I'm trying to determine if drilling a third progression hole on my 48 ida is worth it. I have read where some claim it eliminates flat spots and allows the use of smaller idle jets, giving a more reasonable idle. I have followed the (very much appreciated!) advice of Mr. Hanover in regards to jetting however I'm still not certain what idle jets I should begin with since they were not discussed. This 12a peripheral port will be mainly street driven but tracked on occasion.

The problem with periphery ported engines is that they often have little manifold vacuum near idle because there is a lot of overlap and some reverse flow that causes very poor idle vacuum, and very little fuel flow into the idle circuit. The early open late closing is just like a wild cam in a piston engine, and many of those sound just like ported rotaries.



So the owner screws in the idle speed adjustment screw to keep the engine from dying often. Screwing the idle stop screw in opens the butterfly a bit, and the edge of the butterfly moves above the idle mixture holes on the carb body.

(the two to five very tiny holes). Once the idle stop screw is turned in just a few turns the butterfly is well above the idle mixture holes and any hope of changing the idle jets having any affect is lost.



The idle circuit only works if the down edge of the butterfly is beside the idle jet holes in the body.



What most people call the idle air screws on the base of each throat are actually total mixture screws. They increase and decrease the total amount of mixture (fuel and air) being let into the throat below the butterfly. So when you screw in the idle speed stop screw you let in a whole bunch more air and screw up the idle even more.



So, drop in the biggest idle jets you can find and start there. Run the total mixture screws in to zero. Then back them out one full turn. All of the adjustment should be available between one and two full turns. But just for testing you can turn them out further to see if idle performance improves.



For example, let us say that the idle stop screw is one full turn in from butterfly fully closed. The idle is there but wanders all over. You install a bigger idle jet and the idle improves a bit but the engine still dies once in a while. You turn the total mixture screws out further and idle speed comes up and becomes solid, nearly acceptible. But the screws are out 4 turns.



turning the screws in to get back to 2 full turns causes the engine to wander again (too lean). Go bigger on the idle jet until it runs well at 1-2 turns out.



Even three turns is fine, but stretch the springs a bit so the screw doesn't fall out and get lost. The holes in the butterfly idea came from Daryl Drummond

who has built hundreds of rotaries for the Mazda pro series. Usually reserved for engines with all 4 irons "J" bridgeported. If it is bridgeported it is not going to idle like a stock engine. We don't tune for idle. My idle is 2,200 RPM.

The butterfly is well past the mixture holes. So, it makes no difference what idle jets I have.



Plus on a long spooldown from top speed, the fat idle jets keep the lubricant in the fuel coming in to help seal ware.



Lynn E. Hanover

wow..... this makes so much sence with my 48IDA on my 13B-PP and why the idle is so screwy... I'm running a 1900rpm with 80F10 idle jets. I can't find anything bigger.... I'm ordering in some f11 emulsion tubes to replace my f8's hope to see some more power.

The emulsion tubes have no effect on idle. More mid range and top end. The emulsion tube mixes air from the air correction jet with fuel standing in the emulsion tube well. All of this activity is only to get fuel to fall out of the booster or auxilary venturi. The big tube that hangs down the center of the carb into the choke. There should be no fuel dripping from the booster at idle.



More holes lower on the tube mixes more air and makes the fuel lighter so any vacuum in the booster lifts more fuel, for better bottom end of the mid range. Fewer holes low on the tube, and it is richer on top. Thinner tube=richer. Fatter tube= leaner, and so-on. The air corrector limits the amount of air the tube gets at top speed, so the air corrector can be thought of as the top speed mixture control. Two air corrector sizes is about the same as one main jet size change. The main jet controls all of the fuel use in the carb, but has little effect on idle just because so little fuel is used for idle. You can also raise and lower the float level and make a 2 main jet change. Just be sure the boosters don't drip at idle.



Bigger main jet= richer everywhere (but idle) Smaller main jet= leaner everywhere (but idle).



Bigger air corrector=leaner top end (full throttle top RPM). Smaller air corrector= richer top end.



For top end full throttle NA only, tune for 1600 degrees EGT and about 12-12.7 on the Fuel/Air meter.



At full tilt there are some items to change to get best power out of the Weber. The 48 is just a bit on the small side for a screaming rotary. A 51 or 55MM is better Berg has them as well as any jets you might need. They also have folks who speak Weber over the phone.



http://www.geneberg.com/



Lynn E. Hanover

I will purchase larger idle jets and follow your recommended idle tuning procedure https://www.nopistons.com/forums/pub...IR#>/smile.gif

i'm still mucking around with the best placement of the idle speed screw, i want to find the weber instructions, but so far have just come up empty.



anyways, 12a PP 1 turn in on the idle speed screw = 1100rpm idle



80f10 idle jets with mixture screws almost all the way in = 70f10 idle jets mixture about 1/2 turn out. give or take



80F10 is what the mazda comp book says to run, but they have no muffler



i just changed the exhaust from a 50ish inch header to 72" and it made a bigger difference than the jetting

This is what you would expect when changing the tuned length of intake or exhaust pieces. By changing the header length, you altered the amount of back pressure, and that alters the amount of overlap flow between the intake ports and the exhaust ports.



Rotaries tune like 2 cycle dirt bike engines. The periphery port engine is the closest in tuning, because of the amount of overlap, and the Renesis is only slightly like a dirt bike (with no porting) because it has no overlap at all.



So each change you make requires a new look at idle performance/tuning.



Lynn E. Hanover

i noticed! its so weird.



i need more tuning time, but i think the 70's are going to work out ok.

Great info!

Any recommendations for pump jet size? I'm also going to have to fab the header soon. The material I have is fairly thick walled (3/16") stainless with an inner diameter of 2 1/8"

Is this overkill? It has just been my understanding that PP's love to breathe and I thought the thick wall may help keep the noise down a tad, at least until further downstream. Not to mention I picked it up cheap.

I also have a Racing Beat street header for a 13b with much smaller runners. I could oval the mounting holes and port it a tad if necessary in order to make it work with the 12a if that's a better option. If I went this direction I would cut off the collector and continue the dual runners to the recommended length. At least I could use this until fabricating a better option.

I used whatever pump jet that Daryl Drummond put in the carb. It is of little use in our engines as we are using them between 7,500 and 9,600 RPM. The "J" bridgeport is worse than the periphery port as the j port is open longer, and is more snsitive to some tuning items. The stock carbs come with a 50 jet, and they go up to 100. The bigger the hole the shorter the time it will run and the richer the stroke. Imagine a car sideloaded at close to 2 G's bouncing across an apex as the throttle comes off idle and goes to full on in the blink of an eye.



The surface of the fuel in a one g turn is at a 45 degree angle. Nearly 50 for a two g turn. So in one direction the mains get extra pressure, and in the other the mains are uncovered and getting nothing. In the braking area where we can really pile on the "Gs" the front rotor housing goes super rich, and the rear housing goes dry. Main uncovered again.



The long shot time of the stock jet helps on starts where you can give the throttle a few pumps and seal the rotors with fuel, and a bit of volume jacks up the compression ratio. And bang, instant starts.



I will only suffer poor starting on the very first start after a build. Then it is instant or the engine comes back down.



The idle right after starting is nonexistant. Idle fuel is in droplet form rather than a spray. Once the engine warms up, the carb and intake picks up some heat and that heat overcomes the latent heat of evaporization (the chilling effect from fuel vaporizing) and the effect is that the idle mixture looks richer and richer with more heat in the intake. The rich idle jet helps here.



The idle mixture keeps going into the carb at higher RPM and throttle openings as well. For street use a leaner idle jet will help mileage. For great high RPM power, a well tuned Pport will get about 4.5 MPG at full tilt.



My headers are 1 7/8" ID. I use 1/4 thick (.120" wall) Mild steel headers. In racing the Racing beat street headers are just OK, but only last about one season. Thicker is better and stainless is better yet. The most broad power band will be with two header tubes all the way back to a collector, then a 180 degree bend in 3" tubing into a (big as will fit) muffler, then another 180 back out under the bumper. The Pport has dismal low end torque, but is rather smooth and not snatchy like a bridge port. You can just get it rolling and let the clutch all the way out and drive away, almost like a stock car. A short system is race car only.



I could drive our Pport to the false grid even with a usable first gear. Like a tall second gear in a street car. The same car with a "J" bridge port must be towed with a little tractor.



More than 2" ID is a bit big, but give it a try.





Lynn E. Hanover