Intermittent miss under load where to begin search?

[Dan Epperly]

Drove my '63 coupe to work today and experienced an intermittent miss under load. At idle or cruising speed it hums along fine, but as soon as I begin to accelerate, especially up hills, there is a miss. I just tuned the car up 200 miles ago, meaning adjusted the valves, adjusted the points changed oil. set timing. Plugs were changed out about 2000 miles ago. It ran fine until today. I figure it is either electrical or fuel, but which is likely to be the culprit? Where would you guys begin the search? I have several hours to mull this over until I get home today.

[C J Murray]

points closing
moisture in fuel
dirt in idle jet
crack in cap
bad ign wire
bad condensor
bad coil

there is a start

[David Jones]

Check that the ignition wire from coil to distributor cap is secure. I once lost a race because the coil wire was not pushed home and as soon as I hit 5k under load it misfired. A 1/4" gap between connection and plug wire will do that.

[charlesblankinship]

one or more bad spark plug wires. Check ohm's of each wire. Check to see if one of the wires is
arcing. 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 .

[Dave Wildrick]

A fuel pump diaphragm beginning to go out can also do that. You are O.K. until you get on it a little bit, and the effect gets worse with time.
Crud in the fuel tank and do it too, but gets more noticeable when the fuel level is getting low.

[Jason McCandless]

[quote="Dave Wildrick"]A fuel pump diaphragm beginning to go out can also do that. You are O.K. until you get on it a little bit, and the effect gets worse with time.
Crud in the fuel tank and do it too, but gets more noticeable when the fuel level is getting low.[/quote]

Hope I'm not hijacking the thread, but Re: a failing diaphragm... my pump failed recently, and prior to that it had similar running issue as Dan (the car ran well but sometimes had a mis under load/when stomping on it.) When it finally stopped running, I found there was fuel in the line to the pump (none in the upstream filter) and little to no suction at the pump inlet (engine cranking and a finger over the inlet side.) When I took the pump apart, the diaphragm had no tears or holes and it wasn't leaking gas. The diaphragm material was still flexible, not dried or cracked.

So, what does a worn/failed diaphragm look like? Or, is it best checked by a volume/psi test? No suction on the pump inlet side and no tears in the diaphragm makes me think something else is open/faulty with the inlet valve of the pump.

[Martin Benade]

The pump casting has the 2 phenolic flapper valve seats which can get worn after 50 years and give a poor seal. Inspect the seat areas closely, especially if the diaphragm seems OK. Pot metal doesn't wear forever.

[Jason McCandless]

Thanks for your reply Martin...I forgot to note that my pump is the later style, 64 motor. Are the 2 flapper valve seats you mention associated w/ the early pumps, the hex pieces? Is the equivalent part in the later pump the thin silver flap/valve, the pump tensioner?

I put in a new diaphragm and springs tonight and still don't have suction on the inlet pump. Air does pump out of the outlet side. The pump tensioner in my late style pump has a bend/kink to it, but when in place it lies flat against the housing... I do have a new one from Stoddard that is completely flat. That's the next thing to try on my end...

[Martin Benade]

Yes, I was referring to the earlier type, but it sounds like you are looking at the right part on the one you have. If the pump is clean enough to do this, you should be able to blow into the inlet and air should come out of the outlet, but you should not be able to suck air out of the inlet if both valves are sealing, although this is not a great test as if either of the valves are working, it will act the same as if both are working. I think the kinked part is probably the problem, and if you remove the "exhaust " valve one then the suction test will tell you if that is leaking.

[Dan Epperly]

My points are fine, didn't detect anything odd with the wires, so the pump is my next suspect. I have read the directions for rebuilding the later style pump and it seems pretty straightforward. I have a NOS diaphragm in my spare parts so I will rebuild the pump tonight. No where did I read that you have to "preload" the pump when rebuilding it, which seems odd, every VW pump I'v rebuilt requires a preload. Is this correct? You just tighten the top down and your done? Seems to good to be true...

[C J Murray]

It is easy to preload it a little to flatten the diaphragm while tightening the screws.

[Dan Epperly]

It is easy to preload it a little to flatten the diaphragm while tightening the screws.

When you guys rebuild them do you reuse the factory springs or use new ones? Reason i ask is that I have had problems with the tension on rebuild kits for VWs not being to spec.

[C J Murray]

I never had a problem with a new spring.

[Jason McCandless]

Thanks Martin, I will try those tests before putting in the tensioner, hopefully that sorts it out.

Dan, I found this post about different sizes of springs that were found in rebuild kits. It suggests there is a range of springs that would work. The new spring in my kit was approx 10mm longer than the one on the pump (maybe it has just aged). In any case, I used the new springs in the rebuild.

viewtopic.php?f=1&t=36687&hilit=fuel+pump+rebuild

[Brian R Adams]

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.