An observation on oil

Discussion in '2-Stroke Engines' started by BSA, Oct 14, 2008.

Tags: Add Tags
  1. BSA

    BSA Guest

    i had been having trouble with jetting my HT engine. I had been using synthetic oil at initially 16-1 the engien smoked a lot and th eplug was black so I tried 32-1 but my plug was still just as bad and there was still as much smoke and oil dripping out the exhaust (this was with the micture c-clip at it's leanest position BTW.

    i thought carefully about the jetting on these engines and I realised that in china they probbly only use the cheap non synthetic oil. I put some dino oil in the tank at 20-1 as the manual says, and she runs swell. Toasty coloured plug and hardly any smoke.
    So if you think you night have to rejet your carb, try using standard oil at 20-1 trust me youl see a big improvement!
    I think the reson for this is that synthetic oil is quite a bit thinner. Thicker oil will make the bike run leaner as less air can get through the carburetter jet (I think)

    Last edited by a moderator: Oct 14, 2008

  2. QuadManiac

    QuadManiac Member

    Just an FYI (from 2 stroke motorcycle and outboard experience).

    As you increase the amount of oil in your fuel mixture (eg go from 32:1 to 16:1 mix) the air/fuel ratio becomes leaner. The same amount of liquid gets pulled thru the jet (or maybe even a bit less because the mixture is sligthly more viscous) and is mixed with the same amount of air. But, since more of the liquid is now oil, the burnable fuel (gasoline) available has decreased, making the a/f ratio leaner.

    When you try to adjust the needle to optimize a/f mixture, it may be that you will want to go leaner when you use a smaller amount of oil (more gas available to burn) and richer when you use more oil (less gas available to burn). Example - 16:1 - set needle richer, 32:1 - set needle leaner.

    Of course, the difference between 32:1 and 16:1 is small - about 1/32nd or about 3% difference in a/f ratio... but, this takes the mixture towards the edges of good operation. Where the ideal air/fuel ratio is 14.7 to 1 (air to gasoline), taking the fuel/oil ratio from 32:1 to 16:1 will move that a/f ratio from 14.7:1 to 15.1:1, just a bit leaner.

    Now, assuming that HT carbs can't maintian even close to 14.7 to 1 over the full throttle and RPM bands anyway, this may not make much of a difference... but it's still food for thought.

    Good luck, good rides!
    Last edited: Oct 14, 2008
  3. Mountainman

    Mountainman Active Member

    old fashioned 2-cycle oil may be best

    from 14.7:1 to 15.1:1

    is that enough difference to be seen or felt ???

    as I have though in regards to this thread

    in these cases -- seems the old fashioned 2-cycle oil may be best...

    to -------------------- ride that thing
  4. QuadManiac

    QuadManiac Member

    No, i don't think it will make a heck of a lot of difference.... BUT if you're already running marginally rich at 16:1, then you'll be running even richer at 32:1. This is where a needle clip change to leaner might make a difference.

    Conversely, if you're running lean at 16:1 gas/oil, then the change to 32:1 gas/oil may well fix the problem.

    BTW, maximum power is acheived somewhere between 12.5:1 and 13.3:1 air/fuel ratio, so a bit rich is better from a power standpoint.
    Last edited: Oct 14, 2008
  5. BSA

    BSA Guest

    Yes is the answer to that problem, black plugs at the lenest setting on the carb needle with 32-1 and a toasty coloured plug with 20-1 dino oil at the second carb needle is what I had.

    The fact is the HT carb has been jetted for use with standard oil around 20-1. If smaller jets were readily availible we could use synthetic.

    Last edited by a moderator: Oct 15, 2008
  6. Mountainman

    Mountainman Active Member

    custom jets possibly made up -- if needed ??


    have you thought about having this size custom made up ?

    or is there truly that much need for you ?

    just had a couple of thoughts...

    have a blessed day and much fun when you -- RIDE THAT THING
  7. Pablo

    Pablo Motored Bikes Sponsor

    You guys are on to something.

    After riding that thing for just a little while I decided that while everyone is trying to go fuel richer, I thought a little different....and easily found the correct jet at the Delorto web place. The scary thing is now after 4 or 5 months I can't exactly remember which size works best..... .64mm or .65mm as I recall...........:p

    With that I run synthetic oil and synthetic/castor all over the map and they all generally work fine.
  8. BSA

    BSA Guest

    Yeah if I can get a cheap jet, thats the right size I'll install and experiment.

  9. BSA

    BSA Guest

    Does size 64 jwt mean .64mm jet?

  10. Pablo

    Pablo Motored Bikes Sponsor

    Typically, yes.
  11. JemmaUK

    JemmaUK Guest

    It seems that the HT engines seem to like the rougher grades of fuels and oils because thats what they are happier with (pun not intended).

    Some of the failures might be attributed to that - because I remember that using modern oils in my humber made it run noisy and leak - but if I put the stuff in that I thought (being young) was thick enough to weld with - the engine ran sweetly and the oil pressure went up by 5psi

    Old tech needs old fuel apparently.

    Jemma xx