The Stovebolt Site Forums The Shop Area Engine Shop popcorn popper engine

When i drive ten minutes, stop for five or ten minutes, drive for a few minutes, stop again for a bit, start up, and drive, a seemingly random popping beging. It sounds like it's coming from inside the engine, carb. This is a '54 235 with hydraulic lifters/camshaft. I had it in the shop about two years ago for carb rebuild and valve adjustment, also an exhaust gasket. I recently took it back to the shop, carb adjust and gap the points, undo what I did. Now it runs great, but still popping. They found metal dust in the distributor, and say that may be worn out. This problem happens after the engine has been running, so I think it's heat related, I don't know how that would effect the distributor. Any ideas?

Try changing the condenser in the dist.

Need to verify some things. When the engine starts popping, does it still run well? Or does it get jerky or tempermental? Does it ever pop when you first start it up? Or only after it's fully warmed up? Have you ever driven it continuously for more than fifteen minutes without stopping? Did it start popping while you were driving? Or only after you stopped and then restarted the engine?

The distributor might be getting worn out, but if it is, you'd notice erratic behavior because the timing would change randomly as you drive. So it might be running well one minute and then poorly the next.

The popping sound - is it metallic? Hollow? Deep sounding? High pitched? Is it rhythmic? Random? Stuttering?

New points, condenser, rotor and cap are cheap maintenance and should be a regular on you maintenance schedule. Remember, these were not like the new cars, a plug cleaning could be expected every few thousand miles and a tune up at about four or five thousand was normal. Most cases in 2009 have engines that are not at the top of their game, which calls for even more PM than when they were new fifty or sixty years ago.
But don't discount the coil as your culprit in this case just because the engine starts and seems to run halfway decent.
The coils are filled with oil as a coolant. Mounting them upside down like they did from the factory not only makes it hard to get at the terminals when you need to but over time it also allows some of the oil to seep past the rubber gasket under the socket. A coil that appears good from the outside but is drying up will perform ok at startup but after the engine warms up it can cause erratic behavior.
As mentioned, inverted the terminals are an inconvenience, although it is incorrect as far as originality, you often see the coils mounted terminal up, which actually makes more sense to me.
Denny Graham
Sandwich, IL

Are the valves adjusted correctly, that's heat related.

Make sure the intake manifold is properly seated/sealed and not sucking air. A small leak will make alot of popcorn.

I need a little more information here. Is the popping mechanical(tapping) or some type of backfire (loud pop)?

If it is a loud pop & it happens when you release the gas pedal after driving you could have a exhaust gasket leak.

Of course by now we have told you to completely rebuilt your truck.

Just give us a little more info & we will narrow it down for you!

This is a backfire. I haven't driven more than fifteen minutes, since this problem started. It seems to happen after I have driven and stopped three times. I did lean the fuel and it started popping right away. Now it's running smooth and strong, it seems like it looses power each pop. It's mostly random noise, but last time this happened it had a few steady sounding pops. It sounds like it's popping inside, throaty, not like a backfire out the carb. But the air filter cover is on so it may be that sound or it may be in the manifold. This happens when I'm driving, not when idling, so it's hard to hear where it's coming from.

OK - backfire. Backfires are caused when the spark goes off while the intake valve is still open causing the fuel air mixture to ignite in the intake manifold & blow out the carb.

There are several things that can cause this. I'd make a list & start from the top.

Ignition - ensure points are good (sounds OK as the truck runs)
adjust the timing. If your timing is off you can have all sorts of problems.

Once that is good adjust the valves as mentioned above. These old trucks need this on a regular basis unlike a modern car that adjusts these automatically you must do this yourself. A valve that is out of adjustment may not close in time for the pop.

If you are still having the issue after that look deeper. Since you state the truck runs well when cold you may have carbon build up inside the cylinder. A carbon hot spot can pre-ignite the gas causing this as well.

Check the compression. If you have a valve stuck or out of adjustment this will tell you which one.

Process of elimination. Hope this helps you narrow it down.

Consider examining the exhaust manifold inside the hotbox. This sometimes burns a wee hole thru behind the flapper valve, that then affects the mixture (more likely t0 burn thru if the valve is seized).

I'd check for a bent pushrod.....

I checked the distributor, and the cap contacts are gouged on one side, so I'll replace the distributor and see if that stops the popping. Although it runs good and strong, I still think there's something else going on.

Seems like everybody has hit on the "usual suspects" but your comment about "metallic dust" makes me wonder if you might have/had a dry lube pot on the side of the distributor. Giving it a turn with each lube job is often overlooked. I'd give the distributor/oil pump shaft a side-to-side wiggle and see if you don't have slop between it and the distributor housing.

A worn distributor shaft can have your rotor gouging out metal from the plug terminals......might have created enough metallic dust or a hairline crack in the upper part of the cap to cause a crossfire. Also effects the dwell/points setting. If everything is snug and your lube pot is pushing a little lube onto the distributor shaft, I'd take Denny's advice and pop in a new coil.

Dave

I put in a new distributor, that came with points and condenser. That didn't get it. New coil, still didn't stop the popping. I did go 20 miles at 60 mph with no problems. today I finally drove and stopped three times to create the problem again. When I got home I popped the hood and revved the engine by hand. It would randomly pop. I started pulling plug wires from the distributor, and revving to get a pop, and reconnecting the wire. The engine didn't pop when I disconnected the fourth wire. I had the truck at the mechanics before this started. New manifold gaskets, and valve adjust. I suspect that some of the hydraulic lifters may have collapsed and only fully expand after the engine core temperature goes high enough, this could cause an intake vale to not seat fully. The mechanic suggested some '70's cams were notorious for developing low spots, that causes the exhaust valve to not open fully, leaving exhaust gasses to blow out when the intake valve opens. So now I could adjust the rocker, but I should check the lifters at least. What do you think?

Another area to look at is the spark plug wires laying on eachother. If some are cracked, they may induce voltage to the other line and misfire a cylinder. Try separating the wires to rule out. Look at each one carefully for broken boots and cracks.