cutarug

I converted a Huasheng 139F39cc 4 stroke engine and I get 6000 rpm with a Bolly 21 x 8 , but need a bit more performance, even 200 RPM would do it.
The engine is reliable and does not overheat like Saitos's and alike and run on straight petrol.
Are there hot ups I can do to get some more performance.

kamlooper

My Feedback: (1)

What do you have for exhaust? Try a different carb?
Find av8tor1977's pthreads on here and read up, Quite a bit of good info to be had. He doesnt post here much anymore though.

cutarug

I tried a 2 stroke walbro No24 and used crankcase pulse for pump but it could not deliver the fuel required so went back to original carb
Exhaust is a short pipe 50mm long.
I have 3d printed a new cam in ABS with higher lift and longer duration but have not tried it yet.
Yes av8tor1977's posts are full of good info especially with Ryobi , 2 strokes.

Tidnab

My Feedback: (51)

What kind of carb is on it ? Is it a rotary or diaphragm and what number is it ? I usually get more rpms by using a bigger venturi carb,but not too big.

1967brutus

If anything, I would first check whether all parameters are in order, like valve clearance and ignition timing, mixture strenght and such. Because 6000 RPM on a 21 x 8 indeed seems a bit low to me.

I am a bit amazed about the 3D printed cam, I don't think ABS will last very long, to be honest.

You do not give info on what the original carb is, and whether it has adjustable jets or needles or not, what kind of venturi diameter the original carb has vs the Walbro you tried, so it is difficult to assess the situation from here.

Jesse Open

Conversion to Methanol fuel should easily get you that magic 200 RPM

RC-Cola

Without a compression increase you won't get anything out of methanol.

1967brutus

Untrue. CR has fairly little to do with it.
It has all to do with the amount of fuel that can be burned in a given amount of air. By weight, gasoline contains about twice the energy compared to Methanol, but in a given amount of air, you can burn about 2,5 times more methanol. And since 2,5 x 0,5 is 1,25, you get about 25% more power from methanol (provided the jetting is corrected for the fuel), because you can burn 2,5 times "half the energy of gasoline".

CR does not have much to do with that,

Increasing CR does increase Thermal efficiency, which allows for an even better performance, but the gain is there allready without increasing CR.

Jesse Open

Yes, plus methanol allows one to run even higher compression ratios for yet more power output. But just converting over to methanol alone will bring a good power boost.
Need a little more power, tip the nitro can a bit.


The OP said he was looking for a couple hundred RPM. May or may not be a road he would care to choose. Just an option to weigh.

RC-Cola

The WCM 1/4 scale oval cars tried alcohol for GX31s over 20 years ago and got a marginal increase in power,no where near 25%.The power increase was so pathetic they scrapped the 4 stroke program.
Just switching fuel won't do it.
I speak on experience not guessing.

1967brutus

What do you think I am speaking on?
Introducing myself...

I can replicate those numbers by simply leaving the ignition timing and carburation unchanged, and the engine will indeed run like total crap.
That is meaningless.
So far, every engine I have converted showed that 20~25% difference in output. But you have to convert them properly.

RC-Cola

25% with no changes? get an engine and show us.