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Hi all,
Hope you can help me…I’m a bit out of options.
After my engine rebuild (1600 normal) last year new problems are popping up. For that reason we’ve rebuild the carbs and fuel pump, changed to 123-ignition (electronic) but at the end the engine still stutters. When accelerating and driving at normal speeds it keeps stuttering.
Today we’ve also changed the 123-ignition again, tried a electrical fuel pump, double checked every carb adjustment, replaced the sprak plugs and wires but can’t find the solution for this problem.

Did somebody experienced this before? And did you find a solution?

Thanks!

Too many possible causes to try to troubleshoot long distance. You will get a couple dozen suggestions about individual things that might cause that, but I would go through each system carefully to make sure it wasn't a contributing factor. Causes could range from a bad coil to a camshaft that was installed incorrectly.

Did it ever run right after the rebuild? The best clue you have is from the last thing that was done to the engine before the problem appeared.

Wes Bender wrote: ↑Fri Mar 29, 2019 1:42 pm Too many possible causes to try to troubleshoot long distance. You will get a couple dozen suggestions about individual things that might cause that, but I would go through each system carefully to make sure it wasn't a contributing factor. Causes could range from a bad coil to a camshaft that was installed incorrectly.

Did it ever run right after the rebuild? The best clue you have is from the last thing that was done to the engine before the problem appeared.

Yes, it did run right after the rebuild. The carbs were mainly rebuild because of leaks. The same with the fuel pump.

Need more info. Does it idle OK, is it only there under load, what happens on the overrun when you take your foot off the gas, does it do it in every gear? Can you describe the stuttering better? Can you feel it or is it just a sound? Can you hear it when you rev the engine out of gear.

David Jones wrote: ↑Fri Mar 29, 2019 2:20 pm Need more info. Does it idle OK, is it only there under load, what happens on the overrun when you take your foot off the gas, does it do it in every gear? Can you describe the stuttering better? Can you feel it or is it just a sound? Can you hear it when you rev the engine out of gear.

Hi David,
It idle's ok. It is only there under loud (that's when I feel it). Not especially during a take off but think it is there all the time. It drives but there is a constant (sort of) shock that you can feel. Drove it for 200km's today and it was there all the time. You can't hear it clearly.

Check the head bolts. You may have a loose head and that you can only hear under load. Do a leak down check with at least 100 psi of pressure.

You haven't mentioned - have you swapped out the coil yet??

Bruce Smith wrote: ↑Fri Mar 29, 2019 3:36 pm You haven't mentioned - have you swapped out the coil yet??

Yes, I replaced it at the same time with the ignition (123-ignition).

Quick and easy check for loose head nuts is to remove the valve covers and the rockers and see if they are loose. Don't ask how I know to check them for the reason for the sound you are hearing. One other thing is that if the leak down test shows a large leak you will have difficulty checking the head to cylinder for the leak as it is hard to get the tubing to that joint to listen for the leak.

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

Put the original distributer back in and see if the issue goes away.

So, stuttering like a miss? Is it back-firing at all? Pre-firing? What do the plugs look like?

I would start with a compression, leak down and valve lash check -- just to get them out of the way. Sounds like you have replaced all the ignition bits, but is there a chance you have an intermittent short to/from the coil? Just a loose connection? You have double checked the timing? That change with any point/electronic ignition change. Is the ground to the engine good? Does it stutter while reved with no load or only under load? Next, I would throw a fuel pressure gauge on the line out of the pump to be sure that is in spec.

If you have unburnt fuel, you should be hearing a bit of popping and backfiring. If none of that, and the plugs look light brown (or one or two do) start down the lean route. You rebuilt the carbs, but are they completely clean? All passages? DId they get soaked and then blown out with carb cleaner and then air? There are a bunch of little holes everywhere... Needle and seat holding? Floats,... floating?

Can you remove one plug wire at a time and isolate to a single cylinder? If so, that is either electrical or that carb barrel is way, way out.

Just keep performing tests to box it in. Think about how each part affects the system and make sure it is doing what it should do by affecting it to see an effect.

Franny

You have done every thing but the obvious to me, a parcially plugged fuel line or filter. Check the petcock, and the short piece of fuel line that goes through the firewall
To the tunnel. Don't ask, why I mention it!!

G Thies

I'd follow Franny's advice and, as advised on other threads, start over. Begin with a full tune up, then check fuel system front to back including float levels and electrics. I had a problem with intermittent "stuttering" that I chased for a long time. It wasn't until I disassembled the car for restoration including a new wiring loom that I found the problem. I have an electric tach but, someone in the past, instead of using the "spare wire", ran a separate wire with a splice in it that had worn through the tape and was grounding out.

Does the stuttering feel lime fuel starvation or is is a sharp cut off like you turned off the ignition switch?

Mike

You mentioned you had the carbs rebuilt; Which make--and were they set up to specs like float levels, injections; etc.?
Zenith injections should be straight down; Solexes should be directed to the edge of the butterfly openings and hopefully the
throats aren't excessively worn on one side (on single shaft version), and the worst condition in having "warped mounting feet"
leaking air past the gaskets! Zeniths don't have that problem, but its check valves can. Ditto check valve & its screen on Solexes.
BTWay; Solex injection nozzles have a ball check and can be blocking its flow; I've had some of those apart and cleaned them including the tube itself. Finally, make sure all the jets are matched and make sure the 0,7 power jet isn't in the wrong place!

I test rebuilt carbs on the bench for above tests and make sure output pressure is set to 2.5-2.8 PSI max. (w/a 0-to-5 PSI gage)
Only the proper carb balancing is done @ approximately 2500-3000rpm adjusting the vertical links, then the idle balance (after the mixture screws were set to 1-1/4 turns outward @ the beginning, and minor adjustments can be made for smooth running.