Stumbling down the road
 

Trying to track this down for a couple of days now. 66' 327 300. New plugs and wires... cap and rotor. Just finished rebuilding Holley 600 cfm. Timing set at 12 deg. BTDC. New fuel pump.
Engine starts immediately... and idles perfectly with no hesitation.

But... the minute I put it in gear and take off... it begins to hesitate and stumble. Still drivable... but definitely feels like a miss.

Rechecked everything... wires etc. Everything checks out.

Checked timing... all good.

Checked distributor and no play. Tight.

What's next? Vacuum advance? Springs?

Looking for some help. Thanks

A few questions for ya:

1. Auto or manual?
2. Is it just an initial stumble or does it keep doing it at all rpms/throttle openings?
3. How does it perform when you give it throttle in neutral?

Do you have access to a 'dial back' timing light? Might check your total timing, and see what it is and where it comes in.

Need more information. Does it surge and 'trailer-hitch', or does it chug at all speeds like it has a dead cylinder? Would be a huge help to get it on an old-school scope to get an actual visual on the ignition pattern. Right now, we need to determine if it's fuel or ignition. Did this problem exist before you performed the tune-up work? If not, it has to be something you did: crossed ignition wires, carburetor issues, etc.

Don't do anything until you recheck the plug firing order and spark plug wire routing....
If you mix things up the car may idle half-way decent but miss badly on take off - very deceptive..

Is the fuel at the proper levels in the bowls?

The car is a 4-speed manual.

I can throttle it in the garage... no hesitation... no stumble. The "trailer-hitch" phenomenon mentioned is a perfect description.

Don't have access to a specialized timing light....just a regular one.

Fuel pressure is at 7 pounds. And a new mechanical AC Delco fuel pump.

Also a new coil.

Car purrs like a kitten at idle and when you goose it. But bucks like a donkey once it's in gear and reaches 2000 to 2500 rpm as I accelerate up to 2nd and 3rd gear. Cannot sustain smooth acceleration.

Fuel bowls all set right at sight hole threads... no spill over. New needles, gaskets, springs.

I keep thinking something may be wrong with the distributor.

I run a Pertronix ignitor with a coil with the ballast resistor removed.

Been that way for several years with absolutely no issues.

But how do I check the vacuum advance canister. Doesn't seem to hold pressure with my vacuum pump for more than 1-2 seconds with engine off.

Distributor has never been rebuilt... and I can move the springs (a little loose) with my finger... but they won't fall off.

If you draw a vacuum on the vacuum advance control are you saying it will not hold a vacuum.

If that's the case you have a vacuum leak and it's creating a lean condition causing the surge.

if the vacuum advance wont hold a vacuum replace it with one that follows the two inch rule

Sounds like that might be my problem. The vacuum advance won't hold a vacuum with engine off.... and when I remove the hose to it with the engine idling... the RPM's go up a bit.

Vacuum leak?

Also... what is the 2 inch rule? Please explain that one.

Thanks for all the help!

Hmmm - rpms go up when vac advance is disconnected? Normally would go down due to less advance when disconnected. That would indicate to me that the vac can is bad.

Are you plugging the hose or at least putting your thumb over the end at that point ? If not it will create a vacuum “leak” and idle speed will vary.

Also, 12* static advance seems a bit high for your engine. IMHO 8* would be more appropriate unless I am missing something.

Yep... at idle. When vacuum can is disconnected idle goes up until I plug the hose... then idle returns to normal.

I will replace the vacuum advance canister... then recalibrate the timing and report back. Thanks to all.

The 2" Rule -- Measure the engine vacuum in inches of Hg at idle. Then select and install a vacuum advance unit that reaches maximum advance at 2" below that value. That makes sure you have maximum vacuum advance at idle (provided you're connected to a manifold vacuum source), rather than partial advance or a varying advance.

You can plug the vac advance rubber line and try driving the car....it should idle and accelerate smoothly. Vac advance is most effective at cruise speeds (max vacuum). If the car runs well that's your problem; if not its something else...

I recently discovered a ruptured vacuum advance diaphragm on the Mallory Unilite distributor in my 56 Chevy.

However, when I disconnected the hose from the carb to the advance unit, and put my finger over the hose, the idle increased and it smoothed out.

I had a 100% manifold vacuum leak. :ack:

One of my friends.. and forum member rebuilt my distributor, which turned out to have numerous other issues. He ran it and set it up on his Sun Machine.

Car runs like a top again...

So, I'd be checking that distributor, Pertronix or not..

This is what I was going to say next, but Frankie beat me to it. I'll add that IMO, a vacuum leak the size of the advance hose should have zero impact on drive quality in terms of jerking/surging, etc. Not a big enough leak. But disconnect and plug the hose, and see how it runs. There is a very good chance that it'll run much better after your replace the vacuum advance can with a good one. Keep us posted!!

OP says it starts right up and idles correctly. If wire routing was off it would not act that way

Yep. Depends on which wires are crossed. Varies from engine to engine as firing orders vary. Seen it myself, oh, 200 times in the past 40 years and 5,000 cars.

Again - Thanks for all the help. You guys are a wealth of useful info as usual.

I installed the new vacuum advance and it seemed to make things slightly better. But the problem persists... and the car is still stumbling and misfiring.

I checked for ohms and connectivity on the plug wires because it's really mimicking a classic misfire/crossfire situation more than anything else.

Feels like spark is scattering somewhere... can't find it with lights out or trying to see any arcing with water spray etc.

I'm going to install another new set of new plug wires tomorrow on the off chance that I got a bad set of Taylors. $60.

We'll see. I'll let you know.