Need Help with Carbed 13B NA 1986 RX7 race car!

[Pink Racer]

Here's the situation.

1986 RX7 GX race car, NA 13B converted to Weber 45DCOE side draft carb, very aggressive Bridgeporting, Race engine with 285 hp. It was built on the dyno, and worked fine on it, but only ran 1/2 lap in the car before the previous owner blew the clutch, then parked the car for 10 years until we bought it (he drove it around the yard a bit, and winterized it, drained the fuel and the carb, etc).

Engine has turbo rotors and carbon apex seals. Runs on leaded 101 octane race fuel (VP Racing M105L) with 80:1 Amsoil premix oil (the OMP has been removed due to a previous leak causing an engine to fail).

Here's the jetting in the carb (this may matter later in this post)

mains: 210
air: 170
Emulsion tubes: F16
accel pump jets: 65
Idle jets: 65f9
Venturis: 45 (I think)

Here are the symptoms:

In neutral, the car will rev right through to 8,000rpm. When driving on the track, it will rev to 8,000rpm in 2nd gear, but in third gear, when it reaches 6,000-6,500rpm, it sounds like a rev limiter (but there is none on the car), it stumbles and cuts out and goes crazy until you let off and let the rpm come down. In 4th gear, it's even worse, only gets to 5,000-5,500rpm.

Also, in second gear, it will only go that high on gradual throttle, if you step on it really quickly, it won't get up that high.

If you sort of slow down and stop, the spark plugs are pitch black. Also, it was fouling the plug in the front chamber so badly, it was dropping rotor #1 at a couple times. If you go flat out on the track and shut down the engine as soon as it starts to act up and check the plugs, they have a slight white cast on them, and the bowl in the carb is almost empty. Also, it may be unrelated, but the racing beat intake manifold piece is getting so cold when you're at idle and playing with the throttle that it gets condensation and you could chill drinks on it, but the carb isn't cold, it's luke warm.

So here is what we've done already:

-new ignition coils
-new plugs
-new ignitors
-better ground on distributor
-cleaned pick up in distributor
-new cap/rotor button
-new plug wires
-played with timing a little bit, but it didn't help, so we put it back to where it was

-new Holley Blue fuel pump, 110 gph volume)
-new Holley fuel regulator, set at 3.5psi of pressure
-new 3/8" fuel hose from the pump to the carb
-new fuel filter (clear, we can see fuel inside it)
-Took carb apart and cleaned everything up, set floats to spec, checked all jets and needle seat, etc.


Here are some other jet combinations we tried on the weekend to no avail:

Main: 185
Air: 170
Emul tube: F16
Accel pump jet: 50

Main: 210
Air: 185
Emul tube: F16
Accel pump jet: 65

Does anyone have any ideas for this? Do you think it's a jetting issue or something else?

[seckerich]

Do you have a fuel pressure gauge you can read from inside the car? Empty float bowl and symptoms sound like it is running out of fuel for lack of volume. Spend the money on a wide band O2 setup to get the mixture right and rethink the fuel pump situation. Never had anything but problems from the Holley pumps and regulators. Seen better results from a twin set of facets. The O2 sensor will let you see exactly what is happening when it stumbles.

[Pink Racer]

Do you have a fuel pressure gauge you can read from inside the car? Empty float bowl and symptoms sound like it is running out of fuel for lack of volume. Spend the money on a wide band O2 setup to get the mixture right and rethink the fuel pump situation. Never had anything but problems from the Holley pumps and regulators. Seen better results from a twin set of facets. The O2 sensor will let you see exactly what is happening when it stumbles.
[/b]

In the process of putting in new gauges that will record up to 8 inputs on a memory card. O2 is going to be one of those inputs, just getting the Bosch sensor we need right now for this application. (A friend of ours is an automotive electronic engineer who does R&D for custom gauges and electronics for race cars through his company). The pump we put in is brand new and we can see it pulling a ton of volume through the filter, but I'll look at it.

Another thing people have said though is that it could be the 5th and 6th ports not opening, because the valves are completely load dependent, and have these symptoms. As well, we don't have anything hooked up to them, so I'll have to look and see if they're wired open, if not, I'll have to do that.

[noisycricket]

Another thing people have said though is that it could be the 5th and 6th ports not opening, because the valves are completely load dependent, and have these symptoms. As well, we don't have anything hooked up to them, so I'll have to look and see if they're wired open, if not, I'll have to do that.
[/b]

Even if the valves were not opening (bridge ported with 6 port end housings?) you should still be able to pull well beyond 5000rpm in any gear.

Your description of the problem screams a lack of fuel flow. In addition to watching fuel pressure, what size needle valve are you running? No experience with the DCOEs, but on the IDAs you want to run the largest needle valve available and remove the screen on the inlet. These carbs were designed to only have to flow enough fuel for 100-150 horsepower per carburetor, so when you run only one on a rotary, you can run into fuel flow problems. Incidentally, why only a 45 on a 13B? Those are almost too small for a stock 12A.

[Pink Racer]

The carb choice was the previous owner/builder's choice.

After much research, it looks like we're simply under-carbed. We will need to upgrade to a Racing Beat 51 IDA (a customized 48 IDA), with a 300 needle valve and 235 fuel jets and 165 air jets. The carb is $830 from Racing Beat, plus $252 for the intake manifold. :(

Oh well, we'll have to see if we can get our hands on it!

[sirchadd]

I never have luck with any widebands I have tried on mine RX-7, get better luck with EGT. Car definitly sounds like it is starving under a load though.