1979 250 I6 Running Rough

[KSBandman]

Hello, I read rrent’s thread on the backfire issue.

My ‘79 250 I6 starts easily, but runs rough until it is warm, and seems to lack the power it should have. When shifting, I am hearing popping (but not a true back-fire). Upon shut-off, it “diesels” a bit (engine chugging while the key is off).

This engine has vacuum lines running everywhere, and I replaced obvious cracked and broken vacuum lines, checked the PCV valve (it is ok). The exhaust smells like all the fuel isn’t burning.

After reading rrent’s thread, i pulled the diagram someone posted (thank you) and will compare it to what I see. I don’t recall seeing a “decel” valve on mine, but will check.

Any thoughts on the next steps? I thought about new plugs and wires. It has 67k miles on it.

Thanks in advance.

KSBandman

 

[Charlie]

 

[CorvairGeek]

Deceleration valves were typically seen more on manual transmission vehicles (on GM 4 & 6cyl engines). The high vacuum situation on deceleration (which causes a lot of fuel to be pulled through the carburetor) caused a rich condition (high CO and HC). This was especially bad with manual transmissions because of the direct coupling, and with each upshift.

 

[bucket]

Have you pulled the spark plugs? That's where I'd start.

Mine used to foul a couple plugs if I started it and moved it around without actually driving it. It would run similar to how you describe until I pulled those plugs and cleaned them. Iirc, I ended up using anti-foulers on those two plugs and I had much better luck with it after that.

 

[KSBandman]

Update: So, according to the vacuum hose schematics, the engine is the 912F2 Fed. Light Duty. I have diagram, but it is difficult for me to decipher where what goes.
Easy: Canister, Carb, Air Cleaner, PCV, EGR valve, Distributor,
Difficult: VAC BRK TVS, TVS, Delay Valve, Manifold Vac Source, EFE (Early Fuel Evaporation) TVS.

The military and working in education did not train me for these acronyms.

One photo below is of the vacuum diagram. The line (I assume) should go from the canister to the EGR valve is plugged with a small bolt.

Nothing is attached to the EGR valve.

The next photo is of a small piece attached to the air cleaner housing. One of the nipples is bare.

I will pull the plugs tomorrow to see if any are fouled.

Thoughts?

Thanks in advance.

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[KSBandman]

Update: New spark plugs (same brand and type).
Replaced two vacuum lines.
Replaced the cracked PCV grommet on the valve cover.
She starts right up and purrs. The timing and/or the carb still needs some adjustment, but it no longer diesels after shutdown. A friend has a timing light and we will tackle it next week. I also found out my neighbor had a very similar truck long ago.
He has experience with this carburetor, and his age and lack of hair seem to back up the claim.

I appreciate all the advice.

ksbandman

 

[idahovette]

Hey Tobias no more cracks about the hair!! I'm losing a little but it's due to the medication !! That's my story and I'm sticking to it. I'm "only" 69