[formulabird428]

Car had been running great for years, now is backfiring intermittently through the exhaust and idle is rough. It originally stated out when I was slowly winding the engine out (4000 rpm) and going into second gear.

I have a AFR on the even cylinders, and it didn't seem to indicate an issue during the backfire. I can double check that again.

Engine is completely modified:
Holley elec fuel pump, aero regulator
Holley 950hp
Victor intake
11.25 : 1
KRE 320cfm D Heads
Headers with X-pipe
MSD 6AL with Blaster 2 coil
Solid cam with HS 1.65 rockers


Here are some ideas for the problem and things I'm going to try:

Coil issues, change out
Rockers lash (unlikely, but I'll check)
Bad fuel? Replace? (It's 93 pump, but maybe with COVID the fuel went bad? How long does that take to happen?)
Verify timing


Any else you guys think could be the culprit?

I have a short video clip, I'll see if I can upload it.

Thanks!

[Lemans64]

Bad plugs or wires?? If it idles bad you probably have a misfire. Can isolate by pulling wire and check for idle rpm difference. Also check for distribtor cap for
carbon tracking, or corrosion inside cap. Burnt center electrode in cap. Just to name a few,

[Gary H]

Pull the spark plugs first to see if any look significantly different than the rest. That will tell you a lot as to which cylinder is causing the issue. If they look good, I would check the float level next.

[1965gp]

I would think something broke- probably ignition. I will say that we didn’t drive my wife’s car for like three months and the fuel pump went out at 33k. I think it’s because the fuel sat in there for so long.

Is it only at high rpm? Possibly lack of fuel?

[geeteeohguy]

Originally Posted by Lemans64

Bad plugs or wires?? If it idles bad you probably have a misfire. Can isolate by pulling wire and check for idle rpm difference. Also check for distribtor cap for
carbon tracking, or corrosion inside cap. Burnt center electrode in cap. Just to name a few,

This. Misfire. Put a vacuum gauge on it to rule out a burnt exhaust valve. When my '67 did this same thing years ago it was a burnt exhaust valve, and I've seen it happen quite a few times. More likely a bad ignition wire, etc. Vacuum gauge will point you in the right direction!

[formulabird428]

Wires were changed a couple years ago, only has about 1000 miles them. Same with plugs.
However, I'll pull one at a time and inspect plugs and look for rpm difference.

The MSD has always had corrosion issues, but the rotor was changed 2 years ago. Cap not sure when, so I'll inspect.

Vacuum reading if burnt valve would be too low?

Thanks for the suggestions.

[Joe's Garage]

Although usually the intake valves and backfire into intake. A can of Seafoam in the fuel and fresh gas fixed those.

Also had one more like yours with exhaust backfire. Started out intermittent, gradually changed to constant. Plugs looked good. Wires, cap, rotor OK.

Flat cam lobe....... Checking the lash will identify that quickly.

When you isolate cylinders by unplugging one plug wire at a time, if the miss doesn't change much (might get a little rougher) but the backfire goes away, you've found the cylinder. I'd suggest trying that first - at least you will know which valve cover to take off.

Good luck.

[Half-Inch Stud]

Ahem. uhm, a shorted plug can occur, and occurred to our 70 GTO long ago.

[Half-Inch Stud]

Uhm, could be a wiped Exhaust Cam Lobe. So Plug short or fallen-off Plug wire is a rosy prognosis.

[formulabird428]

Hopefully not a cam lobe. I do have the oiling lifters with the tiny feed hole. Hopefully that's giving me extra protection to guard against a cam issue.

Going to start checking tomorrow.

Thanks for all the suggestions.

[Lightfoot]

With the backfire in the exhaust, I would think a burned or sticking exhaust valve is more likely than a bad cam lobe.

Maybe there's carbon build-up in more than one valve seat (exh).

[PunchT37]

Check all the primary stuff on the ignition system. Including wiring. Check dist shaft for side slop or vertical slop.

[Two_70_Converts]

I'm going with the burnt valve suggestion. Been there.

[geeteeohguy]

Originally Posted by formulabird428

Wires were changed a couple years ago, only has about 1000 miles them. Same with plugs.
However, I'll pull one at a time and inspect plugs and look for rpm difference.

The MSD has always had corrosion issues, but the rotor was changed 2 years ago. Cap not sure when, so I'll inspect.

Vacuum reading if burnt valve would be too low?

Thanks for the suggestions.

Burnt valve would give you a rapidly fluctuating needle on vacuum gauge. Another way to do a quick check to determine a cylinder with weak compression is to pull the coil wire and crank the engine. If all the cylinders have good compression, it will crank evenly. If you have a burnt valve or weak cylinder, the engine will speed up when that cylinder comes up on TDC compression. So good compression= wah-wah-wah-wah-wah....
Bad compression= wah-wah-wah-WAH-wah.....
Takes 30 seconds to do this test to verify a sound (or unsound) engine.

[Formulabruce]

Your MSD box potted? Have seen them partial fail, and complete fail due to no air cooling it. (Potted electronics suck) . At least one each year fails on the hot rod power tour because it gets too "Hot" .

[GT182]

I never like the MSD setup... too many problems. I changed out my 66 points distributor and went with the stock Pontiac HEI distributor. Never looked back and have not had any problems sense I installed it several years ago.

[Jason67]

Any updates on what you found?