Out of options..engine keeps stuttering

[Matthew Devereux]

If you have access to an Innovate air/fuel meter that could help a lot. It will tell you whats going on with your air/fuel ratio. I bought a used one a few years back for selecting carb jetting. I have an O2 sensor bung welded into the muffler as well as the tailpipe sensor holder.

https://www.innovatemotorsports.com/products/lm2.php

[Eric Wills]

We had a car in the shop that would idle OK, but stumble while driving around. Turns out the fuel tank vent was blocked, creating vacuum. Pulled the gas cap slowly, heard a hissing sound and watched the tank expand! Ran a lot better after opening the plugged vent.

EW

[Geoff Ver]

Thanks for all your responses!

@David: will double check for loose head nuts next week!

@Franny: it feels like it doesn't get enough fuel (but double checked the fuel pump and tried a electric one). It's a constant stuttering when accelerating and normal driving.
I will do the compression test next week. Also replaced the coil and checked all connections. Certainly re-checked the timing.
But some plugs look black...and black again after each long test drive. Not all of them.

@Dick: I've got Zenith's. what would you recommend to test again?

[Geoff Ver]

We had a car in the shop that would idle OK, but stumble while driving around. Turns out the fuel tank vent was blocked, creating vacuum. Pulled the gas cap slowly, heard a hissing sound and watched the tank expand! Ran a lot better after opening the plugged vent.

EW

Thanks Eric, to be sure I will test this as well!

[Matthew Devereux]

Do the accelerator pumps output measure correctly, sounds like its going lean.

Yeah I would be checking this next.

[David Jones]

I cannot see the accel pumps having anything to do with it if it is happening in cruise. No accel pump injection will just make for flat acceleration and not affect mixture at all.

[Mike Wilson]

I'm leaning toward an electrical issue. Question: mechanical or electrical tach?

Mike

[Martin Benade]

Have you tried a jumper wire to feed battery voltage to the hot side of the coil? That would rule out the ignition switch and wiring harness. And as alluded to, if you have an electric tach leave it disconnected.

[Steve Newland]

This reminds me of a problem I had about six months ago.
Driving my 356B from Brisbane to Sydney, I’d happily travelled about 800 km (of the total about 900 km trip) and pulled off the freeway to refill the fuel tank. Wind driven rain was pouring down and the contents of my trunk and myself got saturated. After setting off again, all seemed OK until I reached the next hill on the freeway. The car started violently stuttering and I lost power and speed. My immediate diagnosis was ‘rain water in the fuel’ from the drenching we’d just received. This seemed to settle after about 20 km of relatively flat travelling and I thought that the water had been cleared from the system and I completed the trip without further issue, but still in pouring rain.
After the car had been on display with Porshe’s Mission E for two weeks, I returned to Sydney to drive it the 900 km home to Brisbane.
I had no issues with it for the first 150 km and then stopped to refuel. Setting off again, the stuttering returned..... Had I stirred up some residual water in the tank?
Then I remembered what my dad used to say: “If you think it’s fuel, then check the electrics (and vice versa)”, so I pulled over to check the electrics, but couldn’t find any loose connections.
I drove on and it seemed to settle until I reached a hilly area again. Eventually, the little car failed completely and I was now stuck on the side of the road ‘in the middle of nowhere’. I decided to check the electrics more thoroughly, due to the sudden and complete shutdown, and when I checked the connection from the ignition onto the coil, it broke off in my fingers. I repaired that connection (with duct tape!) and drove home like that with no further stuttering issues.
So, I am leaning towards Martin’s and Mike’s views.

[Geoff Ver]

Mike, Martin, Steve, thanks for your responses! I will put this on my list for next week. I did check the coil and all connectors because I know they can get loose. After my engine rebuild I"ve put new connectors on to be sure this wouldn't be a problem.

@Steve: nice roadtrip! Glad I can't get in the middle of nowhere in The Netherlands

[Mike Wilson]

We're happy to help by offering suggestions to solve the problem. I'm sure you'll keep us posted.

Mike

[Jules Dielen]

exhaust leak?

[Felix Macaluso]

My stuttering problem was on the interstate above 50MPH. Thought it was fuel -- turns out it was the distributor rotor. Looked fine but the contacts were corroded. Emory paper on the rotor tip and cap contacts cured the problem.

[Geoff Ver]

We're happy to help by offering suggestions to solve the problem. I'm sure you'll keep us posted.

Mike

Yes, for sure! Next week on Friday and Saturday I've got enough time for new tests.

[Geoff Fleming]

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.