Zenith Running Rich

[printjunky]

Timing's been set by ear, by beer and by gear (light) and is as perfect as it gets. I've actually never had a Rover kick on this quickly. As soon as the key clicks, it's running. Hasn't gotten REALLY cold here yet, but that's a good sign.

Problem is, the black smoke with the STRONG gas smell, and more importantly, the plugs getting so sooted up in just about 10 hours of running that I had a no-start situation. Advancing the timing a bit didn't help enough to notice.

And leaning the mixture all the way out didn't do it, either. It's still carboning the plugs badly (brand new ones were full-black in less than 5 minutes), so it's likely gotta be the float level (or a hidden catastrophic fault in some other part of the carb). I recently did a rebuild on the Zenith (proline parts - yeah, I know, but $25 vs $200!!!!) and my measurments could have been off. So time to redo the float level.

This time I'm gonna pull the linkages and crack the carb right on the car.

My question (finally) is, anyone think I'll need a new top half to bottom half gasket? If so I want to get it ordered before I start the operation.

[gudjeon]

Probably the top carb gasket and the bottom only if its suspect. These are not hard to make and I have even made the carb gasket with taking time and care. I set the float level 1mm lower than the book suggests but only do so if your pump is good. It does not have to be up by much to run rich and sooty. I had a brand new genuine Zenith and the float had to be reset.:sly:

[Nium]

Try tightening the three screws that hold down the triangular cover on top of the carb. It's the cover for the economy device IIRC. I've had the screws on that work themsleves a 1/4 turn or so loose and it has caused poor running.

Adjust the points if you have them.

[printjunky]

ok, nudged the float down a bit, polished the pump piston, double checked everything. Checked the condition of the mixture screw. Reset everything, and it's running like a top. Incredibly quick starting. Smooth, quiet idle, and a quick zip around the block indicates good accleration and better power than before (though that's a quick test.)

But is there any other way to tell if it's still running rich, besides an exhaust gas test? Clean then check the plugs again, I suppose.

It seems better, but it does still seem to be pouring out some pretty gassy black smoke (though less). And it's definitely running better. I've reset the mixture more, and tried leaning that out a bit from the regular 1 1/2 turns. (out is leaner, right?)

[Nium]

Idle mixture screw only controls the fuel mixture at idle and has nothing to do with the running mixture. All the way in is the leanest it can go because the tapered end of the mixture screw plugs the hole the gas goes through to get out of the carb. Anything out from that will give a richer idle mixture.

It should be possible to cause the engine to stall, at idle, if the mixture screw is fully seated. Never force the mixture screw all the way in or seated as you can damage the hole into the carb.

If you're still getting black smoke and it's warmed up I'd think it's still runnin rich.

[ybt502r]

This may be stating the obvious, but has this Zenith been "warp treated"? You can check the histories in this forum, as Zeniths have a long-standing reputation of sometimes developing a warp between the two halves of the carb over time (due to improper annealing of the metal when cast). I had a new (it was unused in the box at least) Zenith on my SIII, and it ran great (replaced a Weber, and I've never regretted it). Drove the Rover from Colorado to Calgary (three hard days) and it started running rich...i.e. warped the carb.

As I understand it, the body of the carb when warped does not seal, letting gas through to the plugs, resulting in a rich mixture (black smoke, dirty plugs). The fix is basic (and well documented) - regrind the two faces with emery paper on a sheet of glass. I did that but ended buying a rebuilt (and dewarped) Zenith anyway (the mixture screw broke off in the body, my fault). It's been just fine ever since.

As you describe it (runs fine, but rich), it sounds very much like the problem I had. And as Walker points out, the mixture screw only is for the idle and has absolutely nothing to do with the running of the carb under throttle.

[printjunky]

Yup, did the warp fix on the first rebuild. Had some other problem so a second rebuild was in order (all before the car was running!). The second rebuild resolved what I suspect were combined clogged jet/orifice problems as well as some unknown pump piston problem.

It is running rich at idle. Maybe throughout the power band, too, but I'm only addressing idle at the moment. I've tested the joint between the halves by spraying some starting fluid around the joint while running. Not an absolutely perfect test, but it should indicate a problem.

I did find a MAJOR vacuum leak in the form of a tiny emissions/vacuum pipe not being connected to anything. Capped now.

And the other big fail was getting the idle mixture screw operation backwards. IN for leaner! Duh. Controls fuel NOT air! That should help.

Gonna go clean the plugs and see what happens over the next few hours of running. hopefully they'll be dry and tan!

Any other indications I might look for for leaner running? What's gonna indicate if I go too lean? What's the perfect mixture anyway? I assume, as I said before, hitting that can only be guessed at wtihout an O2 sensor, right?

[printjunky]

Bumping this to see if anyone knows what I should look for to check for lean running.

[Nium]

You could slap a hand held vacuum gauge on it to check if the idle mixture is out.

Couple of links on vacuum gauges if you're unfamiliar with them...
http://moodle.student.cnwl.ac.uk/moodledata_shared/cdx%20etextbook/dswmedia/toolsEquip/hpt/measuring/vacgauge.html

this one has pictures of the gauge with the explanations
http://www.secondchancegarage.com/public/186.cfm

Cheers

[printjunky]

Walker, I have been working on cars for ...lesee ... 28 years (!). Since rebuilding the 289 in my first car, a 65 Mustang. That also includes a major rebuild on a previous Series III. And I, of course knew about vacuum gauges, but only as a means to find a vacuum leak. Never knew about the diagnostics that could be performed with one! Of course it makes perfect sense now that I read that second link.

Thanks a ton! That was a quick, immensely valuable lesson (that I wish I'd learned 20 years ago).

Adding a vacuum gauge to my arsenal tomorrow, for sure.

Best regards,
- Shawn