Finally working out how to get carb heat on my weber carb. Built a muff for the exhaust and a simple airbox around the air filter. My airbox has an inlet door ar the front and a scat tube connection at the rear. The idea being that with the door open the carb gets ram air from the front and with the door closed heated air will be pulled up from the muff. I have yet to drill the inlet holes in the muff. Im going to drill them in the foward side so they receive ram air pressure as well.

What are your thoughts on how munch inlet area I need in the muff to feed the carb?

Chris

What are your thoughts on how munch inlet area I need in the muff to feed the carb?

Well, at least as much area as the carb throat and I'd guess even more is better. I made an exchanger similar to yours but longer, 10 inches long. Yeah, mine is bigger than yours ; ),  Results? I can hear no detectable RPM drop nor see any on the tach. Rev-B. Added another 10" exchanger on the other exhaust stack and run the pair in series, (one feeds the other). Results? Still no RPM drop. Currently how it sits but will have to revisit it someday. Best guess is really need some kind of baffling inside.  Or check it with some kind of temp gauge to see if it is doing anything.  I have a concern that too much baffling and scat tubing length may make a restriction and create a partial choke effect at which point "undesirable" side effects may occur?
Please share any results good or bad that you get.

I'm trying to figure out how to do mine.  My current idea is (since my air is fan cooled) to tap off the hot air exhaust vent at the top of the engine, cutting another hole in the cooling air shroud and putting an air valve in to route air to the intake.   How hot does carb heat air need to be?  The air coming out of the cooling exit is I think something like 150-160 degrees when the heads are nice and toasty.

ITman496, What engine and carb are you running? Sounds like a rotax 2 stroke? In all my time flying 2 strokes I have only carb iced one once and it was 12 years ago on a cool damp morning with 35deg air temps and 60% humidity, all it took was jockeying the throttle a few times and the cycling of the slide knocked the ice right out of it and got her purring again. I just never give carb ice on a 2 stroke any thought, its quite unlikely to happen given the proximity of the carb to the engine (lots of heat transfer) and if it does a simple cycling of the carb slide cleans it right up again. I have done as you suggest though for cabin heat on a Tbird, capturing the heated cooling air works great for that, just make sure you don't restrict the air flow at all or you will have cooling problems.

Flydog, I plan to get my setup finished thursday and if the weather breaks and the rain stops I will be sure to give a report!

Chris

I am using a kawasaki 440a with blower cooler on it!

It is a 2 stroke, however the carb mounts to the manifold with a rubber bushing that I imagine keeps temps quite low?  I am unsure.  I have an intake air temperature sensor so I'll have to keep an eye on it.  Also, its a slide carb so maybe those de-ice easier?

ITman,
That's the same setup rotax uses. On 2 strokes there is Alot of hot air that gets aspirated back into the carb which also helps to keep it warm. I wouldn't lose any sleep over it, unless you intend on regularly flying in cold damp weather you'll be fine.

Chris

Oh!  That's a good point, I forgot that two strokes pushed air forwards out of the carb during their cycles.  Which is why I can't get fuel injection.  Thank you, you just saved me a bunch of time!

ITman496 said: "Oh!  That's a good point, I forgot that two strokes pushed air forwards out of the carb during their cycles.  Which is why I can't get fuel injection.  Thank you, you just saved me a bunch of time!"


You can fit fuel injection to a 2 stroke, however, it depends on the engine. If it has reed valves in the induction then you don't get the blow back through the carbs, so you can fit fuel injection.

I've seen a design for fitting an injector into the side of one of the transfer ports, but that meant the engine had to have oil injection as well, in order to lubricate the bearings.

Mine does not have the reeds, as its the kawi 440a.  I heard the only injection it can have is basically 'throttle position, rpm, and thats about it'  Alpha-N?

Carb is fine.  Just wish it was less finicky.

UPDATE:

Ive flown around 2 hours on this setup now. I have been playing with the size/number of inlet holes in the muff as well as wrapping the exhaust with screen door springs for better heat transfer. While running at cruise power with a remp probe inserted in the airbox I am reading a 15ish deg rise in temperature of the intake air when carbbheat is applied along with a 50-75rpm loss at power settings above 3k rpm. Below 3k rpm, temp rise is about 10deg and there is no measureable rpm loss withbheat applied.

Chris