good thing in every way, power, efficiency, and loudness. Well, they are quieter which may NOT be better depending on what you like. Their downside is that they can be prone to cracking. That being said, I replaced the 750 style pipes that were on my 850 when I bought it with a set of OEM 850 Norton crossover pipes some years ago. I've have had no issues at all and the cobblestone local roads are VERY hard on such things!

    FWIW, back in the day with my original '71 750/19 tooth sprocket, my typical fuel mileage was in the mid 30's! Admittedly, as a kid back then I mostly operated as if the twist grip had two positions - idle and WOT. ;)
     
  10. maylar

    maylar VIP MEMBER

    Joined:
    May 13, 2007
    Having bought the bike new, I was not cognizant of the rattle effect caused by those stupid exhaust lockring tab thingies. The rose nuts would loosen and chew up the threads, and somewhere around 1981 the exhaust ports needed to be sleeved. The overall job was done well, but the right side was a bit "crooked" and my crossover pipe wouldn't align any more. Been using 750 pipes since. I miss the efficiency of them but don't miss the carbon mess (they never sealed well).

    Sorry, OP - nothing to do with Amal vs Mik - just tossing out what mpg is possible with stock parts.
     
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  11. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    Q; has any body used K& N air filters on a Commando and noticed an increase in MPG at 50-60 MPH?
     
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  12. 84ok

    84ok

    Joined:
    Aug 10, 2014
    can't wait 'til (hope) fullauto chimes in lol
     
  13. MichaelB

    MichaelB VIP MEMBER

    Joined:
    Jan 14, 2004
    Thanks for posting that. That's what I remember back in the day with pure stock 73, 850 with 21 counter. 60 mpg.
    I used to run with a 750-4 Honda at the time. I could out pull him and got better mileage.
     
  14. illf8ed

    illf8ed VIP MEMBER

    Joined:
    Dec 30, 2003
    22T Is OK for flatland. Wonder if it will hamper the climb over the Rockies.
     
  15. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    If you are only changing one set of jets and keeping one air-cleaner serviced, you are much more likely to have your motor tuned to the optimum. I would not use a single carb for racing, but for a road bike, it might be better than twin carbs. I think this stuff about twin carbs using more fuel because they make more power is a bit nonsensical. Half a thou of an inch wear in a needle jet when using petrol should take the edge right off performance - much more than the difference between twin or single carb.
     
  16. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    In a lot of the wilderness areas of Western Canada and the US the gas stations are great distances apart.
    People often say there's no point in having a big fuel tank when the rider needs to get of at 100 miles or so for a break anyway.
    The assumption is that there will be open gas stations wherever you choose to get off.
    I stop for a stretch every so often but really appreciate having that big Interstate tank on there to give fuel range. The 4 US gallon fastback tank is about as small as I would want to tour with. Sometimes you encounter a big headwind and the fuel range drops from its normal number by a third or so.
    Don't have so much fuel on the Vincent and it's a constant worry. It has roughly the same capacity as a fastback tank, a smidge over 4 us us gals.
    The 3.0 US gallon Roadster tank here would be of little use.
    At best it would require a tremendous amount of route planning and verification of fuel sources before heading out on a big tour.
    Or maybe carry spare gas.

    Single to twin carb I doubt would make much difference.
    My single carb 650 BSA does about as the same as the Commando which is same as the 650ss or the Vincents.
    The only bike I have that breaks out of that 50-60 mpg per Imperial gallon range is the modern Triumph Thruxton. It will get closer to 70 if ridden at speed limits or just above.

    Glen
     
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  17. CanukNortonNut

    CanukNortonNut

    Joined:
    Aug 8, 2005
    The 22 will lower your RPM and your MPG will increase at crusing speed. Twin Amal when set up perfectly will give good mileage. I've done testing on numerous long distance trips. I have re-sleeved Amal Mk1s.
    Cheers.
     
  18. CanukNortonNut

    CanukNortonNut

    Joined:
    Aug 8, 2005
    Just drop a gear if you are not on the cam. I've never experienced any issues when travelling through the Blue Ridge Parkway in NC.
     
  19. concours

    concours VIP MEMBER

    Joined:
    Dec 29, 2011
    I have run my 850 with the 22T for 32,000 miles, in Smokey, White, Appalachian Mountains. A lot of it with saddlebags & my 240lbs., no wish for different gearing, ever. I chose the 22 because I like long legs, not tractor gears.
    That said, it’s hard (unsafe)to go 65 when traffic is rolling 80. I’ve seen 50 mpg (twin Amals) when on a Parade ride. Low 30’s at 80mph.
     
  20. Madnorton

    Madnorton

    Joined:
    Jun 9, 2008
    I would imagine that an Interstate with twin Amals will have a greater range than many modern tourers. Not sure if they carry a similar fuel load to the Hardley Dangerouses, but would expect at least to go mile for mile on range.
     

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