Jaclark93

so i got a misfire at idle. timing set to 8 degrees btdc, proform hei distributor, msd wires, acdelco plugs gapped at .045, 4 barrel carb. i noticed that the cap and rotor both had excessive wear that would not come off with sand paper, almost appeared to be burnt i am thinking that that could be the problem. any help would be appreciated.

70sgeek

Assuming the distributor is known to be good and properly installed (i.e. not a tooth off), replace the cap and rotor with good brand quality parts, advance your timing a bit maybe up to 12* (which I believe is the factory setting for 4-bbl motors) to see if runs better. It may still accept more advance (for instance, my 350 initial timing is running 15* with HEI and the motor LOVES it).

Make sure you're using good quality wires - minimum 8mm if possible (I know most would say 7mm is fine on stock motors but I like the extra measures). MSD, Taylor, even Accel make decent wires in both universal or specific Olds 350 sets, make sure you look for the '74 and later HEI sets (beware though, the engine-specific sets typically don't come with a coil wire)

If your plugs are fouled, either clean them really well or better still, replace them maybe with good AC rapidfires or NGK plugs.

oldcutlass

There is no such thing The engine does not care what position its installed as long as the firing order and the timing are correct. I agree that a cap and rotor replacement is probably warranted due to the excessive wear. I also agree that 8* btdc is not enough advance, try 15/16* to start.

If replacing the cap and rotor do not solve your issue, look down into the carb and see if there is fuel dribbling at idle. Make sure all your spark plug wires are pushed all the way on at both ends. Make sure your carb a/f adjustments are correct. What do your spark plugs look like?

Sugar Bear

Is the misfire new or has it been there?

A burnt cap and rotor can be a sign of bad spark plug wires. If replacing the cap and rotor monitor the new ones if not replacing the wires.

Check to see that there is battery voltage to the primary side of the ignition system with the car running. Trying to determine if the resistance wire to the distributor was leftover from the points set-up when electronic ignition was installed. This would supply low/incorrect voltage to the ignition.

Double check the firing order.

Good luck!!!

Jaclark93

I know for a fact that the firing order is correct triple checked it, the plugs looked like it was running rich, adjusted f/a mixture screwsets, im going to try and advance the timing a bit more and will let you all know the results. Thank you all so much.

Jaclark93

Could the plug wires not being in harnesses cause crossfire between the wires, they are ziptied together going up to the distributor

oldcutlass

Plug wires that are zip tied together like in a harness can create cross firing, worse if they are leaking.

Sugar Bear

X2, zip tied not preferred

72442conv

Look down the carb and make sure that there is no fuel coming out of the boosters while at idle. If you have no fuel coming put of the bosters at idle a quick check for the plug wires is to get inside your garage at night and start the car with the lights off or really dim, look inside the engine compartment and see if you see any spark arcing from the wires. I would throw in a new set of plugs, they are cheap to make sure you do not have a bad plug.

Check for vacuum leaks. I just had an idling issue and I found 2 vacuum leaks. One being the brake booster, which I just changed, and one on the AC vacuum hoses.

Through the years I have seen so many misfires be simple stuff. I have seen wires that are just loose on the plug, they are barely holding onto the plug. Coil wire loose, a wire or mutiple plug wires loose. I just had a misfire in my olds after I had a shop pull the engine to paint it. They pulled the distributor to paint the engine and I guess that when they pulled the wires on the distributor one of the terminals moved inside the boot (HEI distributor cap). So the female terminal that is inside the boot, which clips onto the male terminal on the distributor, was not where it should be, it was about a 1/4 inch inside the distributor boot. So you could put the distributor boot on the distributor, and it would stay there, but the terminal was deep inside the boot and would not touch the terminal on the distributor. It was running on 7 cylinders. The terminals on the plug end can also get disconnected from the wire, they can sometimes also move inside the boot.

Carbon core plug wires deteriorate over time, you can measure the resistance to see if they are good with a mutimeter. Here is a video:
To start I would buy a new set of plugs and put them in there. They are cheap and you get one thing out of the way.

Jaclark93

Okay so how much resistance are msd wires suppose to have most/least cant seem to find anything online.

oldcutlass

From MSD
By checking the resistance of each wire you will determine two things: That there is continuity (not open) and if there is too much resistance in the wire. Use an Ohm Meter and connect the terminals to the spark plug wire terminals. Use care to make sure there is good contact or the readings will be inaccurate. On MSDwires the resistance should be: Heli-Core Wires (Blue): 150 - 1,200 ohms per foot 8.5mm Super Conductor (Red): 40 - 50 ohms per foot

Jaclark93

Okay i did the tests and one plug wire didnt even have a reading? One read 650 another read 1400 so i believe that is the problem.i appreciate you all. P.s. they are red super conductor wires

Jaclark93

Okay maybe im not understanding this correctly or maybe im just missing something right in front of my face. The timing at idle is now set to 12 btdc installed new wires with 500 ohms per foot ( msd street fire). Cleaned cap and rotor as best as i could, but still some black build up on the brass pick ups. Could the cap and rotor be bad even though they are cleaned? The misfire is very small and very random. You should be able to put a class with fluid on air cleaner and it is suppose to remain sound correct?

Vintage Chief

I don't know, about the only items remaining are the HEI coil (of which I believe there are two inside the unit - a primary and a secondary coil) and the ECM. Remove the coil and examine for any visible arcing or possibly that plunger in the middle for any corrosion/wear?

oldcutlass

Bump your timing to 16*. Did you adjust your a/f mixture screws? If you had 1 wire that was open, that plug is fowled. Without seeing your cap and rotor its hard to judge whether it warrants replacement but your description says yes.

Jaclark93

This is after i cleaned it up and ran it for a short time.

olds 307 and 403

Heck bump the timing to 20 at idle. A dial back timing light is good to check how much timing you have total. Even 40 degrees base and mechanical advance is acceptable on a low compression 350 like yours. It depends on what you call a miss, could be a super lean mix did you adjust the mixture screws?

Jaclark93

Yeah i have been messing with the idle adjustment screws got it to where i think is good. Misfire is more noticable when i pop it into gear. And sometimes it almost sounds like its surging. If anyone would be willing to message me on my cell phone i can send a video of how its running that would ve super helpful im sure 5172606144 cant get my tach dwell tester to work properly im assuming its just old.

Jaclark93

I just put the timing light back on it and dialed it back to 0 with the vacuum plugged and it is jumping around in between 4 and 0 is that okay?

Jaclark93

Also i should mention the green wire for the tachometer is just sitting there not hooked up to anything could.that be causing problems?

oldcutlass

The jumping around is not much of an issue however it needs to be up around 15/16* with the vacuum plugged. The surging is probably due to the carb a/f mixture screws not being set right. The tach not being hooked up has nothing to do with anything other than it will not work as long as its not shorting. You can set your carb with a vacuum gauge.