Getting more power, high compression 66cc...

[MAFirst4]

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

 

[Deleted member 12676]

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 read more...

 

[MAFirst4]

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?

 

[MAFirst4]

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 read more...

What size jet do u recommend for the mikuni? Vm18

 

[Deleted member 12676]

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

 

[Deleted member 12676]

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

 

[Deleted member 12676]

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.

 

[Deleted member 12676]

Jetting for the Mikuni is listed on my site

 

[MAFirst4]

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....

 

[Deleted member 12676]

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