Stock carb four-stroking...

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kingislnd32

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Aug 10, 2013
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Hi, new guy here.
I am currently using the stock carb on my 80cc bike engine kit. When i go at a steady speed at around 20 MPH at 1/2 throttle or so the engine starts four-stroking. It accelerates fine but once i start going a steady speed it starts happening. It will happen at wot too. I have the needle set at its leanest setting. I have checked that the choke doesn't accidentally come on and there are no air leaks.

The one way i have found to fix it is to put the fuel on/off valve halfway on. My guess is that i need to adjust the float in the carb. I feel like i have read that 21 mm is the best float level.

My question is what is that measurement?
How to i measure that?
If that isnt the correct level what is?
Anything else i should try to fix this.

p.s. my spark plug is a good color

Thanks,
Isaac
 
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kingislnd32

New Member
Aug 10, 2013
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MotorBicycleRacing said:
There really is no "stock" carb on the bicycle engine kits as they
come with a few different carbs now.
Which carb are you using?
NT
"Speed" carb?
CNS?
Delorto clone?

Also your 80 is a 66 cc.
Click to expand...
Sorry, i am using a NT carb
I know its technically a 66cc but i thought most people call them 80cc so...
 
crassius

crassius

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Sep 30, 2012
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rule of thumb - with carb upside down, normal setting is with main jet just peeking thru center of float - richer is more sticking up, leaner is with jet below float
 
maniac57

maniac57

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Oct 8, 2011
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Drop the needle until it pulls strong up to full speed (usually second or first notch from the top of the needle), then drop the main jet size until it four-strokes just a little bit at a full rpm. If you go any leaner, you risk shortening the engine life. Keep an eye on the plug to make sure you don't go too lean.
Don't scream it's brains out, just get to full speed with the slide fully raised and the main jet controlling mixture.
Keep in mind a little four-stroking at cruise is better for these little gems to prolong engine life.
Give her plenty of oil and she will make you smile, lean her out too much and she will start tossing parts.
 
Last edited:
2door

2door

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Sep 15, 2008
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How many miles does the engine have on it? If less than 200 to 300 don't worry too much about performance until it does. I think you'll be surprised how your engine will respond after you reach that magic 200 mark. They are notoriously slow to break in and will not give you the best performance until they do.

Break in procedures are as varied as the number of people who will respond on that subject but most will agree that the engines run better and better as they accrue miles.
Be patient and wait for a bit before doing too many changes to the stock configuration.

What fuel to oil ratio are you running? If you're reading the manual that came with the engine and using 16:1, that's too much oil. Drop your oil ratio down to at least 24:1 then move down to a 32:1 mix after a few tanks of fuel.

Tom
 
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kingislnd32

New Member
Aug 10, 2013
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America
Tom,

I am at about 150 miles.
For the first few tanks i used 24:1 and i am just switching over to 32:1. My plan is to eventually move to 40:1 after it is fully broken in. Is this ok?
 
maniac57

maniac57

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Oct 8, 2011
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That's what I run in all mine.
It usually takes these engines 250-300 miles to fully break in and run consistently.
I'd wait to start tuning until it's settled in.
 
2door

2door

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Sep 15, 2008
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Littleton, Colorado
32:1, even 40:1, if it is good oil is okay. There are alternatives such as a couple of the exotics that will let you go to 100:1 but do some reading on that before you try it. Use good quality oil formulated for air cooled 2 strokes and you'll be golden.

Keep in mind that the needle clip in the NT carburetor only effects fuel flow at or below about 3/4 throttle. Above that the main jet orifice size controls fuel flow to the engine. If you are still running rich, 4 stroking, after a couple of hundred miles you might want to explore a smaller jet size. But as said above, give it some time.

Tom
 
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