No carb heat at cruise speeds

Another dumb question from the new owner. I have carb heat via one of these heat muffs, connected straight to the opening of my airbox (no standoff type adapter, since there's no ram air I suppose).

On the ground and at low speeds in flight, it's effective -- I see several degrees of temperature rise on my carb temp gauge. But at cruise, I don't see any temperature change at all.

Is it possible the ram air speed at cruise is forcing the door open? Or does the brain trust have any other ideas? Flying around here in the PNW with no carb heat in winter sketches me out. I should never have installed that temp gauge, then I would have remained blissfully ignorant (I don't think I've ever actually had ice in it)!

Any photos of your air box? I had a similar issue with a Sam James intake. The ram air was preventing heated air from making it to the air box. I had to re-work the door to seal better. now I see a 30 degree temp rise.

Not specifically of the door/flap assembly, but here are a couple.

Right now it's baffle material in front of aluminum. It seems to seal well enough on the ground but I wondered if the force of the slipstream was kind of bending the baffle material backwards at the edges. How did you rework yours?

Not the inlet door

The problem is likely not the inlet door. That is simply not a large enough heat muff to be effective. I had the same issue even in Georgia so your Canadian cold air is much more of an issue. If you will use a full size muff that fully encloses the exhaust pipe like the ones for cabin heat you will see the results you seek.

Hmm, I hadn't considered that angle. The carb heat muff Van's sells is even smaller, but that doesn't mean it's effective, I suppose.

One thing I can't square away with this explanation is why it works on the ground and at slow speeds, especially since EGT would be lower. Is it that the higher airflow through the cowling and past the cylinders is robbing the muff of the warm air it's trying to collect?

Is your carb temp sensor mounted in the carb?

Yes sir, in the usual location (the pre-drilled hole) in the throat.

One of the guys on the field has a similar issue with carb heat on his O-360 powered 6A. The inlet door opens a bit even during an 1800 RPM mag check. Same thing in the air. If he keeps pulling on the carb heat knob the door stays shut and he sees his carb air temp rise. Not sure what style muff he used.

On my 7 I switched to a push button style locking Bowden cable and the same Cessna style shroud you have. The locking style cable has a slightly heavier wire. Mine stays put and always gives a carb air temp rise when in use. I initially used that Vans half pipe style carb heat muff and a regular Bowden cable. Terrible. No carb heat at all.

carb heat

It would be best to use a control cable with a locking function. Also the arm on the door looks a little short. Would be best to make the arm length so that it uses the full travel of the control. That would provide a little extra leverage against the airflow at high speed.

I was worried about that "forcing open" issue, and I don't have the locking type of cable, so when I'm giving it a shot of heat in cruise I've developed the habit of holding the knob, same as RV7 To Go's buddy. Doesn't seem to make much difference so I'm wondering if the whole assembly is flexible somehow, allowing the door to open slightly due to ram air without actually moving the control.

I think I may have to attack this from both ends -- beef up the door/control to make sure it's not going to flex/open, and replace the Cessna style muff with a cabin heat style one.

I'm just south of you ironically (in Sequim, WA) and my carb heat doesn't do much for me. The functionality of the cable/door system always checks out during my condition inspections, but does not seem to do much in the air. I'll be curious to see what you discover. My setup is very much like yours (I built it exactly the way Van's showed in the plans). Didn't care much when I lived in TX but do care now that I live up here!