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Modern fuel?

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  • Modern fuel?

    My Viper has been out of use for a long time, although the engine was rebuilt just before I stopped using it. This included unleaded valves and seats. So, how do such machines run on modern fuel? Do they need jetting changes, ignition timing changes etc? I'd like to have it set up on a rolling road dyno but can't seem to find one local to me, west mids area UK.

  • #2
    Not sure about the fuel ratings in UK and US, but the consensus here (Oz) is not to use the highest octane, go for a mid rating, something like 95oct. (We have 91, 95, 98 unleaded here).
    And perhaps retard the ignition 1 or 2 degrees, say from 38btdc to 36btdc. It has to do with the flame-front moving faster with modern fuels, causing pre-ignition.
    And a lot of people are finding the newer fuels 'go off' in the tank quicker and become less efficient. 'Norm's Technicalities' has info on this, I'll check later when I get a chance.

    Just to elaborate a little here. A lot of this has to do with the newer petrol formulations for fuel injected engines. (The petrol companies ain't the least bit interested in anything that has a carbutettor hanging off it). Additives are used which degrade the fuel's performance when used with our old very basic Amal carbs. For example they add detergents that might keep all the injection system running nicely, but don't do a solitary cracker for a leaky old new Amal! (Hope you got that snide little joke). Plug heat range can also be affected with the use of the 'new' fuels.
    Ultimately the old-fashioned 'plug-chop' testing method will be the best guide. I know it's a pain but.

    Thanks to 'Norm's Technicalities' for the above very abbreviated information. Norm has lots of pages devoted to this subject in his 'bible'.
    Well worth the price of admission! (Should be available from your local VOC). Sorry I can't be more definitive Singlething, but, well, if all this was easy we probably wouldn't be interested eh?
    Last edited by joolstacho; 12-30-2019, 02:11 AM.


    • #3
      Originally posted by joolstacho View Post
      It has to do with the flame-front moving faster with modern fuels, causing pre-ignition.
      Well, shit. That's something I hadn't heard before, but it makes a lot of sense. I'd been getting obsessive about setting the timing as absolutely bang-on as I could, but it always ran better with less detonation when I first built the bike and didn't know how to do it properly.


      • #4
        Having said that, I do use Silkolene pro FST in all my bikes. On the zx7r and LeMans2 it stops the carb icing, and on the Viper it seems to help with hot starts and detonation, but i also have a lovely clean fuel system on all bikes, and they all do go noticeably better with it than without it


        • #5
          The issue with different burn rate on modern fuels I did now about, which is why I'm concerned with jetting and timing on the Viper. Going stale in the tank if left standing is an issue I have experience of, I use fuel stabiliser in any bikes stored over winter. I've also helped a number of friends sort out their bikes, mowers, strimmers etc that won't start or run badly if they do, on the first start of the season. An additional problem we now have is virtually all modern petrol has bio methanol included, this absorbs water and is therefore prone to causing corrosion in fuel systems, particularly in vehicles used infrequently, another reason to use fuel stabiliser.
          However, going back to my original question, plug chops are one way of checking mixture, but it doesn't help with timing. I know trial and error can help, but both these methods are crude at best. I might try and persuade Hitchcocks in Solihull, they have a dyno but it is mainly used for their Royal Enfield development work. I might ride over on the Velo and try to get them to make an exception!


          • #6
            Hope they'll help. Luckily they don't add Methanol to Oz petrol (though you can get it).


            • #7
              You're lucky! The methanol thing here is all to do with this stupid climate change nonsense! Bring back full leaded 5 star I say!!