Stumped! Possibily solved!

[Explorer]

Grab a cup of coffee, long post. Son's 66 Mustang is giving me a headache. Pertronics went bad, replaced with points and ran great. It got to where it would start missing, barely run and suddenly straighten up and run fine. Replaced Load-o- Matic with rebuilt 68 distributor with centrifical advance and capped off vacuum advance temporarily. All was well, long test drives and ran the best since we owned it. Parked and shut it off and problem is back full time. Will start and idle sometimes(barely).
Here is what I know: Compression is 145lbs all cylinders. Fuel system is fine, good stream of fuel when accelerator pressed, rebuilt the carb less than year ago. Brought #1 TDC compression stroke and timing marks line up so don't believe a problem there. There is a little slack in timing chain. Voltage to coil is good, swapped out with known good coil. Plugs cleaned, regapped and have fire to each plug. I have cleaned and ohm'd out all grounds, fixing a few along the way.
Any suggestions? I've ran out of ideas.

 

[66 E100 Pickup]

Did you change the carb to a non-SCV version? An SCV malfunctioning could be common ground for both setups, as well as creating the wrong timing profile for the 68 dizzy.

John/Fresno

 

[Explorer]

It is a Autolite 1100 SCV carb. Plugged the vacuum port at carb and it ran great for a day. Timing set at 12BTDC give or take, adjusted to where it will idle for 10 sec or so(very roughly). Not saying the SCV didn't go bad. Acts like one does when firing order is off(wires crossed) but I've checked that so many times it's ridiculous. Don't have a spare carb to try. Even thought of something in valve train, but all cylinders read the same, only reason I didn't pull the valve cover. Keep the suggestions coming.

 

[bubba22349]

I would take a look at the carb's float to see if it's dropping (too heavey).

 

[rbohm]

8) does it run good when cold, the go south when warned up? if so then you have a fuel boil issue. if it runs bad all the time, then it sounds like a carb issue, perhaps an internal leak that allows too much fuel to be fed into the intake.

 

[Explorer]

Past the point of what I'd call "running". Occasionally on the first or second try, it will run roughly(like hitting on three cyl) for maybe 10 seconds at idle. I'll break the carb down tomorrow and check float level etc. I developed "tunnel vision" on ignition so time to look elsewhere. Carb could be very well be an issue, as problem was intermittent till now. Will report back.

 

[rbohm]

8) dont forget to replace the adapter to manifold gasket when you do the carb. that is often a source of vacuum leaks that will drive you crazy trying to find.

 

[Explorer]

Grandkids here today so work is slow. Just got the top off carb and nothing obvious. Power valve not sticking, float not sticking, still nice and clean. Jet "may" have had something in it, blew on it and it disapeared, could have been a drop of gas. I'll get into it more after the kids leave. Won't forget the gasket, think I have an extra one.

 

[Explorer]

No joy. Cleaned all passages, blew out with compressed air, float level right. Found a tiny bit of crud, but everything seems fine. No change whatsover. When it does fire, I"ve been noticing a screech, but have been thinking it is the fan belt as it will do that occasionally on first start. Anymore suggestions before I pull the valve cover?

 

[bubba22349]

Are all the grounds wiring to engine and body clean and snug? Weird one I had once was a bad alternator that shorted out you could unhook it and see if it helps or take to be tested free at most auto parts stores.

 

[Explorer]

I'm a stickler for grounds. Every single one cleaned and and ohmed. Pulled the valve cover and found nothing wrong, all valves moving, double checked timing marks watching valves. Even wired all the plug wires to 180 out to double check myself. I'll disconnect alternator and give it a shot. Still not convinced on the carb, plugs should be dripping wet by now and they aren't. Tired of screwing with it today. I've never had an engine whip my butt like this.

 

[bubba22349]

Do you have good fuel pressure? Some other things on fuel system hose at tank is it good condistion? and the pickup in the tank (sock) they can deteriate. Good luck

 

[Explorer]

Unhooked alternator, no effect. Fuel pressure good with everything new from gas cap to carb. This will wind up being something stupid simple and I will freely admit it, just need to find it. I've even tried bypassing everything by wiring coil straight to batt and using a remote starter switch.

 

[bubba22349]

I was going to say maybe a wire broken inside the insulation had that happen more than once with an old harness only found it with moving the wires or probing with a test light. But you said tested with ohm meter and now with seperate wire run. Had a bad condensor a few times or one that was not grounding good to the distrib's point plate.

 

[bmbm40]

I am not much of a diagnostic mechanic but was the Pertronix actually defective? Does this Mustang have that ceramic resistor on the firewall? Seems like I had to replace one of those about 20 years ago on something. Can't remember what the symptoms were but seemed like it was an intermittent condition.
Good luck.

 

[Explorer]

Magnets came out of reluctor on Pertronics. Two jammed vacuum advance and it had a slight leak in diaphgram so I upgraded the distributor. No ballast reistor, but a resistor wire is wired in original from switch to firewall plug, 1.4 ohms of resistance. One reason I temp bypassed to eliminate that possiblity. Can't leave it that way long, but good test to eliminate one possibile problem.

 

[woodbutcher]

Hi Explorer.Many years ago,I had a similar problem.Some how,the INSIDE of the dizzy cap got really nasty.Have you cleaned the inside and the outside of the cap?It gets dirty and it WILL ground to the dizzy body.Also moisture will have the same effect.
Good luck.Have fun.Be safe.
Leo

 

[Explorer]

Installed new cap and rotor with distributor.

 

[Explorer]

Last three days nothing has made a difference. Got up this morning and swapped in a spare set of points I had and it fired right up, all cylinders firing, idling smooth. Slowly started missing within a minute till back where I started. Rechecked point gap and it hadn't changed. Ordinarily I would suspect coil breaking down, but I have tried different coils. Monitored voltage to coil and that did not change. Only thing I cannot prove good or bad now is the carb. Hate to buy a new carb just to verify. There is a Mustang junk yard close that I may be able to find a used one. Any more ideas?

 

[JackFish]

Voltage regulator? Alternator?

Things that work ok and then get weird usually make me suspect electrical.
And you've swapped the coil so that's not it.