How do I set up a Mikuni 44 blow-through carburetor on a Mazda RX-7 FC?

Setting up a blow-through carburetor on a rotary is an Expert-level difficulty project. Rotaries do not tolerate detonation, and if your air-fuel mixture goes lean under boost, you will destroy the apex seals almost instantly.

To convert your FC RX-7 to a blow-through Mikuni 44 setup, you are essentially ripping out the factory EFI and replacing both the fuel and ignition management systems. Here is how you need to approach it based on platform consensus and standard blow-through practices.

1. Intake Manifold and Mounting

First, you need to completely disable and remove the factory fuel injection system ¹.

If you are using a 6-port 13B (the naturally aspirated block), you can use the Racing Beat upper manifold (Part #18101) to mate a sidedraft carb to the engine ¹. If you are using a 4-port Turbo II block, you will need a manifold specifically cast for a 4-port to mount the Mikuni. You will also need to fabricate a custom throttle linkage or cable setup to actuate the carb.

2. Carburetor Preparation

A standard Mikuni 44 cannot handle boost out of the box. Standard practice for any blow-through carb setup requires:

• Solid Floats: Brass or hollow plastic floats will crush under boost pressure. You need solid nitrophyl floats.
• Sealed Throttle Shafts: Boost will leak out of standard throttle shafts, causing massive vacuum/boost leaks. They must be machined for O-rings.
• A Carb Hat/Plenum: You need a sealed bonnet that bolts over the face of the Mikuni to pipe in the charge air from the turbo.

I don't have verified, exact jetting numbers for your specific engine, as that depends heavily on your turbo size, boost level, and whether the engine is street-ported or bridge-ported. However, one community member running a Mikuni 44 blow-through setup was sourcing size 200 main jets and upgraded E-tubes as part of their tuning process ². You will absolutely need a wideband O2 sensor to dial in the jetting safely yourself.

3. Fuel Delivery

Since you are removing the EFI, you will need a custom fuel delivery setup ¹.

Standard practice for a blow-through system requires a boost-referenced (1:1) fuel pressure regulator. Carburetors only like about 3.5 to 4 psi of base fuel pressure. However, if you run 10 psi of boost, that air pressure will push back against the fuel needle and seat, starving the engine. A 1:1 regulator adds 1 psi of fuel pressure for every 1 psi of boost, ensuring fuel can always flow into the bowls.

4. Ignition System

Without the factory ECU, you no longer have computer-controlled ignition timing. You cannot run locked, naturally aspirated timing on a turbocharged rotary. You will need to either:

• Retrofit a mechanical distributor from an earlier 12A/13B and modify it to physically retard timing under boost.
• Run a standalone ignition controller (like an MSD 6BTM or a basic standalone ECU just for spark) to pull timing as manifold pressure increases.

Take your time with the fuel pressure and ignition timing. Start rich, start with conservative timing, and read your wideband carefully.