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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

75 psi compression sounds pretty low Lee, especially if you just recently had the head redone. I’m sure you are aware of the original specs, 110 pounds plus, and all within 5-10 pounds. Mine runs up around 120 to 130 pounds when I put her away hot and wet. That’s probably because I had the head flattened out when I did the valve job and the little bit they took off brought the compression up a tad.

Don’t discount the fuel that we get today, it is definitely different than the fuel that was compounded back when our engines and fuel systems were designed. I’ve had a stumble in mine since the day I brought it home and a little skip at idle.

My 1950, 3600, 216 runs just fine and dandy when the choke is out a little. Right now I’m running a meticulously rebuilt Carter W-1 carburetor and prior to that, the original Rochester ”˜B’ rebuilt the with the same care. The engine performed the same with both carbs. And contrary to my trouble shooting instinct I then dug into the ignition side of the equation. If the compression was up to snuff and it wasn’t carburetion, then it must be ignition! After all new ignition and once again, a meticulous rebuild of the distributor…..the performance was same ”˜O, same ”˜O! And I run my timing on the ball at idle (500 rpm) and there are no vacuum leaks, (I spent a decade as a vacuum tech before retiring from a national laboratory) and the engine vacuum is steady at around 20inHg.

I truly believe that if I re-jetted the carburetion a step or two richer it would perform as it did when new. However at 14 mpg now, with either the Carter or Rochester, I’m afraid that if I go richer to compensate for the alcoholic fuel that we get, my mileage would drop off to 12 or 13 mpg.

And it’s certainly not going to hurt to re-torque your head, and if you haven’t done it since the valve job, I guarantee you that it needs it now. Watch the breather, if it’s puking frothy greenish gray stuff onto your garage floor after you park it, then that is a purty good indication that the head gasket it leaking some. Might not show up in your oil if it’s just a little.
Denny Graham
Sandwich, IL

While the low compression readings will not cause a miss I would suspect there is another problem. Could be the gauge. I have two different gauges of the same brand and they vary by about 20 pounds. Also the cranking speed of the engine is important. If too slow you will get a lower reading.

cranking the engine with the throttle butterfly open or closed will alter readings. the fact it's uniform across the board shows there isn't an indicated problem there

Actually you have not said anything about adjusting the valve lash, or the distributor timing, or that you have checked out the valve timing, the vacuum advance, or whether you did change out the sparkplugs, plug wires, rotor, cap, points & condenser, or the ignition coil. The rebuilding of the carb, did it include cleaning all the circuits in the cast iron base as well as in the pot metal parts of the carb? plugging the vacuum line to the wipers? You may need to take an orderly approach to the problem or problems. Or maybe you are too critical of the engine? You mentioned winding out in low gear and second. Please!..... Don't wind out in low and second, this old long stroke engine only needs to rev up a little in low and second. The torque in high gear is so much more at lower RPMs that there is no need to wind out unless you have a huge load or a steep hill to climb.

He has mentioned most all of the above in previous posts

Thanks, Chevnut - MrMack, I have done essentially all the things you mentioned above. The next gas I get will be regular and put in some lead additive stuff that the auto parts store recommended (but I don't have any trouble using 10% ethanol in my tractors (Allis-Chalmers).

I'll re-torque the head bolts asap. Certainly by this coming weekend.

Thank you all for your comments.

Lee Prairie

Gosh, i am glad that you have felt fit to come here to our humble site. We don'r want to seem to be really aggressive about our ideas and advise. We really want you to enjoy the old 49 the way we enjoy our various models of vintage Chevrolets. I know that you had considered makeing all of these changes but was not sure that you were doing them as you went along.
By the way, You won't need to add any lead additive to your 49's tank unless you have really changed the old six with high compression parts, or you will be hauling very heavyloads or a large trailer at near maximium power output. Leaded gasoline isn't required or needed. The only advantage for the lead additives on the shelves is the profit that is made by it's makers and sellers. But don't take my word alone for that, others here will say the same thing....save your money for tires, batteries Etc. and also to pay the registration fees to attend some of the excellent VCCA tours for our vintage Chevrolets. We have fun and we meet some of the finest folks that VCCA has to offer! I am glad to see that you have some of the best built tractors ever put on a farm Allis Chalmers
Have you ever heard of MyTractorForums.com?
Try it.

Mr Mack hit the nail on the head. Lead substitute additive is nothing but a colossal waste of money.

Mr mack is correct. The 52 has 15,0000 miles on a valve job. Just grinding seats no hardened seats at all. On real long trips I add the recomended amount of Marvel Mystery Oil to the tank as a top lube but that is all. The machinist that did the head advised me NOT to mess with hardened seats on the low compression cool running 216.

A lead additive is completely unnecessary and the last thing you need.
My 1950 has gone over 50,000 miles (most at high speed) and the head has never been off and I don't use additives.

Lead additive is a joke and a total waste of money! Your engine doesn't need it. Lead additive is a gimmick that is sold to naive car dudes just to make a buck.

By the way, how did the timing check come out?

It seems to have been the timing. It is out there purring like a kitten now.

I was dreading the timing, since my experience is with old tractors (Allis-Chalmers) where you time the engine by taking off the inspection plate that covers the flywheel and turn the distributor until the timing marks line up. But it is a lot of 'up-and-down' work because you're on your back on the ground looking at the timing marks in the strobing timing light and you have to get up to access the distributor in most cases.

But, this Chevy can be timed from under the hood. Of course, I read the manual which says to turn the distributor until the ball lines up with the pointer. So I crawled under the truck and found the flywheel cover weep hole and thought... jeepers, I can't see anything through that little hole. Then I looked at the flywheel housing on the distributor side, and once I cleaned away the gunk I saw the viewing hole and pointer.

There is some play in the distributor and the relationship of the flywheel ball to the pointer is very sensitive. I don't know if this is wear or if it is supposed to be that way. But, she's running smoothly now and I'm going for a test (joy) ride now.

Lee Prairie

Glad you finally got it to purrRRRRRRRRRRRRing. We will not stop until you do in getting it to run right. Sometimes it takes a little time to get the gremlins to uncover stuff, put back tools or keep their teensy fingers off those critical parts.

Cool! Glad that you have the engine running correctly. It took us many days and several postings to finally convince you to check the timing. You always start with the easy stuff first and the timing is usually the first thing to check.

I thank everyone who has and is contributing to this discussion and the board in general.

I'm the "Allis-Chalmers guy" that everyone comes to (usually just to shoot the breeze) in regards to their tractors. This is the first Chevy truck for me... I "restored" a 1946 Ford Maxim firetruck, but it only had 3000 miles on it when I got it and so it was essentially a new vehicle. 60 years of passing time had taken its toll, but it didn't need much other than the usual brakes, fuel pump and ignition system.

The Chevy 4400 was (and still is) a working girl with 64,000 miles on her. She lived her life before me on a corn-soybean farm in eastern Nebraska. She only has about 100 miles on the engine overhaul. I would like to get a '48 to '52 half-ton to restore for Sandra. She likes the old Chevy trucks as much as I do. A salvage yard about 2 miles from me just went out of business - they cut up and crushed 15 or so Chevy trucks in that range of years. 5 of them were like mine. I was going to get a few of them, but was too late. Man, I hate to see that happen. History is vanishing before our eyes.

Lee Prairie

But, Dog, I didn't touch the timing. It was running like a little sewing machine when I first got it except that it smoked, fouled the plugs instantaneously and developed a stuck valve (causing a regular miss on cylinder 3). So, I only slowly started suspecting the timing (since the timing seemed to be great before I touched it).

But, when I got it the vacuum hose to the wipers was old, brittle and not connected. My theory is that the previous owner set the timing so that it would run under those conditions. But when I got the wiper motor going and replaced the hose, the timing would have been off for those running conditions. That is the only thing I can think of that would explain everything.

Many thanks,

Lee Prairie

"It is always good to have a planned event come out according to plan, and sometimes I am surprised to have a plan work out."

A hint: I have several old cars with 6 volt systems. I have a very modern timing light that I use on everything with a distributor. I use a 12 volt booster battery to power the timing light. It works lots better than the old timing light that fired the light with the sparkplug, It was so dim I had to close the doors and pull the shades to see the strobe flash. The new light working on the 12 volt booster battery is really bright.

It really doesn't matter if you didn't touch the timing because you need to have a place to start when diagnosing a problem. You were only assuming that the timing was okay, but without checking it you really didn't know, and with some of the changes done to the engine no doubt the timing was affected in the interim. Even when the points are adjusted the timing will change. When working with the type of problems you encountered, don't take anything for granted. Start at the beginning and check everything as you go, and the timing is an excellent place to start.

Again, glad that everything finally worked out for you. Cool!