strange throttle response issue

[CZ-75]

Ever since I bought this car, it has had a strange throttle response issue that only occurs when I punch it while in a higher gear.

For example... rolling along at 35-40MPH in 3rd gear, low RPMs.

I stomp on it, and the motor almost dies.... coughs, then fires back up. Thsi doesn't happen all teh time, only intermittent, and happens both early on (not warmed up yet) and also may happen after I've been driving for a while.

I have been fiddling with timing and carb tunig, but haven't been able to find the gremlin.

It's a '73 351c 2V w/ Performer intake and carb

I have good and steady vacuum, right at 20 hg at idle.

Initial timing is between 15-16 Total advance is in the mid 30's (if memory serves).

Vacuum advance works, and is connected to manifold port on the carb.

I'm beginning to think it may be an ignition thing. I recently switched from an HEI dizzy to Duraspark, and set up the plug & cap wiring exactly like it was before the swap.

It's also important to note that this issue has been fairly persistent from the beginning (both with the HEI dizzy and the duraspark).

I have been playing around with different wire looms, and while doing so, stumbled across a plug wiring diagram that suggests my wiring may be incorrect.

Can someone point me to a really good wiring diagram that shows both the dizzy and plugs in the same diagram?

And would a wiring problem allow the motor to operate fairly normal, but cause the type of problem I'm encountering? Or am I chasing wild goose?


Also... How do I tell which position is #1 on teh distributor? The cap I bought isn't marked.

My #1 plug is connected to the terminal that is circled in this pic:

 

[72HCODE]

did you inspect the accelerator pump?

do you have a manual transmission?

rolling in 3rd at cruise, you stomp on it, the vac drops the fuel goes lean that would explain the almost stall.

if you have a holley you could try a power valve change or a accelerator pump cam swap or shooter change, or pump upgrade from 30 to 50cc

if you have an edelbrock then you need to play with the metering system.

if you have an automatic is the kick down working?

Also what are you gapping the plugs at?

with the electronic ignitions they tell you to increase the gap .05 or more. however each car is different and some cars cannot tolerate the larger gap that will cause lean misfire under heavy acceleration at any speed. if you have .042 or .045 gap i would recommend lowering it to .035 or .040 and see if there is a change.

for example on my car i once tried going from .035 to .045 gap on autolite 24s with electronic ignition. i started to get ping at cruise with light acceleration and an overall feel of loss of power. rather then fatten up the throttle i regapped to .035 power came back no more ping.

check the plug color are they too white? that would indicate you are on the lean side and should fatten it up a little on fuel.

there are a ton of little tricks you can try also like retard timing 1-2 degrees and go for a drive,retarding timing can compensate for the engine going lean also.


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Also... How do I tell which position is #1 on teh distributor? The cap I bought isn't marked.

My #1 plug is connected to the terminal that is circled in this pic:

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well the cap doesn't care where #1 is as long as the firing order is correct. 1,3,7,2... etc

usually #1 is located at 1 o'clock on the cap. when you see #1 starting somewhere else it means the distributor might of been installed with the gear offset 1-2 teeth.

from a maintenance standpoint this is not correct because another mechanic may come along and reset #1 over 2 and then the engine will be completely out of time. so they will reset the distributor correctly moving it 1-2 teeth and then reinstalling it.

if your distributor has #1 starting somewhere else that means it was installed offset and you had a lazy mechanic. for example you could install the distributor 180 degrees out.

then just move the fireing order 180 degrees and the engine wouldn't know the difference because it will still fire off the cylinders in order. its a bad example but it shows you can compensate for a mistake like that.

also sometimes you install an after market A/C compressor and discover it is hitting the vacuum advance on the distributor, you can rotate the distributor but then your timing might be 20 degrees off, so rather then pulling the distributor you can offset the ignition wires by 1 and then return the timing back to normal without pulling the distributor. that allows you again to have a full range of timing adjustment compensating for the a/c compressor taking up space on the engine.

so its not correct from an esthetics standpoint since a mechanic may be expecting #1 to start at 1 oclock and go counterclockwise. but the engine doesn't care as long and the firing order is correct and the initial timing is in spec as well.

some caps have a 1 molded on them to help keep things in order. the HEI cap doesn't so sometimes people are nice and write a #1 on it so it helps a future mechanic know whats up.

 

[CZ-75]

Thanks, H-Code!

I'll check the plugs as soon as I can.

 

[droptop73]

Possibly the secondaries are opening too quickly. This will create a lean condition. It can be corrected by more accelerator pump shot or slowing down the secondary opening.

 

[Totalled]

Classic edelbrock carb cough. Needs more pump shot.

 

[caspianwendell]

I'm going to agree with the others and say accelerator pump

 

[72HCODE]

Good post on the secondaries.

You can do tons of stuff tuning them as well.

If you have a carb too big for a motor you can tune the secondaries to not open all the way, this keeping the fuel mix richer. A carb too big will always run lean, so its a trick to richen it up. Or you can have the secondaries open later keeping the fuel air mix richer long enough for the gap to be filled with a power valve or accelerator pump overlap.

Even more tuning can be done with a distributor curve on the mechanical advance.

Basically tons of ways to solve a lean issue. The basic rule is if the engine is leaning out either you hold it back or dump more fuel. Or you need to hold it back at different throttle points to maintain a consistent air fuel mix.

It really is amazing how much tuning you can do.