Will Claypole’s Experience using a Stock 12A

DELLORTO 48 DHLA CARBURETORS

This is a rundown of what I've learned from my own personal experiences with Dellorto carburetors on RX-7s. This carb isn't always as easy to work with as it was meant to be, but IT IS possible to get a Dellorto to work almost as smoothly and reliably as the stock Nikki carb. As opposed to some other aftermarket carbs, when it comes to Dellortos, power DOES NOT have to come at the expense of drivability.

I picked-up a used Racing Beat Dellorto (12A) carburetor this summer and convinced myself that I could make this setup work "this time". This wasn't my first experience with a Racing Beat Dellorto carburetor... I had one on my car (83 12A RX-7) for a couple years in the past. Unfortunately it was stolen out of my car by a crooked mechanic, which, ironically, didn’t matter because I had basically given up on this carb and had already decided to return to the stock Nikki.

Anyway, here I am, 2 years later, with another used Dellorto and I’m convinced to make it work properly this time.

MY REBUILD

The first step was to rebuild the carb. This included inspecting every jet for damage and proper sizing. David’s site came in handy... they where EXACTLY the same.

230 main fuel

230 air correction

7772.8 emulsion tube

75 Idle fuel jet

7850.4 Idle jet holder (air jet)

.1 auxiliary venturi

95 starter jet

90 accelerator pump jet

Checking these are an important step... it’s very important to know whether the carb has been rejetted for some reason. The next step was to blow compressed air into all the passages/circuits/jets to make sure they where free of contaminants. The exploded diagram was very useful while doing this... there are many tiny parts inside the carburetor that have to be removed before blowing air through it. I also decided to replace every gasket (the kit was sold to me with most replacement gaskets). I had to make a few gaskets... the mid manifold gasket, upper manifold gaskets and that gasket "washer" that keeps the pump jets aligned (it was mysteriously missing). I made these gaskets out of manifold gasket paper available at most car shops. It’s kind of hard to cut, but a sharp exact-o blade did the trick. The pump jet "washer" is an extremely critical part, as Dave has explained. I used Dave’s technique of making a mark on the head of the pump jet to ensure that it was aligned with the passage molded in the carb body. Finally I polished the entire carb with a Dremel tool w/buffing wheel and aluminum polish. I really should have adjusted the floats and accelerator pump while the carb was off the car and "dry", but I skipped that step and regretted it later when these adjustments proved to be very critical.

RECOMENDED FUEL PUMPS

MANY fuel pumps are available for this setup… Carter, Holley, Facet and others all make fuel pumps suitable for this application. RB recommends a pump that puts out 7-10psi and has a flow rate beyond 30gph. This setup would require a regulator and some kind of fuel pressure gauge. Its important not to go too big when buying a pump, because a bigger pump needs more fuel running through it to cool it down. A 150hp rotary doesn’t require a big pump with a huge flow rate… the bigger the pump, the quicker it will overheat and fail. Regardless of the pump chosen, fuel pressure should be set between 4-4.5psi and the pump should be good enough to maintain AT LEAST 2.5psi at higher RPM. I was about to go to a Carter setup when I got an email from Jim Langer at RacingBeat telling me he had one last Facet set-up in stock. I chose to go for this setup since its been designed to fit on an RX7. It also included a Purolater "Pro Fuel" regulator. RB warned me that the pump was "loud". No matter, so I thought, and I decided to test try the pump on my stock carb. The installation was easy but awkward. Not all the mounting hardware was provided in the kit, so I had to improvise. When I finally had the pump and regulator in place, I set the ignition to "on" to see if it was working... "tickticktickticktickTICKTICKTICKTICK" (think fish tank air pump). This thing is LOUD. Maybe a little too loud. I think one of the pumps is hitting the chassis when it’s vibrating causing an even louder "TICK". I will have to look into this. Apart from the noise, the pump works very well. I set the regulator to "3.5" for the stock Nikki, and I could feel a nice difference in high rpm response. I would recommend a new pump to anyone still running the stock one on a stock carb setup. I set the regulator to "4" for the Dellorto.

INSTALLING THE DELLORTO

This is, without a doubt, the best aspect of the Dellorto... it is so simple to install. Once the clunky Nikki carb and emissions are out (not a very hard job), the Dellorto simply fits right on. I used a stock Mazda intake manifold gasket and Mazda O-rings. Only use Mazda O rings and not generic ones. These "cheap" rings can deform and shrink when heated by the coolant around them causing an external or internal coolant leak. If you want to forget about O rings and leaking coolant once and for all, a quarter fits in the coolant passage perfectly. A little JB weld is all that is needed to seal it up. When bolting on the carb, it’s much easier to install the lower portion of the intake manifold first, and then bolt the carb up to it once it's in place. I recommend tucking all the emission wires under the air filter... cutting them would be a bad move if you ever had to go back to the stock setup someday.

The oil metering pump rod supplied by RB had a tendency to rub against the manifold, but a bit of bending solved that problem. A sturdy coat hanger can be bend up if the RB rod is missing (a common occurrence when dealing with used carbs). If the RB oil lines are also missing, it’s possible to use the original oil lines from the Nikki setup with a slight extension on the rotor 2 line. I recommend heating the bends in the lines with a lighter and straightening them out. Just make sure not to melt the line closed, or open.

The throttle cable housing bolts right up to the bracket. Its important to make sure the throttle actually opens all the way when the gas pedal is floored. The part of the bracket the cable slips into might have to be bent forward slightly to achieve WOT. Also, its important to set the PEDAL adjusters properly to ensure that when the pedal is floored, it is exerting pressure on the adjuster bolt and NOT the carburetor and cable. Your leg is capable of enough pressure to snap the throttle cable and bend the bracket (or carb!) under WOT situations. It’s also crucial to leave some "slack" in the cable when the carb is in the idling position.

The choke cable is easy to install, but it has a tendency not to want to return to the closed position. I recommend placing a spring between the cable housing bracket and the cable end bracket. The narrow/long throttle assist spring on the Nikki is perfect for this application.

Connect the fuel line, oil filler neck vent hose, brake booster, cap the fuel return line on the firewall and that's it. It's ready to go.

Starting the dry carb is pretty straightforward. Leave the key on the "on" position for a few moments to fill the floats bowls. Floor the gas and turn the key. Keep your foot down the whole time... it usually takes 3 cranks at 5 seconds each. Once it fires up, slightly blip the throttle until the carb runs smoothly. Warm the engine to operating temperature and adjust the idle to 900rpm. Take your car out for a very hard run… make sure the engine is very hot when adjusting the mixture adjustment screws. I recommend turning them out almost all the way, and then turning them back in until the engine starts to stumble, then back out about 1 1/2 turns. If this is done when the engine is at normal operating temps, the next time you drive hard, your car will idle badly until it cools again.

ACCELERATOR PUMP ADJUSTMENT

The accelerator pump is one of the parts RB had to modify to make these carbs compatible with rotary engines. They added a rubber spacer under the pump body to increase its capacity and to provide better clearance for the pump arm and rod to function properly, but even that wasn’t enough. It’s essential to get as much pump arm travel as possible (at least 13/32").

Here's what Jim Mederer, Racing Beat’s co-founder and chief engineer had to say on this subject:

"The measurement of the accelerator pump stroke is critical and difficult. In short, it needs to be the maximum possible to avoid stumble. To obtain maximum stroke, loosen the pump cover slightly, depress the pump arm, and tighten the screws lightly. Now adjust the linkage to obtain maximum travel. The motion of the tip of the arm near the adjustment nut must be 13/32". In some cases, slight bending of the rod may be necessary." (Thanks Jim!)

Without bending the rod, the carb will not properly return to the idle position when the arm travel is within spec. Interestingly though, I have found that going much beyond 13/32" can actually be detrimental to accelerator pump operation.

CARBURETOR DEFECT? RACINGBEAT CONSPIRACY?

I found on both my Dellorto carbs that turning sharply to the right (w/ low rpms) causes stumbling when applying throttle upon exiting the turn. As far as I've been able to tell, this happens no matter how properly the throttle pump is adjusted. Float levels seem to be directly be related to this problem... the more the bowl level is increased the less the engine stumbles. Racing Beat recommends float levels of 15mm from the gasket in the closed position and 28mm in the wide-open position. It’s important to realize that the 15mm measurement is taken BEFORE the needle spring is compressed. These measurements provide a good compromise between the right hand turn stumbling and flooding the carb by over filling the float bowls. After a lot of experimenting and testing, I've found that a 14mm float height provides improved response and less stumbling than the RB recommended 15mm height and zero flooding. True this setting will cause the carb to run slightly richer (due to the fact that the emulsion tube is more submerged in fuel) but I think that this richer mixture accounts for the smoother operation and near zero stumbling while cornering to the right. Going much beyond 14mm results in even smoother operation, but the instances of flooding after hard cornering or sudden braking also increase. I'll be experimenting with this for a while, and I'll make sure to report back to Dave with the results.

SO, HOW DOES IT WORK? IS IT WORTH IT?

A properly tuned Dellorto is MUCH MORE sensitive to on-off throttle application than the stock Nikki. At times this is beneficial, but in certain situations this can become downright annoying. When calmly driving around, throttle application is best described as “feathering”… the weight of your foot on the pedal is usually all it takes to get the car going from a stop and to a cruise speed. This is also true on the highway, where maintaining speed normally consists of “lightly touching” the gas pedal with your toes… this can get tiring on long drives.

This carb also doesn’t like to be rushed at low rpm. Quickly flooring the throttle at anything below 2500 rpm usually results in a “BWWWAAAPPP (I’m not going anywhere)” sound and this is sometimes accompanied by the car jerking back and forth (I’d like to go faster, but I can’t seem to make up my mind). Of course, the Dellorto really begins to shine if the next lower gear is selected and the pedal is floored. That’s when you notice that this ain’t no stock Nikki. Once floored above 4000 rpm, acceleration is delivered in a solid snap. Whereas the Nikki seems to build speed progressively in WOT situations, the Dellorto gives you all it’s got up front… and it’s got a lot! The rush the Dellorto provides as its building speed more than makes up for its sensitive throttle and the sound it emits is pure aural delight. Shifting quickly can also be tricky, requiring a quicker release of the clutch because of the instant power delivered when the throttle is once again depressed.

Although I haven’t actually compared acceleration times between the Nikki and the Dellorto, I can safely say that the Dellorto provides more power and faster acceleration in pretty much all situations. One exception would be top-end power. Maximum speed and power over 7000rpm does not seem to be much improved over the Nikki. I’m assuming this has something to do with the intake manifold restrictions that Brad mentions in his write-up. This doesn’t affect a stock 12A that shouldn’t be seeing 7000+ anyways, but any kind of porting or internal strengthening would prove useless because the Dellorto requires you to shift at or before 7000rpm to maintain speed.

Is it worth it? The stock Nikki carb is very hard to beat when it comes to smoothness and overall quality, but it lacks the airflow potential of the bigger aftermarket carbs. If you’re looking for an aftermarket carb for a street driven stock or street ported 12A or 13B, the Dellorto is probably one of, if not your BEST choice. It’s not as smooth as a Holley carb, but it is infinitely more flexible and more reliable in tight corners. Webber makes good carbs and excellent Webber side draft kits are available for rotary engines but the Dellorto carb improves on the basic Webber design in almost every way. Many more carbs are available out there, but for the $, an RB modified Dellorto is very hard to beat. Dellorto setups also shine on 13B 6 port engines. The RB upper manifold bolts up directly on to a GSL-SE lower 6 port manifold. RB 13B Dellortos are available out there, and I would recommend using one of them rather than re-jetting the 12A units.

Another excellent aftermarket carb option would be a Paul Yaw modified Nikki. I have yet to test one of these out, but from what I hear, they might just be the new carb of choice. Hopefully in the near future, I’ll be able to post-up a Dellorto vs. Yaw/Nikki comparo.

Will Claypole

For any questions or comments, I can be reached at rxcetera@hotmail.com. Thanks for reading!