So as the title says it never seems to end. Very Long story slightly shorter, rebuilt 302 in my 78 F100. New everything short of wiring (did replace battery and grounds with heavier gauge new wires), changed distributor to HEI dizzy due to problems within the duraspark system, was running great for about 3 months. Had some slight issues but carb adjustments fixed them. Carb is a new (about 9 months old holley street avenger 350 cfm). Well the other day I started getting a rough idle and dieseling every so often on shut down, tried adjusting the carb to no avail. Thought maybe a vacuum leak but was pulling 18-20 pound vacuum on the gauge, pulled and plugged all lines just to be sure and not only did it not help but now my exhaust manifold seems to be smoking. I replaced the doughnut gasket and manifold gasket, still smoking. I did replace the valve cover gasket less than 2 months ago so I doubt its that but I may still have to check it. Im lost at where to go from here. Any ideas? Its my daily driver so really trying to get her back running good. Thanks in advance and Im sure I missed mentioning something so if you need anymore info Ill make sure to post back.

 

Re-snug the VC bolts... the VC gaskets have prolly achieved their crush and the bolts are likely a lil loose.

Dieseling is a timing issue... double-check those vacuum lines and fittings. Maybe something got mis-connected, knocked off, or vibrated loose.

 

^^^What he said!

Valve cover gaskets typically need to be re-torqued, but without over-tightening. That's probably the source of the smoke

Use some carb cleaner on the vacuum hoses to double-check for leaks.

While you're at it, double-check the timing. The new distributor hold-down bolt might be loose

 

Ill definitely check the cover bolts and re check all the lines. As for timing it was all good set at 12 initial then about 32 total, however when I hook up the vacuum advance it will advance past 32, probably about 38-40. is that normal or do I have a defective vacuum canister? Thanks.

 

There shouldn't be any ported vacuum at idle, so that shouldn't be affecting your idle...assuming the distributor is hooked up to ported vacuum

 

As stated, dieseling tends to be a timing issue, or putting in too low an octane of gas. I would get a timing light on in again. If timing hasn't changed you need to try a higher octane fuel. But if you are using 87, try 89, and if that doesn't work try premium.

Smoking is likely coming from oil leaking out from the valve covers. See if you can see oil coming out somewhere around a valve cover. Tighten all the valve cover bolts down again. Check to see that oil is coming out a valve cover vent or the PCV valve.

 

So the dist is hooked up to the carb, which if I remember right is the ported vacuum? but when I hook it up, it definitely advances it to full advance. As far as oil from the cover above, I see nothing that even looks liked where oil could have come from but Ill have another look, same with checking the pcv valve.

 

There are probably multiple places on your carb to get vacuuum. The lower connections, usually below the throttle plates, will be manifold vacuum. The upper connections, usually above the throttle plates, will likely be ported vacuum.

There are arguments in favor of both, but the distributor generally gets ported vacuum, which should be nothing at idle. That's why the distributor shouldn't advance at idle. That's also why the books say to disconnect and plug the line-just in case it's manifold vacuum, or in case the idle speed is so high that you have vacuum on the ported line (that's bad mmmkay)

 

Thank you all for the help, I think (knock on wood) I have it settled back down. I tightened everything back down which ended up being a little loose. I also found a cap that was blown off a carb vacuum port on the back. It was completely hidden by the EGR valve so next to impossible to see and also probably why it was blocked by starting fluid and didn't rev the engine up. One more issue though is the ported vacuum. It is the one toward the top of the metering block and is pulling about 12 pounds at idle. This is with idle in park at 1100 RPM. When in gear that drops to about 600 which is why its set there. Is that normal? Any lower the truck seems to want to stall. I ask because like was stated ideally you wouldn't want any vacuum, hence no advance, at idle but that does advance the dist at idle. Thanks again.

 

600 is probably ok for idle speed in gear-you should have a sticker that shows the factory spec. I think mine is 650 with all electrical load turned on (stereo, lights on high-beam, a/c on high)

1100 in Park seems high. Generally, we like about 200 higher than "in gear", so about 850 or maybe 900. As I understand, setting the speed too high makes the torque converter unhappy

Perhaaps with that vacuum leak fixed, you can dial back the idle speed