My 57 356A is missing at Low Speed Head Scratcher

[GregDumond]

Hey Guys,

Could really use your help on this head scratcher situation. A little summary of how it all started. I was on the freeway headed to a car show about 5 miles away, weather was only around 75. Got off the exit and the car started lurching as if it was running out of gas. Took a quick look at the on lever and gas gauge, everything was fine. Fortunately the show was only a half mile away so I limped there and barely made it and parked the car. Spent about an hour and a half checking out the cars while wondering how I was going to get back home. Fortunately the car started and ran perfectly back home. No missing, no backfiring just as smooth running as can be.

The next day I tried starting the car. I could barely get the car running. It was backfiring, missing and it just kept sputtering for 30 seconds until it died out. So my mechanic and I have been working on it for a few weeks. Here is everything we checked for both Fuel Flow and Carburetor adjustments. I have an electric fuel pump to prime the Carbs. It's turned off before starting car. Did a Manual check... Fuel is flowing fine to Carbs. Also drained gas, Replaced with fresh gas.

Ran a pressure check on the fuel pump. It was low at 1.2 PSI so I had it rebuilt to the proper range of around 3 PSI
Replaced the Spark Plugs, Rotor, Condenser, replaced Coil wire. Cap looks fine. Bruce Smith sent me a completely restored distributor a few years ago. Car has been running flawlessly up to this recent problem.

Adjusted timing
Adjusted Zenith Carbs for Idle Speed (Carbs where rebuilt by Carb Rescue)
Changed oil and filter

Symptoms I'm experiencing now when I drive the car, it's missing/hesitating only when I drive at low speeds 0 to 45 mph with low to medium gas pedal pressure. I can also hear a light popping sound when it hesitates at low speed. That goes away when I give the engine full throttle in all gears. Engine runs perfectly in all gears at full throttle. What am I missing? (No pun intended)

Appreciate any thoughts

Greg

[Jules Dielen]

Did you clean the carb bowls and jets? Sounds like plugged jets to me. Does not help to get fuel to the carbs if it can't get to the combustion chamber.

Driving it got crud in the jets. Parking it cleared them up again until they really got plugged.

Time for an ultrasound bath!

[Vic Skirmants]

Idle jets.

[GregDumond]

Thank you for your comments Jules and Vic. One thing I didn't mention. We took out the 4 # 30 jets and replaced them with 35s. The idea was to get the car running a little richer. That didn't seem to make a difference. I'll check out the Bowls and Idle Jet next and let you now if that makes a difference. Hoping I don't have the disassemble the Carbs.

Greg

[Steve Douglas]

How about the condenser? Is one of the new ones with a green wire? Find an old Bosch with the yellow or black wire, with a soldered connection.

[Martin Benade]

It’s unlikely that the #30 jets suddenly became too small to function correctly.

[GregDumond]

Steve we replaced the condenser with green wire part number 856 which was made in Turkey. I was very surprised to discover that. The new condenser doesn't have a brand unless its covered up by the clamp. It does have a black wire. I can't tell if it's soldered. It has a hard rubber tip sleeve covering the wire into the condenser

Martin this was a suggestion by my mechanic to try out the 35s to make the carbs run a little richer. I haven't noticed any difference? I spoke to Candace at Carb Rescue she suggested staying with the 30s but the 35s where fine for low mileage driving.

Overall the car seems to be running well now. Someone gave me a tip to add some Marvel Mystery Oil to the gas tank. Maybe that helped get rid of the possible remaining crud in the carbs?

Thanks for your comments,
Greg

[Steve Douglas]

Find an old condenser, yellow of black wire, Bosch. Use your Volt Ohm Meter, here's a link to a good test procedure. https://mgaguru.com/mgtech/ignition/ig129.htm
Attached photos: left pair have a rubber seal, green and black wire, the green one failed after 500 miles the black one was less than 250 miles. Right pair are old Bosch with solder connection these are good and last for a long time

[GregDumond]

Steve these photos are perfect! I will check with my 356 Mechanic see if he has any of these Bosch laying around.

Greg

[Al Zim]

Send the carburetors to Carb Rescue they should be able to diagnose a faulty design in the carburetor. We are aware of this situation but do not know when Zenith modified their carburetors to correct this situation . Probably with the 1963 production.

[Charles A Rockwell]

Before pulling the carbs off, make sure the synchronization is equal on both carbs at idle and at 2000 rpm. If one carb is pulling harder than the other it can cause your current symptoms.

[Martin Benade]

An intermittent problem would not be a carburetor faulty design or a synchronization issue.

[Steve Douglas]

If it is intermittent, look for an electrical issue. I had a fuse "filament" crack, looked ok but very small crack, in cool weather car ran fine, then if it got above 70 it would barely run at lower speeds. This was on a trip dow highway 1 to a North South gathering.
Did everything, couldn't find it, then on Saturday afternoon it wouldn't start, it was 80 degrees. I used a jumper wire from the BAT on the regulator to the hot side of the coil thus by-passing the ignition switch, fuse box and dash wiring. It ran perfect, unhooked the jumper, it wouldn't run, measured 2.5 volts to coil. Replaced the ignition switch, no help. Finally looked at the wire diagram and saw #1 fuse is in the circuit. Cleaned and replaced the fuse, everything was good. So check your voltage at the coil terminal #15 / +.