Carburetter problems

I have 1970 4.2 E-type OTS that I can’t get to idle smoothly unless I open the choke valves fully. Since the fuel jets are set at the factory, I tried to increase the fuel mixture by shortening the metering needles. I shortened each needle .005 inch at a time five times for a total of .025. All had no effect on the idle. I also checked the vacuum at the check valve on the reservac tank. It was 15 inch at 1200 - 1300 RPM. Is that a normal vacuum?

Can anyone advise a corrective action?
Thank you in advance, Ron

I’d suggest buying new needles, install them and then come back to troubleshoot. The factory setting for the jets is the baseline from which their levels are fine tuned. Have you checked for air leaks, particularly around the throttle shafts?

I assume this is about ZS carbs. Have you checked the floats and float levels or for sticking float bowl needles ? This is the first thing I check If I develop idle problems.

68 E-type FHC

First thing to look for when you’ve twiddled all the controls: have a look to see if the bodies are worn where the shafts enter/exit the carb body, and if the shafts are worn. Needing to enrich the mixture is a symptom.

Unmetered air entering here can wreak havoc when the butterflies are closed… it’s like a manifold leak.

Second: intake leak elsewhere.

Ah. Yes. Ignore above, except for the possibility of an air leak.

Spray some carb cleaner around the throttle shafts. This is the most common place for a vacuum leak. If they’re leaking, idle speed will change as the cleaner is drawn in.

That would be my guess too. There are lot’s of places on the Stromberg setup where air can leak in and mess up your idle unless you add some choke. In addition to the main throttle butterfly shafts, you have the bypass valve(s), the temperature compensators, and the secondary throttle shafts. A spray of starter fluid or propane around the suspect area is often proposed - though you need to be CAREFUL to avoid a fire, AND HAVE A FIRE EXTINGUISHER HANDY…

Thank you all for your responses. I should have initially mentioned that the float levels have been precisely set with a dial Vernier and I have sprayed carbo cleaner generously around the complete engine compartment, especially in the carb throttle shaft areas. There was no increase in engine RPM. I am still curious if 15 inches of vacuum is typical, even considering no air leaks were indicated with the carb cleaner test.
I hope more suggestions forth coming. Thank you again. Ron

If you are suspecting vacuum leak try to check any area that might grant access to the intake manifold. Several years ago I had poor idle that stumped me for awhile. Turned out to be gasket for the crossover pipe where it mates to the top of the exhaust manifold was leaking.

Try not to fixate on the carbs if not obvious. I have mis-diagnosed ignition problems for carb problems before.

68 E-type FHC