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Figured I should make a new thread instead of adding even more issues to my last one. Also, this is just one problem (hopefully), so it should be less cluttered.
This last tank of gas has been the first full tank the car has seen on the road since I've owned it. I know I can't expect amazing gas mileage from a 38 year old rotary, but honestly... 10 MPG?!? on a stockish car? To me that seems a bit low, and I was curious if there was anything to check right off the bat. The car has always smelled rich, and the plugs were nothing but black but I figured that was just from all the idle testing. While attempting to pass smog, I found that turning the mixture screw on the carb counterclockwise (out, or richer) made the HC's go down. This is a 1982 carb by the way. As of now, I believe the screw is about 12-14 turns out from bottom. I know that's a lot, but it seemed the only way I could pass smog. I've left it alone since and now that I know the mileage I'm curious if this could be part of the problem. Yes it's the idle mixture, but does it affect anything else, even in the slightest? I'm going to assume there's another issue somewhere, as when I did the whole carb swap and all, I could only get it to idle with the mixture screw about four or five turns out. That is opposed to the "base line" of three and a half if I remember correctly.
The other big thing I know isn't helping anything is my lack of vacuum advance. This is something I need to do some troubleshooting for, but essentially the solenoid isn't getting any power. I've heard that the 2nd from front vacuum port on the carb spacer is ported vacuum, although I haven't tested that myself yet.
Something else, that is partially unrelated is the trailing ignition. For the early cars, '79-'80, with the thermal reactors (which I had originally), the trailing plugs would shut off on decel/coasting in order to keep the thermal reactor hot enough to work. By mistake I left a connector unplugged to the ignition box and found that it affected whether or not the trailing was enabled. Would it be worth it to leave it enabled all the time? As of now I believe the trailing is shutting off just above idle, but I'll have to check that. Would that cause better/worse mileage or performance?
To clarify then: 79 with 82 carb? and no stock exhaust?
Presume the usual stuff like new plugs/points/timing check have been done?
Since the Trailing shut off is a stock setting, one would "expect" it would not affect claimed (by Mazda) mileage. I believe Racing Beat and others have suggested wire-around (don't know it ) that actually keeps both sets of plugs firing all the time...
The car is actually an 80, but yes, the timing is dead on and the rotor and cap are practically new (as opposed to the 79's points system). The plugs are still the same, so I really should replace those, although if the issue is something to do with mixture I don't want to trash new plugs. When I bought the car it had an later 12A engine and therefore the thermal reactor and other smog equipment would not work properly. Hence the swap to the matching "exhaust", intake, carb, and rats nest. It's not the most beautiful solution, but I did the research as to what would work still with the 80's ECU and so far it's been running and driving fine, although not very efficiently as it would appear.
The exhaust still has the original header or box that bolts to the engine, but the rest has been replaced with some steel pipe and a 3+1 catylitic converter.
The idle mixture is also basically the same as when you have gotten up to speed and are cruising with light pressure on the gas. So if it idles rich, it will cruise rich. The 2nd port from the front is ported vacuum and should be hooked up to both vacuum advance pots with a tee and some tube. It will make the car feel a lot peppier on take off.
You should check the basics first. Is the float level correct? Check it before starting and then check it while running. Make sure it stays at the center line on both bowls. If the levels are too high it will run rich and the mixture screw won't make a huge difference. Pull the plugs and clean em with some carb cleaner and a wire brush. All that black carbon on there can really make the spark weaker than it already is with the stock ignition. Also is its running as rich as you say, that cat isn't going to last very long.
My hogged out nikki gets ~24 mpg and its an 83 carb/intake in an 80 SA on the stock motor. No ECU used at all. Computers are evils says the me, a software developer.
Wow, I never thought about the idle mixture as also affecting cruising. I guess ya learn something every day.
Floats are good running and not. I just did a full carb rebuild a couple months back (I think it was in one of my other threads, not sure though), and the float levels are nearly spot on. They are actually the ones I took from my 80 carb since the 82's kept flooding for some reason... I have noticed that since my "swap", the car has seemed much less peppy for certain. Perhaps that's because the SA's ran one of the advances (leading or trailing, I forget which) straight from the spacer block, and the other from the solenoid. Now both aren't connected at all. I also recall checking the 2nd from front port and finding that it never produced vacuum at any time. Although, that was just sitting and revving in the driveway so perhaps that would be different while driving? I'll look into it. I did make sure all those passages were clear before I put it back together so you would think it would work.
Any ideas on why the car doesn't run at the baseline settings (besides vacuum leaks)? Also, why would making the mixture richer make the HC's go down; perhaps the cat was just getting hotter?
And again, wow! I thought 10 mpg sounded low... That's impressive. If Oregon ever moves the smog year up - it's 1975 right now - I'd sure be glad to remove the remains of my rats nest. I might look into making a "custom computer" from an arduino or something to tidy things up. Just to manage functions I still want, if there are any that is.
Well if that's the case, my mileage is definitely sub par .
As for the fixes, yesterday I figured out why I don't have ported vacuum. It turns out that the throttle plate I had installed did not have the two extra holes for the ported vacuum source. One of my spares did so I spent last night cleaning it up. I have to say, reassembling a throttle plate is not the most fun thing ever. Those secondary butterflys really hate lining up... Anyway the carb is reassembled now so today I'll put it back on the car and retune it. Turned out the mixture screw was only 8 and 3/4 turns out from bottom, so not at much as I had thought. I did put it back at baseline so maybe I can figure out the issue and tune it properly. I also noticed the new throttle plate has one more jet that runs to both primaries. My carb body doesn't have it so I suppose it shouldn't affect anything.
Here's a photo of the reassembled throttle plate. Not modified yet...
Definitely need to be careful mixing and matching carb bodies to throttle plates. They vary slightly by year and as you discovered have extra holes in some and not others. Thats why when I did my nikki I grabbed the intake, tbody and carb in one chunk at the boneyard so I wouldn't get an issue like that when I was done.
Jeff20B has some really informative musings on this subject in several threads. Just do a search on his userid and start reading up.
Well I can definitely see the issues that a mismatch could cause. And speaking of issues... I've got the carb back on and it "idles". I'm having issues with it bucking at idle and up the rpms. I also can't get it below 900ish rpm for idle, however it did idle a lot closer to the baseline setting this time. I've rechecked the float levels and they are darned near perfect, both at idle and running it up. I can't find any connections or hoses I forgot so.... I might just take it back apart and double check everything is good. Maybe the butterflys aren't lined up just right causing them to be open slightly. Are there any things to check right off that cause bucking? It almost sounds more like a miniature backfire, but not quite.
On a more positive note, I have vacuum advance now
Well yesterday I took it all apart again and just for the heck of it, cleaned the entire carb again. Put it all back together and I still can't idle below 900-1000. My car also doesn't seem to like vacuum advance a whole lot. It backfires more when you let off the gas and it "bumps" while you're just cruising down the road. Disabling the advance is as easy as plugging that hose, but it's the idle that's bothering me. The car is also more difficult to cold start (with choke) if that indicates anything.
As for the tuning: Mixture is about 6 turns out and the throttle stop isn't even touching the linkages. Surely I couldn't have made such an idle difference just by cleaning the butterflies before I installed them (steel wool/sandpaper) The amount of light that I could see between the plates and bores were minimal to none, but I suppose that could still be it? Unfortunately I can't just swap throttle bodies back quickly as I had to disassemble the original for some of the parts for the new one. How far open is normal for the primaries to be at idle? From what I can find, 6 turns out for the mixture is still really far. I'm about to go look at it again to see if I notice any stupid mistakes.
08-22-18, 07:43 PM
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