'77 Garelli won't idle

My buddy has a 1977 Garelli moped. When we got it the cylinder was rusted to the piston. We eventually ended up boring out the cylinder and buying a new piston. So we got a little more power out of it, nice. But for some reason it won't idle now, i guess it never idled anyways. We thought it was in the carb. We've taken it apart several times and cleaned it real good. Anybody have any ideas. Maybe you have to adjust some things that i am overlooking.

Josh

Re: '77 Garelli won't idle

a good idle is a final indication that the carb and intake system is in good shape.. everything's gotta be perfect to have a steady, smooth low rpm idle..

a dirty air filter, a misadjusted float level, a fuel leak anywhere in the carb, any clogged or restricted internal carb passages, any vacuum leak, loose nuts and bolts, a old dried out gasket in the intake system, improper choke/throttle cable adjustment, etc, etc can prevent a good idle..

A carb has an independent idle system.. this includes some tiny passages as well as an small idle jet.. idle is also dependent on proper float level.... If the float or float needle valve has problems and float level is not steady and correct, a good idle may be impossible.

And the orifice through an idle jet is miniscule.. without finding it and poking a very thin wire through it and then using compressed air to blow everything out it may not be clean and open..

Spray some Starter Fluid around intake manifold gaskets, listening for a change in RPM to detect air/vacuum leaks.. repair or replace gaskets and use a coat of Permatex 2 (non hardening gasket sealant) to make them air tight.. check bolts all over and around the engine for proper torque ..

Idle adjustment is the very last thing that gets done after a thorough tuneup, including checking ignition timing and everything else.... and do it only on the hot engine or it won't idle when hot.

Re: '77 Garelli won't idle

Leon Swarmer /

Did you change the jet when you increased the combustion chamber?

Re: '77 Garelli won't idle

We didn't change out any parts when we did the bore. Would it be bigger or smaller.

Re: '77 Garelli won't idle

A small bore job should not change your jet requirement.

Does it have the Dellorto 14-12 carb? This carb has no idle circuit so the only thing you can do is screw in the idle stop screw. This raises the slide. The one problem is that the idle speed might become too high causing the run clutch to drag. It is a fine line.

Re: '77 Garelli won't idle

Did you mean to say it has no idle circuit?

I can understand many carbs may have no idle mixture adjustment (screw), and only idle speed adjustment is available by directly opening the throttle, but to be clear about it, i think there's gotta be an idle "circuit" of some kind...

..meaning the carb has special passages to provide air and fuel at idle.. Perhaps an idle jet or orifice is positioned so that even very low speed air can draw fuel at idle.

As i understand it, at idling no fuel can be drawn up the main circuit.. the venturi's vacuum "signal" is just too weak to draw it up..

So, a separate idle-air intake hole is provided, perhaps a tube or a hole in the air intake horn.

This air travels through an internal carb passage and, acting like siphon, draws a small amount of fuel up and into this idle circuit. This mixture is then fed into the throat of the carburetor, usually through one or more tiny holes drilled into the side of the throat somewhere near the throttle.

but maybe i just don't understand how this carb works..

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