Engine dieseling diagnosis…

[rusted nuts]

Exactly what I was going to say it has to get fuel and air to diesel anyway you covered it. I've always thought sea foam was like snake oil. I use water myself ,have for over 40 years. I suggest it once on here and everybody acted like I told him to douse it with gasoline and throw a match in. Water works great. O.P. is this truck stone stock all the way down to the fuel pump. If your fuel pressure us too high it can diesel.

Water is good as long as You don't use to much.

 

[GTX63]

Seafoam works, but if the issue isn't carboned valves or a gummy quadrajet, then you might think it is useless.
Personally, when I get a motor that has been sitting, or driven in a poor and unmaintained condition for an extended period, I start with a full can of Berrymen B12 in the gas tank.
And carbon deposits can become hotspots, which will cause dieseling.

 

[1STLS1]

When under the hood and tipping the throttle, when it bogs, try whiffing it with some carb cleaner and see if it picks up, it would verify it's a fuel issue. When it's bogging, spray carb cleaner around the base of the carb, intake manifold mating area to the cylinder heads and any other place there could be a vacuum leak.

Be careful because carb cleaner is flammable, a bad spark plug wire can light it up right now so keep it away from them. I keep 2 small fire extinguishers available when working under the hood. It can happen to the best of us in a second, please be careful

If it continues to falls over once the throttle is depressed, verify the accelerator pump is functioning correctly and is providing the correct shot. If you don't find anything there, verify the EGR is coming in at the correct time and isn't stuck open causing a vacuum leak.

 

[dkraven]

Sprayed carb cleaner looking for a vacuum leak, found 2 spots where the rpm jumped around 80 when the carb cleaner hit. One is where the pcv valve goes into the manifold. The other is around the springs on the secondary throttle shaft on the driver side. pcv valve is brand new so maybe the rubber gasket where it inserts into the manifold is bad? And I'm wondering if the carb has a leak where the shaft goes into the base plate. I didn't drill it out and put in bushings when I rebuilt it.

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[Grit dog]

@Bextreme04
Lol, Jesus clip! Haven’t heard that in a coons age….
Funny story, in high school me and a buddy had 2 girls with us road tripping to his sisters college to party one weekend. My 70 Impala blew fiber carb spacer out somewhere a half hour out of town. Drove it into town no vacuum, having to keep it revved up. Hit up the Napa and got a new base gasket and proceeded to fix it in the parking lot. Buddy knew less than nothing about cars but wanted to “help” so he didn’t look like a dummy Sittin in the car w the girls. First thing I did was pop the Jesus clip off the choke or throttle can’t remember. Told him to hold it and NOT lose it. Because it was very important. Of course he asked what it was. I said Jesus clip and proceeded to get the carb off and back on. Asked him for the clip. He had it safe and sound in his hand still, I saw him checking a few times to make sure. (We were drinking… lol).
He gave me the clip and I proceeded to f up and it sprung off, hit the hood and went straight down the throat of the carb. Of course I yells Jesus Christ among other things. He gets this mortified look on his face and I was like **** it we don’t aactually need it….I wasn’t going on a hunting expedition. Had better things to do with my night. He was a little pissed off for a bit but got over it. I think it ended up getting digested in the #8 hole as there was a scratch inn the cyl wall when I pulled the heads to do a top end a year or 2 later!

 

[dkraven]

New information! Decided to check my foundation and used a piston stop to check tdc. In the picture the nicely outlined etched line at top if what I was using for timing. The two black sharpie marks are the two piston stop points and between them is a barely etched line that is closer to actual tdc though it's nearer the top sharpie mark. Plan is to make a new timing mark and then time the engine from there.

 

[Snoots]

IIWY, I'd replace the harmonic balancer before I started messing with the timing. That thing is TOAST!

 

[mxer147]

yea, looks like a good idea to replace it.

 

[dkraven]

Seems good advice. One of those projects where the problem keeps going one layer deeper! But I already have the fan off to get to the crank so it's not that much further to get to the damper, just drop the $70 and borrow the tool from Autozone. Then make a good timing mark. Then tune up the distributor w new cap rotor and wires. Then set the timing!