just to update i rerebuilt the carb while being very picky. then i drilled a small hole in the throttle plate (0.035"; too big and the carb wont work anymore, too small nothing happens)



and now it will happily cruise down the street @2000rpm in 2nd, no bucking or snorting it drives just like a stock side port. idle is very strong around 1000rpms, rump rump



i also went thru my idle jets, 60 and 70 made no difference, 80 the mixture screws are all the way i, so i used the 70's. its actually idling on the 1st progression hole, so the mixture needle doesn't do much.



the rest of the jetting is still right out of the comp book, and it feels way rich, but i am running mufflers, so that's kind of expected. i'm also running the dizzy locked @18degrees



exhaust is rb header into 2x2" pipe, into 2x2.25 glass packs, into a 3" borla. the borla is going to get replaced with the RB muffler. it sounds like some out of control garden equipment....



i had a good week, big progress! next step, main jets

 

Be careful... playing with engines like this is how I started to get a reputation as a "Carb Guru". When tiny changes make huge differences in drivability, it can be very frustrating. But at the same time, if approached with a scientific attitude, the educational possibilities are phenomenal.



With respect to plate drilling. I prefer progressive carburetors specifically because you can tune the additional air by adjusting the secondary plates' idle position. High-zoot carbs come with them actually adjustable and that capability is wasted by people who don't Get It yet.



Lemme tell ya about the time I got phenomenal drivability with a four-corner-idle carb, and then the guy goes and evens all the adjustments out because, well, they weren't EVEN. It can't be right that it was idling mostly on the secondaries!



And then he wondered why it drove like crap again.

 

carb guru? i hope not i have enough grey hairs....



i only have gotten really interested because there is no instructions for the damn weber... and ive made a couple observations which makes me think for a bridge/pp the 48 IDA is actually a mediocre choice...



observation #1. the PP idles on the transition circuit. this leaves no transition circuit to transition, depending on header length you can choose when the lean spot occurs, but due to the design of the carb its going to be hard to overcome without running 10:1 AFR's the rest of the time. a rich mixture = bucking snorting all the pp things you hear are true...

road racing this is fine, as they only use this in the pits/paddock. on a normal piston engine car they would be running at least 2 carbs so the carb needs to flow more air at idle than the carb was designed for.



observation #2. we were watching the 7's only spec 7 things (its a 13b PP ist gen with an FD body) run around thunderhill. we went to turn 14-15 to watch our car, but i was also watching these things, and there is a bump after the apex of 15, car hit bump then the engine stumbles...



i ordered more jets today, and i have a few more things to try, but right now the challenge is to get the 2000-3000 area where it goes from the idle circuit to the main circuit to be better.

 

carb guru? i hope not i have enough grey hairs....



i only have gotten really interested because there is no instructions for the damn weber... and ive made a couple observations which makes me think for a bridge/pp the 48 IDA is actually a mediocre choice...



observation #1. the PP idles on the transition circuit. this leaves no transition circuit to transition, depending on header length you can choose when the lean spot occurs, but due to the design of the carb its going to be hard to overcome without running 10:1 AFR's the rest of the time. a rich mixture = bucking snorting all the pp things you hear are true...

road racing this is fine, as they only use this in the pits/paddock. on a normal piston engine car they would be running at least 2 carbs so the carb needs to flow more air at idle than the carb was designed for.



observation #2. we were watching the 7's only spec 7 things (its a 13b PP ist gen with an FD body) run around thunderhill. we went to turn 14-15 to watch our car, but i was also watching these things, and there is a bump after the apex of 15, car hit bump then the engine stumbles...



i ordered more jets today, and i have a few more things to try, but right now the challenge is to get the 2000-3000 area where it goes from the idle circuit to the main circuit to be better.

 

The idle and transition holes are fed from the idle jet (Fuel) and the hole in the jet holder (Air). So the total amount of mixture for both is changed with a jet change or holder change. One way to see which way you might want to go, is to remove the springs from the adjustment screws and back them way out one turn at a time. The screws control a mixture of fuel and air. The jet and holder control the ratio. 12.2:1, 13.7:1 or whatever.



So backing out the screw adds more (total) mixture. Not just air, and not just fuel. So as the screw backs out total mixture goes up. Now reducing the butterfly opening cuts off air flow through the body, (and increases vacuum over the idle circuit hole) so you will go richer. If you open the butterflys with no other changes, you will go leaner. (More air through the body and low vacuum over the idle circuit holes.



Once you see which way to go, the adjustment screws should be between one and three turns out from zero flow. With the springs back on the screws.



The nozzle size for the accelerator pump can be reduced so the stream lasts longer so it helps the transition from off idle.



The 48 should have no trouble with off idle, transition and RPM up to about 8,000. Steady state wide open throttle above 8,000 RPM requires some tricks, as this is actually above the flow rating of the carb. Where rules allow, you would see the Gene Berg Weber replicas in say a 55MM body in similar applications. The Weber has more problems shifting at 9,600 and higher because it needs 6 or more pounds of fuel pressure to keep the bowl full.



Once you get it right, you will love it.



Lynn E. Hanover

 

i'm at the point now, where ive seen how nice it can be, its just a matter of getting it thru its transitions. the exhaust header length change made a big difference too.



the idle circuit thing is simple, the engine wants air! if i back off the throttle plates enough to use just the idle hole its a 400rpm idle. another half turn of the idle "speed" screw, gets me 900ish rpm idle, but its also on the 1st transition port. if it wasn't a PP i don't think this would be a problem....



drilling the plate makes a big difference in intake vacuum, but a small difference in flow. so by the time you drill a big enough hole the carb doesn't work! a small hole, reducing intake vacuum a little actually seems to be a good thing, however.



so the idle jet holder testing is next....



the other important thing is the fit of the aux venturi to the carb, if its loose the signal to the main jet is lost....



i got some new jets in the mail, its just a matter of playing with em hopefully!



right now i'm running this



48 IDA.



43 choke

70 idle jet, depending on the holder test, it might go to 75

120 holder, which may turn into an 80 or 100, pending further tests



f-8 em tube

130 air

main is 230, but i bought 210-215-220, pending testing



fuel pressure is about 4.3, 89 octane with about 100:1 premix.



timing right now is locked @about 18degrees L&T. once the fuel is sorted i might unlock it, it does idle better around 10btdc



exhaust is 72" long (i showed the fab guy where the collector should be but i forgot to TELL him it should be 90"....) 2 glasspacks in the primaries, and rear muffler is a borla 3" its almost quiet enough to street, the borla is probably getting replaced with an RB unit. going from 50" to 72" changed the carb's stumbling point from 3500 to 2500rpms...

 

update! i closed the throttle plate holes, and soldered the idle jet holder down to a 100 from 120.



idle circuit went from 1500ish rpms to about 2200ish, which i think is all i need.



the throttle plate having no hole though, turned it back into a bucking snorting stinky hard to drive POS.



next step, redrill hole, after that the gas station. i'll have to look at my notes, but i think the 0.025" is about right. i also happened to notice that the factory EFI throttle bodies all have a big hole in the primary throttle plate, its for a different reason, but still

 

well progress is progressing. its unusually humid (27%) and hot too (95), so its acting rich all everywhere, and the W/B wasn't playing very well either, BUT i can floor it @1100 and it will just start pulling, no hiccups hesitations or anything



still too rich i think, mid 11's @3500... i need to get the W/B a better voltage source and keep going.



cooler weather would be nice too. black car in the summer = burned hands!

 

today's update!



so i decided to move the MSD to BOTH leading and trailing, previously it had only been hooked to the leading. attempting to start the car resulted in zilch. i'm not sure when the leading ignitor died, but it was. i think it died right before the last update. it was grumpy that day, but not the day before....



anyways. driving it with MSD on all four plugs, it ran fine up to about 2200rpm, then from 2200-3500 it was just a big stumble, until it hit 3500, and then it took off.



so after puzzling over why i could floor it @1000rpms, and it would pull to about 2500 then not work, then start working again over 4k, for a couple of days i decided it was either too rich or too lean in the stumble spot.



soOOOoo after going back and forth i decided to go leaner.



60 idle jet is all i have, and it works. its a little lean at idle, but at low throttle its mid 14's. it makes the same bird chirpy pinging noise the rx8's do. the 70's drove better, the 60's are a little touchy



the 70 idled rock solid, but cruise was mid 12's. i think a 65 might be about right.



the main jet was swapped to a 170. surprisingly car runs almost perfect! going WOT @1000rpms, afrs drop and then it just pulls smoothly, around 4k its about 13.2....



i put the 175 in there, i'd like low 12's... but it looks like i need to either open the exhaust up more, or buy more jets.... i've got nothing in between 175 and 210....

 

Ah, jet swapping... reminds me why all of my cars are fuel injected nowadays. It's only expensive initially, after that tuning is free.