My car was in storage all winter. Some weeks ago I took it out and it ran fine on the ethanol free race gas it sat all winter with. seemed fine. I don’t think I drove it hard at all.

I put pump hi test in it and parked it. When I went to connect the battery tender I touched the positive terminal and got a big spark.
The heater hose clamp had slid down towards the battery box. I had inadvertently connected the positive battery box through the tender’s alligator clip and heater hose clamp to the battery hold down bolt. I moved the clamp, plugged the car in and left it overnight.

The next day the started hard and idled terrible. I suspected the e-spark was fried the previous owner put in. I took it out and replaced that with points and condenser, coil and coil resistor.

It still ran bad, maybe not quite as bad. Hard to tell. Bad just at idle. Misfires, dark exhaust. Probably down a couple cylinders.
I put new plugs and wires and a rotor in it. The idle seemed ok. The dwell was 30 and timing with vacuum plugged was at 4 btdc. It ran rough under throttle, backfires etc.

Another set of points, a new distributer cap and another set of wires in it today. Moroso wires. I didn’t trust the shielded “correct” wires. Dwell and timing set. The idle seemed nice and I could at least get on down the road a bit. Once I get into the throttle though it misses or hesitates again and wants to backfire, just not as bad as before.

I am a lot better off than I was at first. Clearly the grounding I caused fried something like the e-spark plus whatever else.

I’ve replaced basically everything related to ignition tuneup. The stumbling under moderate throttle is the issue now. Bad gas? Carb secondaries not functioning, something with the timing?

 

Power valve. Same thing just happened to my 3367 in my '66, found a bit of puddled fuel in the intake manifold primaries. Replaced the PV, cleaned the plugs and we're back to normal. I had to think back to 1986 when it was always the power valve.

Ray

 

Doesn’t it seem funny tho that it ran fine the first day out before I shorted it and had put the pump gas in it?

I don’t know how to replace a power valve but I do have a brand new carb ready to put on it. The existing one was a 3370 from LIC the previous owner put in. I have a Chicago Corvette restored correct 3370 from march ‘66 ready to go.

 

well good news is Phill at Custom rebuilt carbs should have rebuilt that. the bad news is you had to deal with Chicago Corvette

 

I drove mine around the neighborhood on Saturday with no problems. The very next day as I was loading it onto a trailer for some paint correction out of town I exhibited the same drivability conditions that you stated here. Nothing really changed over night. Got it back home, pulled the plugs that were fuel soaked, removed the carb to find the puddling of fuel in the intake manifold and repaired it as stated above. You can check the PV with a vacuum pump, it's an $18 part and it's easily serviceable, it's one of those repairs that is a pretty simple one but the satisfaction of repairing it yourself is worth ten times the effort spent on it. Pull those sparkplugs first and see what they look like.

Ray

 

You all are describing a pissing carb, stuck needle or blown valve, and the requisite black smoke and rough running, just as it was in 1974 so too it is today. These are old cars with old car problems.

dan

 

As a cheap precaution, I would pull the carb and shoot all the air bleeds, idle passages, vents, etc out with carb cleaner in case something stopped one of them up after to gas in the tank was stirred around. Or simply try your other carb. While at it, see if it looks like gas has been pooling in the intake.

 

I wonder if a power valve blew out then after the initial short caused the (e-spark) car to run so badly, backfiring etc.
Sounds like the PV doesn’t like backfires and it sure did a lot of that after that first start after shorting it.

 

A backfire will generally blow a Holly carb power valve. Especially on the older carbs.

Larry

 

Remember guys, the car ran fine until I shorted it. No carb issues at all although I did fuel it just before getting home - and then shorting it.

I then just focused on putting her back to an original single points car and chasing possible electrical causes.

 

So let’s review: car ran fine, you shorted it causing the car to run poorly due to an ignition component, before you replaced it the car “coughed” or backfired as we say taking out the power valve, you then replaced the faulty ignition component only to have it continue to run poorly now due to the bad power valve. You’ve gotten great advice above.

 

"Dont be afraid" to tackle a Power valve, just because you have never changed one. You want to change the advice items instead of getting so deep into changing other stuff, that now your one problem becomes compounded, changing stuff, increasing your odds of causing a new issues. A guy shy to change a power valve, thinking the carb should get changed definitely increases the chance that the new carb will be less tuned for your application, than what you have on it now Yes I always blow out the bleeds on my airhorns first. The fuel passages rarely clog. It is generally the air sensing bleeds. Before guys blow a bunch of gumout down the idle circuits, you can probably 1/4 or 2/3 open the idle screw to disprove a blockage in a fuel passage related to the idle.

 

Sticking needle valve or a float with a hole in it. Most Holly’s that have been rebuilt have a power valve blow back check valve installed. Holly’s are known to leak internally if they are warped. A few heat cycles may have loosened it up enough to flow.

I would remove the sight screws and see where the fuel level in the bowl is, proceed from there.

I don’t think your battery mishap has anything to do with your issue.

 

Don't forget to simply replace your fuel filter too...

 

What type of engine? The poster never mentioned?

 

Profile says 66 427/390 HP.

Larry

 

Agreed, power valves are an easy swap.
Wish I knew more about the big blocks - wish I had one of those too. Is that a transistorized ignition?

 

Pull the needle seats out of the carb and inspect the o-rings.

One of mine went out, , just like that on the way home from the shop. The car ran fine at and around the shop but started flooding out in just a couple miles on the way home. After sitting for a couple years, the o-ring for the secondaries had crumbled to dust (car has never had ethanol) and fuel was by-passing the seat flooding the secondaries. The other o-ring on the primaries was perfect, so maybe quality control issues in the rubber?

 

You are not far off from what happened really but I just would say my reasoning, as one who is learning the car, is worth outlining again.

I shorted the ‘66 390/427 E-spark car at the battery the same day I fueled it with pump gas.

It ran very very poorly the next day. The guidance I received from various places was that the E-spark was damaged.

Removing the E-spark and going back to points etc was a big job for me. Distributer out, disassembled etc, coil, ballast, points, condenser etc.

When it still ran bad I did some backtracking on how I did the distributer and installed it and tried new points etc while I had the distributer out again.

Continuing issues led me to believe I had a spark plug wire issue. The car did have the wrong dated wires in it so I wanted to replace them anyways. The plug wires were very difficult to remove and I have almost no doubt I damaged at least one pulling on them. Hindsight.

I still had not learned about a carb pv yet.

A bad plug wire tester and weird ohm’s readings was further confusing for a few days. I’ve been educated on plug wires by the great folks at Moroso and while I was sorting that I learned about the possible pv deal.

I had a new carb ready to go, so what better time to put that on - again, in my reasoning. I thought I’d sorted the plug wires. Maybe not..

Today, the issue continues with the new carb.

I am almost certain that early on removing a plug wire that was hard to get off I may have damaged at least one wire and that perhaps was the issue ever since I rebuilt the distributer. I perhaps just never sorted the plug wires issues.

Tomorrow I will lay the new replacement 1Q66 wires out and test them and check the crimps. If they are good I will silicone them liberally and get them on without question correctly.

I hope my test run today didn’t damage the pv valve in the new carb. If it runs bad after the wires go back on tomorrow and I don’t shoot myself, I believe at that point it may be a pv valve todays outing damaged and I will tackle that.

I’m learning. My troubleshooting route or logic isn’t beyond criticism. Did I say I’m learning?
Everything I’ve worked on has been at least a little new to me and they have all been things I wanted to learn and change.

I hope tomorrow it gets sorted finally.

 

I have a 93k miles '66 427/390 with all original dated components. I had the same symptoms that you had. And I also replaced all ignition components including a few coils. There's always a slight improvement, but then it would fall on its face and stutter on hard acceleration.And backfire. I rebuilt the Holley 3370, but the real difference was when I sent the 50+ year old distributor to Dave Fiedler at T.I. Specialty.

But there is one major change that I had to make and that was the timing. You said that you had set it at the factory recommended 4* btc. That won't work. And after Dave Fiedler sent my distributor back, he recommended that I set it at around 8* btc initially. Then I timed it the "right way"and that was using my dial timing light to set timing near the peak rpm power curve. I'm trying to remember the specifics. Someone please correct me if I'm wrong (and I'm sure you will!) But try setting it at 28* at 4,000 rpm, lock it down, then let the timing fall where it is at idle. I idle at 600-650 rpm and timing is at 12* btc with the vacuum advance line plugged. After I made this change, the response was immediate. And it's run perfect since. Starts right up, no detonation, and screams in every gear.

I did notice that you had this same issue last year when you posted on this subject. And then you also pointed out the factory timing setting (which you can forget forever). https://www.corvetteforum.com/forums...te-427-bb.html

And also change to AC R45XLS plugs. Big blocks run best on this hotter plug. My big blocker NEVER overheats (stays @180) either, even with the timing change.