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Trouble shooting flathead V8

 
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Old 04-18-2019, 07:50 PM
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Trouble shooting flathead V8

I have done a few tests to determine why my truck is running a little rough. Stockish 1950 flathead V8, Red's headers, Mallory ignition, new NGK spark plugs under 100 miles, and electric fuel pump with regulator. The temp stays at 180, no back fire, and oil pressure is good as measured by a new VDO gauge. Appears the issue(s) are worse or limited to certain cylinders, so am I looking at valve issues rather than carburetor which would affect all cylinders? Are the rings ok if the compression is roughly within 16% of each other?
Compression test:
1-100
2-110 Spark plug Carbon fouled
3-100
4-100
5-120
6-110 Spark plug Carbon fouled
7-105
8-120 Spark plug Carbon fouled
Vacuum tes performed on the manifold outlet:
jumped between 5-15 psi erratic
 
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Old 04-18-2019, 08:26 PM
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What is the history on the motor? Is this freshly rebuilt, with new rings, or is it an engine you're trying to bring up to snuff?

Those numbers are pretty good for compression. You have good compression in the cylinders that are fouled. Are you sure you have the firing order correct? Check for spark at each plug, could be bad wires. You could have valves sticking but usually you'd get a backfire or popping thru the exhaust, and your compression numbers wouldn't be so good.

Is the Mallory a dual point or electronic?
 
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Old 04-18-2019, 08:32 PM
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I'm not sure of the history, PO said it was rebuilt but it sat outside in NC for 9 years after that was performed. I'm pretty confident in the firing order as a shop did the distributor upgrade, but I will double check. The vacuum gauge reading struck me as surprising, 5-15 and rapid fluctuation.
 
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Old 04-18-2019, 08:35 PM
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I tested for spark and used a gauge that shows spark going to the plug. This is the second set of plugs with the same carbon fouling at the same cylinders.
 
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Old 04-18-2019, 10:03 PM
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If you turn in the mixture screws at idle, does it reduce RPM as you get near the bottom? If there is no effect, the carb's power valve may be failed, dumping gas in there.

You might try adding Marvel Mystery Oil or SeaFoam to the gas, to hgelp lubricate the valve stems, they could be hanging up a little. Some vigorous driving may clean up the valve seats and re-seat the rings.
 
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Old 04-19-2019, 07:28 AM
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Vacuum should be steady, I'll bet you have valve leakage problems. Maybe just stuck...?
 
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Old 04-19-2019, 07:38 AM
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Originally Posted by ALBUQ F-1 View Post
If you turn in the mixture screws at idle, does it reduce RPM as you get near the bottom? If there is no effect, the carb's power valve may be failed, dumping gas in there.

You might try adding Marvel Mystery Oil or SeaFoam to the gas, to hgelp lubricate the valve stems, they could be hanging up a little. Some vigorous driving may clean up the valve seats and re-seat the rings.
Like we used to say in the old days, "Get on it to clean the carbon out."
 
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Old 05-07-2019, 06:03 PM
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UPDATE: Put Marvel Mystery oil in the gas tank, and in the oil (full, not over full). Ran it like it belonged to my ex for about 30 miles. Smoked, and started to run rough, especially at idle. Manifold Vacuum still jumped erratic from 5-15. Drivers side seemed to run rougher, and it takes twice as long to heat up (180) as the passenger side. Pulled the intake, and driver side head. I bumped the starter with a remote and watched the valves open and close, seemed normal. This is a real mystery? One correction I have a MSD distributor (not Mallory as noted earlier).

Driver side head

Driver side block

EAB heads
 
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Old 05-07-2019, 06:05 PM
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And I did turn the mixture screws at idle, and it reduce the rpms as they got close to the bottom.
 
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Old 05-07-2019, 06:21 PM
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Like Ross suggested, with those compression readings, that engine should run fairly well. Did you follow each spark plug wire from the distributor to the plugs and for being correct positions? And, how old are the wires and the distributor cap? How much fuel pressure do you have dialed in to the carb?
 
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Old 05-07-2019, 07:55 PM
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Yes, I followed and labeled each spark plug wire, and determined that it is the correct firing order. The cap and rotor are less than 1,000 miles (when the distributor was installed). I checked each plug wire for spark, all good. Fuel pressure is 4-5 as measured on the fuel pressure regulator gauge. I have an electric fuel pump installed. The performance has been going down, and the idle rougher for about 100 miles.

I am thinking that the issue is at a cylinder because of the vacuum reading, and 2, 6, and 8 having high carbon build up. I also noticed that the push rod for the unused manual fuel pump is still in position. From reading older posts I believe I can just leave the push rod out on the 48-53 engines.
 
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Old 05-07-2019, 08:15 PM
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Did you clean the head on the chamber for #8, or is that just a trick of the camera that it's shiny and clean? The exhaust valve for #8 is also dark, indicating it is not seeing high-temperature flow. A clean chamber frequently indicates a water leak into the cylinder, any sign of that?

Yes, you can leave the FP pushrod out, it won't hurt anything. It will rattle around and hit the underside of the intake if left in.

Did you re-curve the MSD mech advance per their instructions? Are you using the vacuum can that came with it? They may have changed since I bought mine, but mine came with a can for a Chevy 350, and it gives HUGE advance.

What kind of FP regulator do you have? If you are running the stock Holley carb, 4 - 5 psi is too high, need to knock it down to 2 psi or so.
 
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Old 05-07-2019, 08:28 PM
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I did not clean the chamber for #8 when I took that picture, I have since cleaned it. There is no evidence of water, and the spark plug is carbon fouled. I would think in a water leak situation the spark plug would be clean also. The block looks good, and the gasket looks good.

I had a mechanic install the MSD and it ran great. I think the vacuum fluxuation maybe affecting the advance currently. I took the reading at the manifold port that feeds the distributor. The erratic vacuum pressure with good compression really has me stumped. I was almost hoping to find a valve issue, just to determine the problem.

I have a Speedway fuel pressure regulator that feeds an Edelbrock 1151 carb. I can back the pressure down.

Also I noticed that the pistons, after cleaning are .060
 
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Old 05-07-2019, 09:15 PM
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It's still possible that valve(s) or lifter(s) are sticking, maybe not every time they cycle, or are being held open with carbon that is popping off the surfaces. What are your valve clearances? Soak the lifters and guides with Kroil or something that will dissolve hardened varnish.
 
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Old 05-08-2019, 01:09 AM
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Not that it would give you the symptoms you've described, but it almost looks like there's a crack from the valve port to the cylinder on #6 exhaust. Did you notice anything there while cleaning? - Bob

 

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