Hi all,

New 383 build with around 600 miles. getting intermittent backfire through exhuast at idle and deceleration around 2200 rpm. Pulled two plugs on each side. Left bank looked normal. This is right bank 3 & 5. I am not well schooled in reading plugs so thought I would ask the experts here what they think.

 

Why not all the plugs?
More details on the engine, heads, cam, intake, carburetor, timing?

 

Better if the pictures were showing the whole end of the plug not a side view. Brighter light would help too. I always take a carboard box or something to make a spark plug holder and label which one came out of which cylinder for troubleshooting purposes such at this.
Hard to tell but it looks like oil to me but again the picture is not too telling. Something is off.

 

Not the best photo's or information.
How many miles on these plugs? Mileage on the engine. Type of fuel you run. Do you consistently run the same fuel?
Some fuels do leave deposits on the plugs.

 

What plug model are these? ll i can see 12YC but not what is before the 12.
I am wondering if your plugs are too "cold" and not burning off deposits as well as they should. With modern fuels and regular town and country driving many of us go with AC Delco 45 or 46 or equivalents.

 

Dean,

Backfire during deceleration is often a small vacuum leak.

When number 5 is involved with a misfire, check the plugwire routing with # 7 also.
Never let #5 & #7 wires cross each other or run parallel in the wire looms. Due to the firing order, Inductive Crossfire is a somewhat common issue.

But first, remove simple possibilties.
Remove 3 -4 plugwires from the passenger side of the distributor cap. This will allow you to lay the cap over on the drivers side.
Wipe the inside with shop towel to remove carbon tracking. Inspect cap terminals and rotor. Put it all back together.
Double check every plugwires tightness in the cap and reroute 5 & 7 if needed.

Your 383 likely has aluminum heads. You knew that steel sparkplugs need anti-seize on the threads. Too much goop prevents a excellent GRD at the plug.

I do not believe this fouled plug issue is oil, coolant, rich carb, related. But rather lack of full voltage on #3 & # 5.

 

Hard to tell from those pictures the plug on the bottom looks to have a chunk of the insulator and center electrode missing. It also appears to have Little Dots of aluminum on them like it's pinging. .

 

It appears that a "little" oil is being burnt in your engine. The little particles are likely the result of the gasoline and its detergents being used in combustion or even the ZINC that is added to the oil during break-in. I would not be too concerned about the oil being used in the first 600 miles as the rings are being seated. I have seen much worse plugs from new engines, these are really not that bad.

Remember that a newly rebuilt engine will run hotter than normal until the parts are seated and everything is happy. This is why they tell you to not put ceramic coated headers on a freshly rebuilt engine, the higher temperatures can damage the coatings and cause them to fail. On my 427 with high compression the headers burned off the first few inches of coatings on all 8 exhaust pipes, this is normal for the lower temperature ceramic coatings being used commercially. If you need the extra protection they do offer higher temperature ceramic coatings that can be applied to the headers.

Have you been using a good non-detergent engine oil for the break-in period? I don't switch to synthetic motor oil until I have some miles on the engine and there are no oil leaks.

Take an infrared thermometer and verify that your 8 cylinders are all running about the same temperature. I usually pick a nice spot and use it on all 8 cylinders to ensure consistent results. You are looking for consistency, anything out of the normal should be addressed.

 

If the threads are oily, the motor may be burning oil on those two cylinders.

Jason

 

Top plug looks OK...a little dark, indicating a 'rich' condition. The bottom plug looks to be "fouled" due to excess oil burning. Many possibilities as to why. Could be simple poor ignition issue; could be oil ingestion or more difficult mechanical issue. If the car had been running well, most likely a spark plug, plug wire, or ignition component problem.

 

Second pic looks like it is fouled between ground and positive electrodes, plus more foul material around the positive electrode

 

First of all, Thank you all! Just got back from a work trip and will pull all of them and post. Here are the specs on the engine:

383 10:1 Comp
Generic aluminum heads- 64cc
Melling 22136 Cam- 215/230 110LSA .047/.048 lift
Lars built quadrajet
Those are Champion plugs from Summit
Running 91 octane gas

The motor runs strong, but there is a small amount of blue smoke during shifts (5sp) and on startup. Then most recently the backfiring.

I really need to get it on the lift and pull all of the plugs and check them all. I will post as soon as I do.

Thank you all again!

 

What number plug?
Why do you need a lift to remove the plugs?
You are sure those are the correct plugs for those heads as far as length of thread?

 

Champion 12YC
I don't need a lift but I have one and its a hell of alot easier to get to them
No I am not. But that was the reps recommendation from Summit. Need to verify

 

Are you saying the generic heads are from Summit or the guy from Summit just guessed?
Can you look down the plug hole and see more than 3/8" of thread?
The reason I'm asking is because of the darkened plug threads near the electrode end.

 

Are there any letters before the 12?

12 is maybe a little "cold" where 14 may burn off deposits better (if the engine is in good tune).

 

RC12YC is the Standard plug that all aftermarket cylinder recommend or the equivalent of , I prefer NGK's myself and chocolate ice cream

 

Ah, yes, a man I could get along with... TP tag over the top of the roll or under? Silverware handle up or down? WIX or Baldwin?

 

3&5 is on the left bank. do you mean 2&4 on the right bank?
Pat

 

Yes sir, my mistake. Those two came out of 4 & 6. Going to pull all of them and will post them labeled.