M20 Sticky throttle
 

I have a M20 with a Walbro carburetor that runs like a champ except the throttle valve sticks at idle when the engine is running. The governor is working like it should, but don't have the strength to move the throttle. Stop the engine and the throttle is free as a breeze. It seems like the vacuum makes it stick. In 65 years, I have never seen this before. Anyone have any ideas how to fix it?

I don't understand what you mean by the governor doesn't have the strength to move the throttle? When it's at idle, the governor is basically relaxed and holding it at idle. When you move the throttle on the dash, are you saying that it won't move off of idle? I just don't know what you mean....

Is this an adjustable Walbro? If so, maybe you have the low idle screw, not the fuel screw, the throttle stop set too low. If you do, the butterfly will stick in the throat of the carb. It needs to be setting on the screw. If it's a non-adjustable, it will still have a throttle stop screw to set idle. If you are sure that is not the case, then you likely just need to clean the carb. I would think the linkage is far more likely to bind that the carb throat being so dirty it sticks. I'd check the stop screw.

Hi Jon, I'll try to explain the situation better. It is a Walbro with adj. idle jet, idle speed is set at about 1200 rpm. Start the engine, throttle up to 3000 rpm, back the throttle down and about 2000 rpm is where the throttle stops unless you take your finger and push it against the stop screw. I disconnected the governor linkage, held my finger on the throttle keeping it closed and started the engine. I can move the throttle and it stays where I put it. It feels like it has Locktite in the bearing. Stop the engine and it free. The manifold gets very cold when it is running. In my 65 years of working on engines, I haven't seen anything like it.

So, if the engine is off, the throttle moves freely from idle to WOT? :Unknown:

3000 RPM is too low for high idle no load. Should be at least 3600. I prefer to see 3600 with a deck on (engine under slight load). Ok, that's a lie, I set my engines to 3800, but whatever. :biggrin2:

1200 is about right for low idle.

Normal for an air intake to get cold.... nothing new there.

If the throttle plate is free with the engine off, sounds to me like you need to adjust the governor.

Throttle is free with the engine off but as soon as it starts it does the sticky thing. The governor is not binding in any way and is set up per the manual. 3000 rpm was not the max governed rpm, it was just a guess as to the speed when I blipped the throttle.

Adjust the throttle linkage then.
If the throttle plate is free when it's off, it should be free when it's running. Has to be an adjustment that isn't right.

Are you sure the gov is working properly? Those engines are known
to gernade the gov and put holes in the block.

I've installed a new governor and it is working like it should. The throttle butterfly just sticks when the engine is running.

So.... if you unhook the governor, using your hand to operate the throttle, being careful not to overrev it, it still sticks when running? :bigthink:

Did you try freeing it up? A little WD on the shaft to see if it helps? Anything? Maybe it's just a little "sticky"? All else fails, take the carb apart.

It still sticks with the governor disconnected. I thought about spraying some WD-40 on it but the engine has 800 hours on it so it should be wore in by now. Tomorrow I'll take it off and try to determine why it sticks and lube it up.