The Stovebolt Site Forums The Shop Area Engine Shop Spark plug Question

I'm running a 68 250, bored .030 over. The head requires 5/8" nut plugs (narrow passage on the head). The distributer is from a 75, electronic ignition, separated coil. Now, with all that in mind.
What plugs and gap should I be using? I found plugs to fit, but gapped them to the 68 standard (.035). Should I be gapping these to .045 (or higher?) because of the E ignition? Currently, the truck performs ok (power), but gets poor mileage (about 12), and diesels at shut off. Hopefully, Chiphead or one of you other gurus has the answer for this one.
Thanks.

question 1: What's the timing set up at?

question 2: What's the carb setup?

I think these will help get an answer.

I've found that there are as many opinions on the correct gap for HEIs as there are guys runnin 'em. Supposedly, when they first came out, GM recommended a gap of .080. That seems a little wide to me....and it must have to GM too, because now most of the time the parts computers show a recommended HEI gap of .035 to .045. I've tried running both or either and I can't tell any difference at all between the two.

As far as the dieseling, 90 percent of the time I've found that it's because the idle is set too high. And many times it's because the builder/wrench can't get it to idle smoothly at anything less than 900-1000 rpm. In most of those cases, the culprit turns out to be a intake vacuum leak.

Just my experience.

Irwin,
1) 4 degrees BTDC
2) a boring Rochester Monojet. (wifey wont let me buy the progressive 2bbl)
A thought just occured to me: Is this engine suppossed to have a spacer under the carb?

I am not feeling like this an ignition question, but like Asylum getting the impression that its more of a carburation issue. Has carb been gone through and set?

I really don't think with that set up the plug gap difference would cause the truck to get bad economy or diesel.

Dieselling can be caused by a number of factors - if the truck runs really hot - or is really carboned up - then that crud can carry heat and make the thing continue to run. Trucks will run hot with too much advance, but you don't have that problem at 4 btdc. If the carb is dumping fuel, that might be an issue.

One thing you can try is to see if the truck trys to stall if you start quick or stop quick - which would indicate sloshing of fuel in float chamber - i.e. floats not floating or set wrong. I just went through that.

Also check your plugs after use. They should be light brown. If your plugs come out black and wet all the time and its not appreciably burning oil - that would indicate too rich of a mixture. Too rich can also be caused by a dirty air filter acting like a choke, thus richining up the mixture. Engine starved for air - bad economy.

Your carb adjustment on idle screws should just be to adjust to highest idle, set about 1/4 rich from there, then re-adjust idle speed. If it won't idle, like Mr. Asylum sez, then there could be a vaccum leak causing the mixture to get goofy. Then you'd be leaning on the gas to make it go, thus, bad economy.

I really don't think .030 overbore would change things very much with stock fuel system and stock exhaust. HEI should just give you a hotter spark, which should help burn more fuel, but, not make a huge huge difference as these things were ok spark-wise stock.

Anyhoo - anyone else got better ideas? That's all I can think of off hand.

Well, based on your (both of you) advice, I stopped for a fresh intake/exhaust gasket. It does idle fairly smooth at 700 rpm, but sure is cold blooded in the mornings.
The carb is a fresh rebuilt, the engine is new (ok 6000 miles on rebuild ). It does run a bit hotter than the old one, about 2/3 up on the temp gauge, but I have the 180 thermo (book recommended)in it. If the new gasket (and retune)doesn't cure the problem, I'll try a 165 thermo.
Here's a thought. Just before I changed engines the old radiator got holed (don't drop a screwdriver with the engine running guys!). I had to replace it with a new single core (all that was available locally). Since I only have the single row, shouldn't I be using a cooler thermo so it opens sooner?

not necessarily. If the engine doesn't overheat, then its not an issue. Thermostat will open up when water is hot enough and if the truck always goes to the same temp and stays there, regardless of outside load or temp, then you are ok.

If it gets hot out and the truck does overheat, then the radiator might not be able to remove enough heat. The only thing that would help at that point is to increase cooling capacity by either a helper fan, or, more radiator. There maybe some additives like I used in my race bike that can help too (like water wetter).

The thermostats all let through about the same amount of water. Its just when they are going to open up. An old trick is to remove the thermostat, but then the truck will run cold, and take longer to warm up.

What do the plugs look like?

I haven't had a chance to do anything yet. Too bloody cold out here :mad: . 15-25 degrees outside, snow everywhere, and no garage..... Lord, I can't wait 'till I move.