Help diagnosing a rough running 292

[M_R_Q]

Hello,
My truck is a 68 chevy K10 SWB with a 292 and a 4 speed. Its an old farm truck thats been in my family for years. The last time I drove it was probably 10 years ago. It was running rough, then just got worse and worse to the point that it was parked. Im not much of a mechanic but Ive been trying to get it running again, and I just cant figure out the issues.

Symptoms: Runs really rough, misses badly and coughs out the carb and backfires out the exhaust. Barely has enough power to move without stalling.

I have a good running 71 with a 250 that I stole parts off of to help find the problem.

Ive swapped the coil, distributor, points, condenser, cap and plug wires with the 250. The 250 still runs well on the 292 parts, no change on the 292. I pulled and cleaned the 292 carb. No change. I swapped on the 250s carb. No change. I pulled the valve cover and all the valves seem to be moving at the same rate. I did a compression test, and compression was all between 120-130psi which I understand is normal, especially for a motor that was fairly tired to begin with.

One thing I have not changed are the plugs. I gave them a good cleaning, but since the plugs are one thing that cant be swapped from the 250, i opted to not buy new ones, mostly since I wouldnt think the plugs would cause the backfire issue.

Could it be the timing gears? when I roll the engine over by hand, then reverse the direction, there is a bit of play (maybe 5 degrees) until the rotor starts to move.

Any other thoughts on what I should try? Can bad plugs cause a backfire/coughing issue? To me it sounds like a timing issue, but I dont have the knowledge or experience to say for sure, and Ive changed everything related to the timing except the gears.

Thanks

[Swar]

Did you check it with a timing light? It might just be off a little.

[landy]

Have you checked the timing? Truck has been sitting for a while? Vacuum leaks, bad gas? Maybe carbs needs to be rebuilt?

For problems like this, do one thing at a time and see what changes.

[primetime]

i would check timing first as well

[65standard]

I would install new spark plugs.

[hooverfish]

Check inside of distributor cap for hairline crack. Maybe swap the one from the 250.

[Wrenchbender Ret]

Those have an aluminum timing gear & no chain. I would rule out timing gears. They are fool proof. The 292 is notorious for manifold gasket problems when used in heavy duty trucks.

[loom34432]

I'm with 65Standard, I'd change to some new plugs. This can make a difference.

[factorystock]

Originally Posted by Wrenchbender Ret

The 292 is notorious for manifold gasket problems when used in heavy duty trucks.

I agree, bad intake and exhaust manifold gasket or warped intake or exhaust manifold will cause a major vacuum leak resulting in very rough operation.

['68OrangeSunshine]

Make sure Distributor is set at #1 Cylinder [pointing to #1 Contact and the front of the block] when at TDC. Could be a tooth off.

[M_R_Q]

I didnt check it with a timing light (I can never make sense of them the few times Ive played with one) but I did try adjusting the timing. Usually I can get it pretty close just by feel. Not perfect, but certainly usable. I also cant get it to stay running well enough or long enough to bother with the timing light. When I do adjust the timing it doesnt seem to make any difference in the way it runs.

The problem originally popped up while still being in use, so sitting wont be a big factor. Ran it on fresh gas and replaced the carb with a known good one off the 250 with no change, so I put the old one back on. It could be a vacuum leak, I suppose, but I dont have a gauge to do that test. I doubt its vacuum though, based purely on my gut

Distributor cap has been changed. The whole distributor has been swapped, and the old one works perfectly fine.

Its my understanding that the 292/250 had a nylon/fiber timing gear that is fairly well known for losing teeth? At least that is what Ive read. Perhaps thats just on the 250?

The problem happened while driving it, and not all of a sudden, so I doubt the distributor is off a tooth. Ive also pulled it and set it back in a few times with no change.

When I get around to it, I may buy some new plugs for it. I just wasnt sure if a bad plug could cause backfiring/coughing. I can see it running rough from that, though. I guess Ill just have to try it and see.

['68OrangeSunshine]

Originally Posted by M_R_Q

I didnt check it with a timing light (I can never make sense of them the few times Ive played with one)

[Find a buddy who has one and knows how to use it. It's easy, you point at the timing marks on the HB and Timing indicsator on the Gear Cover, pull the trigger and read the blinks.]

but I did try adjusting the timing. Usually I can get it pretty close just by feel. Not perfect, but certainly usable. I also cant get it to stay running well enough or long enough to bother with the timing light.

[Is your fuel pump working OK?]

When I do adjust the timing it doesnt seem to make any difference in the way it runs.

The problem originally popped up while still being in use, so sitting wont be a big factor. Ran it on fresh gas

[Have you changed the Oil yet?]
and replaced the carb with a known good one off the 250 with no change, so I put the old one back on. It could be a vacuum leak, I suppose, but I dont have a gauge to do that test.
[Get one, they're about $20. but you can check for manifold and carb base leaks with a spray can of carb cleaner. Spray around these places, and if the engine changes RPM then the carb spray is getting into the air/fuel mix.]

I doubt its vacuum though, based purely on my gut
[Gut is only reliable when shooting dice.]

Distributor cap has been changed. The whole distributor has been swapped, and the old one works perfectly fine.
[Have you run a resistance (continuity) check on the plug wires? Did you try swapping out the coil, too?]

Its my understanding that the 292/250 had a nylon/fiber timing gear that is fairly well known for losing teeth?
[Yes some L6s had a nylon/glas cam gear. Crank gear is steel. I think '68s had an alumin(i)um cam gear stock, but 47 years later who knows if some budget-minded person or shady rebuilder didn't slip in a plastic piece. Visual inspection is the only way to know, but that requires pulling the Radiator, Fan, Belts, Harmonic Balancer, etc. just to get at it.
The cam has to be out of the block to install a metal gear. The alum. gear is put in an old saucepan covered in light motor oil and superheated with a blowtorch while the whole cam has been chilled in a freezer. When the cam gear is cherry red it's slipped on over the chilled cam end and the assembly is allowed to cool.]

At least that is what Ive read. Perhaps thats just on the 250?

The problem happened while driving it, and not all of a sudden, so I doubt the distributor is off a tooth. Ive also pulled it and set it back in a few times with no change. [Fuel filter clogged ?]

[Without verifying that Cyl #1 is at TDC, we have no starting point.]

When I get around to it, I may buy some new plugs for it. I just wasnt sure if a bad plug could cause backfiring/coughing. I can see it running rough from that, though. I guess Ill just have to try it and see.

[Plugs are cheap. $2 to $6 @, depending on quality/hype. Copper plugs work well enough. Platinum tips can't be freshened up with a small file].

Hope this helps.
Good luck.

[M_R_Q]

Thanks for all of the input.

I do have a timing light, I just havent been able to successfully "read the blinks". I havent tried it more than a time or two though and not on this truck

The fuel pump does work, but there is always a chance its a bit weak. Ive never had a failing fuel pump with these symptoms, though. It also runs rough with a fully primed carb the second it fires, so it doesnt act like a starvation issue.

I have sprayed carb cleaner all over the place to see if I could detect a vacuum leak but I didnt notice anything, but that could be lack of experience.

I think my grandfather had the engine rebuilt some time in the 80s or early 90s, so perhaps at that point they put on a plastic gear. It really hasnt had much for mileage on it since then though. Id say 20,000 tops, which seems a short time for a gear to strip out, I would think.

I have read from a few people that have managed to do the gear swap without pulling the engine by heating the gear, cooling the shaft and tapping it on with a dead blow. Not ideal Im sure, but Ive read it from enough sources that Its likely doable.

I switched the coil and plug wires with my 250s, and the old ones check out. No change.

I can blow through the fuel filter with ease, but I havent replaced it.

It reads as being at tdc. The rotor points at #1 and the timing marks line up. I havent pulled a plug and actually checked to see if the piston is where it should be, though.

Plugs would be the cheapest and easiest change. I feel annoyed at myself for not changing them out already. I just needed to have someone tell me that its a possible cause before spending any more money (even if its a small amount) on getting a truck that cant be driven running. Especially when my non dice playing gut has been saying they arent the problem.

Thanks again.

[pritch]

No one has said it yet, so let me be the first: HEI! They are all the same from any GM I6 and with the new plugs, ignition will not be the problem.

[65standard]

Bad spark plugs will be weak or not fire under compression. They can surely cause a backfire or misfire.

[69chevy lwb]

My vote is plugs. Mine was burning oil and fouled the plugs. I put a new ignition system in it as I knew it was the issue but I didn't think to check the plugs. It turned into a very expensive plug change. Back firing through the carb is usually timing related or due to lean conditions. The plugs will also tell you this.

[Daner]

The only thing I can think of that hasn't been mentioned is make sure you have the firing order correct. Although I guess if symptoms began while it was running that wouldn't be the problem.

['68OrangeSunshine]

Originally Posted by Daner

The only thing I can think of that hasn't been mentioned is make sure you have the firing order correct. Although I guess if symptoms began while it was running that wouldn't be the problem.

Yup. And that's 1-5-3-6-2-4.