Idle curosity

Is there any trick to getting an 80 cc engine to idle. I'm still at the start of my break-in period and I've played with the idle screw, but it dosen't seem to make much difference, as soon as my hand is off the throttle the engine dies. It's not much of a problem since the motor will restart after a couple of pedal strokes. Would adjusting the throttle linkage help or will the problem go away after the engine is broken in?

Getting all air leaks out of the intake system will help. I started using an old MTB thumb shifter with adjustable friction for a throttle, making it easy to tweak the throttle at idle.

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if the idle set screw does not work,you have a air leak.
put a little gasket maker on intake pipe(dont get in carb) or use washer(9/16?).
put a little gasket maker around where throttle cable threads into top of carb.
put a piece of black tape around throttle cable goes into the top of carb.
make sure your intake gasket is sealed.
now your idle set screw sould work.

Thanks all. I will start sealing it up and see what happens.

This is a Manic Mechanic "How To"

The Idle screw is not for the conventional, Air/fuel, mixture adjustment that you may be accustomed to! It is actually a stop for the carb slide. These carbs do not have a high/low circuit.

DO NOT attempt to make this adjustment with the engine OFF...You may create a burr on the Carb slide!

DO NOT turn the idle screw without cracking the throttle open...("the righthand-side twist handle that was provided in your kit for the throttle").

It is difficult to describe how to do this, but it is so easy to do!

Ideally a helper will hold the clutch lever. If a helper is available...simply keep the engine running, (Rev the engine!), and turn the screw clockwise as though you were looking at it head-on. At some point you will be able to return the throttle to idle and the engine will not die. If the idle is too high, back the screw off counterclockwise until the idle is correct...done.

No helper:

Simply ride the bike cracking the throttle at a low cruise speed and turn the screw clockwise as though you were actually looking at it head-on.

Turn the screw in half-turn increments and return the throttle to idle. When you get to the point that a return to idle keeps the engine running, pull in the clutch and stop. The idle will likely be high...back the screw off, (counterclockwise), until it idles normally.

Jim

With a warm engine…

1. Make sure your twist throttle is turning freely. Try twisting it further toward the off position and see if it’s hanging up and holding open the throttle. If this doesn’t work…

2. Make sure your throttle cable jacket ends are seated properly and the cable isn’t adjusted too short at the top of the carb or at the twist throttle. If it is, it may be holding the throttle open. If this doesn’t work…

3. Take off the top of the carburetor and take out the barrel (the cylindrical part that is attached to the end of the throttle cable and that goes up and down when you twist the throttle). If I remember right, there is a small slot on the side of the throttle barrel that has to line up with a small pin in the side of the carburetor to work properly. When I first assembled my carb, I misaligned the pin and the engine idle had symptoms like you describe. Once I reassembled the carb correctly, it worked fine.

[I’ve only built one MB and have experienced all of the above.]

4. I don’t think I had any air leaks to speak of, but I sealed the carburetor onto the intake manifold by putting a little gasket paste from a tube onto the flat round end of the manifold (see photo) before finally installing the carburetor – just in case. That’s the only place I could see where there might be a leak.

As I understand it, the end of the idle set screw is tapered and the throttle barrel rests on it when in the idle position (assuming the cable isn’t too short and moves freely). When you turn the idle set screw in, the throttle barrel rests on a wider part of the taper allowing the engine to run faster. If everything is working and installed properly, you should be able to set the idle low enough for the engine to quit and high enough to be annoying.

Unless the connecting parts between the carburetor and manifold are damaged, misaligned, or defective, I don’t see many places where much of a leak could develop.

Good luck!

Attached Thumbnails

 

Very Good Earthman!!!

Jim

Motorized Bicycle Carburetor install and rebuild
look here in page 2 permalink 12
Norman

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After the "break in" period, I found this to be an excellent mod. for idle improvement...

Stock carb mod

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48cc Grubee Starfire Round Head......

Thanks again all. I spend most of my time yesterday running a Ryder hot air engine, a little big for a bicycle, but an interesting machine designed to pump water during the 1800's. Today I'm on grandparent duty, but I plan to get back working on the bike tomorrow and start using all of your good suggestions.