Brad Page’s Experience using a 12A Street-Port

I’ve had a Dellorto 48 DHLA on my streetported 12A since January 1999 and have been tuning it extensively. The jetting that Racing Beat installed on the carb was designed for a stock port 12A motor and is inadequate for a streetported motor. I did find an improvement by going to larger main fuel and smaller main air jets, as well as larger idle fuel jet (this helped the progression stage of the carb a lot). The problem with the Dellorto setup is not the carb, it’s the horrible manifold that Racing Beat designed for it. It’s a 2 piece manifold, the upper piece is the same for 12A and 13B models. The bottom piece must be purchased from RB for the 12A; and the upper piece bolts to the stock intake manifold for the 13B. This was done for ease of install and lower cost for the 13B owners, which in a nutshell resulted in restriction. Read on for details.

Some people say Weber is better than Dellorto, but the Dellorto is a fine carb. At European Motorworks, the local carb shop I go to for my jets and carb parts, I see just as many Dellorto’s as Weber’s. Dellorto took the basic design from Weber and improved upon it and used more modern manufacturing techniques. It has more progression holes, non-brass floats (like Weber), a wider range of jets with more choices within the range, and several small details. The 48mm DHLA can supply my thirsty 12A streetport with pleanty of fuel and make good power.

The jetting that Racing Beat put in their carb, as intended to be installed on a stock port 12A with RB exhaust and RB intake manifold is:

230 main fuel

230 air correction

.8 emulsion tube

75 Idle fuel jet

.4 Idle jet holder (air jet)

39mm chokes

.1 auxiliary venturi

95 starter jet

80 accelerator pump jet

Keep in mind the .8 emulsion tube has had several holes drilled in it by Racing Beat, to facilitate a leaner mixture above idle. There are about 20 holes total in the e-tube. A normal .8 emulstion tube will be too rich. You can still buy the RB .8 e-tube from RB. What I ended up with on my streetported 12A with RB’s intake manifold RB dual presilencer exhaust system and primaflow muffler is:

240 main fuel

210 air correction

.8 emulsion tube

80 Idle fuel jet

.4 Idle jet holder (air jet)

39mm chokes

.1 auxiliary venturi

95 starter jet

80 accelerator pump jet

3 ½ turns out on the idle fuel mixture

The 80 idle fuel jet was too small, which made the motor run real lean on the progression circuit. My A/F meter pegged on the lean end of the meter fora sec or two during hard acceleration; and I tried 85’s which were way too big. I think 82’s are a good compromise. I also tried a 90 accelerator pump jet and it seemed to help a little, but I’m not sold on it.

I noticed the RB manifold, which is U shaped and has 2 sharp bends, is very restrictive. A shop that flows and ports manifolds told me this too :-] It’s throttle response is not all that good, and it’s rather lazy off the line. Also it flows like shit above 7000rpm. At the track I was shifting at 7K. Anything above that and I was just spinning the motor. A streetported rotary has it’s powerband from 5k - 8k. The manifold never let me get into the top of my powerband. This is the reason why Racing Beat couldn’t make a good street port Dellorto setup. NOT the carb. Try blowing air thru a bent straw and you’ll know what I mean. No one can not seriously tell me that Dellorto’s are the hot setup on 914’s, air-cooled VW’s, and Mercury Capri Karmann built 6 cyl, but for some mysterious reason it falls on it’s face when bolted up to a rotary. March 1999 I installed an intake manifold built by Lake Cities Rotary. It’s 4” long, horizontal with NO BENDS, allows the carb to have a straight shot at the intake ports like a true sidedraft should, and is the only manifold positioning the carb beside the motor and not on top. Unfortunately it’s also above the headers which are real hot, but I adapted the stock heat shield to fit over my headers, by building a bracket so it’ll bolt onto the two upper header bolts which cooled down the carb and manifold a bit. Also pointed one of my cold-air induction tubes towards the manifold. All this really reduced under-hood engine temps. Needless to say the manifold flows great, and absolutely positively screams over 6k. MUCH BETTER THAN THE RACING BEAT MANIFOLD. Day and night. I never knew my motor could revv up that quickly. But that was with the 39mm chokes, which were way too big for this manifold and my low end torque sucked (below 5K not much oomph). This manifold was originally designed for a Weber 45 DCOE with 36mm chokes.

I have just started to rejet it with 37mm chokes and this is what I have so far:

190 main fuel

220 air correction

.8 emulsion tube

82 Idle fuel jet

.4 Idle jet holder (air jet)

**** 37mm chokes ****

.1 auxiliary venturi

95 starter jet

80 accelerator pump jet

2 turns out on the idle fuel mixture

Here I am using cheapo 30mm tall velocity stacks and a 90mm tall foam ITG air filter. The stacks inside diameter are a little small and restrictive. I gotta try different air/fuel jets but this is a good start. Much better low end torque than with the 39mm chokes, about what the RB manifold has, and with the LC manifold it revs much better over 6K. Really allows the enlarged engine ports to flow. I think this manifold added at least 5hp. Frey Racing dynoed the manifold as adding 15 more horsepower to a streetported 12A, but I think that’s a little optimistic. BTW the LC manifold is not made anymore thru LC. Atkins Rotary sells it for $200.00 but if you post ads on the internet someone might respond with a used one for less$.

June 2000 I now use the 39mm chokes with the LC manifold since I only use the rx7 on the racetrack now for drivers schools. I am now using TWM induction 50mm tall velocity stacks and a K&N 3 ¼” tall air filter. The air filter barely fits inbetween the carb and thr passenger side frame rail/unibody. Not much torque below 4000rpm. The motor doesn’t wake up till 5000rpm but that’s fine for a track car!!! The carb’s jetted like this:

180 main fuel

210 air correction

.8 emulsion tube

80 Idle fuel jet

.4 Idle jet holder (air jet)

39mm chokes

.1 auxiliary venturi

95 starter jet

90 accelerator pump jet ** seems to like the 90’s.

2 ½ turns out on the idle fuel mixture

January 2001 I just installed some 40mm chokes since the motor didn’t wind out as much as I wanted it to over 8000rpm with the 39’s and that’s a bad thing since my peak horsepower and shift point is at 8500rpm. I will start tuning the carb next month. I’m having a close ratio tranny built to keep the motor in it’s narrow rpm range.If you want to buy different jets, try cbperformance.com or European Motorworks @ 1-800-722-8678. The latter has really helped me out.