12a Bp With Weber 48 Idf. 38mm Chokes?
12-30-2004, 03:07 PM
#1
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I have a 12a BP in a 72 RX-3 with a Weber 48IDF with 38mm chokes. This car is for drag racing. Am I going to gain anything by going with bigger chokes? I got the carb from a road race guy, and appearantly they were limited to 38mm chokes on the rotary cars. Would switching to bigger chokes net me much? Is this carb really restricted right now? Just wondering if anyone has any input.
12-30-2004, 03:56 PM
#3
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Originally Posted by Stock Josh' date='Dec 30 2004, 01:06 PM
I have a 12a BP in a 72 RX-3 with a Weber 48IDF with 38mm chokes. This car is for drag racing. Am I going to gain anything by going with bigger chokes? I got the carb from a road race guy, and appearantly they were limited to 38mm chokes on the rotary cars. Would switching to bigger chokes net me much? Is this carb really restricted right now? Just wondering if anyone has any input.
Yep.
Drop in a set of 44MM chokes. Start jetting with 235 Mains, .100 airs (The smallest they make). and F-7 tubes.
Open the bottom end of the unused accellerator pump well, and notch the wall at the top to increase bowl volume. Run 4-6 PSI fuel pressure at the carb. Keep adding fuel pump until you can hold that pressure all the way down the track.
A Holley works nice because it has two outlets, so one can be for the pressure gage. We use two Carter sliding vane pumps.
Remove the filter under the brass nut in the fuel inlet. It will never flow enough
fuel. Drill out the land where the filter sits.
Raise the float until the boosters start dripping with the pumps on. Then lower the float about .030".
Install a shield between the intake manifold and the header. Blow cool air on the carb.
What RPM are you shifting at? Do you have hardened gears? Headers?
Bolt on air horns and a cold air box with air from the top center of the grill opening.
Lynn E. Hanover
12-30-2004, 10:02 PM
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Thanks Lynn!
Well, heres the rundown on the car. I bought the engine, trans, flywheel, intake manifold, carb, clutch, header, and cooler from one guy, all at once. This engine was supposed to be going into a datsun, from what I understand. The engine was supposed to brand new, never been run (which I believe to be truth). It has new rotor housings, and new side housings. New rotors, and a bridgeport. The intake looks like a 12a mani for a IDA, but my IDF bolts right to it. Its missing the bar across the center of the carb flange to allow the rod for the accell pump to swing. Moving on. The header is a RB unit, with 2 inch outside diameter with the super thick pipe that RB uses. I then had exhaust put on. The exhaust consists of extending the primaries 2 feet each with 2 1/4 inch, the gently collecting to a 3 inch pipe, through a Dynatech muffler (3 inch) and promptly dumping under the car roughly before the rear tire (72 RX3).
I got a little off topic there. The gentleman I bought it from put me in contact with another gentleman named Chris Schroeder. Chris is appearantly the individual that did the porting on the motor, but not the assembly. Assembly was done at a place known as "Rotary Engineering" in southern CA. I spoke with Chris, who does a lot of road racing, and travels the country on some road racing team ( I am vague because I am not well informed) and he was impossible to get a hold of. He tells me that the engine I had is an "older" road race combination than the new stuff he runs (rule changes). He said he was comfortable telling me what was invloved in it because it was no longer a "legal" setup. I dont know much about road racing, so I am relying on what he said. He said its a "small bridge". He then explains that a "small" bridge requires only minor chamfering of the rotor housing. I am at a loss as to how large or small this may be, but she needs to idle roughly at 1100 to 1200 to not die when the fans kick on. Nice and smooth at this rpm (for all the brapping). I just got it running, and at 25 degrees total (firing both coils off one MSD, triggered off the lead ignitor) she "feels" like she makes power to about 8200. This is with pump gas, running premix in the tank. The flywheel appears to be some sort of lightened unit, because it is severely cut back when compared to a stock 85 12a I have in my garage. I still have the stock rearend (3.90 gears?) and 22.5 inch tall tires. I have no experience with rotaries, this is my first, so I have no idea where this puts me in terms "size of the bridge" and "power potential". He didnt know what apex seals would be in it, and said that the gears "should be" hardened, but Im not gonna take that as gospel.
Im feeding it with a Holley blue and a low pressure reg at 3 1/2 psi right now through 5/16 line. I know the needle valve is a 175, and the emulsions are F11's, but I cant remember the main jets off the top of my head. It has air horns, they are about 2 1/2 to 3 inches tall. I know that tuning a carb via the net is impossible, and thats not what I'm asking. I was just wondering if stepping up the choke size is gonna benifit me at all, or if Im chasing the wrong idea. The car is plumbed for nitrous, and when Im comfortable with the tune on the motor I'll be proceeding to hitting it with a 100 shot, and more if she likes it. Id like to know if I should swap up to bigger chokes now, instead of wasting time on the current 38mm ones. From what you are saying Lynn, its obvious I could be cheating myself of some power, so I already have my answer, but I thought Id present this information in case you had any other advice you would care to give. Thank you very much for your time. I appreciate it very much.
Well, heres the rundown on the car. I bought the engine, trans, flywheel, intake manifold, carb, clutch, header, and cooler from one guy, all at once. This engine was supposed to be going into a datsun, from what I understand. The engine was supposed to brand new, never been run (which I believe to be truth). It has new rotor housings, and new side housings. New rotors, and a bridgeport. The intake looks like a 12a mani for a IDA, but my IDF bolts right to it. Its missing the bar across the center of the carb flange to allow the rod for the accell pump to swing. Moving on. The header is a RB unit, with 2 inch outside diameter with the super thick pipe that RB uses. I then had exhaust put on. The exhaust consists of extending the primaries 2 feet each with 2 1/4 inch, the gently collecting to a 3 inch pipe, through a Dynatech muffler (3 inch) and promptly dumping under the car roughly before the rear tire (72 RX3).
I got a little off topic there. The gentleman I bought it from put me in contact with another gentleman named Chris Schroeder. Chris is appearantly the individual that did the porting on the motor, but not the assembly. Assembly was done at a place known as "Rotary Engineering" in southern CA. I spoke with Chris, who does a lot of road racing, and travels the country on some road racing team ( I am vague because I am not well informed) and he was impossible to get a hold of. He tells me that the engine I had is an "older" road race combination than the new stuff he runs (rule changes). He said he was comfortable telling me what was invloved in it because it was no longer a "legal" setup. I dont know much about road racing, so I am relying on what he said. He said its a "small bridge". He then explains that a "small" bridge requires only minor chamfering of the rotor housing. I am at a loss as to how large or small this may be, but she needs to idle roughly at 1100 to 1200 to not die when the fans kick on. Nice and smooth at this rpm (for all the brapping). I just got it running, and at 25 degrees total (firing both coils off one MSD, triggered off the lead ignitor) she "feels" like she makes power to about 8200. This is with pump gas, running premix in the tank. The flywheel appears to be some sort of lightened unit, because it is severely cut back when compared to a stock 85 12a I have in my garage. I still have the stock rearend (3.90 gears?) and 22.5 inch tall tires. I have no experience with rotaries, this is my first, so I have no idea where this puts me in terms "size of the bridge" and "power potential". He didnt know what apex seals would be in it, and said that the gears "should be" hardened, but Im not gonna take that as gospel.
Im feeding it with a Holley blue and a low pressure reg at 3 1/2 psi right now through 5/16 line. I know the needle valve is a 175, and the emulsions are F11's, but I cant remember the main jets off the top of my head. It has air horns, they are about 2 1/2 to 3 inches tall. I know that tuning a carb via the net is impossible, and thats not what I'm asking. I was just wondering if stepping up the choke size is gonna benifit me at all, or if Im chasing the wrong idea. The car is plumbed for nitrous, and when Im comfortable with the tune on the motor I'll be proceeding to hitting it with a 100 shot, and more if she likes it. Id like to know if I should swap up to bigger chokes now, instead of wasting time on the current 38mm ones. From what you are saying Lynn, its obvious I could be cheating myself of some power, so I already have my answer, but I thought Id present this information in case you had any other advice you would care to give. Thank you very much for your time. I appreciate it very much.
12-30-2004, 11:40 PM
#6
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Location: Central Ohio (Hebron) Zephyrhills Fla.
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Originally Posted by Stock Josh' date='Dec 30 2004, 08:02 PM
Thanks Lynn!
Well, heres the rundown on the car. I bought the engine, trans, flywheel, intake manifold, carb, clutch, header, and cooler from one guy, all at once. This engine was supposed to be going into a datsun, from what I understand. The engine was supposed to brand new, never been run (which I believe to be truth). It has new rotor housings, and new side housings. New rotors, and a bridgeport. The intake looks like a 12a mani for a IDA, but my IDF bolts right to it. Its missing the bar across the center of the carb flange to allow the rod for the accell pump to swing. Moving on. The header is a RB unit, with 2 inch outside diameter with the super thick pipe that RB uses. I then had exhaust put on. The exhaust consists of extending the primaries 2 feet each with 2 1/4 inch, the gently collecting to a 3 inch pipe, through a Dynatech muffler (3 inch) and promptly dumping under the car roughly before the rear tire (72 RX3).
I got a little off topic there. The gentleman I bought it from put me in contact with another gentleman named Chris Schroeder. Chris is appearantly the individual that did the porting on the motor, but not the assembly. Assembly was done at a place known as "Rotary Engineering" in southern CA. I spoke with Chris, who does a lot of road racing, and travels the country on some road racing team ( I am vague because I am not well informed) and he was impossible to get a hold of. He tells me that the engine I had is an "older" road race combination than the new stuff he runs (rule changes). He said he was comfortable telling me what was invloved in it because it was no longer a "legal" setup. I dont know much about road racing, so I am relying on what he said. He said its a "small bridge". He then explains that a "small" bridge requires only minor chamfering of the rotor housing. I am at a loss as to how large or small this may be, but she needs to idle roughly at 1100 to 1200 to not die when the fans kick on. Nice and smooth at this rpm (for all the brapping). I just got it running, and at 25 degrees total (firing both coils off one MSD, triggered off the lead ignitor) she "feels" like she makes power to about 8200. This is with pump gas, running premix in the tank. The flywheel appears to be some sort of lightened unit, because it is severely cut back when compared to a stock 85 12a I have in my garage. I still have the stock rearend (3.90 gears?) and 22.5 inch tall tires. I have no experience with rotaries, this is my first, so I have no idea where this puts me in terms "size of the bridge" and "power potential". He didnt know what apex seals would be in it, and said that the gears "should be" hardened, but Im not gonna take that as gospel.
Im feeding it with a Holley blue and a low pressure reg at 3 1/2 psi right now through 5/16 line. I know the needle valve is a 175, and the emulsions are F11's, but I cant remember the main jets off the top of my head. It has air horns, they are about 2 1/2 to 3 inches tall. I know that tuning a carb via the net is impossible, and thats not what I'm asking. I was just wondering if stepping up the choke size is gonna benifit me at all, or if Im chasing the wrong idea. The car is plumbed for nitrous, and when Im comfortable with the tune on the motor I'll be proceeding to hitting it with a 100 shot, and more if she likes it. Id like to know if I should swap up to bigger chokes now, instead of wasting time on the current 38mm ones. From what you are saying Lynn, its obvious I could be cheating myself of some power, so I already have my answer, but I thought Id present this information in case you had any other advice you would care to give. Thank you very much for your time. I appreciate it very much.
Well, heres the rundown on the car. I bought the engine, trans, flywheel, intake manifold, carb, clutch, header, and cooler from one guy, all at once. This engine was supposed to be going into a datsun, from what I understand. The engine was supposed to brand new, never been run (which I believe to be truth). It has new rotor housings, and new side housings. New rotors, and a bridgeport. The intake looks like a 12a mani for a IDA, but my IDF bolts right to it. Its missing the bar across the center of the carb flange to allow the rod for the accell pump to swing. Moving on. The header is a RB unit, with 2 inch outside diameter with the super thick pipe that RB uses. I then had exhaust put on. The exhaust consists of extending the primaries 2 feet each with 2 1/4 inch, the gently collecting to a 3 inch pipe, through a Dynatech muffler (3 inch) and promptly dumping under the car roughly before the rear tire (72 RX3).
I got a little off topic there. The gentleman I bought it from put me in contact with another gentleman named Chris Schroeder. Chris is appearantly the individual that did the porting on the motor, but not the assembly. Assembly was done at a place known as "Rotary Engineering" in southern CA. I spoke with Chris, who does a lot of road racing, and travels the country on some road racing team ( I am vague because I am not well informed) and he was impossible to get a hold of. He tells me that the engine I had is an "older" road race combination than the new stuff he runs (rule changes). He said he was comfortable telling me what was invloved in it because it was no longer a "legal" setup. I dont know much about road racing, so I am relying on what he said. He said its a "small bridge". He then explains that a "small" bridge requires only minor chamfering of the rotor housing. I am at a loss as to how large or small this may be, but she needs to idle roughly at 1100 to 1200 to not die when the fans kick on. Nice and smooth at this rpm (for all the brapping). I just got it running, and at 25 degrees total (firing both coils off one MSD, triggered off the lead ignitor) she "feels" like she makes power to about 8200. This is with pump gas, running premix in the tank. The flywheel appears to be some sort of lightened unit, because it is severely cut back when compared to a stock 85 12a I have in my garage. I still have the stock rearend (3.90 gears?) and 22.5 inch tall tires. I have no experience with rotaries, this is my first, so I have no idea where this puts me in terms "size of the bridge" and "power potential". He didnt know what apex seals would be in it, and said that the gears "should be" hardened, but Im not gonna take that as gospel.
Im feeding it with a Holley blue and a low pressure reg at 3 1/2 psi right now through 5/16 line. I know the needle valve is a 175, and the emulsions are F11's, but I cant remember the main jets off the top of my head. It has air horns, they are about 2 1/2 to 3 inches tall. I know that tuning a carb via the net is impossible, and thats not what I'm asking. I was just wondering if stepping up the choke size is gonna benifit me at all, or if Im chasing the wrong idea. The car is plumbed for nitrous, and when Im comfortable with the tune on the motor I'll be proceeding to hitting it with a 100 shot, and more if she likes it. Id like to know if I should swap up to bigger chokes now, instead of wasting time on the current 38mm ones. From what you are saying Lynn, its obvious I could be cheating myself of some power, so I already have my answer, but I thought Id present this information in case you had any other advice you would care to give. Thank you very much for your time. I appreciate it very much.
Well, then, just run what you have until you have a handle on this rotary stuff.
The small bridge port should be good for 220 HP. Stay under 8,500 RPM until you find out what apex seals you have. The stock iron seals will go above 9,000 with a little work, but the housings suffer a bit and wear faster. The carbon seals will survive above 10,500 RPM. Get a rev limiter on that MSD. Check to see if your feet would be in danger if the flywheel turns into a kit on a missed shift.
Once you feel good about the learning curve, you can jump in and do a "J" bridge port and 44MM chokes for an easy 260HP. I should have added that the Weber needle seats are too small also. Use a Gross jet 350 or 400 needle and seat.
We get about 250 HP with a big bridge and 38MM chokes
Lynn E. Hanover
01-19-2005, 11:17 AM
#7
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Join Date: Oct 2003
Location: Reno, NV
Posts: 121
Update. I bought the Innovative Wideband O2 kit. With 38mm chokes, 180 mains, 175 air bleeds, and 65 idle jet the car A/F at idle was in the low 11's. At cruise it was in the low 10's and would stutter and spit badly. Drivability was poor. A change to a 55 idle jet chnaged it to high 12's low 13's at idle and cruise. At WOT the A/F went to high 16's. I upped the mains to 200's and the A/F is in the 14's and begins creeping to over 16's as it revs. Makes me nervous. I didnt think that an A/F that lean would even light, but I guess the MSD will light anything. I'm gonna drop the air correction a couple sizes, and up the mains again, but should the A/F mixture be climbing 4 points (from 14 to 16 and higher, I let off) because of the air bleeds? Seems extreme. When the motor hits 6K it has a big change in tone (powerband coming on) and thats when the A/F starts climbing. I'm figuring thats because the motor is coming into its efficiency range, but the jetting seems pretty big for 38mm chokes and the fact that I live at 4200 feet.
01-19-2005, 03:39 PM
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Well, another update for anyone following along. I changed the jets out on my lunch break. I upped the mains to 210's and drop the air bleeds to 160's from 175's. The A/F dropped to the 13's at WOT, and climbed to the 14's by 8000. Im gonna drop the air bleeds 2 more sizes to see what that does, and maybe up the mains to 220's. I'll try the air bleeds first to see if that gets the A/F's in the 12's. The lesson here. Buy a wide band everybody. They are the greatest thing on earth.
01-20-2005, 12:14 AM
#10
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Originally Posted by Stock Josh' date='Jan 19 2005, 01:38 PM
Well, another update for anyone following along. I changed the jets out on my lunch break. I upped the mains to 210's and drop the air bleeds to 160's from 175's. The A/F dropped to the 13's at WOT, and climbed to the 14's by 8000. Im gonna drop the air bleeds 2 more sizes to see what that does, and maybe up the mains to 220's. I'll try the air bleeds first to see if that gets the A/F's in the 12's. The lesson here. Buy a wide band everybody. They are the greatest thing on earth.
Two sizes of air corrector is the same as one size of main fuel jet. You are right in the area where you may be running into a bit of a wall. If additional changes do not get the top end as rich as you want (you did take out the filter right?)
you want to go up on the fuel pressure. It is probably lowering the fuel level and staying lean as you go smaller on the air corrector.
Changing the main jet changes everything through the range, starting with idle.
Changing the air corrector only changes the top end.
We run 6 pounds of fuel pressure with a Gross jet. The stock needle may not hold that much back.
Ideal power is 50 to 100 degrees rich of peak EGT. So look for (a very safe) 1575 to 1650 degrees. Where that is on your gage I have no idea.
Hold each specific speed with your left foot on the brake, so the reading stabilizes for a second or two.
One of my drivers got to drive the Buckeye Bullet to a new world record of 315 MPH. On a practice run he hit this 3 mile marker at 275 MPH.
Lynn E. Hanover