Weber 48 IDA help with jetting please...
 

I have a 13b in my Miata that I will be using for road racing at the local tracks like BW and WSIR. I am not very experienced with carb setups so I have a couple of questions and looking for suggestions as well.

The set up is:
90 6 port block with a large street port (primaries are pretty big)
9.7:1 rotors
Ignition is stock distributor with 2 msd blaster 2 coils.
straight exhaust (dual all the way till about 1' away from single muffler)
Holley red Fuel Pump with Malpassi Regulator set at 4.5-5psi
Weber 48 IDA
40mm venturis
F-ll Emulsion Tubes
220 fuel jets
195 air jets
65 F-9 idle jets
300 Needle valve

As is I made 157 rwhp and 117 ftlbs of torque I made peak hp at about 7,800 RPM. I run 89 Octane with 1oz premix to every 2 gallons on the street. But will run 1oz/gal for the track.

Right now my exhaust is restricting some power, because of the way it was done. I will change it later on. But I want to know what jets are best to use? I want to run some bigger ones to make a little more power and be on the safe side and run a little richer. During the dyno you can see I ran right at the border line on my AFRs at one point. I will be running 91 and advance the timing a little from now on.

I have a set of 42mm and 37mm venturis as well, but I have not tried them. Which would be best for my setup? I would like some low end to get out of the corners and get back up to speed quicker. But I would also like to have some top end just for fun. :)

Any suggestions? tips? tricks? etc...

Here is the dyno graph and video for reference.

Attachment 706463

http://www.youtube.com/watch?v=MjwYhoR0YQw

Send an e-mail to 01Racing from the Canadian forum. He has a ton of experience with weber carbs on BP and PP engines. His car runs very well and he can give you some advice on effects of changes and what he has used as starting points. The main concern is that there is no exact set up that people can transfer from car to car. Weight, porting and gearing make a huge difference.

I would suggest as alternatives is to talk to Dave Lemon at Mazdatrix or the guys at Racing Beat. They have all the knowledge in the world on using these carbs on rotaries.

Eric

I run a 2nd gen in SCCA with a set up similar to yours. I changed the exhaust to a 3" single into a Magnaflow at the rear. I have an RB header with a merge collector after the flange. My #s are about 50 more than yours. I just dynoed at Topeka in October. When I get home tonight I'll put together my tuning specs for you. I suspect you are losing a lot with that exhaust set up. I also run Sunoco 93 premixed 1 oz per gallon.

Why are your AFR lines all different? Were you changing jets between runs? If so, what line represents the current jetting?

I got a little confused when I saw the post last night as well.

My jetting:

race port 12a Weber 48 IDA (SCCA EP specs 38mm venturi)

F11
A/C 185
Main 250 (rear runs lean, now at 270, this is a work in process)
65 F9 idle
300 needle

SDJ header magnaflow, premix, no dyno but guessing 190+hp at rear wheels

The porting and exhaust are the key.

From my research on this forum, a 13b streetport with 40mm chokes in IDA came up with these combinations:

200 fuel
170 air

210 fuel
180 air

200 fuel
150 air

we were playing with the timing, the jests remained the same.

you want to shoot for 13:1 AFR's for best power 12.5 to be safe. I made 3 more whp just leaning it up from 12.8 to 13.1 above 5k by switching from a 140 a/c to a 165 a/c with a 205 main. Now I'm in a stock port 13b, but you get the general Idea. Get a baseline jetting in then toy with teh timming the go back to jets and then a final timming tune. Your best bet is to rent the dyno for an hour. Your tq curve is really flat and looks good, but you should be making more near 125 ftlbs, your engine might be a little tiard, or you're just running way too rich down low.. you should try out 205 main and 170 a/c and see how you like that.

If you want max power then use larger chokes and richer emulsion tubes. I would start with 45 + chokes and use f8 or f7 emulsion tubes and then play with jetting to get the desired a/f as mentioned above. A good start would be 240 main and 110 air and go from there. Rotaries love alot of fuel and air. One thing alot of guys dont use is a EGT meter- very important in order to tune properly and see what is going on.

I use an Air/fuel ratio meter in my race car. It is very important to make sure you are not leaning out at top end. Also, it is hard to make a Weber run lean at top end, if you have a good ignition system.

per the posted dyno runs: It sure is odd that if all you were changing is spark timing, then why the mixture was changing so much run to run. Something else is going on.

What heat range spark plugs are you using? What gap?

When you change timing your wide band indeed will read different afr values. That is not a suprise.

Thats why you jet it, time it, then jet it again

would i lose a lot of low end if i went with 42mm venturies, 230 mains, and 75 idle, and advance the timing a tad?

well i'll let you knoiw I just went from 37mm to 43mm chokes in my 48ida, and my butt dyno says there wasn't a loss down low but a huge gain up high. i'll let you know after the next dyno run.

i can help you with the 48 IDA i know everything about it. i also have a good p port motor with a 48IDA with a real nice street tune has allot of bottom end torque.

Tell me more about this. What size are the intake ports on the peripheral port. What size chokes in carb. How driveable is it on the street really? Does it kick or buck under 4,000rpms? MPG? Dynosheet? What is the exhaust setup header to tailpipe?

okay my car dirves perfect with no stumble anywhere. 13b small street port pulls all the way to 7500 rpm now.
I haven't gotten it on the dyno yet but I'll post results when I get them
43mm chokes
0 bypass
70 idle
100 idle holder
220 main
120 air corrector
f11 emulsion tubes
set at full advance no vac just mech advance L25*btdc T18*btdc
remember when ever you change timming, choke size, or emulsion tubes then you have to rethink all your other jetting.