Yeah, I know. This topic has been covered to death, but I can't find a direct answer to my questions by someone who has actually observed the habits of a healthy metering oil pump on a 12A! I've been mixing at about 256:1 (1985 12A, stock) only the last year just in case the damn thing isn't working properly.

So here's the deal. I rebuilt the pump about three years ago with the Lowe performance kit. If my car sits overnight, I see some air, (especially at the rear rotor housing feed tube at the carb since it is a longer run) or if it sits a few days, the tubes are almost dry inside. When I start the car, I can see the oil creeping up in little steps so it must be pumping.

I don't remember if my '79 SA was like this or not. I'm wondering; what have others actually observed happening with their oil metering system? The engine doesn't use nearly as much oil as my RX8 anyway. (but then, my right foot gets heavier when I drive it)

Also, does anyone know if there is supposed to be a check valve in the pump? I found a small steel ball in my parts washer (about 1/8" dia) a few months after rebuilding the pump, and I hope like hell it's something I didn't miss it when reassembling the pump.

 

I've never rebuilt an MOP. I once saw an engine that would constanly have bubbles in the tubes. I assume it was because the front cover gasket was not seated properly and the counterwait was forcing a small amount of air into the MOP oil feed passage every time it would rotate past that part of the front cover. Or maybe it was somethng else. Anyway, this doesn't sound like the problem you're having. It sounds to me like the missing check ball may be what is allowing your tubes to gravity-drain back into the MOP and out through the MOP over-pressure hole on the inside of the front cover adjacent to the MOP drive axle shaft.

 

Hmm my lines have always had air bubbles... never worried me

 

Well, my MOP lines have *never* had air bubbles in them, so I don't know what to tell you. Part of the OMP metering function is the vacuum lines that come off of the 'spider' over the intake manifold - this spider gives fluctuating pulses to the metering heads on the IM, and if you don't have them hooked up to a vaccum source, I'm not sure if they'd work correctly. They also help to 'squirt' the oil into the intake air path.

I rebuilt my OMP last summer and don't remember a check-valve ball of any type. If you're seeing the oil move up the lines, then it is working. Pre-mix never hurts, but may be unnecessary if you can see the oil moving up the lines. HTH,

 

Duck, 12As and earlier 13Bs didn't use vacuum lines on the MOP line connectors. They instead went right into the carb.

 

I've always had air bubbles in my MOP lines. Motor now has 197,000 miles on it and still running strong (knocks on wood) so I don't guess it would be a problem....

 

Hey thanks for the input. Yeah, I have read the EGI 13B MOP uses a different plumbing configuration.

The air space would seem to be a problem only until the engine has run a minute or two and the tubes fill, but that wouldn't seem to be the ideal situation.

As far as the steel ball I found goes, it could have been from my friend's scooter I was working on at the time, or something else.