SBC miss. Trying to get the old girl back on the road. Chevy gurus please help.

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My insurance ran out in February 2020 on my 40. With everything shut down for COVID I decided not to renew and instead do my P/S conversion. When I parked it the recently installed 1972 350 4-bolt main SBC was running perfectly.

Fast forward to now. After reinsuring and getting it back on the road, I noticed it had developed a slight tapping noise under certain conditions at 2000+ rpm. My instincts were that one of the valves was a bit loose.

I’ve now adjusted all the flat tapper hydraulic valves to zero plus 1/4 turn. The tap is gone. When cold it seems to have a bit of a miss. At idle the vacuum gauge has quite a noticeable flicker. It seems to go away under light throttle. As the engine warms up the miss seems to get worse.

When trying to narrow down the issue I replaced all the relatively new plugs. All looked good with a light tan colour except for #7 which was black with possible oil residue.

After a 1/2 hour drive the new plug is also black. I previous found that pulling the plug wires out of the distributor cap made a difference to the idle for all cylinders except for #7.

Any thoughts of what went wrong while it wasn’t being driven? I’ve also added Seafoam to the oil to try and loosen things up.

Motor is out of a ‘72 Chevy pickup that has 72k miles on it. I installed a lower mileage RV cam into it along with the lifters that were with that cam. Also a new timing chain as it was loosing bits of nylon gear.

Could one of the used lifters be failing? Did I loose a cam lobe? What should I check next?
 
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whitey45

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Did you keep all the lifters in order with the lobes? Sounds like #7 might have wiped the cam lobes. Quick intake removal will tell the story. You cannot mix up the lifters as they each break into the cam lobes differently.
 
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Did you keep all the lifters in order with the lobes? Sounds like #7 might have wiped the cam lobes. Quick intake removal will tell the story. You cannot mix up the lifters as they each break into the cam lobes differently.
I believe I did as I was careful about swapping it across and know that they need to stay matched... but to be honest, more than three years later, and given how the last couple have been, I can't say for certain.

My gut feeling was that it could be a cam issue... but I was hoping I was wrong and it was something less involved.

I was thinking I'd pull that valve cover and compare rocker movement for that cylinder to the others.

It's a slippery slope if I need to pull the intake. If I have to go that far, I might as well cancel the insurance, swap in a new cam and lifters, install the TPI intake manifold, hook up the higher pressure fuel pump, stand alone painless wiring harness, distributor, and speed sensor. All of which means it would be parked for most of the summer.
 
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Your problem may be a bad plug wire on #7. You can check resistance with multimeter or simply move it to another cylinder and see if it corrects the #7 problem which has moved to the new location. If you have a spare plug wire that you know is good replace the #7 wire.
Thanks, that’s a great thought.


The spark will jump an inch to the cap when it’s running… but that doesn’t necessarily mean it’s getting all the way to the combustion chamber. I’ll check it out.
 

YODA 88 62

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Similar issue happened recently and I thought my plugs and wires were good. Behind the heat shield was several cracks in the plug wire. Replaced with $40 AC Delco wire kit and it runs like a champ.

51F9DD63-9B17-4631-869C-B74A9747C91B.jpeg


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65swb45

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You haven’t mentioned if you did a compression check. Whatever position the engine is shut off in, a few valves will be open…to atmosphere, indirectly. And you don’t live in the most temperate climate.
 
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You haven’t mentioned if you did a compression check. Whatever position the engine is shut off in, a few valves will be open…to atmosphere, indirectly. And you don’t live in the most temperate climate.
I haven’t done a compression check. I did start it every couple months and move it around.
 
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Pulled the #7 lead to measure its resistance. Discovered that my Fluke meter only measures 0-400 ohms. Both #7 and #5 were above that in resistance. No visible damage to the lead, but I’ll have to dig out my other meter to measure its resistance.

What I do know is that it does conduct electricity through it. I (+) to (-) on the battery is 13.0 volts. If I add the ignition wire to the circuit that drops to 3.91 volts.

I’m too tired at the moment to figure out what the resistance would be… especially without knowing the amps.

Off the top of my head I don’t know if the lead is good or not. Apparently up to 5000 ohms per foot. Thereby making 6000-6500 ohms a very possible normal wire.

Any thoughts?

Edit:
For comparison sake, the #8 lead tests at 4.14 v. It appears to be the same length as #7 so I’d have to say the resistance of #7 is somewhat higher. However, I suspect not enough to cause the miss. When I put it back together I’ll swap the leads and see if #7 is still missing or if the miss is now at #8.
 
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Dug out the other Ohm Meter.

#7 with the miss 11,460 Ohms

vs

#8 that's good 9,930 Ohms (and it's the same length)

Could an extra 1,530 Ohms be enough to create a miss with a high voltage Coil?

I'll swap them side to side next to see what happens.
 
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I swapped 7 & 8 and it ran better till it warmed up. After a few miles it was running like a three legged dog. It was gutless and the miss was getting worse.

Here are the plug wires I took out…
10BE8150-8730-4964-99DA-B4367F566972.jpeg
A7B2BFDE-2FB6-42B3-A1FB-593E73BD658C.jpeg


Visual they look fine. No visible cracking or wear. When I was checking things out, I didn’t even get any shocks thru their insulation.

The ohm meter told a somewhat different story. #8 was 9,930 ohms and #7 was 11,460 ohms even though it was the exact same length. Roughly 4500 ohms per foot for 8 and 5500 ohms per foot for 7.

Solution…
image.jpg
image.jpg
image.jpg

These are rated at 500 ohms per foot making for leads with 1/10 the resistance. They are a cut to size kit that comes with a basic crimper that does a decent job of securing the ends.

I’ve left the wires long for now and crimped on the male ends at the distributor cap. When I swap in the newer distributor I have I’ll use the alternate female ends for its newer cap. At that time I’ll trim them a bit shorter once the exact routing is determined.

I’m pleased with the quality and would recommend them for a “stock” look. For a more stealth look, solvent can be used to remove the white lettering on the black wires.

Initial test drive results: better throttle response, no noticeable miss, way more torque, no tendency to stall easily. It’ll need some tuning once it’s fully warmed up, but it’s 100x better already.
 
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Similar issue happened recently and I thought my plugs and wires were good. Behind the heat shield was several cracks in the plug wire. Replaced with $40 AC Delco wire kit and it runs like a champ.

View attachment 3025653

View attachment 3025654

I owe you a drink… :cheers:

I’m not certain it’s at 100% but the wires definitely were the biggest part of the issue.

The morals here are:
1) wires can be bad even when they look like they’ve been recently replaced.
2) aftermarket wires can deliver a hotter spark than stock… less resistance = more voltage to the plug.
 
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Skreddy

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I’ve been using Taylor cut to length wires on my recent Chevy engines. Pleased with them. I also like using a good quality wire loom to keep everything tidy and wires away from headers / manifolds. Pics are RM I think? Saved on my phone but I’ve used similar wire holders. That’s my unsolicited thoughts on Chevy plug wires
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pb4ugo

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I usually run my wires below the headers or manifolds. I think its a cleaner look and easier to work on. I've never had an issue with burnt wires.
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I usually run my wires below the headers or manifolds. I think its a cleaner look and easier to work on. I've never had an issue with burnt wires.
View attachment 3040353View attachment 3040355
All you need now are some stock heat shielding. Here’s how mine are run:
8559D3C4-F16C-4BCD-9C2C-4D9433A3A2FD.jpeg
FA5D5503-9F55-4656-9C27-555847745618.jpeg
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Pulled the pictures out of this older thread:
 

pb4ugo

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I'm familiar with the heat shields, I remember having to deal with rusty heat shields falling apart back in the day when I was working as a mech. I personally have never run heat shields.
 
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