Duke! Some help with "afterburn" please

[Gary Wilkerson]

I recently purchased 66 BB, 427/390. It has a carb from a 425 horse so it tends to run rich. I need to look for another carb. I haven't driven the car more than a couple of miles because I need to work several major issue.
One big problem I had yesterday was when the engine was rev'd it seemed to run fine but after it returned to idle or close to it a tremendous boom came from the exhaust. The second time this happen it literally split the passenger side muffler. I assume I'm looking at afterburn.
On a yearier post you said afterburn could be caused by a plug not firing, lean condition or a valve thats leaky. I assume I'm not looking at a lean condition because of the carb I mentioned and it did smell like it was running rich.

I thought I'd approch it this way:

1. Look at the plugs on the right bank (all eventually)for signs of ignition issues.
2. Run a compression check (I cannot perform a leak test myself)
3. Check the valve adjustment to see if an exhaust valve is too tight, not allowing the val;ve to seat.

I'm I going at it correctly?

Gary

[Mark #28455]

Re: Duke! Some help with "afterburn" please

If the motor is running too rich, you should also consider the power valve in the carb. Holley carb power valves can crack over time or get blown by a strong backfire and cause the motor to run extrermely rich.

Also check your timing if you haven't already done so.

Good luck,
Mark

[Duke W.]

Re: Duke! Some help with "afterburn" please

Back in my mischievous youth, one of the "tricks" we would pull (not with my SWC, but with buddies beaters or parents cars) was to turn off the ignition switch for about five seconds and then turn it back on - KABOOM! The unburned fuel air mixture would fill the exhaust system and then ignite from the hot exhaust when the ignition was turned back on.

You have a good test plan, but I would also include a thorough check of the secondary ignition components, particularly the plug wires. Carefully inspect all the terminals to ensure they are making good contact and measure end to end plug wire resistance. While measuring resistance, twist and bend the wire, especially near the terminals, and watch for a steady reading on the meter. I don't have a resistance spec, but it should probably be no more than about 5K ohms per foot, so look for that consistency across all four wires. Also check the cap and rotor for mechanical or resistance defects. If it's only happening on one bank then the primary ignition components and are probably okay, but if the afterfire occured on both banks then the primary side is suspect, too.

The amount of unburned mixture to split a muffler is substantial, so you should be able to detect a noticeable misfire. Can you hear it misfire? The L-36 should have a very steady and stable idle at about 600.

Duke

[Clem Z.]

Re: Duke! Some help with "afterburn" please

a leak in the exhaust system allow air to get in will cause a explosion that can damage a muffler.