I believe I have a misfire which is noticeable at idle, but may go away with increased RPM- but of course, with having 8 cylinders it's hard to tell if it's truly gone away. I tried pulling plug wires off the distributor one by one to listen for a change in idle, but the difference was too subtle, they all seemed to react the same.

Is there any surefire way to determine if a misfire exists, or which cylinder? What are the most likely culprits? The spark plugs are new which make it difficult to tell by that alone.

The truck idles like it has a race cam in it, and I see puffs of black smoke which indicate to me that the raw fuel is getting into the exhaust.

Correction on the title-- truck is an '85 with 96k miles. Original engine/drivetrain AFAIK.

 

If you had a 'dead' cylinder then that runner of the exhaust manifold would be noticeably cooler.
If you have an inductive timing light it will not flash when on that wire.

Are you sure it is just not too rich at idle?
You should notice a change by pulling off the wires one at a time.

But if your floats were too high or the powervalve is leaking it will be fat and lopey at idle and clear up with increased throttle.

Does it pop on deceleration?
If you take the cover off the air cleaner can you see fuel coming from the primary boosters while idling?

 

It does pop sometimes on decceleration, but nothing major. Overall it behaves like it should, other than at idle where I get black smoke and rough running. Above idle, it starts to go away with increased RPM.

Not really sure what primary booster refers to, as I'm not really carb-saavy. I pulled the cover off the air cleaner, and looking into the carb, I don't see any fuel whatsoever until I depress the throttle. When I press the throttle, a quick jet of fuel comes from what looks like a screw at the front of the carb, but then I only see it coming from the two circles right above the primary throttle plates.

Throttle tip-in is damn near perfect with no stumbling or hesitation when changing throttle positions, so it appears that portion of adjustment is good. I do see a small amount of fuel leaking from the front of the carb, there appears to be a small cover with a gasket that is leaking.

 

The small cover directly beneath the fuel inlet?

That is the accelerator pump, the "gasket" is the pump diaphragm.

I still think you have a leaky powervalve or bad cam timing.

 

Yeah the small cover I refer to is mounted from the underside of the carb, and you can see the gasket sandwiched between the cover & base of the carb. It appears to be leaking slightly at that gasket, or maybe the fuel inlet connection above it.

Can you tell me where the power valve is, and how I can eliminate that as a possibility? How much trouble is it to replace the timing components with one that's not 8 degrees retarded? Any easy way to check the cam timing, or do I just check for ignition timing jumping around, since the distributor appears to be driven by the cam?

Thanks for all the help.

 

If all the wires reacted the same it's probably not an ignition problem.

How's your mileage? Sounds like you're rich, and if your carb is 27 years old it could be due for a rebuild.

I had a lumpy idle too until I went through mine.

 

The Powervalve is between the primary metering plate and the body of the carb itself.
You can see part #58 in the exploded view here: http://www.holley.com/data/Products/...9R10060rev.pdf

I don't know how to eliminate it as a possibility without disassembly, but you can make it a certainty by bottoming both idle mixture screws while the truck is running.
If the engine is still able to idle it is getting fuel from the enrichment circuit.

http://www.holley.com/data/Products/.../199R10645.pdf

Brnfree is right. just get the 3-1346 kit and rebuild it.
Your truck will thank you.

Replacing the timing chain is a pretty involved job.
You need to remove everything from the front of the block.
Radiator, fan, alternator, power steering pump, A.I.R. pump(s), water pump, backing plate, harmonic balancer, fuel pump (if mechanical), timing case, etc...

Use the later roller chain timing set intended for a fuel injected engine.
These are not retarded and will last a lot longer.

At 96k (or 196,000) and 28 years old, that plastic cam gear is shot and all the little nylon shards are clogging your oil sump screen.
I was amazed at what I found when I first cleaned mine out.
It took me almost an hour in the parts washer to get the pickup clear.

 

Okay, I'll put those two things on my to-do list.. The carburetor will be a learning experience for sure.

The timing chain replacement sounds a lot like changing the timing belt on a modern V6/V8 engine, which is time consuming, but not too difficult aside from getting the harmonic balancer off. Are there any how-to's or helpful tips regarding this procedure?

For the part, I looked on Rockauto for a '96 timing set, and they show a double roller set; is this the correct one to get to get back to base timing? Price range from $40 to $65, I suppose that's reasonable.

 

Yes, the double row set will work fine, I put a later one in mine when I changed the timing cover to the EFI cover. Check your balancer condition too, it is a 27, almost 28 year old piece, mine had started to come apart.

 

It's important to pay attention to the obvious.

If you've got black smoke coming out of your tailpipe, then you're WAY WAY too rich, and you've got carburetor problems.

Pull it, rebuild it, put it back in, set it with a vacuum gauge, and check your timing.

It'll run like a scalded ape after you do.

Except it's much easier! Changing timing belts on a motor that has an overhead camshaft can be a challenge. Changing a timing chain on an old school V8 is as easy as getting to it, pulling the old one off, and putting the new one on, and buttoning it back up. You just have to line up the dots on the timing gears.

 

Glad to hear that! Any idea what size bolt on the harmonic balancer? Might be the only tool I need to get if it's rather large.

Half the front end of my Nissan pickup had to come off, just to replace the timing belt. Dealer wanted almost a grand to do it, no thanks.

 

Not that large.
15/16"