Light white smoke and backfire at high RPM or under load

[iflyfisher]

Getting a ‘73 K20 running that had been parked since 2005. I rebuilt the carb, replaced the plugs and ignition wires, checked the points and condenser, and have the timing set for 4 BTDC. I also changed the oil, drained the old fuel, and ran fresh gas through the pump to push out any old gas before reconnecting the fuel line. The truck starts fairly easily and runs okay. It emits a bit of whitish smoke and the tail pipe splatters oily carbon spots all over the garage floor when I pop the throttle. Out on the road, the truck seems okay until I run up the RPMs fairly high or put a load on the engine. At which point the engine backfires.

I am no mechanic but willing to roll up my sleeves. I wonder whether the engine has a stuck valve or two. I have only put about 10 miles or so on the engine since I got it running. I’m wondering if I should put some Marvel Mystery oil in the oil and put some more time on the engine; or should I look for the cause now? And if the latter, where and how best to start?

Thanks in advance.

 

[rusted nuts]

Getting a ‘73 K20 running that had been parked since 2005. I rebuilt the carb, replaced the plugs and ignition wires, checked the points and condenser, and have the timing set for 4 BTDC. I also changed the oil, drained the old fuel, and ran fresh gas through the pump to push out any old gas before reconnecting the fuel line. The truck starts fairly easily and runs okay. It emits a bit of whitish smoke and the tail pipe splatters oily carbon spots all over the garage floor when I pop the throttle. Out on the road, the truck seems okay until I run up the RPMs fairly high or put a load on the engine. At which point the engine backfires.

I am no mechanic but willing to roll up my sleeves. I wonder whether the engine has a stuck valve or two. I have only put about 10 miles or so on the engine since I got it running. I’m wondering if I should put some Marvel Mystery oil in the oil and put some more time on the engine; or should I look for the cause now? And if the latter, where and how best to start?

Thanks in advance.

Look for it now if it is a stuck valve You could burn it by running the engine

 

[PrairieDrifter]

Pull the valve covers and check for sticky valves or really loose rockers. If everything seems okay, put some actual miles on it and give er some rpm! Seafoam, marvel mystery oil.. use em both! Run it till it either gets worse or better.

Im willing to bet its valve seals are all dried up. Do you know anything about the engine? A compression test would be a good thing to do.

 

[GTX63]

A gummed up valve train, rotten oil seals, stuck piston rings, a rat's nest in the muffler, loose rust and carbon in the exhaust.
A gen 1 motor that has sat since 2005 needs a little time to wake up from a nap that long.
Yes, Mystery oil, berrymens B12 in the fuel system, Lucas Oil Treatment, etc. Drive it easy but drive it long, meaning 30-45 minutes or more to build engine temps and loosen up the joints.
I wouldn't close the book on the timing just because it is set to specs. Timing chains stretch, carbs, vacuum, advance springs/distributors all had a mind of their own.

 

[Ricko1966]

A stuck valve would be most noticeable at idle, put a vacuum gauge on it see what the needle does, look for vacuum leaks,cracked plug wires,cracked,cap,carbon tracking on the cap. A easy test for ignition leaks,wait until it's dark out spray the plug wires and cap with salt water watch for arcs,power brake it,watch for lightning. I'd do a power balance by pulling plug wires 1 at a time, but most are afraid of getting bit by the plug wires. I can do it,you can too. Take the girdle off the plug wires. Engine running slowly and carefully wiggle and pull the plug wires off the cap one at a time. Grab them a couple inches back from the cap and just pull them up about 2 inches,pay attention to the spark at the cap,and changes in idle.Pulling the wires at the spark plug end is a sure way to get bit.

 

[iflyfisher]

@rusted nuts, I'll do some more digging. Wish I knew more.

@PrairieDrifter, Good advice. I will check the valves and compression over the next day or two and let you know what I find. The truck was purchased by my Dad new back in 1973. He was fanatical about oil changes and other general maintenance. I even have his old book where he logged all the work he had done to the truck. Points and condenser were replaced about 4K miles ago). The truck has just over 100K miles, but hasn't been driven since 2005. Most of the miles were put on over the first 10 years. The truck probably got about 1000 miles a year up to 2005. So it really hasn't be driven much at all over the past 40 years.

@GTX63, I picked up some Lucas oil fuel treatment today and will add that to the tank. I dumped my last of the Marvel Mystery oil in with my previous tank of fuel. I'll get another bottle and add to the oil mix. Even though I changed the oil only 20 miles or so ago, my plan is to change it again at about 100. I'll get the Mystery oil in before then. I had the timing advance to about 12 BTDC and still had the same backfires and missing.

@Ricko1966, The truck idles okay, but it is idling a touch high. I have backed out the idle screw to the point the throttle cable limits the blade from hitting the idle stop. Maybe the cable has shrunk a bit over the years. I need to look into whether there is a way to extend the reach. You note to put on a vacuum gauge and see what the needle does. I have no idea where to hook up one or even what to look for. I'll look for some YouTube videos. There should be something out there. I did have a carbon track on the previous dis cap, which I discovered when trying to get engine running. I think I got it when I washed the grime off the engine. I tossed out that cap and got a new one. I put on new plug wires and coil wire. But I'll try the saltwater spritz one evening. What do you mean by "power brake it and watch for lightning"? When pulling the wires to test for spark, I assume I do one cylinder. Turn off the engine, reinstall the wire, then prep the boot to pull the next wire, repeating till all 8 cylinders have been tested. What exactly am I listening for? I presume the idle should drop when running on only 7 cylinders. If one cylinder doesn't drop, I might have an issue there?

Thanks for the help. I'll let you know how the weekend goes.

Ken

 

[Ricko1966]

Nope you do it running and you listen for the idle to drop,you'll get an ear for it after 1 or 2 cylinders. But if you notice 1 that doesn't change your idle speed and characteristics that one is a problem and needs investigating. As I said do it at the cap end and keep your fingers a couple of inches from the cap. If you try it at the plug end you are guaranteed to get shocked. This has been one of my first go to and favorite diagnostic aid for decades As for vacuum find a port on the manifold hook up a vacuum gauge and watch the needle. I'll try to find you a chart online to compare your readings okay I just read your reply a little better,if you changed plug wires go back and double check,then triple check you didn't get any out of border because that will cause the exact condition you described. As for power brake and look for lightning that means put one foot hard on the brake,truck in gear givebit some throttle with your other foot. Not enough to over power the brakes but enough to put a load on the engine. If there are arcing ignition components powerbrake,salt water in the dark,it will be very obvious. And a vacuum leak will cause a fast idle make sure all hoses are in place and not cracked.

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[iflyfisher]

@Ricko1966, thanks for the clarification. I did notice when putting the carb back on, the vacuum hose connection going to the carb from the vacuum advance was very hard and stiff. I’ll get a vacuum gauge and check. I’m not clear on how I can be in the cab driving, doing a power brake, and being able to see under the hood if there is any arc’g.

I appreciate everyone’s time and patience!

 

[iflyfisher]

Just read the linked article. Dang, lots of good info in there. I’ll try and work through some of the tests. Definitely going to save that one. Thanks for digging it up.

 

[Snoots]

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[Grit dog]

Old distributor still you said, I believe? I’d be looking at total ignition advance being too low. Either vacuum or advance springs or both. Seems that could cause a backfire under those conditions otherwise running normally.
White smoke? If you’re sure it’s not just steam or cold weather, is generally coolant leak into cylinders aka head gasket.
Valve guide seals/rings etc let oil pass into cylinder and blue smoke.
Black smoke is running rich. Very rich in a gasser.

Just suggestions.

 

[iflyfisher]

@Snoots, thanks for the diagram. I am going to chase down a vacuum gauge and take some readings. I might pick up some fresh vacuum line and replace the old stuff.

The truck still has the original Quadrajet. I think it is a 4MV. Where do I hook the vacuum gauge for manifold vacuum? In looking through the manual I have, looks like the vacuum brake for the choke is manifold vacuum. Do I "T" the vacuum gauge in there? Looks like the vacuum advance is ported, so don't want to use that for the gauge.

@Grit dog, I was talking about the backfiring/missing issue with a friend last night, and he wondered the same thing. I have no idea how to check total advance. Something else I need to research. I could bump the throttle while the timing light is on and see if the light picks up an advance on the harmonic balancer. The scale only goes up to 14 BTDC I think.

The smoke is usually when the truck is warming up. When I first changed the oil, I looked for any contaminants in the oil and didn't see anything. I haven't pulled the dipstick since I put in the fresh oil. The exhaust is still spitting out oily carbon dust, though it doesn't seem as bad as when I first got it running. I am hoping most of this cleans up with additives and run time.

Thanks again, Gents!

 

[iflyfisher]

Quick update. All the valves are functioning fine. I turned each of the rods, and all turned fine, so they should be straight. No excess slop around the rockers. I also put the timing light back on and checked the advance when I blipped the throttle. The timing mark advanced as the RPMs increased. I'm off to NAPA to get a vacuum gauge and fresh set of throttle return springs. When I got the truck, it had no spring on the throttle return. I don't know if had rusted and fell off or my Dad was doing something and forgot to put it back on. I put on a spring I had laying around, but it isn't strong enough. I checked the exhaust again, and it is still very dirty. See attached pic.

 

[DoubleDingo]

Quick update. All the valves are functioning fine. I turned each of the rods, and all turned fine, so they should be straight. No excess slop around the rockers. I also put the timing light back on and checked the advance when I blipped the throttle. The timing mark advanced as the RPMs increased. I'm off to NAPA to get a vacuum gauge and fresh set of throttle return springs. When I got the truck, it had no spring on the throttle return. I don't know if had rusted and fell off or my Dad was doing something and forgot to put it back on. I put on a spring I had laying around, but it isn't strong enough. I checked the exhaust again, and it is still very dirty. See attached pic.

Looks to me like normal cold condensation in the pipes. That means you have a leak-free exhaust system.

 

[Grit dog]

Looks to me like normal cold condensation in the pipes. That means you have a leak-free exhaust system.

Yeah old engine, no cats to trap stuff. The 86 spits black chit out just like that. And it runs just fine.
Again no cats, the old 86 is a smoke (steam) show when the weather is cool. Until the engine warms up.

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