Surfer69

I bought new cap, rotor, points. Pulled all the plugs and cleaned up and checked gaps. Still going down the road you can hear this obvious misfiring cylinder off again on again, even though you cant really feel anything wrong. Rev the engine and it still sounds strong.

So wondering what tricks there might be to try and isolate whats causing this. Maybe its fuel related but don't think so. Any ideas appreciated.

SH-60B

The plugs would have identified the misfiring cylinder, use new plugs and drive it, then re-check the plugs. The timing is all set up? Vacuum advance works right? Speaking of vacuum, have you checked the intake and carb base for vacuum leak? A quick check of engine vacuum can diagnose general engine condition, as well as a compression test.

Roughrider

It does sound like a vacuum leak might be the likely suspect, but don't overlook a spark plug with a cracked ceramic...the spark could be going to ground rather than the firing tip.

toddalin

I was having what may be a similar problem with the '64. The engine pulls great with good dyno numbers (over 280 RWHP), but at idle every now and then you would hear/feel a cylinder(s) not firing properly. You could actually see it at idle in the fan which would give a "hiccup/shiver" and the belt would go slack when it would occur.

Mine was cured with a new set of valve springs. The engine is now much smoother and the problem seems to be gone. Seems that the old springs had lost about 30# and I guess that one would occasionally not seat all the way, or bounce, or...

dan1495

I had the same issue until I replaced the repro dated coded plug wires with some new Delco ones.

mysixtynine

Buddy of mine had 3 and 5 wires switched on his car recently.

Car ran fine ... just missed.

Use a straight pin and a jumper lead to the carb stud and short out each plug at the cap ... the one or ones that have no effect on the engine idle is your missing cylinder.

He also had 1 bad wire... I ohmed the wires out and still could not figure out what was up with the one wire... must had a bad spot in it that was arching while in the car.

Surfer69

Interesting. I have been noticing more popping on deceleration lately than normal which might mean vacuum leak. Also I hear a slight ticking which I haven't isolated as either lifter or exhaust leak but that might be part of this. Redid the solid lifters few months ago but should be no issues there unless something came loose.

I just bought new expensive wire set not too long ago, so I would hate to think one of those went bad. I will try the short out a wire trick and look for vacuum leaks first. Thx

JimT

That ticking noise could be a plug/plug wire jumping to ground. I found a bad(cracked) plug by starting the car at night and turning out the garage lights - what a light show!!

pltmgr

Make sure all the wires are firmly seated in the cap and on the plugs.

leadfoot4

I did that once, with the engine on a test stand., and as you said, it ran good, just an occasional "pop".....

sug

Is your choke working properly. My 69 was back firing and poping. The choke was not opening up fully. Once I adjusted it the car runs great . Just another thing to look at and eliminate.

BLUE1972

Mist the wires and plugs with water from a Windex bottle and look for sparks.


Check the coil and capacitor for output.

A weak coil / fuel pump will cause a miss-fire.

Check the fuel filter for dirt.

When we go to "summer gas" I have to richen up the idle 1/4 turn.

Hope you find it ...

bruiser

My old motor had a miss when idling(original motor & cam) . Turned out to be a lobe on the cam worn out. My new motor has a roller cam so this should not happen again.

gungatim

take it to a competent "old school" mechanic/shop that still has a scope. that is what they are for. and by scope, I don't mean an ap on a phone, or an o'scope program on a laptop. you need a guy that knows how to use a real suntune setup to find out what is going on...

cardo0

U can try a simple test at night. Open the hood in the dark and look for sparks and arcing - might try and rev it a little to. Works better with over the v/c wires as under header wires not as easy to see. Some guys will spray/mist some water on them to - use your own discretion for that.

Hope it helps.

Surfer69

OK found one problem for sure. Intake gasket has blown itself out both sides at number one and two cylinders across from each other. Pics below. Vacuum leak for sure. Not sure what would cause this other than both breathers get clogged and pressure builds up and pushes gasket out?

Surfer69

So I temporarily put some Permatex 2 on the edges of the blown out areas to stop the vacuum leak.

Sure enough the misfire seems to be gone. Idle is up.

Just need to figure out how to stop this in the future. Buddy says maybe using silicone on gaskets might have rotted and made them slip out and try dry next time, not sure if I did that or not.

REELAV8R

Over torquing may cause that to happen. It appears that your valve cover hold downs are bent too. Might want to hammer them flat or flip them over.