In over carburetored situations such as this a typical slide carb (be it flat or round) will bog the engine when floored because the instant the throttle slide is lifts the carb goes to the needle jet/jet needle circuit and the main circuit. However, as this happens the instant it does the air flow is the same as you had when you smashed the throttle - so at idle you will have a little bit of airflow then when floored you get tons on fuel in and this floods it.

Now this is why CV carbs are good (flat side CV would be awesome!) because the throttle slides lift exactly at the rate the engine needs it (even when floored).

My Mikuni BS34's flow as much as 42 and possibly 45mm Webber DCOE as the venturi in a Webber is steeped down. (42's generally have 34mm venturies and so do 45's depending on the jetting). But after jetting my BS34's correctly you get the flow of a decent Webber and If I want I can set the idle to like 300rpm (you can watch the fan tick over lol). Plus even at 300rpm when you floor it the engine spools instantly with no bog or hesitation.

The only problem I'm having is that my idle is slightly rich. Im using a 42.5 pilot and 160 air (stock Datsun A12 is 43 pilot and 220 air), but after drilling my air jet to 220 its worse (even more rich - go figure). Removing the air jet defies logic as the fuel air is so rich the car hardly idles.

Fun and games - tell you what though, as much as I love a carbie for its mechanical simplicity and durability, changing the injector pulse duration and using a wide band O2 to tune seems a hell of a lot easier; I reckon I'll give EFI a go in the future.

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Are you saying it idled poorly with 160 air? did you get an exhaust readout with 160 and 220?
160-180 should be about right on the air jets but is it on an a12? maybe the pilots need to go down further? say around 38-40.

Well the manual for the A12 says 43 fuel, 220 air for the pilot circuit.

I was running 42.5 fuel and 160 air and it was rich. I drilled the air jet to the datsun spec 220 and it was RICHER - does that make more sense? My idea was that the pilot air jet was to lean the idle fuel air as the size got bigger?

Yeah my next idea was to put the air jets to 160 (make sure the air passages are real clean to, they could be dirty still but I doubt it) and then try smaller pilot fuel jets. A 38 pilot would flow about....20% less then 42.5! When thinking of pilot sizes I thought that going from a 42.5 to 38 would only change the flow about 10% tops but if you work it out its about 20%.

Ive got a number 79 drill (0.368mm) which once drilled should leave about a 0.38mm hole AKA #38 jet. Time to break out the solder.

And I did get an exhaust reading too with the jets lol, I was choked with grey/black exhaust smoke (so yea it was a tad richer).

First Inkjunkie: yes I do think 39's are overkill for a street A12, I think 32-35 would be a better fit. My car would be very streetable but I suspect the gas milage would be down a good bit also..........here in Las Vegas we actually have to smog any car 68 or newer (all the other counties have no smog) so as rich as the car is at idle it'd be iffy.

No as far as the bog due to air velocity, as I play with needles it gets less and less. I'm currently on a OCFBR needle with the clip on the lowest postion and bog is more of a flutter and oh so slight. At anything above 3K you can roll on the gas very quicky. You still can't slap it to the floor ( but you can stuff your foot in it faster than you ever would on a race course. I have some OCGFR needles which is the next step steeper taper, I only ordered these so I could put the clip back in the number 3-5 position. As with my dirt bike carbs there are times when you move the clip up or down in for heat or cold.

diymark................holy cow a 220 air jet!!! I don't know how the Mikuni compares to the Keihin but I'm running a 50 Pilot Jet with a 100 Slow Air jet. There is a 145 main with a 175 main air jet. Fuel screws are two turns out.
There is a website Factory Pro.com that has a tuning guide for the FCR and the recommended steps are Main jet, Main Air, then needles then Pilot and Pilot air jet..........which I've found to be very much the case. I suspect I have one step to big on the pilot trying to offset the needles. .

For anyone tuning these you absolutely need to get dyno time or an air fuel ratio meter. Mine were so far off that at one point I had the 145 mains in them but it still didn't feel right, I went back the other way once and then worked my way back to 140's which finally made the car run halfway decent. I went up a few steps on the pilot and raised the OCEMV needle all the way up and that seemed very good but then at the dyno I made another round of changes. just going up one main jet size made 3hp. Like I said previously another dyno trip is in order.........might even get to 75whp.

As for webers don't have much experience with them so can't say how they compare, now the oft maligned Hitachi SU's are a great all around carb but obviously they aren't going to make the same kind of power. The motor would pull 8K with the SU's but the FCR's pull the revs so much faster and do have that snappy rev that everyone always talks about.

Tom

The leaner needle will solve this problem but this is because it lets less fuel until you reach the main circuit which is good to solve the bog problem when you floor it but what if you say your "moderately" accelerating and using the mid slide position (critical needle taper area) will your engine be running lean since you put a leaner needle in? Need a WBO2 in this case really and do some road tests.

Speaking of mains I'm running 235 (yes thats a 2.35mm hole) IDK why is so big but that's what runs good - at WOT and the car pulls darn good (my wide band proves this - best $350 I spent). It maybe an excessive size and maybe the needlejet/jetneedle combo is still acting like the restricting function at WOT and regulates the fuel? IDK, car runs good though.

Actually I'm going to a richer needle , it is just as you say..............steady throttle then going to full throttle is what causes the bog because the needle is to lean. This will be my third set of needles, with each each step richer the condition improves and I end up lowering the clip (raises the needle) to fatten it up. You are also correct in that if you have a needle with to big a root diameter it restricts the flow at WOT (read the stuff at Factory Pro.com)

I went to the dyno because it's really hard to do a proper plug chop / top gear pull on the road. Once I got the proper main jet in everything else quickly fell into place. Often what feels fast and what is fast differ. If you're slightly lean in the mid range there may be no bog but then the car bursts into life at 4500 RPMs it's going to feel faster than if it were properly set up. I'll be heading back to do some dyno runs.................you do need to know the air fuel ratio and have to get a load on the thing.........this is tough to do even on old abandoned highways. It's kind of hard to find a place where you can go 107 mph for 10 seconds.

Tom

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What does a flat-slide CV carb look like (or is the description that obvious) and why the excitement over the flat slide as opposed to the piston slide?

The flat slides are on rollers, less wear and resistance. a CV carb has both the piston and a butterfly........most easily spotted by the diaphragm chamber on top of the carb. The CV carbs flow less, also I seem to recall that a flat slide meters fuel better than the round slide..........I'd have to look it up to say for sure.

I have them on the car because as with all things on my car I got a deal on them and unlike the Weber Carbs I didn't have to move the brake master..............granted flat slide carbs would be me first choice but if I'd got a smokin deal on DCOE carbs well they would be on the car.

Tom