What did I blow up now

[Blue GMC]

Took out the 71 this weekend and she was acting weird. Accelerate from 0-30 fine then sputtered from 30 to 38 then ran like it rear was on fire until 50 then did the same thing until 58 mph. When it was sputtering was blowing black smoke like a diesel. No problems going up and down the rpm band while in park.

Running a 350/350 combo and both were rebuilt 528 miles ago but long enough to have no warranty. Truck does sit a bunch but has never acted up before.

[WhiteCST]

Timing chain intermittently slipping under load maybe?

[RustyPile]

Timing chain can't "slip" and then miraculously "retime" itself.. My money's on an ignition problem.. You say the truck sits a lot?? How old is the gas in the tank??

[Blue GMC]

Gas is about 6 months old but has stabil in the tank

[Ironangel]

What's the initial timing? What carb? And some rebuild details like was a new cam and lifters installed and properly broken in? Pull the valve covers and check the rockers and springs...?

[Rich69shortfleet]

HEI? Cheap HEI module in the HEI?

[Blue GMC]

Originally Posted by Ironangel

What's the initial timing? What carb? And some rebuild details like was a new cam and lifters installed and properly broken in? Pull the valve covers and check the rockers and springs...?

Running an Edelbrock 1405 carb. Engine was ran for the first 300 miles with break in oil then drained and the valves were adjusted and the oil was switched to Mobil 1. Details I remember from rebuild are few as the paperwork was lost last year when basement flooded. But running a mild can upgrade with cast votrec heads bored .60 over. Trans has a stage 2 shift kit and a 2200 stall. Builder said had to have an octane booster added with each tank as 92 octane would not cut it

[Blue GMC]

Originally Posted by Rich69shortfleet

HEI? Cheap HEI module in the HEI?

New AC Delco hei was placed and all wiring replaced.

[Wrenchbender Ret]

Hi neighbor. The black smoke makes it seem like a carb problem. I'm thinking the choke is going off & taking a set. Some of those from that era had a spring around the choke pulloff. If the choke didn't heat up enough the spring would close it.
George

[truckster]

Originally Posted by Wrenchbender Ret

Hi neighbor. The black smoke makes it seem like a carb problem. I'm thinking the choke is going off & taking a set. Some of those from that era had a spring around the choke pulloff. If the choke didn't heat up enough the spring would close it.
George

1405 should be a manual choke. Is the choke all the way open?

[68 P.O.S.]

What is your timing set at? Get your timing set correctly first, then you can move on to the carb.

[Blue GMC]

Originally Posted by 68 P.O.S.

What is your timing set at? Get your timing set correctly first, then you can move on to the carb.

That's now the plan this weekend then follow up with the carb. Vehicle is at my in laws right now so has to wait a few days. Miss read the carb model(dang bad eyes) and it is the 1406 with the electric choke.

[hjewell2]

Originally Posted by Blue GMC

Gas is about 6 months old but has stabil in the tank

was it stabil 360 by chance?

[Steve78LT]

I had an HEI coil start to go bad on me and act the same way. Lots of backfiring though as the raw gas which made it into the exhaust pipes would get lit off when the coil cam back on. Ultimately traced it to one of the wires which plugs into the bottom of the distributor cap being loose.

[GASoline71]

Originally Posted by Blue GMC

Running an Edelbrock 1405 carb. Engine was ran for the first 300 miles with break in oil then drained and the valves were adjusted and the oil was switched to Mobil 1. Details I remember from rebuild are few as the paperwork was lost last year when basement flooded. But running a mild can upgrade with cast votrec heads bored .60 over. Trans has a stage 2 shift kit and a 2200 stall. Builder said had to have an octane booster added with each tank as 92 octane would not cut it

Was the cam properly broke in if it is a new flat tappet? And unless you're running 11:1 compression or higher you won't need octane boost. I have a very snotty 355 in my truck, and at 10:1 compression with a wicked cam for the street, I can run 92 octane right out of the pump no problem.

Gary

[prodjay10]

Dirty gas tank?

[Rich69shortfleet]

Change the fuel filter at the carb and check out the HEI cap, rotor and coil for problems. Make sure the ignition module has the proper heat sink goo on the bottom of it.

[garyd1961]

Originally Posted by Blue GMC

Running an Edelbrock 1405 carb. Engine was ran for the first 300 miles with break in oil then drained and the valves were adjusted and the oil was switched to Mobil 1. Details I remember from rebuild are few as the paperwork was lost last year when basement flooded. But running a mild can upgrade with cast votrec heads bored .60 over. Trans has a stage 2 shift kit and a 2200 stall. Builder said had to have an octane booster added with each tank as 92 octane would not cut it

I would drain the tank and put fresh fuel in it.

[BrianG]

Originally Posted by GASoline71

Was the cam properly broke in if it is a new flat tappet? And unless you're running 11:1 compression or higher you won't need octane boost. I have a very snotty 355 in my truck, and at 10:1 compression with a wicked cam for the street, I can run 92 octane right out of the pump no problem.

Gary

I agree. Based on the description of the rebuild, I would think it should run fine on even 89 octane. Also that seems way too soon to switch to Mobile 1. But I’m sure there are many different opinions on that. I’d pull the plugs and see what they are telling you. Post a pic or two of them.
Edit... also what is your TOTAL timing? I think the Vortec heads require it to not be too high.

[Rich69shortfleet]

Originally Posted by Blue GMC

No problems going up and down the rpm band while in park.

Change the fuel filter.

[garyd1961]

Old gas, water in tank?
See it quite often at work

[GASoline71]

Originally Posted by Rich69shortfleet

Change the fuel filter at the carb and check out the HEI cap, rotor and coil for problems. Make sure the ignition module has the proper heat sink goo on the bottom of it.

Seen a lot of ignition modules burn up because cats will use dielectric grease instead of heat sink grease.

Gary

[stsalvage]

Did you for get to put the distributor bolt in and tighten it down

[Blue GMC]

Thanks for the help gives me a good place to start looks like next week I will start with:

1. Dump the gas tank(where to put 17 gallons of fuel that will be fun)
2. Replace fuel filter.
3. Go through hei and check wires ect.
4. Pull plugs and check probably just replace.
5. Check timing

[Rich69shortfleet]

Originally Posted by Blue GMC

Thanks for the help gives me a good place to start looks like next week I will start with:

1. Dump the gas tank(where to put 17 gallons of fuel that will be fun)
2. Replace fuel filter.
3. Go through hei and check wires ect.
4. Pull plugs and check probably just replace.
5. Check timing

Don't bother dumping the gas. Do the other things (change the fuel filter!) first and see how things go. I've used gas that is two year old and never had a problem. Bad gas certainly wouldn't make it run bad at certain RPMs and okay at others. You may have had junk in the tank which is why you want to change the fuel filter. Oh, did I mention, change the fuel filter. Then check and snug down all the HEI connections. Also, does the HEI have a clean 12v connection? If you connected the HEI via a resistance wire or the wiring you are using is weak or not well connected, fix that right away.