Misfiring all over the place.

[EdwardSams]

About a week ago I took my 70 out for a ride and it was running fine for the first hour and then after shutting it down to grab pizza for the kids it started right up but was running rough.

Got it home, let it rest, and the next day I fired it up and it ran rough from the start. Sounded like it was missing so I asked a few friends for advice who suggested I start with wires and plugs.

I swapped the plugs and wires and it’s still running rough. Tonight I did notice a missing header bolt and another was loose. Tomorrow I’ll get new bolts but can that be the root problem?

A friend suggested check vacuum lines. All look ok.

Now One friend is saying it’s timing. Another is saying cap and rotor(I’m running a Mallory HEI and it’s a 350). Another is saying carb (Edelbrock 1406). I’m kind of lost.

This newbie would appreciate any suggestion of appropriate next steps.

[Lancialonnie]

I just had the same issue with an HEI distributor cap. The center electrode was cooked and cracked, it must have barely conducting the energy. A new cap and rotor and all is good again! I hope you get it figured out.

[67ranchhand]

Start with the small stuff like cap and rotor or vac hoses and vac caps. Then move on to the pricey items. Most of the time it’s the little parts that cause havoc. Good luck.

[Sheepdip]

Just located and fixed a "Missing" rough running flathead V8 for a buddy, it's equipped with 2 Stromberg 97's. He changed plugs and wires with no improvement.

I took a squirt bottle with gas in it and with him revving and holding the engine to a higher RPM I could get it to smooth out quite a bit on the forward carb, pulled the jet's...plugged with debris both sides, cleaned and blew out both carb's jets, changed fuel filter and back to purring like a kitten.

[Greasey Harley]

...Ignition module?

[bill3rail]

Many possibilities and you must eliminate each one until you have solved the issue. I would also check to see if the wires are installed correctly. 153624 if I recall correctly.

Bill

[rpmerf]

I have the vacuum advance fail on me. Vacuum leak Caused a vacuum leak and the timing wasn't right anymore.

Couple thoughts.
Cap and rotor are cheap. Start there.

Check idle vacuum. Should be around 20*. If it is lower, look for leaks. High idles is also a good sign of a vacuum leak. Cap off anything unnecessary to eliminate possible issues. Could individually test the components if you have a vacuum pump. Spray carb cleaner around the base of the manifold and where the intake meats the head and listen for vacuum leaks.

If it runs better with the choke on (or your hand over the carb inlet), you are running lean. On the Quadrajet, you determine running rich by disconnecting the accelerator pump and giving it a rev. if it stumbles a bit, but is OK with the accelerator pump attached, it's good. If it revs fine, without the accelerator pump, it's rich. Not sure if you can disconnect the accelerator pump on your Holley easily.

Read your plugs. Dry black is rich. White is lean. Tan is good. Wet black is oil. Clean is coolant.

Put a timing light on it and see where the timing is. Make sure it's not wildly off. Also note if the light is consistent. Put it on each of the wires and note if the timing light doesn't light up consistently. If the light seems to skip, that should point a miss. Could also use an inline spark plug tester.

[Steeveedee]

I'd blame the carb last under the current scenario, and a vacuum leak won't make it run that badly, or you'd be able to hear it. As has been mentioned, check the cap to see if the carbon electrode is wiped. It should have the plug wires connected in the proper order, since it started before you went after the plugs and wires.

[demian5]

My son had the same thing happen in his 86. It was the canister full of gas and the purge valve went bad. Pinched the line off, ran great. Replaced the canister with another, runs amazing.

Happy motoring.

[EdwardSams]

Thanks for all the advice and ideas - it was exactly what I was hoping for.

I'm going to start with getting the header bolt situation resolved - it's a 350 with Doug's Headers. Suggestions on gasket type?

At the same time I'll swap out the cap and rotor but here's the thing, it's got a Mallory 8548201 for which I can't find a replacement cap and rotor on the web. I'm thinking that MSD offers a compatible set up so tomorrow I'll call a few auto parts places to see what I can find.

And I'm going to pick up a timing light so I can confirm where it's at.

About checking vacuum, I found this video - what do you think of it? https://www.youtube.com/watch?v=NrnzD7PgoHI

[rpmerf]

I use the high temp white gaskets:
https://www.summitracing.com/parts/m...make/chevrolet

Video is decent, wish they finished it though. Yes, basically any manifold vacuum port.

If you have something like this that can pull a vacuum, you hook it up to the vacuum devices individually (vac advance, trans, booster), pull a vacuum and watch that it can hold the vacuum.

At least from the picture on Summit Racing, the dist looks like it uses a standard HEI cap.
https://www.summitracing.com/parts/maa-8548201

[Grumpy old man]

Before you start throwing parts at it check for timing chain slop , Your symptoms sound like timing chain slipped So I'm going to bet a donut it's the timing chain .

Easy to check doesn't cost anything unless it's proven to be bad

[RichardJ]

I really don't think someone switched his plug wires or the timing chain wore out while he was eating Pizza.
After fixing the obvious, the exhaust gasket, diagnosis with a timing light and a vacuum gauge is the next logical step.

Here is a page with animated vacuum gauges.https://therangerstation.com/Magazin...acuumLeaks.htm It's pretty cool.


Here is an image that looks a lot like the Holley video. To bad they cut that one short.

[EdwardSams]

This evening I was able to pick up a cap and rotor and install it. It’s definitely running a lot better but not perfect. Tomorrow I’m hoping to get that timing done so I can check that off the list plus I’ll get the missing header bolt on and new gasket while in there.

I did pick up a vacuum gauge since who doesn’t want to have another tool in the toolbox. The one thing I’m not clear on is which vacuum port should I plug it to? Pull brake booster and use that line?

Thanks for all the guidance.

[EdwardSams]

I was hoping to post a few pictures but I’m getting a security token error message. I noticed that the charcoal canister only has one vacuum line connected to it. It appears to be running down to a fuel line which I guess comes from the gas tank. Should there be any other vacuum lines plugged into it? It looks like it has two unused ports that are open.

[Steeveedee]

Originally Posted by EdwardSams

I was hoping to post a few pictures but I’m getting a security token error message. I noticed that the charcoal canister only has one vacuum line connected to it. It appears to be running down to a fuel line which I guess comes from the gas tank. Should there be any other vacuum lines plugged into it? It looks like it has two unused ports that are open.

There should be a line to the bowl vent if you have a stock carburetor, and a line to vacuum when the engine is running. The missing lines will not cause the misfire. I'll bet money that the charcoal in that canister is completely saturated, since it isn't being drawn off by by the engine.

Can't help you with the pics, I've not seen that error.

[EdwardSams]

It’s been a few quick weeks since this thread was active so please forgive my delay in sharing an update.

I’ve got three little kids so I don’t get as much time to spend on the truck as I’d like but I finally was able to change the wires, change the plugs, change the cap and rotor, confirm vacuum is OK, adjust the timing, and tune the 1406 to get the right idle speed. I’m glad to share it’s running like a champ.

Today I took the day off and spent the better part of it out cruising.

Thanks again for all the advice.

[Steeveedee]

Good to see you got it working OK again!

[tino0217]

Check firing order and timing and then check fuel filters