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Eratic miss (not Erotic Miss)

The reason I removed the carb on the 49 Chevy (216 bored to 235) was because the fuel-air mixture screw was broken off in the body of the carb and the engine had an eratic mis-fire.

The rebuilt carb is back in and I still have the problem. Sometimes the eratic miss goes away (under load at low RPM) and sometimes it doesn't (under load at high RPM (e.g. first gear)) and idling. I can adjust the choke in the cab to minimize the amount of missing, but can't make it go completely away by adjusting the fuel-air mix and idle speed.

I normally think of this kind of thing as a carb adjustment issue. But now I'm wondering if I should look at the timing...

The symptoms are: 1) it has an eratic miss while idling. 2) adjusting the choke can affect it, but not remove it completely. 3) I can't remove the miss by adjusting the idle fuel-air mix, but I can make it worse or better. 4) it goes away under load at low RPM but remains under load at high RPM (winding out 1st gear to shift into second). 5) the miss is prominant when I get off the accelerator (as in shifting gears).

Any suggestions are welcome!

Lee Prairie

I always suggest a basic work up on a problem like this. you need to elminate problems with ignition, fuel, and air the 3 things that can cause a problem. A wet dry compression test, attach a vacumn gage and see what variation there is in idle. You do not mention the age of the engine but a sticking valve could contribute to or be the problem.

I had the head and valves done a couple of months ago (it had a regular miss on cylinder 3 which sounded like an exhaust valve that wasn't closing completely. The engine serial block number starts with HAA which is a 1950 engine block, according to the internet sites. The overhaul I did this past winter included bearings, rings, head/valves, valve guides et cetera. Compression tests indicate 85-90 psi in every cylinder.

A faulty cylinder, for whatever reason would seem to me to present a regular miss rather than the eratic miss I observe. The miss sounds like an incomplete detonation of the fuel charge or mis-timed detonation. The engine can idle for some number of seconds before the miss occurs and it is sensitive to the fuel/air mix and the RPMs.

I'll start looking at the ignition this morning.

Lee Prairie

the first thing I would do is change the sparkplug, then the plug wire, then look inside the distributor cap for a carbon arc. Have you checked the intake for a vacuum leak ? :vcca:

Lee,
Start the motor at night or in a dark garage. Look for dark blue sparks along the plug wires. I expect you will find an eratic one along that wire that goes to the problem cylinder. If not the distributor cap and roter will be arcing where it shouldn't be.

I would suspect a vacuum leak. Probably between the intake manifold and cylinder head. Other posible place is a cracked insulator (under the carburetor).

Thank you all for your comments.

I re-torqued the manifold bolts and put half a bottle of Sea Foam in the oil to cover the unlikely situation that my new head job resulted in a sticky valve. I still have the random miss, but I notice that it doesn't occur during idle when the engine is cold, but rather comes on as the engine warms up.

A single occurrance of the eratic miss sounds exactly like a single occurance of the regular miss I get when I pull a spark plug wire off while the engine is idling. So, in the case of the eratic miss, somehow the fuel charge is not detonating occasionally.

I am suspecting something in the ignition. I'll exhaust that possibility before I change the manifold-to-head gasket. The vacuum seems fine.... at least the windshield wipers are nice and peppy. I made a tool for testing the valve sealing in the head... perhaps I can modify it to measure the vacuum from the manifold vacuum port.

Lee Prairie

The most common cause for a vacuum leak at the manifold to head connection. is an intake manifold ring out of place.....tightening will not help. Squirt oil at this location with the engine running. and missing. The oil will stop the miss and cause exhaust smoke if there is a leak.
When its missing at idle pull off one spark plug wire at a time. When you get to the one that dosen't cause the miss to get worse you found the missing cylinder.

Thanks, Chev Nut - I too had the idea to pull each spark plug wire off one at a time to see which one didn't make the eratic miss worse. They all made the miss worse. Thanks for the tip about the oil around the intake ports. I'll do that.

One of the first things I did was replace all the components in the ignition system except the distributor proper. I remember being surprised that the distributor cap has a hole drilled in it (I got it new that way) underneath the place where the posterior clip goes. None of my tractor distributor caps have that. The posts in the distributor cap are all burned and some oil had gotten in there. I'll replace the cap and the rotor and clean the distributor.

Is the dist. cap supposed to have a hole drilled in it? Never seen that before.

Lee Prairie

Some had the hole.

I posted this before... this morning. But somehow it didn't stick.

The vacuum measures 18 inches mercury. So, no vacuum leak. I replaced the coil, plugs, plug wires, distributor cap, rotor, points and condensor. Nothing. Still have the eratic miss.

I can make the miss go away on fast idle with the choke 1/2 way out by turning the distributor clockwise.

When Sandra gave me the truck for my birthday, the vacuum hose to the wipers was bad and split at the manifold port. Could it be that the previous owner timed the engine to this poor condition and now that it is corrected, the timing needs to be reset?

Regardless, I'll check the timing next.

Lee Prairie

Wouldn't the timing have been reset when the engine was overhauled this past winter???

Sounds like an intake manifold vacuum leak, check the intake area with propane.

I didn't touch the timing when I overhauled the engine. I did the head, bearings and rings. And I did it with the engine in place ... the distributor, cam, crankshaft, et cetera were never removed. The timing is the same as it was when I got it.

I don't believe there is a vacuum leak. First, the vacuum tests at 18 inches of mercury at low idle, which is what it is supposed to be. Secondly, I did the oil test that Chev Nut suggested with a negative result.

Lee

The fact that the vacuum is 18" of mercury is not proof that you don't have a small vacuum leak. Plug the port where the wiper hose attaches to the manifold temporairly. I had the small line to the vacuum advance only finger tight and it caused a eratic miss on a 235 engine, also the diaphram in the vacuum advance could be leaking enough to cause a miss. How about the idle circuit in the carb? maybe it isn't right. Have you adjusted the idle jet? If it doesen't miss with the choke out maybe you have too lean of a mixture on idle. Pour a can of Seafoam in the fuel tank and be sure that the tank is full of fresh gasoline. Do you have a inline fuel filter in the line going to the carb?

MrMack What is SEAFOAM I have not heard of it before now.

Even though you never adjusted the timing when you went through the engine, check the timing to verify that it is correct so that you can eliminate that as a possible problem.

if pulling the choke out makes the miss go away it tells me you have a lean situation. either by an intake leak and/or carb problem

Dads31 thank you for the info.

18" of vacuum does not mean there is no vacuum leak.

I believe you may rule out the carb, because it did it before the rebuild, as well as after the rebuild.

I suggested a propane test, you made mention of performing the oil test.

If it is not a vac.leak by the intake, run the propane by the base of the carb, to see if the insulator is cracked, as suggested by Gene.

These engines have been around for a loooooooong time, the process of elimination goes fast.

If pulling out the choke smooths out the engine there is a lean condition. If no vacuum leak is found I would suspect that something is screwed up in the carburetor.

Andy, Seafoam was originally used to treat the fuel in marine engines. I use it in the gasoline in cars trucks, mowers boats, tractors and anything that has to sit for weeks or months without being run, it works as a solvent. much like B-12 carb cleaner but not as harsh of a solvent. I get mine at Tractor Supply or Walmart for around $7.50 a can it is around $10 at NAPA.

Thanks MrMack I will check into it at TSC. I use STABLE now when storing the equipment for the winter.

Propane torch test is a propane torch with NO Flame turned on and moved around the manifold or the base of the carb, idle speed will change if there is a leak.

I used carb and choke cleaner for an additional vacuum check ... Sprayed it around the manifold and carb joints. The test produced a negative result.

All six cylinders test at 75 psi when I put the compression guage on each cylinder (take out all the spark plugs, screw in the compression guage to each spark plug hole separately, and hit the starter for a few revolutions). All cylinders test the same and 75 psi.

I have no thermostat in it presentlhy and I notice a tiny bit of bubbling in the radiator visible when the radiator cap is off. Can the water pump be causing that from turbulance since the thermostat is gone? I'm now concerned that this eratic miss problem might be a leaking head gasket, but, as I mentioned, all the cylinders test the same compression-wise and I can make the miss go away on high idle by turning the distributor clockwise.

If the timing is OK, I'll re-torque the head bolts. If that doesn't solve the problem, then I guess I'll pull the head again and re-install it with a new head gasked. I would like to avoid that!

Lee Prairie