Question about Weber jetting
 

I've got a very lean idle in the REPU, and it's recently started to backfire on the rear rotor. The rear barrel idle setting is 4.5 turns out, while the front is 3.5. I can close off the rear idle screw completely and it'll hum along on just the front rotor, but the back is running very poorly.

I pulled the carb off to test-fit some old Hitachi carbs I picked up, and I brought the Weber in to work. It's a 51 IDA. Upon disassembly, I found that the lid was very loose, thought I couldn't access any of the screws because they're all under the air cleaner. After tracing through the carb again (it's been a while since I was monkeying with it last) I can't find any reason that a loose top cover would cause trouble at idle except that the float would now sit just a little lower, effectively dropping the needle and seat. No puking problems from the bowl, though.

I've currently got the following jets:

195 Main Fuel
F11 Emulsion Tube
145 Air Corrector

45F10 Idle Jet
120 Idle Jet Holder

If I'm 3.5 to 4.5 turns out in order to idle (with slightly red headers, I might add) I'd expect that I need a bigger idle jet. But how much bigger? Should I jump from a 45F10 to a 55? 60?

since you have it off, check #1 is to make sure that the idle screw isnt turned in too far, turning it in too far moves the throttle plate away from the idle holes, into the transfer holes (and into the main circuit if you go too far). once you do that, the idle circuit isnt really working anymore. you can turn the screws however you like and it wont matter.

so once you have that settled, its on to idle jets. since you're 4 turns out, i'd say go for the 55's. on my car going from an 80 to a 70 is a little less than 1 turn of the mixture screws, although its not your engine, and i'm still dialing it in too.

i keep reading that ideally you wanna be about 1 turn out from seated, but obviously for tuning it doesn't matter

Back in my VW air cooled days, I had Weber 44 IDF. Every once in a while I would get a popping in the exhaust during idle. I knew it was time to blow out the idle jet.

The jet is so small, it plugs with the slightest debris.. Easy to check it out.

60f10 idle jet, and 100 jet holder. Trust me... a 45 is way way to damn lean. I'm supprised you can even get it to idle below 1500 rpm. https://www.rx7club.com/1st-generation-specific-1979-1985-18/48-ida-help-3k-rpm-cut-854043/
another thing to consider is that the idle screws do wear out. If there is a difference in wear between the two needles then they will need to be adjusted differently. mine were about 3/4 of a turn off from eachother until I bought new screws and now they're both out 1-1/4 turn with 65 idles and 100 holders.

+1 on the bigger Idle jet. Like hyper said, start with a 60 and go from there.

Well, for what it's worth, I learned a few things about the 51 today after some less-than-helpful discussion with Racing Beat.

When they hogged the bores out from 48 to 51mm, the throttle plate still has a slight angle to it when closed. Now that the plate is bigger (but at the same angle) it now runs into the progression holes at fully closed. It has 1 hole uncovered at full-closed (two holes uncovered once you crack the idle stop 1 1/2 turns as recommended to get some idle air), so I have two little unmetered fuel feeds at idle, which may be why they got away with such a lean jet. Thankfully it's got 4 progression holes so it's still not so bad.

In all the looking I did online it was clear that I was kinda lean. I've also got two slightly-boogered idle screws that I should replace, the worst of which was the one for the rear rotor that was having trouble. I've got a buddy with an IDF that uses the same jets, so I'll order a small assortment of jets, two new screws, and see how it goes.

Thanks for the concensus on the 60 idle. If a step of 10 amounts to almost a full turn, then I'll get a pair of 70s for backup.

your mileage may vary with that. be interesting to see though. if it was like an SU or something, they have it all worked out