Idle RPM

[tartan30cirrus]

All.

Have a new Moyer exchnage engine in...brand new...only about 6 hours on it...at the end of the season I could not get the thing to idle really low rpm...the old A4 was able to idle around 400 rpm. On the new A4 I get it to idle low and then throttle up and bring it back to lowest throttle and engine won't drop back to the same idle...idles around 800 if I recall. Questions: What is the "right" idle rpm? I know how to adjust it with the set screw but I was not getting this to work. Any suggestions for when I try again in a couple days. (I thought maybe the cable is sticking and not letting the idle lever on the engine slip back in place after motoring awhile.)

TX.

Cheers,
Clint Chase
Portland,
ME

[tartan30cirrus]

Any ideas out there? TX.

Cheers,
Clint Chase

[Don Moyer]

Clint,

If the idle RPM does not follow the adjusting screw below 800 RPM, it's usually the case that the throttle cable is limiting the throttle arm from dropping down any lower.

The most important consideration in setting the idle speed is that it isn't so high that it causes undo stress when shifting into forward, and yet not so low as to cause an occasional shutdown when moving the throttle quickly from cruising power to idle. Within these perimeters, 600 to 800 RPM is usually a very acceptable idle speed.

I would point out that the idle system in the carburetor is arguably the most delicate and problematic sub-system of any component within the engine, so I don't think it makes sense to try for an unnecessarily low idle RPM.

For the engine to idle reliably, fuel must be drawn up through a drinking straw sized passageway from the lowest part of the carburetor and delivered through a tiny hole (about the size of a wire brush bristle) just above the main throttle valve in the uppermost part of the carburetor.

In making this journey up through the center of the carburetor, the idle fuel first depends on the main bowl gasket to seal around the "drinking straw" as it passes between the upper and lower housings. The fuel then sequentially gets metered through a tiny fixed idle jet, mixed with air from a rather crude adjustable needle valve, sucked through a fly-specked sized port beside the throttle valve, and finally (on a good day) makes it up into the manifold at a fuel/air ratio that the engine will find acceptable.

Regards,

Don

[tartan30cirrus]

Don,

I am idling around 1000 RPM cold and 1100-1200 wamed up. So i will go for 600.

For the cable, is there a preferred lube? Should I take it off the engine end and fool with it? Or should I mess with the throttle end?

Will be trying this tomorrow. TX!

Cheers,
Clint Chase

[Barnacle Bill]

throttle cable

Clint, I would try losening the cable clamp on the carb just enough to allow the outer cable cover to move forward an eighth inch or so. Usually requires a rt angle screwdriver and just a little cussing. Good luck.

[Don Moyer]

Clint,

Unless your throttle cable is difficult to move, I would stick to the throttle end for now. I would position the throttle to idle (don't bother starting the engine) and then try to visually determine the position of the tip of the idle set screw relative to the idle stop on the carburetor.

If the tip of the idle set screw is not seated firmly against the idle stop, loosen the throttle cable and adjust it so that the idle screw is against the stop. Depending on the configuration of your throttle cable, you may be able to loosen the moveable part of the cable at the swivel on the throttle arm and adjust it there, or perhaps you could loosen the end of the outside of the cable on its stationary mounting bracket and move it a bit toward the carburetor.

If you do decide to lubricate your cable, any medium weight motor oil should work.

Regards,

Don

[tartan30cirrus]

Tx for help, Don. Got her dialed in nicely. Now I seem to have bigger problems. See my post in troubleshooting!

Cheers,
Clint