Intermittent miss under load where to begin search?

[Dan Epperly]

one or more bad spark plug wires. ... Wait till it is getting dark if car is outside or turn off the garage light start engine open rear engine deck lid use throttle linkage to bring engine RPM's until intermittent starts and look at all spark plug wire to see if wire is arcing to some metal part of engine .

I, too, am interested in the result of a "light show" test.

Yeah, I gotta do that tonight. I did not do that test yet. The plug wires are only about a year old if that, but you never know, quality being what it is now days on replacement parts.

[Larry Coreth]

Jason,
On late type fuel pump pictured below:
Have you verified that the inlet valve is opening, closing and sealing ? Note it is the reed valve and if it is bent or kinked it will not seal, thus no pumping action ! if you have removed it, you must insure when replacing it is centered and has tension in the closing direction.

Note also that the spring under the diaphragm determines the fuel pressure so when in doubt use the original, they rarely wear out ! The smaller (dia.) spring under the plunger is of little consequence to the performance of the pump other than pulling the diaphragm back to suck the fuel into the central chamber Again if in doubt using the old one will work just fine, unless it is broken.

[Jason McCandless]

Thank you Larry! I always learn best with visuals.

When the reed valve was removed the kink was readily visible (on two different pumps I have). (On each reed, the kink is in the lower, narrow portion, not in the area covering the hole/passageway. Anyway, once the reed is screwed in place it lays flat against the housing. But, it must not be sealing because there is no suction on the inlet side.

So, thinking it through, I suppose the diaphragm is pulling air (fuel) through the Inlet Valve, then pushing air (fuel) out via the Outlet Valve AND out via the poorly sealed/open Inlet Valve. Thus, little or no suction??

Maybe that's not quite the mechanics to it, but I have the new, flat, reed valve, from Stoddard and will swap that in next. I'll take some pictures of the new and old parts and post them later this week. And thanks for the detail on the springs. The spring under the diaphragm that comes in the kits is identical in size to the one in that was in the pump.

[Dick Weiss]

If there are 2-different springs in pump rebuild kits, the difference is: the short spring wire will be thicker than the long spring wire being thinner; when either is installed, the output should be the same.
Otherwise, add another gasket-shim to suit output of 2.5-2.8 PSI (if pump is on the engine)--easier if
pre-tested on the bench.

Also, follow the directions (in the shop manual) to pre-load the spring action to flatten the diaphram before snugging-up the 6-screws. Aged diaphrams will take a 'set' from the spring tension and will not pump fuel even when properly assembled; manually depressed by hand, you should get a 'croaking/farting' sound!

Dick

[Dick Weiss]

Just checking the plug wires may not show any problems w/resistance, but the terminal(s) may show higher resistance or a 'dead' issue--especially w/the last/later Beru BE 4 terminals which have the wire-wound resistors, or the earlier Beru's w/the moulded-in version.
During the past years, the radios didn't need resistors and they were replaced by same-length piece of copper or brass; also check the rotor resistance and any blackened/arc'd posts inside the cap and no cracks.

Dick

[Dan Epperly]

No light show, but I pulled the pump after work and it is spitting gas out about a foot, so I doubt that is the problem. I ordered a rebuild kit anyway for another pump I need to rebuild for another car, so I have a backup. I also pulled the distributer so I could get a better look at the points which I replaced along with the condenser about 2500 miles ago. Even though the dwell checked with a meter was perfect, the surface of the movable point had a slight rough depression in it: could that cause my issue even though the dwell was fine? I replaced the points with new ones but left the condenser. The engine sounds better but I didn't have time to take it for a test run, will do that today after work. These damn short days puts a kink in my maintenance schedule.

[Jim Kaufmann]

At idle or cruising speed it hums along fine, but as soon as I begin to accelerate, especially up hills, there is a miss.

Dan, You did not mention the carbs. Have found when cruising at speed on the highway, touching the gas pedal for a bit more power and getting a hesitation is usually a result of the float level being to high. After lowering the float you would need to readjust the carb idle screws and air flow through the throttle plates to even things out at idle. Pump injection quantity (to much or to little) can also cause a hesitation.

Jim Kaufmann
356 Carburetor Rescue

[Jerry Garwick]

Has everyone forgotten the Maestro's method of determing which cylinder is cutting out? It still works.

[Dan Epperly]

Has everyone forgotten the Maestro's method of determing which cylinder is cutting out? It still works.

Never heard of it.
I did try pulling one plug at a time and revving the engine and the response was the same across the board, so I am thinking it is something affecting the whole engine and not just a specific cylinder.

[Brian R Adams]

At idle or cruising speed it hums along fine, but as soon as I begin to accelerate, especially up hills, there is a miss.

Dan, You did not mention the carbs. Have found when cruising at speed on the highway, touching the gas pedal for a bit more power and getting a hesitation is usually a result of the float level being to high. After lowering the float you would need to readjust the carb idle screws and air flow through the throttle plates to even things out at idle. Pump injection quantity (to much or to little) can also cause a hesitation.

Jim Kaufmann
356 Carburetor Rescue

Jim, I'm glad you mention this. My Zeniths (stock '59 Super) have this issue - when cruising along smoothly at, say, 60 mph in 4th gear, then barely increase throttle, it starts "hunting". I keep opening throttle more and more, until it finally smooths out and rpms pick up. OTOH if I stomp on it, it steps out smartly w/ no hesitation, so it's not the accel pumps misadjusted.

But I adjusted the float levels to 1. mid-range, 2. low end of range and 3. high end of range, and it never affected the problem.

Sorry for hijacking the thread, but your thoughts may help Dan as well.

Brian

[Jim Kaufmann]

Brian,

There many other issues that cause a hesitation. Electrical and fuel pump problems as discussed in this thread are always possible.

Try setting the float level to 20mm off the top of the carb body. About as low as you can go with the 32 NDIX and has helped performance on some cars. It's also possible the throttle plates are worn, air passes by the plates in the wrong location when worn out. Contact me off line if you need more help.

[Dick Weiss]

Dan,

I forgot to mention that I worked on a couple local member 356s and they had similar problems that you have and the result was renewing (or substituting) another good coil and it solved the problem; coil breakdown under load!
Further, there are some black distributor caps which have aluminum sockets (w/their internal posts) which tend to corrode.

Dick

[Dan Epperly]

Dan,

I forgot to mention that I worked on a couple local member 356s and they had similar problems that you have and the result was renewing (or substituting) another good coil and it solved the problem; coil breakdown under load!
Further, there are some black distributor caps which have aluminum sockets (w/their internal posts) which tend to corrode.

Dick

Its a pretty new coil, but I will check that. I also have the cap with aluminum sockets and posts, I will sub it out and see what happen. I also should get my pump rebuild kit today and will replace that as well.

[Richard Shilling]

All the posts above are headed in the right direction. I don't think anyone mentioned checking the voltage at the coil input. Should be very close to charging voltage measured at the battery. For a more exotic problem, every once in a while they manufacture a coil with the wires switched to the upper connectors. This will cause the coil to fire with reverse polarity - weaker output. As I say, it's rare, but switching the wires is a cheap test and the car will either run better or worse.

[Larry Coreth]

Richard,
Interesting, how did you determine the internal wires were crossed in the coil ? "Reduced" output voltage can be caused by winding insulation break down too, especially on the primary side. Was it a Bosch coil ?