Out of options..engine keeps stuttering

[Geoff Ver]

Perhaps something as simple as the leads to the coil being reversed. I have had both a 12v and a 6v coil go bad, when I used my cars as daily drivers and the engine would stutter upon acceleration. Unusual for the coils to fail but it does happen. Do not exclude anything from suspicion in your search.

The coil and connectors is definitely one of the first tests on my list, thanks!

[Geoff Ver]

Hi all,

After another full day of testing I still haven't found the solution. We checked the following today:
- Checked the head bolts (just to be sure), no problems.
- Did a compression test, all cilinders fine.
- Valve adjustment, only very small adjustments necessary to get them back to specs.
- Changed the coil, didn't help.
- Checked all connectors and fuel lines, everything fine.
- Put the 123-ignition back in correct order.
- Reopened the carbs and checked everything again, they do look fine.

I'm out of option again unfortunately. Could there still be a problem with the carbs?
Thanks again!

[Matthew Devereux]

Did you check the float levels?

[Geoff Ver]

Did you check the float levels?

Yes, we did. It was just a little bit to high so adjusted it to correct specs.

[David Jones]

Geoff, could it be a problem in the transmission? What does the stuttering sound/feel like. I am trying to imagine what you may be experiencing but I guess it is something that must be experienced to understand properly. Is it a surge up and down in rpm or do you feel a variation in drive power? I assume it is not just a sound you are hearing but you feel the car change speed but if you de-clutch does it immediately go away and does it go away on the overrun. You are in the Netherlands so the only way to do a load test as a hill is hard to find is to put the car in 2nd gear with handbrake on and let the clutch engage at 4K rpm and see what it feels like. Maybe if you can find someone with a chassis dyno it may be worthwhile putting it on and see what can be found.

[jule block]

Are you running with a condenser or is it not part of the 123 ignition. I had a new one go bad and led to many funny running issues that the old one solved.

[Matthew Devereux]

The 123 distributor doesn't use a condenser. The red is to the coil terminal #15 + and the black wire is to coil #1 terminal. With the stock distributor the wire goes to the coil #1 terminal. I'm not sure what happens when you muddle these up? It may have been suggested already but you could jumper power directly to the coil #15 to ensure it isn't a low voltage issue at the coil.

[Geoff Ver]

Geoff, could it be a problem in the transmission? What does the stuttering sound/feel like. I am trying to imagine what you may be experiencing but I guess it is something that must be experienced to understand properly. Is it a surge up and down in rpm or do you feel a variation in drive power? I assume it is not just a sound you are hearing but you feel the car change speed but if you de-clutch does it immediately go away and does it go away on the overrun. You are in the Netherlands so the only way to do a load test as a hill is hard to find is to put the car in 2nd gear with handbrake on and let the clutch engage at 4K rpm and see what it feels like. Maybe if you can find someone with a chassis dyno it may be worthwhile putting it on and see what can be found.

Hi David,
There isn't a weird sound. When driving the 356 at normal speed I feel a constant stuttering. It doesn't surge up and down in rpm and there isn't a variation in drive power.
With the stuttering the 356 is still drivable and accelerates fine. The stuttering is just always there.

[Geoff Ver]

The 123 distributor doesn't use a condenser. The red is to the coil terminal #15 + and the black wire is to coil #1 terminal. With the stock distributor the wire goes to the coil #1 terminal. I'm not sure what happens when you muddle these up? It may have been suggested already but you could jumper power directly to the coil #15 to ensure it isn't a low voltage issue at the coil.

I've been testing with two different coils, don't think it is a coil issue.

[Matthew Devereux]

I was suggesting a coil wiring issue

[Mike Wilson]

When it shudders, is it what I would call a soft shudder like the fuel was shut off or it was starving for fuel or a sharp shudder, staccato-like, like you shut off the ignition? Fuel-related vs. electrical.

Have you run the engine in the dark and looked for bad wiring causing arcing?

Mike

[Bruce Smith]

Remove carbs. Confirm that the float valves are good, replace if uncertain. Set 2X float levels and 4X accelerator pump volumes. Reinstall carbs and bypass the fuel tank with a gas can and fuel line directly to the fuel pump. Run from idle up to 3000 RPM long enough to confirm fuel pump is keeping up and filling the bowls. Replace fuel line to fuel tank and compare.

[David Jones]

Geoff, whenever I have arrived at the stage you are at with a problem that I cannot resolve or pin point I find the best thing is to substitute known good parts and at this stage I would suspect the carbs, probably only one and probably only one throat. There are two ways to go. Run your carbs or distributor, coil or fuel pump on another car or borrow a known good part from another car and try it. Do it one at a time. I have one other thought, did you check that the throttle butterflies are tight and also the venturis or any of the other parts that can be removed are tight in their places. Compare one Zenith and base with the other and look for variances in them. Apart from the linkages they should be identical with identical jetting.

[Geoff Fleming]

Along David's thinking, try running the car with one carb linkage removed. Run up to 40 or 50mph, then repeat by only running with the other carb. It may help isolate the problem to one side or the other.

[Mike Wilson]

I'm assuming Zenith carbs. Have you checked the float needle valves? There are some bad ones that hang up.

Mike