First off, if you have seen my other posts here it's obvious I am new to this, there is probably a simple answer but none that I could find via searching previous posts.
So, on my 454 with HEI, I checked plugs as part of my new owner tinkering, and found the NGK YR5s were gapped at 0.035. Car seemed to be running fine, but it's new to me and the only big block corvette I've ever driven, so, based on what i have learned here, I thought that maybe gaping to 0.045 would help,keep the plugs cleaner, maybe improve fuel economy.... Anyways, I cleaned and gapped the plugs and fire it up. Right away, the idle seemed smoother, less loping. Driving however, was disappointing. No power, the occasional mild pop from the side pipes. Even though I did them one at a time, I verified that I didn't mix up wires, that all the boots were making good contact, plugs were tight, etc. No better. I bought a set of Autolite 26s, gapped at 0.035, put them in, and the loping idle and all the power came back, maybe even a little better, had a mild chirp from the tires as the TH-400 went from 1st to 2nd after a satisfying spin and hood lift on takeoff . Anyways, why no go on the 0.045? Should I have to adjust carb with a change like that? Timing? Is it possible the PO adjusted the HEI to prefer the smaller gap?? I'm happy with current performance and won't waste my time again changing the gap unless I might be missing out on something performance wise from a bigger gap. All advice appreciated, thank you

S

 

69427 wrote a nice article about ignitions. Your hei is not a high powered modern ignition and obviously runs out of jam when the spark plug gap goes up. It could be poor quality plug wires that arc through the insulation.

http://forums.corvetteforum.com/c3-t...s-like-me.html

 

Weak coil or plug wires are the most likely cause. Could also be an old cap with carbon tracks.

A wider gap puts more stress on the secondary ignition system and will find any weak spot.

 

Increasing the plug gap also retards the timing. It takes just a little longer for the coil to have enough energy to jump a larger gap.

 

that is why msd call for a bigger gap on the plugs high powered ignition

 

GM built multi-millions of cars with HEI and an 0.045" plug gap. The unit can fire an 0.060" gap if the wires are up to it.

Plug brand is irrelevant.


Are you sure you have HEI and not some points conversion widget?

 

I agree with mike but I'm thinking resistor wire is being used on the HEI
It's a common mistake, people will use the hot resister coil wire that takes 12v and drops it down to 6v. This is enough to get the HEI to work but it will have problems at high rpm and foul plugs. If this sounds like your problem. Run a direct 12v to the HEI and see if it's better?

 

That is definitely a great article!

 

Thank you all. Yea I read 69427s primer on electrical foundation, it led me to ask more questions, which is a good thing. Mum also intrigued by the possibility that there is only 6V going to the HEI, I'll start there as the plugs do seem dirty. The car seems to make good power, but again, I don't really have any muscle car driving experience to compare it too. It also sounds like there is a possibility that the PO retarded the timing, perhaps gre was unaware of then potential to run a larger gap. To answer your question Mike, I think it's an HEI, I don't have a separate coil, just the one unit. I haven't been brave enough to pull the cap and see what's underneath. What would be a good way to rule out a points conversion widget? Thank you all again

S

 

You most likely have and HEI. On the side of the cap you'll see where the wires plug in. Pull the wire maked "Batt" and check it with a multimeter for 12volts. That would be my first move. Then I would try advancing the timing by turning the distributor "counterclockwise " an 1/8 of an inch. Make a mark on the block and dist shaft so you know where you started at. Test drive it. If it's better.... Move it some more. Until it cranks hard or starts pinging under light load.
Every vette has it's sweet spot, it's up to you to find it

 

Won't work. When there's no load on the circuit that terminal will show 12V even if it does have a resistor.

It should be measured with the engine running. You can (temporarily) poke an extra wire into the terminal to measure off.

 

You need to stop correlating engine power to spark intensity. Sooty plugs are not related to spark intensity. Plug gap will not affect engine power utput except on a .000001 percentile all out race engine. The difference in timing between .035" gap and .045" gap is too small to be measured by a timing light.

Post a picture of the distributor.

 

good power = runs well, no backfire or hesitation
Gotcha on the plugs to power not being related.



A little dirty, still sitting on the dealer lot at this point, only pic of dist I have atm.

 

Is that the speedo cable or mechanical tach cable disconnected by the dist??

 

Mechanical tach. I have a gauge from a 75 on its way to match the electrical connection on the dist.

 

That looks to be a bog standard GM HEI. No telling if it's been wired correctly. Run a new wire from the ign. terminal on the fuse panel.

 

Ok will do. Couple days til I'm back in the garage. Mare those terminals for plugging into dist an over the counter part somewhere? Also, 12V right from ignition fuse, 10 gauge wire?

 

Best way t wire it is to remove the resistor wire at the engine side of the fuse panel and put a 10 gauge wire instead.

 

Perfect, thanks all, cant wait to contort my 6'4" 270 lbs in there. Has to be done, need to pull the tach anyways and get rid of the old mech cable.

 

Wow really? spark intensity can be directly related to power output. A weak spark initiates fuel burn poorly or late or maybe not at all vs an intense spark.
If burn is initiated later or more slowly then peak cylinder pressure is reached later and power is reduced.

You will not likely read the timing difference on a timing light by increasing or reducing the spark gap.

Make no mistake the timing difference is present.

In a lawn mower type ignition system (magneto) the only way to vary the timing is with spark plug gap. Otherwise it not adjustable due to a keyway on the flywheel.

Where you set your timing compensates for these differences in actual burn initiation. That is one reason why timing can and will vary from engine to engine.
It's all in an effort to reach peak cylinder pressure at the right moment and delay ignition until the minimum negative work is being done to get the most power from the cylinder, then lighting it to get it to burn quickly enough to reach peak cylinder pressure just after TDC. Approximately 15* after TDC, this also can vary.

Not likely pertinent to the OP's issue, but worthy of consideration.