Zenith Stromberg Running Lean

When I bought my 1980 MGB, it was missing the heat shield, had some starting issues when hot and would idle inconsistently after a drive to the point of almost stalling at a light. I decided to rebuild the carb when I took it off to install the heat shield (plus it was running rich). The only problems I ran into was getting the right mixture needle. Because the engine had been replaced with one from a 1975, I wasn't sure which to use and finally figured out the 75-76 ZS carbs have four long screws and two short in the float chamber cover while the later ones have six long.

The rebuild addressed idling issues and I think the hot start issues, but it runs lean now. At least based on lifting the piston slightly and it almost stalling. Same goes for lifting the damper cap/rod slightly. I checked for vacuum leaks and found a bad one at the EGR which I fixed and still have one at the carb body to the manifold which I plan to fix.

If I adjust the needle all the way rich, it still runs lean. Can the vacuum leak be the cause? The EGR valve leak had to be there before. I pulled the plugs tonight and based on the photos on theautoist.com I'm a #2 (gray with a slight pink tint). I also feel a hesitation at times, not really a skip due to a plug not firing.

Thanks for your advice.

I suspect the leak might be the cause as ZS's almost always run rich - fix the leak first for sure - then check

Yup. A vacuum leak will lean out the mix. You'll need to fix all that before you can get a good setting with the carb.

Thanks guys. I'll pull the carb and try to better address the vacuum leak. I used new gaskets and used some gasket dressing without success. I'll try applying more next time and see if I can achieve success. I thought I would see some improvement with the EGR vacuum being fixed, but I guess not.

Try locating Permatex Hylomar for the gasket. A THIN film should do.

Rumor has it Harbor Freight carries it.

I pulled the carb and figured out the gasket between the heat shield and the induction heater was oriented incorrectly. I had the narrow part to the top/bottom and the contact for the heater was outside the gasket. I reoriented it and put everything back together. No leak confirmed with carb cleaner.

It is still running lean based on the lift the piston test and I can't find any other vacuum leaks. However, I have figured out the anti run-on valve is not working. Could that be contributing to the lean running?

Thanks for your advice.

Maybe put the old needle back in and see what happens? Are you sure the needle is moving when you turn the adjusting allen screw? Can you tell any difference when you adjust the mixture? Also if you changed your air filter at the same time as the rebuild that could be the reason it appears lean now. The filters get dirty from the inside and may have been restricting airflow. If its not spitting back thru the carb when warm (under load) i would think you are fairly close. Just some things to check and maybe get you on track to fix it! Bob

The needle is definitely moving when adjusted based on pulling the piston and resetting it to flush with the piston a couple of times. The air filter was changed back in February prior to any of this and still looks clean.

I pulled the anti run-on valve last night and found it full of charcoal. Once I cleaned it out it seams to be working, but I'm now trying to get the charcoal cannister out so I can rebuild it and prevent the reoccurrence of the blockage.

I have not tried running it since I pulled the anti run-on valve, but I wonder if the valve was jammed partially open it could be causing issues?

If this doesn't help, I'll try the old needle.

Thanks again for the advice.

Rand