Engine Trouble Constant Acceleration problem

[Rickb]

So after doing some reading, I have some ideas of what the problem might be. But I will post anyways. I have the 80cc two stroke engine. Stock carburetor. My low end acceleration is very smooth and fast. When I get up to about 20 maybe, the noise of the engine changes from a purr to a less smooth sound and at the same time immediately stops accelerating. Iwill jump back and forth from smooth to rough sound and each time it changes back to smooth, I have a jolt of acceleration. It seems like I am hitting a wall and not getting enough fuel or air. However, when not under load and just idling, I can rev the engine way up, but the rev is also not a super quick rev, I have to hold the throttle for about 3 seconds before I hit max rpms. I have a new carb coming soon, just want to know what this could be?

 

[crimsonchaos420]

from what iv read its a jet size issue. maybe jaguar can confirm

 

[Rickb]

from what iv read its a jet size issue. maybe jaguar can confirm

I hope he can. Yeah that is exactly what it feels like, the engine is trying to take in more than it can get. I hope the new carb works better.

 

[Rickb]

Just figured it out, I am four stroking, I am going to lower the needle and see if that helps at full throttle. If not then I will try fiddling with main jet. I need more air in there at full throttle

 

[Purple Haze]

Adjusting the needle will do little to nothing to effect WOT performance. Sounds like the needle was set correctly anyway, if off-idle acceleration and part-throttle performance was good. Try a #66 jet, should be close, don't know what altitude you're at, I'm at 900 ft. The rule of thumb is 1 jet size every 1000 ft. All engines are a little different, though. I've got 2 66cc engines, identical with "speed" carbs one runs great witha #66 the other runs great witha #62. Both run best with the needle at the lowest position. (top groove) By the way, the stock jet is #70.

 

[Rickb]

Purple haze,

I lowered my needle all the way, now it hauls ass but hits that wall at a bit higher of a speed. So yes I need a new jet. However I have a performance carb on the way. And after I get my carb, I will then check to see if I still need a new jet. Theoretically, these engines should maintain steady acceleration until they either break or detonate correct? No four stroking? Not saying I would push it but I would like a responsive acceleration from any speed. Lowering that needle though, wow, leaning it out makes a huge difference. My cylinder head was solid black so I was definitely running rich

 

[Purple Haze]

They jet these carbs too rich from the factory. Depending on which carb you ordered, (hope you didn't get the CNS POS!) a jet in the #62 - #66 range will really wake up the engine at full throttle. I run the NT "speed" carb, if you ordered the "RT" it's a dellorto clone, which gives superior atomization, a good thing all around. They take the 5mm dellorto jets, readily available from many sources, including SBP.

 

[Rickb]

Ok I admit, I just bought a performance carb from China (I know, I cheaper out, maybe I should have bought a Waldo carb.) I also ordered a low 60's set of Jets. So I am wondering if just putting the correct size jet in will do just as well as with any other carb with a correct size jet. The speed carb as I understand allows more air in and therefore more fuel resulting in more power. I don't really need more power at this point but I need all my rpms. I commute in the morning and when it gets warm out, I am planning to take this thing to my train station every day. At the moment it isn't pleasant not being able to keep up with traffic on 30 mph roads.

 

[Deleted member 12676]

besides jetting don't forget to check your air filter.
take it off and blow thru it while it seals against your face.
if you feel any resistance to the flow then the filter is too restrictive and will cause the mixture to be richer.

 

[Rickb]

These are the jets and carb I have purchased