It appears that the 10A engine uses two ports per rotor for the intake. Interestingly, one port is closed off at low rpms for better torque. However, if this port were unable to open, it could severely hinder performance above 4 grand. Given that the carb will be dumping in as much fuel as possible but the engine will be receiving only half the amount of air it needs, the incredibly rich condition could be the cause of the smoke. That would also explain why it pulls cleanly until 4,000 rpm.
FYI all rotary carburetor engines operate this way from 10A to 13B.
Rene I'd like to correct a few untruths regarding any 10A engine, probably applied from the L10A to the last of the Rx3 10A's.
First of all pretty much everything seal wise required to rebuild a 10A is still available from Mazda. I know first hand because that is where I purchase my parts.
Owning any classic car is obviously going to need you thinking outside the square, whether it be a first generation Skyline, Savanna Rx3, Mazda R130, Datsun 1600 etc etc... I've been told a million times that I'm dreaming and will never get parts I'm after. I have organised quite a few non existent Savanna Rx3 parts, whether they be rubber, plastic injection moulded parts, engine parts, and my latest is brand new heated rear Savanna Rx3 windscreens including custom order all clear front windscreens (they normally have a blue band across the top). All done at reasonable cost, some parts are one offs which aren't shown anywhere.
From what I have seen on L10B engines there is not much to worry about regarding a rebuild unless it requires new rotor housings or new alloy side rotor housings. The rotors I saw first hand looked the same as R100 or early Rx3 rotors. I see no reason why you couldn't even use better sealing late model 10a Rx3 rotors. If the rotor stationary gear where different a competent machine shop could swap them over.
I've said this a million times to all the 10A detractors, Mazda still sell 90% of the seals required to build a 10A ! That's right you can still purchase 0 ring kits, apex seals, side seals, over size corner seals, water seals all from Mazda !!
While other parts such as apex spings are not available from Mazda they are still easy to find for reasonable money. There is a guy in Australia that sells the better sealing 2 piece 10a apex seals with springs for approx $500 or there abouts. That is cheaper than buying the equivalent 12a and 13B parts from Mazda. Mazda charge approx 400 - 450 for a set of 6 single single piece apex seals so that makes his price a bargain if you ask me. PM Mo's (user ID) if you are interested.
Other parts such as paper gaskets which would be unique on the L10B can easily be manufactured. We do this on for some of the later model twin distributor engines as not all gaskets are available. Not hard to do at all. The main problem with some of the new generation mechanics and the like is unless it comes out of a packet it's in the too hard basket. Rotor bearing and stationary gear bearings can be custom made to suit with little trouble. I see no reason once you know the required clearances, why you couldn't have a set of 12a or 13b bearings machined down.
Again there is nothing hard about building a 10a engine, most parts are available new (sealing parts). For the L10B your biggest concern would be the availability of the unique rotor housings. If they are screwed you will have a hell of a time locating a set. Same goes with the side alloy rotor housings although that would be less of a problem given that I see there is a guy in Europe making his own for a custom built quad rotor motor. Racing Beat also sell custom 12a / 13b alloy side rotor housings. What I'm getting at with these particular parts is that there are avenues available. I can think of 2 Australian engineering shops that could possibly be interested in making one off sets.
For me at least I don't believe in doing engine swaps because something is rare for the future generation to appreciate argument.... screw that !!! If I outlay some serious dollars I'm going to enjoy it for what it is and often as I like regardless of how unique or rare it is. If it blows up I'll worry about it then, there are always options.
Like others have said it sounds like a carb problem, I'd also take Danny's advice and have it properly tuned and not pull it apart until it's well and truely worn out And goes no longer. I just wanted to add that if the rotor housings and alloy side rotor housing are fine you'll have no trouble rebuilding the engine, 10A seal parts are not difficult to find. Joining some Cosmo Sport club would go a long way to prevent you from reinventing the wheel. Last I checked Europe, Australia and US still have good in house fabrication, machine and engineering shops all screaming for business from serious people in hard economic times. No one said owning any classic car was going to be convenient !