I have a 1966 with a 5.0, 650cfm Holley, factory gears and factory 4-speed. I am not sure how to diagnose the problem but I can put the symptoms on here and maybe one of you out there can help me figure out what the problem is:

-Going down the road at low RPMs or shifting through the gears at low RPMs cause the engine to feel like it is completely shutting down...now skipping, shutting off. The car jumps harshly while this is going on.

- I thought it was either vapor lock or that some of my rubber fuel lines were trying to collapse once they got hot, but this morning I had the same problems before the engine could get up to operating temperature...so apparently heat is not the cause.

-Fuel filter is clean and has no signs of blockage.

-The car has only shut off at a red light once since the problems started...and I go through a lot of red lights.

-Three different tanks of gas from three different stations have not remedied the problem so I am assuming that it is not a case of getting bad gas...even tried a fuel additive in one tank.

Now when I say low RPMs, I am not talking about lugging the engine to the point of almost shutting it down...with stock gears and a factory 4 speed I am talking about going 30-35mph in 4th gear and this problem showing up. It is not just 4th gear that it has problems...it can be in any gear. Any help or advice is greatly appreciated.

 

Check timing and dwell. Does the distributor have points? Does it run smoothly at high rpms?

 

No points...it is an Accel Billetproof distributor. The car runs like it is supposed to when it is up to speed but while accelerating and going through the gears the problem shows up.

 

Is your carb squirting fuel into the venturi when you open the butterflies?

 

The carburetor is operating as it should be in the driveway and at idle...but I know that does not account for going down the road. Unfortunately, the only consistent thing here is that the condition is degenerating...now the engine will act up at speed (while cruising, when I am no longer accelerating) though it is not very frequent. I had the engine stuttering at 60mph, then it backfired and started acting as it should. While going 50mph the whole engine felt like it stalled, I pushed the gas pedal all the way down and when it backfired it sounded like shotgun went off...loud as hell. Either my distributor is acting up or I have a float sticking...but I do not have enough knowledge of these two items to where I can properly diagnose what the source is.

It never feels like the motor is skipping, just feels like it is "trying" to shut off...and the problem is not consistent as far as when it starts acting up. Sometimes it will do it while I am accelerating quickly, sometimes while I am taking it easy...most of the time nothing happens while I am cruising but there is no guarantee that anything will. Every now-and-then I get a smell from the engine bay that is telling me the air/fuel mixture is rich, but just like everything else involving this situation, this not a consistent occurrence.

If it helps, the carburetor is a 4150 Double Pumper Holley with mechanical secondaries and manual choke...the engine has run without problems for nearly a year, with of the exception of having to replace the ignition switch several months ago...but that just inhibited it from starting, after replacing the switch it went back to normal. As always, thank you for the help.

 

It can be really difficult to diagnose based on a description. Esecially when the symptoms seem to change. Also, only you would know all the little details that can make a difference.

I have had problems in the past with the clip on style lead wire to the coil. It would work fine except under hard acceleration. You may try checking the wires responsible for coil voltage.

I had some initital concerns with fuel reaching the engine. You can check the acceleration pump by removing the air cleaner on a non running engine, open the butterflies and watch for fuel squirting from the front and rear squirters.
A lack of fuel squirting would cause the stumble upon accelerating. I would aslo suggest you check the fuel pressure. A weak pump may cause some of what you are describing.

A likely culprit would be dirt/junk in the carb. You may have to start eliminating the possibilities 1 by 1. Don't just assume because you have filters inline that the carb isn't dirty. Keep messing with it and keep us posted.

 

I will keep toying with it...the aggravating part to it all is that there is no real consistency in what it is doing so I cannot avoid the activity that seems to trigger it. I have a jump wire for the coil so after I finish checking the butterflies I can straight wire it and take it for a spin to see if that helps. Something will show up eventually.

 

What rpm and throttle position does it do this at? How does it idle? Did the idle recently change when the problem started, like idling higher or rougher etc? Does it get worse or better when you accelerate harder? How does it run at WOT? What happens if you try to rev the engine in the driveway with it in N? What happens in the driveway if you give it just a bit of throttle to try to bring the engine up to say 1,300-1,500rpm?

 

67-The car always worked as it should in the driveway...I can old high RPMs in neutral and it works fine, quick rev's and everything else don not show any issues while sitting still. While moving there is not set RPM or throttle position in which the problem begins; it is completely random...easy and hard acceleration, day or night, or whether the engine is hot or cold does not matter.

The good news is that I found the problem Friday evening and it was an easy temporary fix while the replacement parts are shipping in. I took a screw driver and a scotch-brite pad along with my distributor cap and rotor and polished the hell out of all the copper and brass surfaces...the problem quit immediately. Replacement parts are on the way...I never knew that the cap and rotor would cause the engine to act that sporadically but I have now learned otherwise.

 

90% of carburetor problems are actually ignition problems. And yeah, it can. The spark has to jump a gap from the rotor to the cap terminal just like in a plug. When the cap/rotor get corroded, worn etc the resistance when jumping the gap becomes much higher. Too much resistance and you get very little current to the plug so you get a weak spark. Weak spark = poor combustion.