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Weber DCO jetting, any suggestions?

Old 12-06-04, 12:11 AM
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Weber DCO jetting, any suggestions?

After screwing with my REPU ignition system, looking for intake leaks, and playing with the carb I have decided that my WEber needs re-jetting.
I started with 220 air correction, 210 main, and 65F8 idle jets. The idle was crappy but it ran pretty good at higher rpm.
I found some reference chart for jetting and it looks like I have the proper jetting for somewhere between a 12a bridge port and a large street port 13b.

From a friend I got a 150 main, 200 air correction, adn 45F9 idle jet. I put them in and idle was awesome but I couldnt at all get over about 1500 RPM.

I swapped back in my 220 and 210 air correction and main jets. The idle got rougher, but all the top end came back. There was a horrible stumble at transition though. I richened up the mixture adjustment, turned the screws out to about 3 turns out and it was better but still stumbles pretty bad.

I am certain the idle jet is too small, what size would be good to try? 50 or 55? F8 or F9?
I would also guess that the main jet and air correction are too large.
Any idea where I should begin rejetting?
One big question is how do I know if I need a different air correction jet? What effect does that have on the way things run?


Also, how do I go about getting jets? Is it best to just buy a bunch and return what I dont use?
thanks
Tom


I am running
Weber 48DCO
Racing Beat header
2.5" cat and Magnaflow straight through muffler, very free flowing
5/6 port wide open
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Old 12-06-04, 01:37 PM
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Idle fuel jet delivers fuel for idle AND progression. go bigger.

Ask on the 1st gen forum, I bet someone has done this already.

-bp-
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Old 12-07-04, 07:17 AM
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Look at this thread. https://www.rx7club.com/1st-generation-specific-1979-1985-18/weber-48-dcoe-jet-sizing-help-362439/

I have a similar setup, I ended up at 210 air, 190 main 65 idle, f3 emulsion tube. I did have some issues at the track. I will retune soon and will post my results. My general feeling is that the main is about right at 185-200 range but the air corrector is too big.

I'll post more later. Got to go to work.
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Old 12-07-04, 09:19 AM
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Thanks guys, based on some searches I did I ordered a new set of jets.
I am going to try a 180 main (still have that 210 if I decide this is too small) 170 air and a 55 idle.
cpa7man, I just realized I didnt specify I have a stock port 13b. I dont know how that compares in air flow to a street port 12a but thanks for the info. Reading what you posted I realized I am still confused about emulsion tubes. You mention an f3, the only place I remember seeing f numbers is in the idle jets. for instance, I ordered a 55F8 idle jet. When looking to order parts I only find one emulsion tube, maybe there are different sizes but pretty much everybody uses this one size so that is what everybody sells? Looks like I am going to have to do some more research into this.
thanks
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Old 12-07-04, 06:49 PM
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Originally Posted by Tom93R1
Reading what you posted I realized I am still confused about emulsion tubes. You mention an f3, the only place I remember seeing f numbers is in the idle jets. for instance, I ordered a 55F8 idle jet. When looking to order parts I only find one emulsion tube, maybe there are different sizes but pretty much everybody uses this one size so that is what everybody sells? Looks like I am going to have to do some more research into this.
thanks
My bad, in my haste this morning I wrote f3, should say f2 emulsion tube. Has to do with the size I assume. I did run 180 main with 200 air correctors and I believe it was too lean. Long story that I don't want to elaborate on here. That said I'm going to stick with the 190 main jet and try a smaller air correction jet(make it run richer). If the low rpm to high rpm transition is smooth than your idle jet is good. Rough idle is another issue. There is some good information here http://www.mazspeed.com/faq3.htm under carb facts. If you find any 48 DCO specific stuff please post it, I'm a long way from having this figured out.

In response to your 13b stock port vs. 12a street port I'm just guessing here but I would guess that the exhaust being equal the flow is close. I do run a higher flowing exhaust than the RB.

Edit:looking back at your post I think you need to stick with the larger main. I think the 180 is too small. 190-210 range sounds about right.

Last edited by cpa7man; 12-07-04 at 06:54 PM.
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Old 12-17-04, 05:02 PM
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I have heard running the F11 emulsion tubes. A street port would require larger jetting then a stock port and possibly larger venturis but 38mm are pretty big and a good large size for a 13b stock port. PM me if you have some questions I have a similiar setup Tom, only mine is ported. I run all different size jettings depending on temp. distance, power request etc. I usually keep one thing in mind. I never go higher in my Air correctors then what the Main jet is, which I have heard keeps the motor from running too lean at higher rpms on all Rotaries....
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Old 12-17-04, 10:32 PM
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Originally Posted by therotaryracer
I have heard running the F11 emulsion tubes. A street port would require larger jetting then a stock port and possibly larger venturis but 38mm are pretty big and a good large size for a 13b stock port. PM me if you have some questions I have a similiar setup Tom, only mine is ported. I run all different size jettings depending on temp. distance, power request etc. I usually keep one thing in mind. I never go higher in my Air correctors then what the Main jet is, which I have heard keeps the motor from running too lean at higher rpms on all Rotaries....
Can you post the combination of idle, main & ac jets that have worked well for you when you have time. I would be most interested.

So the ac jet should be smaller than the main jet. This is contrary to the piston engine settings.
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Old 12-17-04, 11:15 PM
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It sounds like it couldnt hurt if you left the air corrector jet lower then the main jet, also I dont know a proper combination since all motors with different exhaust, intakes, to venturis to so many things would make a difference. Your best bet is installing a A/F halmeter into your exhaust and making sure at no points is your engine running lean. F11's work good and you can run jetting as high as you want until you find out your foaling plugs smoke out the back etc. 13b w/ 38mm venturis can run from 200-230 even to 250 main jet all depends the porting and what air corrector you use. I heard that a decrease of .5mm in the main jet is the equivalent to .15mm increase of your air corrector jet. Running a high Air Corrector doesnt do harm if your backing it up with a high Main Jet to cover the chance of excess leaning of your motor. Also the Pump JET(S) need to be properly sized for your Combustion chamber size so every revolution the engine pumps in the correct amount needed. I hope this helps CPA7Man
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Old 12-17-04, 11:48 PM
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Yes it does, thanks! I've got a lot of work to do.
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Old 01-05-05, 01:01 AM
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Tom I think you should gice your carb to me and see what it will do on my car lol
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Old 01-05-05, 11:20 AM
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I will sell you the carb for $3000, and it comes with a free truck
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Old 01-07-05, 03:15 PM
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I have, I think sucessfully jetted my 48 dco. As I mentioned before... Its on a street ported 4 port 13b with a lake city rotaries intake and a racing beat header. The exhaust from there back is a home made colage of 2" and 2.5" and so on.

Im running

180 mains

220 air correction

65f9 idle jets

f15 emulsion tube

I have no idea what the pump jet size is but it works great! I can get plenty of scratch in the first 2 gears and some in third too depending on road surface with my cold kumho ecsta v700's. I detected bit of hesitation once or twice but that seemed the exception. I didnt get a chance to do any long runs with it as school is just getting out and there are kids walking home so bad idea at the moment. I'l try one tomorrow morning but I think I have it nailed.

Im so happy I could wizz my pants :-D
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Old 01-07-05, 07:20 PM
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Great info! That's a big air corrector. You might lean out a little at high rpm. It's great to just establish a baseline than you can tweek from there. I've got a few more weeks till I get my motor back togeather than I'll post my results.
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Old 01-08-05, 12:09 AM
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yeah I know the air corrector is big but it was crapping out at high rpm with the smaller ones so... I think the big emulsion tube has something to do with it. the f15 is way bugger than what ever was in there. Shoot I tell you what, If this carb works Im never going to touch it again lol
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Old 01-09-05, 12:37 AM
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Well cpa you were right. It kinda dropps off at high rpm. I think it has to do with the big emulsion tube. The f15 in there right is huge compared to the f7 that was in it before I started tuning. Oh by the way tom... you can buy jets at Karls Customs VW Shop. Any thoughts on what effect an f7 or cpa's f2 emulsion tube would have on my ability to get more top end?
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Old 01-09-05, 12:41 AM
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KArl's Customs? I called them and they said they couldnt get any in the sizes I was looking for. Maybe I didnt talk to the right person.
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Old 01-09-05, 09:46 AM
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I think the F2 emulsion tube is the one that came stock in the 48 DCO, not that that means anything.
Try a larger main jet and/or a smaller air corrector jet and see what happens. If none of that helps go to a smaller emulsion tube and start the trial and error process over again. At least it sounds like you've got the idle jet (really should be called low rpm jet) figured out.

Good luck.

Most import car centers have the same access to these parts as the so called "Weber experts" on the internet. My local import car center does not even charge freight and gets the parts next day. The other thing is to try race shops in the Phoenix area. All these parts come out of a central place for the entire USA. On the west coast. I was able to order stuff 6:00 pm central time and still have them next day with zero freight. BTW - I'm an ASU alum.

Last edited by cpa7man; 01-09-05 at 09:50 AM.
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