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Hello,

The other day I took my '51 3100 (stock 216) out for a short drive. After about 20 minutes or so, while driving flat out (you know, 40-45 mph), it started backfiring through the carburetor. It seems like it's just one cylinder. At idle there's a pop pop pop from the carb that, of course, was stronger and faster at speed. I haven't started troubleshooting yet. Any suggestions about where I should begin, or the likely conclusion?

This is my first post to ask for help, but not the first time I've taken advantage of the collected knowledge of the group. Thanks very much!

Tim

I would check the timing first.

Pull spark plug wires off the plugs (with some well-insulated pliers) one at a time to see if you can isolate which cylinder has the problem. When the plug quits firing on that cylinder, the noise will either go away or at least change sound. It sounds like you might have a sticky intake valve. Sometimes drizzling a lightweight tuneup oil like Marvel Mystery Oil down the carb throat with the engine running at a fast idle helps, but be aware that you'll create a cloud of blue smoke that will put a mosquito fogger to shame! If you've got snowflake neighbors with lace on their drawers you might get a visit from the fire department or the clean air cops from CARB!
Jerry

Ditto on the timing. See if the distributor is loose, which can happen depending on the situation. If previously removed and re-installed, the distributor can sometimes bind and not go all the way down when meshing with the camshaft. After tightening the holding clamp the the distributor can later on drop in place and the clamp is now loose. When I re-install a distributor, I like to rotate the shaft back and forth via the rotor (jiggle it).

Timing will affect all the cylinders, not just one. A sticky valve or a broken intake valve spring is also a possibility. Once you find which cylinder is missing, pull the valve cover and inspect the springs. That's also a good time to do a valve adjustment.
Jerry

X2
Exhaust cam lobe that is worn down will make it pop back thru carb as well.

So. Ignition timing checks OK. The distributor clamp was tight and no recent work to report (the distributor hasn't been pulled in the time I've owned the truck) that would cause a sudden change to ignition timing. I tried advancing and retarding the timing without any change to the problem.

I pulled spark plug wires one by one with my insulated pliers (note to self: get pliers with better insulation! Yikes!). The number 2 cylinder appears to be the culprit. I think I'll postpone the CARB violation inducing procedure for now and pull the valve cover.

Stay tuned.

Thanks,
Tim

X2 Textbook (second only to bent pushrods) for a sticking valve.

I've pulled the valve cover and found the pushrod for cylinder #2 exhaust (I believe it's exhaust -- 4th pushrod from the front of the engine) sitting at an angle, not engaged with the rocker arm. There doesn't appear to be any damage to it. It looks straight and neither end is chewed up. Thoughts about causes and my next move?

Tim

Sticky valve guide. Has the truck been idle for an extended period of time and then run without a thorough flush or replacement of the gas tank? Stale gas forms varnish, which will accumulate in the valve guides and cause valves to stick on the SECOND engine start. The first one will appear to be good, then once the cruddy gas cools in the guides, the sticking begins. It's a very common problem when waking up an engine after long storage without dealing with the gum and varnish in the gas tank.

If that's the case, it's a roll of the dice whether anything short of a disassembly of the head and reaming or cleaning the guides will help. External guide cleaning with solvent and/or gas additives are seldom effective.

For a definitive check of the pushrod find a piece of glass the right width so the flared ends of the rod hang over the edge, and roll it. A straight pushrod will not show a gap anywhere in the middle as it rolls across the glass. Stovebolt pushrods have approximately the rigidity of half-cooked spaghetti, so they're pretty easy to straighten if you're careful.
Jerry

That could very well be the problem. The truck was idle for a while. I drained the tank of all of the old gas and rebuilt the carburetor. It's been run several times since, but that could still be the issue.

Tim

Unfortunately, your situation is not unique. Several other bolters have experienced similar problems, up to breaking rocker arms and/or pistons hitting stuck intake valves in extreme situations. Due to the design of the head, it's impossible to get piston/valve contact with the exhausts. The intake valves are the ones that point straight down toward the pistons.
Jerry

Thanks Jerry and everyone. Sounds like we're off to the machine shop... and to order a new gas tank.

Tim

Take the rocker assembly off. Using a block of hardwood and a hammer strike the offending valve stems after a good soaking with your favorite penetrating oil. Don’t get too heavy handed but some force is necessary. If you have a fair size collection of 4 leaf clovers you may save yourself a trip to the machine shop. I’m sure someone will poo poo this idea but I have had success in the past and so have others.

Thanks Martin.

Tim

Most parts stores have a valve spring remover that they'll loan to customers. I did the head on my 38 myself after sticking several valves closed by doing a first start with old gas in the tank. I pulled the head, removed the valves, yes I had to tap a couple out using my dead blow hammer, then used a brass bore brush and Chemtool to clean out the varnish from the guides. I then lapped the valves, reassembled and reinstalled the head.

I've seen some home brewed valve lapping jobs by chucking the valve stem in a reversible dill motor and spinning the valve back and forth by pulling up on the stem and alternating directions of spin. It doesn't beat a good valve and seat regrind, but it's better than nothing.
Jerry

Valves used to have two small holes in them that the serviceman would engage with a valve lapper. It was like an eggbeater, but reversed direction automatically as the handle was turned. I think there may be one of those in my dad's garage down in California. I still have a small tin of valve lapping compound from when I lapped the valves on a Model A engine. The business end was a steel disk with a couple nubs on it that engaged those holes in the valve head. Others used a suction cup to hold the valve. Found a pic.

Kevin! You're a Hansen, I'm a Hansen. Your dad's in California, I'm in California. Your dad has a valve lapping tool, I....

I went a little too far a little too fast there, didn't I? Never mind....

Amazon has valve lapping tools with suction cups. On a stick for about $10, and on a drill attachment for about $20. Not too much to spend even if I only use it once.

This might be the way to go for me. I might be fooling myself, but the biggest part of the job, I think, would be removing and reinstalling the head. More affordable would be a good thing. Easier to get spousal approval.

Thanks again.

Tim, The coincidences don't stop there. We both have '51's.

Unfortunately, my dad's been gone since '96, and I'm sure my mom couldn't find that tool if she could even get to the garage. If I happened to be down there, I'd be happy to dig it out and loan it to you. Even that might take an hour or two. Mom's place is not far from you in Carpinteria. I've been thru Buellton a time or two.

Next time I'm down there, I'll have to look you up and we can compare notes.

I have used a suction cup on a stick before as well. It'll work, just not as cool as one of those old eggbeater valve lapping tools.

Kevin,

Looking through your photo album -- I really admire the work you're doing. My truck will never see that level of restoration. I'm sorry to say that I'm way better at starting big projects than I am at seeing them through to completion. "Oh,look. A shiny penny!" My goal is to never have my truck more than a weekend or two away from being drivable, and with this latest issue I find myself at the ragged edge of that limit now. Do look me up next time you're down this way. Be warned, though, that the exchange of information is going to be very lopsided.

My best to Mom.

Lapping is also a good way to check for a slightly bent valve. It used to be the final operation of a machine shop valve job, but they got away from that with new valve grinding and seat fitting procedures that gave a better fit without using the abrasive compound. These days I use Prussian Blue instead of lapping compound to check valve seat position and width.
Jerry