Running on 7 &1/2

[Mickd]

My 8ba is idling just a tiny bit rough lately and feels like it's only running on 7 & 1/2 cylinders.
When I short out the plugs individually at idle, number 3 makes no difference. There's plenty of spark at the screwdriver.
Swapped the plug - no difference.
Did a compression test on all cylinders & number 3 is as good as any of the others and better than some (110 lbs on standard heads).
So spark and compression are ok.
There is no sign of moisture on the plug or looking in through the plug hole to the valves.
Still, I can't help thinking that maybe there's a bit of water seeping in to number 3 chamber. Dunno what else it could be.
I'll take it for a run to get it hot & retorque the heads to see if that helps. If it's only a weeping, I might try some "Stop Leak" or "Chemiweld". Hopefully that might help. I replaced the heads a couple of months back and haven't done a lot of miles since. (I've had them re faced beforehand). I've re torqued only once since, after getting it hot first. Used studs, a couple of which weren't inteference fit so there may be a bit of water coming up the thread on one of them - I did use thread sealer.
Copper/composite gaskets and I coated the gaskets with coppercote.

Any thoughts fellas?

Mick

[deuce_roadster]

Do you suspect a coolant leak because you have to keep adding? Does the tailpipe continue to be pumping moisture out after warmed up and hot? I hope your oil doesn't look like a milkshake. What else it could be is something bad.
I use Best composite gaskets and never have had that kind of problem.
I am hoping for the best for you.

[Mickd]

Quick reply there Deuce! Thanks mate.
The coolant level hasn't gone down. That's why I think if it's a leak, then it's a very slight one. My thinking is, because I'm testing for spark at idle, then there's not much fuel in the compression chamber so maybe if there's moisture in there as well It's more likely to be affected at idle? Maybe? The oil isn't milky, and yep, I used Best Gaskets.

It's winter here in Tasmania at the moment and pretty cold, so I expect to see a bit of condensation moisture at the tailpipes when I fire it up - and I do. But I don't notice it there after I go for a run.

[Mickd]

I've just realised that my little problem's making a mockery of my "signature" statement! Haha!

[aussie merc]

could be a dud plug swap the plug with another and see if it follows the plug and definitely retorque the heads a couple more times

[aussie merc]

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sorry just re read the post check the leads

[solidaxle]

A leak down test maybe in order.

[oldbugger]

Intake leak? maybe more air less gas at that cylinder

[Gene1949]

Originally Posted by oldbugger

Intake leak? maybe more air less gas at that cylinder

I'm chasing the very same 7 and 1/2 cylinder issue. I'm thinking that I also have a slight intake leak around that port.

[Flathead Fever]

Take a propane bottle and run it around the base of the intake and see if the idle straightens out. It is also possible for an engine to have an intake leak on the valley side of the gasket. Put a piece of paper over the oil fill tube and see if it sucks it down. I had a Dodge 5.9L at work do that. I blocked the PCV valve and put propane into the crankcase and the idle straightened out so I knew I had a leaking intake gasket inside the engine (that happened once in 30-years). You can have a cam lobe going flat, You can have a bad valve spring. You can get carbon buildup on an intake valve so that and it can barely flow. Just for fun put a vacuum gauge on it and see how steady the vacuum is.

Do not pull wires like you did if it has electronic ignition, use a test plug to check spark. If you hold the wire further than the spark can jump it will look for another way back to ground through the electronic ignition components. I went to all kinds of schools for electronic ignition back in the day. First thing they tell you is, do not pull a plug wire off a running engine unless the plug wire is grounded or hooked to a test plug. You won't immediately know you have damaged the ignition module, but it could burn a microscopic hole in a transistor that will gradually degrade over time. When it does fail you won't make the connection that the plug wire you yankd off a year ago caused the problem. Never unplug any modern car electronics with them powered up.

[Mickd]

Some good thoughts there guys.
Flathead Fever, you might be on the money with the flat cam lobe.
I removed the intake manifold and checked for valve lift though. But a couple of times in the past I've pulled the intake off and had to readjust number 3 exhaust lift because of a loose adjustable lifter. When I had the heads off, I removed that offending lifter and shaved a washer to lock it between the lifter and the adjusting nut so that it wouldn't screw itself back in again. Those adjustable lifters don't like being adjusted more than a couple of times. They seem to lose their "interference ".
You've got me thinking now. I guess it is possable that the washer that I used may have collasped because there is some tappet rattle (again!) when I first start it when it's cold, but it quietens down.
I'll probably end up pulling the head off again eventually. I don't want to if I can help it because the head gaskets are about $135 bucks a pair down here in Oz!
I did check for possible air leaks around the intake port when I had the manifold off. And I copper coated the reused intake gasket.

I'll have to get myself a vacuum gauge too I think.

Mick

[flathead4rd]

Don't know what type of plug wires you have but if you have a spare one swap it out on #3 and see if there is any difference.

[Flathead Fever]

Even if it has spark when you pull a wire doesn't mean it has spark in the cylinder. It takes more voltage to jump the plug gap under compression. It could have an intermittent or weak spark that would be hard to notice just holding a plug wire and watching the spark. Always try the simplest thing first like a plug wire before I get you thinking about flat cam lobes or other rare problems. We had mechanics at work that panicked and came up with the most complicated weird diagnosis and ended up replacing parts and wasting time they shouldn't have when it was the simplest thing they should have checked first.

It's hard enough to diagnose some of his stuff when you do have all the equipment like scopes and smog machines that you could look at the exhaust readings. At home it's even harder. I finally bought an old Sun scope. I got used to working with that stuff for 30-years, then I retired, and I didn't have access to it anymore to help diagnose stuff and it sucks.

You did the correct test by doing a "cylinder defeat test" by pulling each plug wire. That pinpoints the weak cylinder, that narrows down the problem down in a hurry. It's not an overall problem but specific to that one cylinder. That is the only way to do it without a scope to see the firing voltage of the cylinders. The scope also has a cylinder defeat test built into it. You can push buttons one through eight and kill the cylinders one at a time and see how they affect the rpm. It's a lot better than getting the crap shocked out of you pulling plug wires. I have been shocked many times by plug wires but one time I pulled a wire at work and it felt like a sledgehammer to my chest. I think it went in one hand, across my chest and out other the hand. The guy in the next stall heard the engine stumble and I must have looked stunned, he looks over at me and said, "you all right". No I am not alright, I didn't feel right for about an hour. Now I "never" have my left hand on the vehicle when I pull a plug wire with the right hand. That is not the path to ground I want to give that coil.

Please do go out and buy a vacuum gauge. They are fun to play with. It will make a better mechanic out of you. It has begun a lost art to use one because it's not some fancy electronic tool. They are still applicable to any modern engine when diagnosing problems. Plus, everybody that works on engines needs to understand vacuum. Your engine is eight air pumps, a vacuum gauge tells you how well each of those air pumps are working. Look up how to use one, what the different needle movements mean. Each cylinder pulls a vacuum and together they make a steady reading on the gauge but if one cylinder is not doing the same work the needle will momentarily drop when it's that cylinders turn to pull the vacuum. They can diagnose a lot of different mechanical problems.

[dean333]

How can it run on 1/2 a cylinder?

[Mickd]

Originally Posted by dean333

How can it run on 1/2 a cylinder?

ummm... not very well !

[Mickd]

Flathead Fever, You raise many valid points.

And yes, I know the feeling that you get when you pull a plug lead. You certainly get to know pretty quick if there's spark there !!
My old man used to tell a yarn about a big fella he knew in the army back in New Guinea during WW11. He could roll up his sleeves, lean over an idling flathead V8, rest his arms across both banks of plugs, shorting them out and stalling the motor. What's that old saying.... "No sense, no feeling"!

Sorry, I digress.... My plug leads are repro solid wire leads without rubber caps on the plug ends, so I was able to simply grab a screwdriver with a thick insulated handle and hold it against the lead at the plug end without unplugging it.

I've got an early "Motors Auto Repair Manual" (used to belong to my dear ol' dad years ago before he passed.) which covers American Automobiles from '35 to '49. It's a good read in itself and has a section on the lost art of diagnostics with a vacuum gauge.

Oh, and Dean333, I hope I didn't offend you with my silly little "Not very well" quip mate. I guess what I was trying to say by my "7 & 1/2 cylinders" comment is that the faulty cylinder may probably still be functioning a bit, but not as well as it should - certainly not at idle anyway.

Cheers

Mick

[Mickd]

I know I'm reopening an old thread here but I fixed my suspected "running on 7 1/2 cylinders" issue a few months back.
I was running on 91 octane petrol. Went to 95 and after a tank or so of the higher octane the old bitch is purring like a kitten. Which surprises me - I thought the old flathead would run fine on the low octane stuff, considering the quality of fuel back in the 50's.

[Terry,OH]

Consider checking the #3 Cyl ignition circuit with an ohmmeter and compare it against a good cylinder this should check the plug wire and the distributor cap to the inside terminal for the rotor, for slightly higher resistance. The plug wires should be solid core wire so near zero ohms same as the distributor cap. Don't use resistor plugs.

[Mickd]

Originally Posted by Terry,OH

Consider checking the #3 Cyl ignition circuit with an ohmmeter and compare it against a good cylinder this should check the plug wire and the distributor cap to the inside terminal for the rotor, for slightly higher resistance. The plug wires should be solid core wire so near zero ohms same as the distributor cap. Don't use resistor plugs.

Yes. Thanks for your input there Terry. I did try that (another lead, another plug. But as I said in the last comment, using 95 octane seems to have resolved my problems.
BTW. I have read on a number of sites that we shouldn't use the higest octane (That's 98 in Australia) because it apparently can cause a gel buildup in carby jets and have even read that it will eat away at steel fuel tanks! Dunno how true the latter is, but I can believe the jelling up of carby jets I guess.
A man probably should do a bit more research there, but in the meantime I'll just run with the 95.
I had blokes say to me ... "91 ! I wouldn't put that stuff in my lawnmower!"

[JayChicago]

Maybe they add some extra additives to the more expensive 95 octane gasoline, which helped clear a sticky valve stem. Maybe they put Marvel Mystery Oil in it! Is MMO available down under?

Then again, maybe the engine just cleared itself from some additional driving.