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RV-8A...carb heat seems weak...?

During run up, carb heat check shows a very small drop on RPM, (maybe 10-30rpm), and no other noticeable engine data indications such as a change in EGT, fuel flow, etc. You can almost not tell when it's on. I don't have a carb temp gauge in the plane.

Looking at our setup (pic below. bought, not built) I'm wondering if this is typical or if we should add a little baffle material to the flap there, or make a larger flap to get a more complete seal on the main air intake and force it to take more hot air from the engine bay for the carb heat. Concern would of course be blocking too much air from entering...? Any advice here, or examples of what you have is appreciated. Thanks!

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Carb Heat

If that photo is the full hot position that is the cause of your problem. In the full hot position the flapper should almost entirely. shut of airflow from outside the cowl. The flapper valve and arm should be strong enough to avoid flexing or movement to the cold position. A locking control cable is a good idea. Spruce sells a variety of locking cables.
 
Check the Bowden Cable Connection

From the posted picture, check and adjust the cable end of the flapper arm. I am not sure what is holding the Bowden cable end, could be a stop screw.
 

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Thanks for the info. My concern was about adjusting it or modifying it in such a way that it blocks that intake entirely, but I really didn't know if anything short of that would solve the issue.

I forgot to take a photo of the muff, but I will do that when I get the lower cowl back off.

It does have a locking cable and seems sturdy in flight, but I'll look at the end attachment and see if that is slipping and causing it to open.
 
In the case of carb icing, which is what that flapper door is for, you want to block that inlet entirely. There is plenty of opening above the carb heat flapper to feed the engine. It draws warm air from the lower cowl area, and from the semi-open tube on the exhaust crossover pipe. Newer air box design uses a much more robust sealed heat exchanger box that goes over both crossover pipes and is fed via a scat tube plumbed to a forward cowl inlet ramp for positive pressure. This same heat exchanger also feeds air to the cabin heat valve on the firewall.
 
all good comments above. First make sure that flapper closes fully.
For info, testing carb heat on my ship (standard 2003 vintage air box) during run-up also returns almost no RPM drop, despite the flap being fully closed...
 
During run up, carb heat check shows a very small drop on RPM, (maybe 10-30rpm), and no other noticeable engine data indications such as a change in EGT, fuel flow, etc. You can almost not tell when it's on. I don't have a carb temp gauge in the plane.]

o320 carb, 1992 airbox, my flapper secure and totally closes, same results as you.
 
When you get a chance...

Please remove those AN364/5 NyLock (Fiberlock) nuts and replace with MS21042s.
 
What he said except some fasteners need to have more thread showing. Personally I would just do the flapper upgrade and get rid of all those steel fasteners that have a potential to become undone and ingested. Ever see a piston crown and cylinder head on an engine that swallowed a nut or screw? Not a pretty sight that can result in catastrophic failure.
 
When carb heat is applied, that carb air door should block air entry from the front air inlet. As shown, the door is not closed as far as it should be.

IF THIS SPOT, has been ground out to move past the nut, your door is not totally closed in this pic. Verify you are indeed getting full travel.
 

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For those that get little carb heat drop - is your muff the same as the one at https://rv6aj.wordpress.com/2013/03/03/mounting-exhaust/ - 3rd photo from the end?

If so, that "muff" is notorious. It's not going to do much. You really need something better. Well unless your idea of 'carb heat' is a knob that I pull for show, not function.

Also the original poster - was concerned about fully blocking the intake - think about encountering wet snow. You want to keep that out of the air filter, not let it pack in.
 
The vertical heat muffs are next to useless. I installed a 6" stainless muff around one of the horizontal pipes which was better but did not produce the rpm drop you see in store bought aircraft. I then fitted a carb temp gauge which never went anywhere near to freezing so stopped worrying about the heater muff...
 
that "muff" is notorious

Ok, admit, that is the one installed on my ship. And agree, I think the effect is probably more like having alternate air than a real carb heat.
On the other hand, >700h with my "old" original FAB, some in very icing prone conditions, and some with moderate icing on the airframe, never had any indication of carb icing (disclaimer, this reported experience is for my ship only, and may not apply to yours) or real carb ice (drop of MP with my CSP).

Had a carb gage as well... spent many a flight with the needle in the yellow range, but as reported above, nil carb ice. During the last panel update out went that useless gage, sensor and wiring...

Shall I rename "Carb Heat" into "Alt Air" ;)
 
If the carb heat door doesn’t fully seal the opening when full carb heat is applied, even the best carb heat muff will be totally useless. This is emphasized and detailed in the latest fab airbox install instructions.
 
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