76 Vandura 350 v8 backfiring issue.
 

Hi all,

First off, just wanted to say, I am new to this forum and hopefully I am in the right place.

I just replaced the timing chain/gears on my 1976 GMC Vandura with 350 v8.
Timing is at 10 degrees BTDC.

I was on my break from work and when I was out I accelerated hard from 0 - 80km/h.
After about 5 seconds @ 85km/h there was a very loud backfire with a lot of black smoke, and the engine started to make a very low rumble.

I pulled off the road and listened at idle, no issues at all with sound or backfiring when the engine is at idle.
When I accelerate slowly there doesn't seem to be much loss of power, but the engine sounds very low and throaty (kind of like a muscle car).

If I chomp on the gas, the van accelerates, but a little slower than normal.

If I hit the gas, then let go quickly it pops, sputters, and backfires until it gets back to idle.

--

Engine is a 350 v8
Edelbrock 1405 manual choke 650 cfm carb.
GM stock fuel pump, replaced 500 miles ago.
Timing chain/gears replaced ~ 60 miles ago.
Engine timing 10 degrees BTDC @800RPM without vaccuum.
No ticking, tapping, clicking, pinging or other 'strange' noise other than
the very low sounding rumble.

Any help or pointers are appreciated.

u may have blown out the exhaust or cracked a manifold. the back fire may have been caused by carbon build up from the bad tune it was in from a slacked timing chain, and you blew the carbon out, there may have been a lot and enough to hold the exhaust valve open for a second causing the explosion to travel down the exh., check for cracks on the exh manifolds, if none are visible the turn it on and listen to where it comes from

Thanks for the reply.

Checked the exhaust and there is a leak right where the manifold meets the crossover pipe.

tested with a quick patch job and the noise is gone, and so are all of the little sputters and backfires on decelleration.

Will be replacing the entire exhaust system in a couple weeks anyway so the patch might hold up until then.

if u are looking for a cheap fix you can just replace the "Y" pipe and call it done, but if the exhaust is bad, then give 'er