Getting more power, high compression 66cc...

Discussion in 'Performance Mods' started by MAFirst4, Jan 27, 2014.

  1. MAFirst4

    MAFirst4 Member

    Hey all,

    I have been modifying my 66cc 29inch cruiser this past month and I have just a few questions. Here's where I'm at, I run the speed carb with the clip on the needle second from top. I have a mild expansion chamber exhaust pipe and the Fred high compression head installed. I am running a champion cj8 plug.

    She runs pretty well overall, I'm 220pounds and it does about 27mph on flat over 30 downhill but I feel like it can do much better. I notice after about 20mph it goes from 2cycling to 4cycling. It continues to pick up speed but seems to bog as it does, or not sound like a real 2cycle should.

    Why is it 4 cycling? It's not rich the plug looks great. Do I need to get a better carb? And what carb can I get that is the most adjustable? What about Cdi. I know there are some out there like jaguar, but will they help run my high compression engine and stop 4 cycling, do I need more spark? Different plug?

    any information is helpful. And anyone running the 5.3 original Fred head, let me know your setup with it!!!
    thank you!

    -Brett image.jpg image.jpg

  2. jaguar

    jaguar Well-Known Member

    Did the vendor of the head give you any idea of what your cranking psi would be?
    If it is more than 135 you are asking for trouble. (others may disagree but that is my opinion. High quality motocross engines only have 150psi. These engines are of lower quality.)
    With high compression you definitely need more spark power.
    The chinese put the crappiest cheapest carb they could find on their engine. Buy a Dellorto (not SHA), Mikuni, or Walbro.
    click on my signature link to
  3. MAFirst4

    MAFirst4 Member

    I checked it myself I'm running right around 145 psi. Seems to do pretty well there. I wonder if a new hotter Cdi will stop the 4 cycling? Ii have one just gotta get a coil to hook up to it. I know those carbs aren't best quality but I think they flow enough fuel. Just wish you could adjust more than the idle screw!!!

    What is the best spark plug to run with high compression?? NGK? What model?
  4. MAFirst4

    MAFirst4 Member

    What size jet do u recommend for the mikuni? Vm18
  5. jaguar

    jaguar Well-Known Member

    the standard CDI sometimes is just strong enough for the standard compression (90psi). They vary in output.
    "flowing enough fuel" is not a prerequisite for a good tunable carb.
    A good carb has a circuit just for the idle mixture (which the NT does not have) and can be tuned in six ways (idle air screw, idle jet, main jet, needle, needle height, slide cutaway)
    Since you are generating more engine heat with high compression I would recommend the NGK B7S
  6. jaguar

    jaguar Well-Known Member

    the standard CDI sometimes is just strong enough for the standard compression (90psi). They vary in output.
    "flowing enough fuel" is not a prerequisite for a good tunable carb.
    A good carb has a circuit just for the idle mixture (which the NT does not have) and can be tuned in six ways (idle air screw, idle jet, main jet, needle, needle height, slide cutaway)
    Since you are generating more engine heat with high compression I would recommend the NGK B7S
  7. jaguar

    jaguar Well-Known Member

    the top rpm is mostly set by the height of the exhaust port. For more RPM just dremel away at the roof of the port so that it opens sooner. Go slow. Take 1mm off at a time (which is .04"). Keep the top shape roundish which is best for a broad powerband. It also keeps the rings from snagging if you dont bevel the edge.
    the pipe you put on it has a very short header which is only good for very high RPM, like for racers.
    Put the original pipe on, dremel away at the exhaust port till you have the top speed you want, then put the expansion chamber on after cutting the header in half and reconnecting it all with heater hose. Experiment with different lengths till you find the right length to match the engine porting. Best length produces the fattest powerband. Then buy pipe of the same diameter, cut a section the right length, and have it welded in place.
    Thats too much work for most but that is what is required. Two strokes arent for lazy people.
  8. jaguar

    jaguar Well-Known Member

    Jetting for the Mikuni is listed on my site
  9. MAFirst4

    MAFirst4 Member

    Thanks for all the info. I'll try that plug out and a new Cdi. I think I am going to go with the mikuni vm18. Lots of work to do and tuning! But that's half the fun right....

    Any idea when u will have more Cdi for sale? I saw there was a waiting list....
  10. jaguar

    jaguar Well-Known Member

    unfortunately the wait is 2-3 weeks more. I switched countries which slowed down production.
  11. MAFirst4

    MAFirst4 Member

    So I ended up taking off my expantion chamber exhaust I bought and puting back on the chrome stock one. No more 4 cycling! Way better rpms. Runs great with the high compression. Guess the moral of the story is I know expansion chambers do work, but build one that fits your applications on your engine. Not all ones you can buy will work for you. So that is my next project!
  12. HeadSmess

    HeadSmess Well-Known Member

    NONE of the ones you can buy will really work as well as a properly designed and fabricated one.... ;) making a pipe CHEAP means its made cheap, not properly! no matched ports, no spring mounted headers etc, no removable silencers, no actual dimensions that are similar to what should be used... all the jazz that makes companies like FMF famous... and, noone is interested in paying for any of my pipes, that for $180 or so, means im making them for charity basically. (a local welder charges 90 an HOUR, and he would still take at least two hours, more considering he would make jigs as well)

    PICT0380 (Small).JPG

    obviously this pic was taken before i completed it. yes, that is a watercooled head.

    the guy i made this for tried haggling me to $120, finally settled at $140, plus postage...never heard a word back. making me say you can ALL go and get well and truly. at least the guys with chainsaws PAY. dont ask. i wont.

    usually... a lean condition will get slightly better as the engine warms up, as if its rich, it will get slightly worse (richer again)

    my only, oft repeated criticism of the mikuni vm18... why stick an 18mm throat carb onto an intake manifold and port, that are only 16mm in diameter? work on port matching, removal of daggy bits, cleaning the ports up, etc etc.

    simply reading the plug can be deceptive, requiring the correct heat range to work properly, as well as other factors. i have never seen a HT engine get an insulator any lighter than a fairly dark tan without refusing to run (no danger of lean seizures, trust me)... nowhere near as clean as the plugs in my saws or trimmers etc, that all run MUCH hotter than the HT.

    its always better to determine mixture simply by riding the things and seeing whats working best... at least on anything as low performance as a HT ;)

    tweak all you want...i will still overtake you :jester:
    Last edited: Jan 29, 2014
    MAFirst4 likes this.
  13. MAFirst4

    MAFirst4 Member

    Any tips on getting the clamp on carb with oring to seat better on intake manifold? I got a small leak there nothing crazy but would like to get it stopped. That oring seal is a poor design...
  14. HeadSmess

    HeadSmess Well-Known Member

    erm....get rid of the oring. if the oring isnt in a properly machined groove, its useless. just measure the depth of spigot in carb, and make sure its goes that far on the manifold (within 1/16). smear of silicone/sealant will help, but i dont even do that. you may have to file certain parts so it will slide on all the way. which parts, will be perfectly obvious if filing is needed.

    there is a mikuni vm16.... quite rare, and memory tells me its a flange mount type. i may be wrong. usually am :wacko:

    (and a 17 too!)
  15. jaguar

    jaguar Well-Known Member

    the Mikuni VM18 was suggested because of its price and availability.
    of course if you can make a 18mm I.D. manifold that would be better.
    please dont anyone comeback with the old wives tale of bigger carburetors causing worse running at low rpm. god, will that fairy never die?
  16. Fabian

    Fabian Well-Known Member

    If you make any changes to the exhaust or intake system, you will need to rejet the carburettor to suit the new configuration.
  17. HeadSmess

    HeadSmess Well-Known Member

    nope. not saying that at all. oversized carbs DO cause bogging when opened up wide when operating at low RPM but. no old wives there, just plain, well documented truth.

    im saying that the port only has a cross section of 16mm, 17 at the most. theres not enough material there to try and enlargen it, even by a whisker, and ive made properly matched manifolds before. at which point, the studs and the nuts really start protesting about clearances... then try bunging a weld in there... have to stick that tube to a flange somehow...


    make the port WIDER, and you cant get the flange to match, without some tomfoolery with the studs. oh yes, its possible, but awkward. look closely and youll see where i had to bash it to clear one nut...

    make the port higher, or deeper...there simply isnt enough metal left to create a good seal... i like my gaskets to have at least 2mm of contact all around...

    you can just see that that manifold is curved smoothly, not "cut and welded" through some angle, and the angle it attaches to the flange is the same angle the port itself has... i get awesome flow... but cant improve it with a dremel, at all. other than to take the seam out of the tube, maybe...

    now compare my manifold flange to the best "port matched" flange you can find on the market... ha!

    nothing about worse running at low rpm. everything to do with minimising flow restriction.

    its like sticking a brick behind a high reynold number wing... the wing makes a lot of lift (suction on the jet in the case of a carb with a properly shaped throat, such as the mikuni, which is , really an airfoil shaped as a cylinder) that just gets broken into destructive turbulence afterwards.

    if you can TAPER the manifold smoothly from the 18mm (or even a 20 if you can get the taper right...) to the intake port, then its all good.

    an abrupt step... sharp edges dont have great airflow. its why we port match.

    and whats that bit about sound waves being reflected by sudden changes in diameter? hmmmmm?

    ill stick to my stock standard NT ;) possibly taper bored.
  18. jaguar

    jaguar Well-Known Member

    yeah but no good rider opens the throttle up more than what the engine can handle. Normally the throttle is gradually opened, not full throttle at low RPM. It is called "throttle control". I have an 18mm on my 55cc and I never have any low RPM problem with it. Also I have a custom made adaptor for the RS reed valve.
  19. Fabian

    Fabian Well-Known Member

    That's never been my style. I have only two throttle positions: idle and full throttle; done faster than the blink of an eye.
    MAFirst4 likes this.
  20. Fabian

    Fabian Well-Known Member

    Please do give more info about your custom reed valve adapter?