Feels like a miss but it isn't

[Ted K.]

1966 427/425. Starts good, idles smooth, accelerates smoothly and winds up with no missing, however just cruising along about 20-30 mph,1500-2000 rpm feels like an engine miss. All ignition components new and set up properly. I have had this action with other cars when the spark was too far advanced so I brought the spark down to about 8 degrees. A little better but still annoying.
Any suggestions?

[Mark E.]

Re: Feels like a miss but it isn't

My first thought is excessive ignition advance, as you also mentioned.

Is this happening during high vacuum conditions? If yes, the VAC may be providing too much advance for that speed and load.

As a test, disconnect it and report back.

[Rich G.]

[Duke W.]

Re: Feels like a miss but it isn't

1966 427/425. Starts good, idles smooth, accelerates smoothly and winds up with no missing, however just cruising along about 20-30 mph,1500-2000 rpm feels like an engine miss. All ignition components new and set up properly. I have had this action with other cars when the spark was too far advanced so I brought the spark down to about 8 degrees. A little better but still annoying.
Any suggestions?

So what IS your spark advance map - actual measured not OE specs because is may not be OE?

You could also have an improperly assembled distributor or improper installation.

Are the wires properly indexed on the cap?

Does is still have the OE ported vacuum advance or has it been converted to full time?

What is the ID data on the VAC and does is meet spec?

Look at the following thread:

https://www.forums.ncrs.org/showthread.php?127930-Another-botched-distributor-installation-corrected

Duke

[Duke W.]

Re: Feels like a miss but it isn't

Look at post #4. I'll make a substantial wager that you have the same problem. A REAL L-79 (meaning at least a real L-79 cam) runs fine with the overly aggressive 8" OE VAC and lazy centrifugal curve (30 @ 5100 max).

They run noticeably better with a less aggressive 12" VAC and bringing all the centrifugal in at 3000-3500.

The above is with a properly assembled distributor including proper wire indexeing and proper installation in the engine.

Duke

[Tim G.]

Re: Feels like a miss but it isn't

I assume you are running the original K66 ignition on your car, if so, this fix would not apply. I had the same problem on my 1967 390 horse car when it warmed up and finally replaced the ballast resistor and that fixed it. The original was cracked.

[Ted K.]

Re: Feels like a miss but it isn't

Today I disconnected the vacuum advance and plugged the line, drove the car and little or no difference. I hocked a vacuum gage to that vacuum hose and basically had no vacuum. I don't know if that was a defective gage or not a good source for checking vacuum. I started looking for vacuum leaks and discovered one of the bolts holding the carb down is stripped. I will Heli coil soon, but I can't imagine that little leak, if there is one will cause zero vacuum on the gage. The car runs great under full throttle, no hesitation or misses up to 4800, it wants to do more but I am comfortable at that speed.
By the way I did spray starting fluid around the base of the carb and there was a very slight increase in rpm, hardly noticeable.

[Timothy B.]

Re: Feels like a miss but it isn't

I would fix the threads and make sure there is no vacuum leak in that carburetor base. For a test, warm the engine and close the choke half way and go for a ride and try to make the engine surge like it's been doing. If no surge, you either have a air leak inside the carburetor creating a lean condition or the carburetor needs to be richened up some.

How far out are the idle emulsion screws? Stock camshaft? Throttle blades set at idle so the transfer slot is only a little square? If the blades are not set right at high vacuum conditions you may be lean. Make sure the primary blades are set and the secondary blades are fully closed with the stop screw just touching enough so the blades don't stick in their bores.

These settings are very simple to do and will allow you to troubleshoot and not miss the simple things. I once had a slight hesitation on a old Nova I owned that drove me crazy trying to troubleshoot and it turned out to be a egged shaped vacuum hose connecting to the vacuum advance control which created a lean condition.

[Duke W.]

Re: Feels like a miss but it isn't

Today I disconnected the vacuum advance and plugged the line, drove the car and little or no difference. I hocked a vacuum gage to that vacuum hose and basically had no vacuum.

Well, at least you answered one of my questions in post #4.

Also, when checking idle vacuum you have to tee the vacuum gage into a vacuum source. You can't just remove a vacuum hose and hook it to a vacuum gage because that CHANGES the engine configuration.

Duke