Idle mixture screw.

[hammmike]

The idle air screw on my carburetor is not making any difference when you screw it all the way in. It continues to idle at 650 rpm. Could my primer leaking be the problem. My mechanic is coming down later this week to look over the plane and annual it. It runs fine for the most part . Any ideas.

[Mikek]

This idle mixture screw does not work at low rmp you need to get it over 1200 rpm to see a change.

Go to the www.aeronca.com site and under manuals you will find the one for the Carb, maint. & overhaul look at page? 6-8 it talks about the mixture idle.

Once you adjust, smoothly advance throttle up thru 1500 rpm to see the affect.

[MikeB]

Unless I'm reading the question wrong, the idle speed is set by the idle screw on the throttle shaft. Basically, idle mixture just adjusts the air/fuel ratio once the idle is set.

MikeB

[Paul Agaliotis]

What type of carburetor are you using?
Paul

[hammmike]

I am sorry for the confusion. The mixture is wired open. I am talking about the knurled thumbscrew that you adjust to set the idle mixture. The screw on the side that sets the idle speed is set to 650 rpm. The idle mixture screw is 1 1/8 turn out. It does not make any difference idling if I screw it all the way in. I think the primer may be leaking. I will block it off tomorrow and see if it helps any.

[hammmike]

It is a Stromberg.

[Mikek]

Like the manual states to set this screw you need to be at 1300 - 1500 rpm. it affects the cross over to cruise settings.

[Paul Agaliotis]

There are so many things that could be causing this it's tough to nail down. I would look for an item that is causing a rich mixture at idle. The primer is a good place to start.
The throttle valve could be too far open or damaged. This will allow the fuel to be delivered from a different orifice that is not adjustable. Or the float may be too high, the carb will leak with the engine off and the fuel on. Anything else that could be causing your problem will be inside.
I see a lot of mismatched stuff in the carbs, jets and venturis top the list. Some people will change them in hopes of getting higher performance from the engine. For the most part it just increases the fuel burn.
Paul

[hammmike]

Flew the plane a hour today with the primer blocked off and the engine ran great. The A65 that I replaced also ran a little rich so I guess the primer is bad. Has any one rebuilt these or do I need to get a new one.

[Paul Agaliotis]

Some are easily rebuilt with rubber O rings. The early ones have a small steel piston ring to seal and I don't know of a source for them.
The check valve that is probably the culprit is a steel pin that is at the bottom of the plunger. If it or it's seat is damaged it won't seal or if the spring that keeps it extended is broken the same is true. Most of the PT's had a shut off valve that served this function as the primer was not in the cockpit.
Paul

[hammmike]

Thanks, I will try to post a picture tomorrow. Mike