I need some help, my brand new 390 keeps cracking plugs, last week one was cracked, now this week three more bit the dust. I'm starting to get worried, over all the thing is running pretty good, but it's doing some things that worry me:

1) cracking spark plugs
2) sputter or almost a rattle at lower RPMs, gets drastically worse with more throttle, goes away after the PRM gets above 15-1800 (just an estimate, no tach)
3) seems to be running warm
4) runs better when cold, get's slightly worse as it warms up

Motor specs:

390 .040 over
stock heads, new valves
compression about 9-9.5 to 1


carb:
brand new Holley 2300 500 CFM 2v
#76 jets

timing/ignition:
stock points distributor with Pertronix Ignitor I and 40,000 volt coil
timing set at 12 degrees per machinist's recommendation

Sam

 

Sounds like a real bad timing issue. Do you have vacuum advance hooked up? Is it pulling a bunch more in for some reason?

12* at idle isn't bad. You need to see what your total timing is also. 36* or so is all it should go.

 

You were right, it was a bad timing issue. I threw on a new balancer and checked my timing, turns out the old one had slipped and and told me I was at 12 when I was really at 20 degrees! The advance is working perfectly, 35 degrees is all it would go. I haven't had a chance to drive it yet, but it seems to run much better already.

I threw another set of spark plugs into it too, I gapped them at .040, but could I go wider with the ignition setup I have?

Sam

 

You were lucky. When my Balancer slipped, I did a routine tune-up and adjusted the timing IAW the slipped balancer. Engine wasn't running right. It was fighting being revved up. But good old me thought maybe a plug was fouled and I could clear it by forcing some rpms out of it.

Nope, broke a piston due to detonation from too far advanced timing.

All fixed now though. By the way, I also use a Pertronix 1 and the matching Pertronix coil. I'm gapping at 0.035" and based on some other info I won't go into right now, I think that's close to the limit "keeping in mind I'm at about 10:1 CR".

If you're below 9:1, you might get away with 0.040", but I'm guessing a bit.

Just try the 0.040". If the spark can make the jump under full load, that's great as you'll have more spark energy and a more reliable spark. But under high compression and load, the system might be over-tasked. So if it misfires, close the gap up a bit.

 

The gap needs to be .035.

 

Ouch, melted piston are not something I want in my future, haha! The sound I was getting wasn't a knock sound, more of very faint rattle. Just enough to let me know something was wrong. It was one of those things that's drove me nuts ever since I dropped that motor in. It ran pretty good, just not perfect.

I'll take it for a drive tonight and see how it does.

Sam

 

I drove the truck around a bit last night, I think I still need to go up a size or two in jets, the plugs still look too clean, and it pulls the hills much better with the choke pulled out just a bit.

The timing definitely made a difference though, it runs much better.

EDIT: critterf1, why do you say .035? Pertronix says I should be able to open up the gap a bit, and .035 is stock plug gap for points setups isn't it?

Sam