crx warm air intake
 

my carbureted crx has a 1 3/8" hose going to the exhaust manifold . it is vacuum operated to open when the engine is cold. I am considering fixing it where it is open all the time. the problem is, it raises a flap that closes off the main air tube, which is 2" diameter. I could remove the flap , allowing it to get air from both sources, or partially block the 2" tube, allowing it to draw air from both. any suggestions?

If you are properly hyper miling I don't think you will be restricted much. But opening it partially so that it draws air in from both sources would probably be wisest. Carbs are very sensitive to additional vacuum which acts like a choke. Get some warm air in there without too much restriction.

I removed the flap so it can now draw both cold and hot air. the cold air has no hose on it, so it is getting warm air from inside the engine compartment. the 2" tube should be drawing warm air, and the smaller tube can also supply hot air all the time now.

That flap is thermostatically controlled, so whatever you do, make sure that the air going in to the carburetor is kept at 100F, otherwise your engine is going to run to lean if the air is to cool.
That bit of ducting is the same size of course as the 1.5L engines used in the 4 wheel drive, 4 door wagovan, so I wouldn't worry to much about it in your CRX's 1.3L engines intake, you could look up what size is the ideal size for air intake tubing for your size of engine, but I suspect it will be pretty close to what you have as the intake on my civic vx is pretty close to that size as well with nearly twice the peak HP.

I have an upper grill block, so the under the hood temps should be plenty warm. if I have to, I'll route the 2" intake closer to the exhaust.

Over time it's pretty easy to tell if your engine is running excessively lean... of course you already have a lean burn engine so running even leaner is risky, but if you find that you are burning and bending valves or burning holes in your pistons then you are running to lean, with your engine design your spark plugs are getting a rich charge so it will still start and run ok, other pattern I've noticed with people who've removed the warm air intake thermostat controlled flap is head gaskets don't hold up either, not sure if it's connected, but engines that are running lean tend to burn hotter then designed.

Did you find any charts to see what your ideal intake size is? to large of an intake and you loss momentum and loss efficiency.

I very rarely run it over 2,000 rpm. so I doubt I will have a problem running too lean. I could always choke down the 2" intake to match the 1 3/8" heated pipe. I would guess those having head gasket problems are running much harder than I drive.

The issue with running lean is not to much air but to cold of air, unless you have a remote thermometer probe in the air cleaner box to track the air intake temps because that little flap and the vacuum diaphragm is there to keep the air temp consistent so your air mass can stay steady, it gets rid of the need for a MAP sensor and a bunch of other sensors that you would have if you had fuel injection.
I know I've seen calculators before for figuring ideal intake size but I can't find one at this hour, but that 1 3/8" heated air duct is the same size as the Honda Accord used to supply air to their 1.8L engine of the same vintage as your car, so I really doubt that it's causing a lack of HP or MPG for your 1.3L engine running at 2,000 RPM, I just would hate to see you damage your engine because you ran it lean by disconnecting some of the controls on the carburetor.
On the other hand it might just keep the choke on if the air is not up to temp, so you might be safe from engine damage but would also be wasting gas and reducing the performance of your engine by having the carburetor keep the choke on.

I appreciate your concern. by removing the little flap, I am allowing it to draw hot air from the manifold continually. I might go ahead and block off the main inlet ,and just feed it with the 1 3/8" hot air tube. I seldom, if ever rev it over 3000 rpm, so the loss of the 2" inlet tube probably won't hurt mpg. I could also run a hose down to the exhaust manifold, and have 2 warm air intakes.

I guess I'm at a bit of a loss now as to what you are trying to do, from what I've seen people tend to find their peek mileage when their intake air is around 100F to 120F and your car has a very clever way that keeps the intake air exactly in that temp range.
I would also guess that if you wanted, you could use a chunk of a larger size of garden hose to supply the warm air and not notice a change in performance or gas mileage up until near full throttle.