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I am trying to get a modified Nikki up and working. The float bowl needle valves appear to be new and I am thinking that is why I can't control the fuel level in the bowls. I have had to adjust the float height (back to stock 16mm) and now have one side that seems to maintain the right fuel level (at least for a while). The other side still fills the bowl and overflows through the main fuel jets. I have tried "burnishing" the needles, but am not sure what exactly I am trying to do. Can someone give me some advice on how to do that and also if I need to burnish the inside of the housing (and if I need to do that how to do it). I also tried using a "broken in" needle from another spare carb without success.
I did get the car to run on the car for a short period of time (15 secs), but then it flooded and couldn't restart until I put my old "stock" carb back on. I think once I get the flooding issue sorted the carb should work out OK. And yes, the bowl vent valve has been properly de-activated.
Check the seat, it may have a burr or something that keeps the needle from closing. Also make sure that float is actually floating. Floats can get holes over time and fill with gas.
I have checked the floats and they are both fine. I could not see anything on the seat, but did lightly, manually spin and pointy dremel abrasive bit on the seat. Also rounded the corners of the needle (the square cross-section brass part) a little more. Blowing in the supply side(I think I can blow about 2 psi) and blocking the return, there is no airflow when the floats are setting down on the needles. Must just be catching under some circumstances.
It hasn't stopped raining here in sunny San Diego for quite a while so have not had a chance to retest it. Hopefully tomorrow will be clear enough.
Another quick question. How important are the little wire retainers that hold the needle to the float. There were none in the carb, but I have some spares from another carb. When I put them in though they seem to interfere with the movement of the float. Might have even caused my last flooding incident.
You can get by without them but they are there to allow the floats to positively pull the needles down. Not sure of the technical reason for that but those Mazda engineers usually have reasons.
Another thing I'd add about those clips is that there are different designs, as are there different float styles. Talking about just RX-7 carbs (I don't know much about the earlier ones) there are two different styles; the SA and the FB. While the differences are slight - I wouldn't know the clips apart unless I saw them side by side - if you use a combination of the two there can be some interference. I've found that it sometimes is enough to prevent the float from returning fully before depressing the needle (causing low fuel levels) or get caught and cause flooding.
If you're having doubts about the clips try omitting them for now. If you solve your flooding issue try adding them back in - they are there to help pull down the needles after all like tg mentioned.
Are the original needles and seats still in serviceable condition?
Just (a couple of minutes ago) got the carb to not flood. I installed new float bowl valves from a new rebuild kit and it works fine. It seems like the springs inside the old needles were kind of jacked up. I did get the clippy things installed, but seems like there might be some interference. At least I got to where I could start the car on the modded carb (124mm primary venturies). Currently at idle (if you can call 3k idle!) the A/F is too lean 14.5-15.0. Bringing the revs to 4k smooths things out and drops A/F to 13.0 so I am thinking the idle circuit might be a bit jacked. No idea what the PO did when "rebuilding" the carb. Pulling out the manual now to see how that circuit works. Never had to deal with it before, because only did general jet cleaning and jetting changes on the original carb. Nice to get the flooding out of the way, at least temporarily.
Ah, that reminds me that there are also slight differences in needles too, although if it works it works (or at least is closer to working). Nice to hear there's progress.
Still more work to do, but got the idle decently smooth at just over 2k with A/F = 12.5. Completely closed the Air Adjust Screw, which brought excess air right down. Now need to see what I can do with mixture adjusting screw in combo with the air adjust. Honestly if everything else works fine under load, idle at 2k is perfectly fine, since this is purely a race car.
Getting close to being ready to go to the dyno for carb (stock vs modded) comparison. Managed to get a "private" session with my son at Willow Springs last Saturday to break the engine in. At the end of the day (when rpm limits were lifted to 7k) the car was 3 secs per lap faster (1:43 to 1:40) with the street-ported engine and stock carb. That was on street tires with +30 racing hours on them. So results are promising and hopefully more so with the modded carb.
It is an SA carb. It was the easiest to start with (or at least the most attractive starting point in the pile of carbs). Hopefully I don't run into any issues with the power valve. I pulled the power valve solenoid, which should prevent the power valve from ever actuating. My only concern is a possible vacuum leak in the bellows (or whatever) that actuates the power valve. Not sure how to identify a problem if there is one, but.....
Had another flooding incident after getting everything running smoothly. Took the air horn off and the clippy things on the needles seemed to be interfering with the float so they are now out for good. Based on the attached picture, I don't have any clips that will work with the SA float and I honestly don't really see the need for them anyway. The Mazda carb manual doesn't show them for any of the generations. Looking at the floats you can see that the flanges on either side of the level tang are very small on the FB, but very wide (which is causing the interference) on the SA. Will see how consistently the fuel level will be maintained with the new needles and seats.
Anyway, the idle is adjusted down to 2k, which is fine if that doesn't cause any other problems. A/F is about 12.5, going up a little as rpm is increased to 4-5k. I am guessing the jetting of the secondary will be pretty close, since I didn't change the size of the venturis or jets there. Will see how things look on the dyno, hopefully in the next couple of weeks.
04-09-20, 03:12 PM
