Probably running to lean that's what it sounds like

Lean yes, why? Primer line broke? Crack in casting for Intake rear/above carburetor?? Hose coupler intake front?

keep us informed.

i'm thinking too rich, causing icing. temps low. try leaning? flooding on startup?

Take a look at the carb … if it is really running lean… remove the float bowl … this should be done every 500 hours per the MM… what happens is that Franklin drip is oil dripping from the intake though the carb… some of that oil and contaminants get I to the float bowl … over time it crates blockages

You made me think that the leak may come from the manifold fitting on the carburetor casting. I’ll check tomorrow. Thanks

Could the primer be unlocked?

Cht readings? Someone not to long ago had a rocker break and cause issues.

Could have impulse coupler springs broken on the mag cause retarded ign timing … happen to me… I though it was fuel… but I saw higher egt when it happened

Sounds like an induction leak. Pressurize it through the carburetor with the output of a shop vacuum and spray soapy water on all the joints and tubing looking for bubbles. I had an induction leak that was a crack in the crossover tube, caused by a EGT probe clamp too close.

Is it possible your mixture cable slipped in the carb mixture control arm?

Follow.

I had the exact same problem. The gaskets on the intake manifold were not sealing so the already lean running engine was too lean. I had to manufacture shims exactly the same size as the gaskets to fill the space. Solved the problem.

Just a data point. When I add carb heat, it adds about .4 to .5 GPH to my engine monitor fuel flow. So it sounds like the engine is running too lean. I'll leave to the experts as to why it's running lean. Good luck, we're all counting on you.

If it is too lean, I would expect some backfiring on final.

Michel, un de mes amis de Chibougamau a eu exactement le même problème il y a quelques temps, et son carburant était du super sans-Plomb (mogas) mais due au nouvel norme du gouvernement, son sans-plomb contenait de l’éthanol… c’était la première fois qu’il lui mettait ce genre de carburant… et c’est exactement les mêmes symptômes que tu viens de décrire

intake leak

I’ve installed another carburetor. Beforehand I was losing 200 rpm at cruise with CH off and idle was erratic without CH on. Now idle is smooth with both CH on and off, but the engine starts back firing at 2000 rpm which gets worse at higher rpm unless CH is on. That indicates to me that the engine gets too much air or not enough fuel at high rpm. Hence I’ve come to the conclusion that a newly overhauled carburetor would be the solution unless someone has other suggestions.

After working on this very unusual issue, I’ve got to the following stage. My engine is running very smoothly at idle and up to 1500 rpm. When I press on, it starts running very rough at around 1700 and can’t go any further as it vibrates very badly to the point where I must pull back on the throttle for the fear of creating internal damage. Here’s what I’ve done so far: Changed carburetor, magnetos, spark plugs and all intake manifold gaskets and seals. Checked fuel flow with 35 g/hr, verified all fuel filters and air cleaner. Ran with P leads removed in case there was intermittent short to ground and did a compression differential check on cylinders with all over 76/80. Removed the valve covers and verified valves to ascertain rockers clearance at 0.040. I’m out of ideas. The engine just won’t operate over 1700 rpm without serious self destructive behaviour. Anyone with any more thoughts beside installing a new engine?

Broken valve spring?

Hi Michel. Did you happen to observe each valve as it opens and closes? I'm wondering if you have one or more broken or weakened valve springs? Both times I had a similar situation were valve related. One was corrosion from iron valve guides the other was contamination possibly from sand or media blasting. Could be carbon or lead on the exhaust valves and stems depending on what gas you are using. Just some thoughts. B

Long saga but final solution. Right after my initial loss of rpm on my F150 with CH off, I had decided to change the carburetor with a newly overhauled one, which created a worse situation. After having replaced various parts including the magnetos, intake manifold gaskets and seals, spark plugs and ignition harness, and after checking all filters and verifying fuel flow, the problem remained. The engine would turn very smoothly at low rpm but not rev up over 1700 without severe vibration and roughness. I was getting out of resources to the point where I was ready to install another engine. Yesterday I dug out from my parts inventory an old carburetor MA-3SPA which I was due for overhaul. It was doing fine when removed; so I decided to install it. Well guess what? My engine is now running as intended and was declared serviceable after intensive run up and successful test flight. I can only conclude that it was all a question of stoichiometric mixture which only a serviceable, not necessarily a recently overhauled carburetor can provide. Not enough fuel or too much air can play havoc on an otherwise perfectly serviceable engine.

Have you watched the carb heat flapper move when you turn it on and off?