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79 Midget Missfire

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caseyjones955

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Good day all. I have a 79 Midget w/the stock Stromberg carb and distributor.

I have checked compression, adjusted the valves and set timing in many places (now at 8btc) it runs well above 2200 rpm, before that point it appears to miss on one cyl. After checking plugs it appears to be #2, its slightly gray where the rest appear almost new.

Lumpy idle and runs like a bunk cyl, vibration becomes really pronounced between 1500-2200 rpm, above 2200-ish it smoothes out and gets responsive again. Its very smooth and happy at speed. I had a british car guy once over it and he speculated it was an electronic problem. Im still hunting it down. Please help, this just became my daily driver. Any other checks or common culprits?

Thank you

Tony
 
Off the top of my head, it sounds like you may be way too lean at this point. So, I've put together suggestions based on you having the stock ZS carb, dist, and the assumption that since you have the stock ZS you probably also have some if not all of the emissions equipment still installed.

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First, I would check to make sure the Gulp Valve is not stuck open. It should be a valve in the plumbing between the Air Pump and the intake manifold. The Gulp Valve, if stuck open, will cause the car to be extremely lean, and will cause symptoms very similar to what you've described.

Next, I would check to make sure you're getting spark on all the plugs. You may need help with this, but it should be easy to do by pulling the plugs one at a time, and having someone turn the car over while you watch the plugs.

Next, double check your valve lash.

If there are no problems with any of the above, I'd look into adjusting the carb mixture to make it richer. The plugs should have a cocoa/golden brown appearance to them. A slight grey appearance on one and the others looking almost new, seems to indicate that you're way too lean.
 
Thank you for the fast comeback.

I double checked valve lash, its at .012". I still have the stock ign and carb. Emissions system is gone and there is a header and aftermarket exhaust system. This issue gradually came into existance long after these changes were made.

when I took the car to a local guru he had a special tool to adjust the ZS carb and he leaned it out by half a turn and that seemed to make no difference in the miss but made it a whole lot more drivable after about 2200 rpm, I get notably better milage now also. The plugs didnt appear to be burning to rich but it sure was going through insane amounts of fuel before being adjusted. Im waiting on a friend now, we're going to check for spark.
 
Have you checked the carb diaphragm? The ZS has a diaphragm attached to the piston. If that has a leak in it, then the carb won't run right. You can access the diaphragm by removing the top from the carb (4 screws) and pulling the piston out. The diaphragm should be softishly rigid but not 'flimsy' and should have no leaks in it. I'm not sure what the best method for checking the diaphragm for leaks is though.

Also, have you checked for manifold leaks?

What about the throttle shaft on the carb? Have you checked that for vacuum leaks?
 
Checked spark, each cyl sparks strong and w/o missfire, each plug wire pulled at idle has a reasonably equal effect, manifold gasket is relativly new but I checked for leaks the oldschool way and could find nothing wrong.

This is a strange sympton for a carburator but Im new to the SZ, are they worth rebuilding or should I find some other side draft to put on?

Will check diaphram tonight and see how it is.

Thanks

Tony
 
Check the diaphragm well, Tony: Hold it up to a light source and stretch it a bit "this-way-and-that" all the way 'round to get a GOOD look. They can fool ya! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

HTH!
 
Tony, the carb question is an ongoing debate. I'll try to remain neutral as I sum up.

1) the Zenith is pretty miserable, has a poor automatic choke which can cause problems, but can be made to run OK.

2) Weber DGV downdrafts are a step up, make better power than the Zenith, get good mileage, but are a compromise solution.

3) SUs, as came stock on european cars, are the preferred choice for looks and power. A little more expensive. Small sheet metal mods to the inner fender area are required on later Midgets.

4) Single- or double-sidedraught DCOE-style Webers / Dellortos are beautiful, make slightly more power than SUs, get worse mileage, and are $$$. Great for race applications.

Opinions are like bellybuttons... but I'd say that's as close to 'group consensus' as you'll find.
 
Have you also tried asking this question in the Spridget section? That section is dedicated to Sprites and Midgets. You might get more activity on this question in that section.
 
If it has a Zed-S on it, the best first step (as in: least expense) would be to kit the thing and get the car to run properly. Options for alternative induction devices can come after.

What's a "kit" cost for one-a them poxy things? Less than $50, IIRC.

Just my 'first blush' take.
 
The diaphragm seems to be the issue, upon close inspection it would seem to have been on borrowed time. The local MG guy likely has a pile of various carbs for it. I think its time to go mining for an SU.

Thank you for the wisdom!!

Tony
 
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