Hey, just after a few opinions on which jets to run for this motor. (See pics below.)



Previous setup before she blew was...



13b Bridgeport Rx5 Block

48 IDA

43mm Chokes

F10/90 Idle 120 Holder (F10/70 had been drilled)

230 Main 170 Air F7 Emulsion Tubes

Maybe 90/40 Acc Pump Jets (drilled aswell)



Free flowing exhaust, medium-large exhaust ports. Motor blew due to 1) running an UNLOCKED dizzy and 2) possibly too lean up top due to 170 Air's?



Will be putting it on the dyno once its built, but want a ballpark staring point in re to carb jetting. The previous setup had very good drivability, but was somewhat unorthodox and apparantly too lean up top?



Cheers

 

You didn't say what this is used for? Below is the road racer setup.



The air correctors are too big, but usually there needs to be some work done on the Weber to get enough fuel flow for the size of chokes you have. A 110 air would get you close, jet wise. The 203 mains are a bit big, but again that is not the problem.



The Weber is operating above its design flow rate with chokes above about 36MM. However there are some mods that can increase fuel flow enough to work. Get in touch with Paul Yaw to see if you can buy one of his 6MM needle and seat sets.

Or find a Grose jet from years ago, that uses two ball bearings instead of a needle. The Weber only goes up to 3.5MM and that is not enough flow for the rotary. So the effect is that the needle and seat becomes the only jet, and changing anything else (beside the choke size) just has no affect.



There is a filter under the brass nut at the fuel inlet on the bowl cover. Discard the filter. Drill out the hole below the filter to about twice the size it is stock. Chamfer the ends of that hole. Clean and install very large needle and seat assembly.



In the float bowl there are two accelerator pump wells. Only one is used. Drill a 1/4" hole in the very bottom of the unused well. Notch the top edge of this well so it cannot seal against the bowl gasket. This is to increase bowl volume. In the old days it was common to see a ford float bowl epoxied onto the side of the Weber bowl to double the bowl volume. It would get you further down the straightaway before the engine layed down overlean.



But it is not required.



So now you could dump in enough fuel to keep up with the engine at high revs coming out of a corner, but right after that you need way more fuel as the revs come up to 9,000 RPM or whatever you shift at.



So, adjust fuel pressure up to 6 pounds. Adjust the float level up until the boosters just start to drip with the engine off and just the pumps on. Always start the engine with the pumps off. Then turn on just one pump until at speed on the track, then turn on the second pump. Keep in mind lots of extra fuel will fall into the engine as the car bounces around, and if you get a stumble that is too bad to race with out af a tight corner, then just lower the float a bit.



You need MSDs or similar on leading and trailing to light this much fuel. The rotary will run fine way over rich and have very low EGTs. It will also run just fine way over lean, and have very low EGTs. Makes for a tough read tuning wise. At low speeds you might see a hint of black smoke. Coolant temps will be lower than normal. Oil temps lower as well. Same for over lean..................ouch.



Way over lean will not hurt anything. Less fuel means less heat and better mileage. With a CD ignition there will not even be any missing. However, with overlean, the engine keeps passing through, best power, best power mixture as it leaves transition throttle going to full throttle, and right at the end of accelerator pump fuel supply. (You want a long lasting pump stroke and supply) That is where the apex seals get turned blue and sag in the center.



So let us say you have all of this done and you need jets to start off with. Mains 200 to 220. Air correctors 100 to 160.



Cool morning, low humidity= 220 mains 100 or 110 airs. F-7 tubes.



Hot afternoon lots of humidity= 200 (or smaller) mains. 140 to 160 airs. F-7 tubes or maybe f-11 tubes.



An easy test with no insturments in the car: On a long straight, left foot brake at full throttle, to hold the engine at the test RPM, (say 8,000 RPM) shut off the fuel pumps. look at the tach.



RPM sags away as soon as the pumps are off. Too lean.

RPM hold up for a few seconds (as the bowl empties) then sags away. Real close to right on, jet wise.

RPM builds up for a few seconds then sags away. Too rich. Engine begins to run better as bowl empties.



Make a movie of the gages that include the RPM/FA/ temps and EGT. Top temps not more than 200 water, 210 oil.

100 PSI of oil pressure. not over 1600 degrees EGT. FA in the low 12s. Use Redline 40 wt. racing oil in the sump. Redline racing 2cycle oil at one Oz per galon as a premix. Use a rev limiter. The engine will last for years, assu,ing it is assembled properly.



Years ago, a fellow helped us at West Palm beach, when I was having a devil of a time trying to get the rotary to run properly. He had a tuning gizmo the screwed into the drain nut on the float bowl. It had a clear plastic tube with MM marks on it. So it reported fuel level in the bowl. Pretty clever. He started my engine, and ripped the throttle wide open for maybe half a second. Not quite to 8,000 RPM. What do you suppose the fuel level dropped to when this happened?



It dropped to zero.



So the needle and seat is the only jet involved in tuning in that case. You need a big fuel pump. If you cannot hold 6 PSI of fuel pressure all the wat to the end of the longest straightaway, then you need more pump. Measure fuel pressure right at the carb inlet.



Lynn e. Hanover

 

Thank you Lynn, you reply contains more than ample information. The car will mainly be used as a street car, with occasional forays to the strip for a few blasts down the 1/4 mile, as well as they occasional burnout . The motor and Carb setup is actually out of a Australian Track car that used to race years ago in tasmania, so has the reversed IDA Manifold.



In addition to the abovementioned specs that the carb was running prior to blowing it was only running a stock 2.0 Needle valve.



The motor is back in the car and running now, Put it together just yesterday running Carbons and have locked the dizzy. Leading at 20 btcd trailing 18 btdc. The carb setup currently is



43mm Chokes,

10F90 Idle 120 Holder,

220 Mains 120 Air Corrector,

F7 Tubes,

Utilized the spare acc pump resevoir

3.3 Needle Valve (bored out)

70/40 Acc pump jets

4psi fuel pressure.



Took it for a drive yesterday afternoon (85-95% humidity as it always is downunder in NZ) and the car felt rather flat in comparison to the previous setup. drivability was rather poor, very bad stumble when transitioning from idle to main jets, and when I was on the gas, it felt rather flat (only revved it to 6500rpm)



I though maybe the idle was a bit rich so I had a tinker with the mixture screws, nope it seems fine, both chambers (rotors) only needed 1/2-2/3 turn out from a fully seated position to come to a even aggressive nice idle, 2100rpm (warmed up).



On decel coming from moderate accelleration (4000-5000rpm, the engine backfire frequently, although not worrying, this is differnt than before. What is also differnt is that the owner unknown to me was previously running Bur7eq plugs allround, now im an running B8EVG all round.



Standard coil ign and electronic dizzy.



I will be heading to the dyno int he next few days and we see what is exactly happening.



Brendon

 

Warm and 85%? Jeeezz no wonder you guys talk funny. There is no Oxygen down there.



The jetting shown above is for an all out racer running between 7,800RPM and 9,800 RPM. So it will be dull down low. Plus it will get like 5 MPG. Try about a 160 main and 160 to 180 air, and keep the pressure down around Weber numbers like 3 PSI.



The idle mix is over-rich as well, and excess fuel in the exhaust flow is igniting on closed throttle. I do like the big fire ***** out the back, but the cops do not. So lean out the idle.



If you don't like the rump-rump-rump on the street, you can drill a 5/32 hole on the down side of each butterfly. Then you let the idle screws down so the edge of the butterfly is in front of the transition jet holes in the body, so you can get back some good off idle feel and get away from some of that stumble. If all of the irons are ported, it will be a poor street machine.



Intake opening is close to 110 degrees with lots of overlap. Pissing backwards and popping fire out of the air filter would be common. Very much so if jetted for racing. We don't even try to drive the racer to the false grid. The metal clutch is like a light switch, and the engine has almost no torque off idle. Stock center iron and ported ends would make a nice combination engine. Make up a manifold gasket to close off the end irons. Drive slow and lean during the week. Then change the gasket and jet rich and rev the **** out of it on weekends. With the correct high rev mods of course.



Should be good for 240HP at 9,000 RPM. Strong from 7,000 to 9,600RPM



Lynn E. Hanover

 

Duly noted will lean out the idle and run bigger air correction,



Cheers again



Brendon

 

great info as usual lynn! once my car is in the tuning stages that float level gauge seems easy, and important!

 

This is a problem with any carb used above its upper flow design limit.



You can also raise and lower the float level to make about a two jet Main jet size change or a 4 jet air corrector change.



How much fuel is in the bowl has a big affect on total fuel flow. The higher the fuel column the more pressure forcing the fuel through the main jet. Simple.



Main jets make a small change in overal performance, while air correctors are like a leak in the shop vac hose. Bigger leak less vacuum to pull fuel. Smaller leak more vacuum to pull fuel. The air correctors only affect fuel delivery through the booster directly. So, air correctors make changes in top end mixture.



Of course they then lower the fuel level in the bowl at full tilt, and there would then be little idle fuel available for a sudden closed throttle time. Or a quick off-on gear change. So you might get a lean indication on closed throttle followed by over-rich on opening the throttle, and a real big fire ball and report out the back. So a sudden over-rich spike from idle mix and accelerator pump lights the lean mixture in the exhaust, and you get yelled at. Or, at least the kids remember your car number and wait for it at the end of the straight.



If it just crackles in low voice during all closed throttle, you have an air leak into the exhaust system near the engine.



Make a big change, so you can tell which way you went tuning wise. Make only one change at a time. Making two or more changes is like reading two pages of different books at the same time. Same thing on the dyno. One change at a time.



Everything affects everything.



Lynn E. Hanover

 

although they can be fooled, widebands make this much much easier