So I'll give you the end of the story first. The 360 in my '73 seems to be finally giving up the ghost. It isn't firing well on the number 2 and 5 cylinders. Vacuum gauge bounces all over the damn place. With the engine off, compression is around 120 on both cylinders, while it's running, it's around 20

So I wanted to run the sequence of events that led me here by you guys, because I want to see if it makes sense.

Rewind 3 months ago. The fast-idle cam arm on my Motorcraft 2bbl, was broken and I figured while I had it off repairing it, I'd just rebuild it. Well I put the rebuilt carb back on and the truck didn't run right. It was 90%, but you could hear a sucking under heavy load, and it didn't idle well. Vacuum leak that I couldn't find. So I bought a professionally rebuilt one off eBay for $200. It was from a '72 truck, and the PCV didn't hook up. It was also bigger CFM. Put that carb on, and the truck ran even worse. So I took it to a guy I know who does quality work. Limped the truck over there with the new carb, and when I got there it was dripping fuel from the fuel filter on to the manifold, but still running and idling properly on all 8.

Two days later he calls me to tell me that 2 cylinders are dead, probably valves, and that he adjusted points, timing, and carburetor and got it running about as good as it can.

So I'm trying to come up with potential scenarios of how this could've happened. First off it's a 40 year old original engine that's been used consistently for 40 years. Maybe the carbs were too lean and I burned up some valves?? It ran rich for the past year, but I'm having a hard time putting it all together in my head. Plugs, wires, coil, cap, rotor are all new and have been verified. Both cylinders getting spark.

So I'm in the market for a pre-emissions 390 to replace my old 360, but I wanted to try and make sense of what's happened so I don't F something else up.

Thanks in advance,

-MK

 

If it were a Chevy, I'd say you flattened out a couple cam lobes. If you had a dial indicator, you could pull the valve covers off and see if the cylinders in question had full lift compared to the other cylinders.

A bigger carburetor wouldn't make it lean. If your carb was REALLY lean, it would take a long time to burn up a valve. A customer brought in a '69 Camaro that was SUPER lean, and it was like that for YEARS. That engine didn't burn a valve though.

I think you need to investigate a little more and see what's going on.

 

That's really baking my noodle. What could an honest guy who's been working on carbureted vehicles for 35 years possibly do to make an old engine stop firing on two cylinders. He said all he did was adjust the points, and it ran like that. It 100% ran on all 8 when I dropped it off to him. The carb I had on there wasn't tuned right and wouldn't run well much past 1/2 throttle, but it idled well and ran fine.

I honestly have no idea what is happening/could have happened.

-MK

 

If you have compression when it's off. That sounds like a valve train issue.
My other idea is it may have over heated and sucked the rings on those cylinders.

 

Definitely didn't overheat. Cooling system was replaced earlier this year, and it runs great. Nice and cool.

I'm thinking valve train. Broken springs? The only thing that's really getting me is that it's 2 cylinders, one on either side of the block and it happened at the same time. Probablility alone on something like that happening is low...

-MK

 

Maybe time to have a more serious talk with your mechanic? But.....this may have happened by itself and not be the mechanics fault...hard to say.

One day I was driving my 79 F150 and it was running perfectly like always...heard a noise then it started missing....I drove it home and removed the valve covers and found a broke valve spring on the passenger side. Replaced it later that day and I was off and driving again.

 

Something is bugging me about this statement:

That just doesn't quite add up. If the compression guage is reading 120 while cranking with the starter, it should read higher when the engine is actually running. The starter doesn't turn the engine over nearly as fast as it is spinning at idle. The faster the engine is spinning, the less time there is for the pressure to "escape" past rings or valves that aren't sealing.
Your results are totally backwards from what would be normally expected. weird.

Now, you say it's #2 & #5 that aren't firing right. These are fed by the same side of the carb on any dual plane intake. I'd strongly suspect a vacuum leak at the base of the carb, or uncapped port on the carb that has been over looked.
Many motorcraft 2bbl carbs have a vacuum port that is pretty easy to overlook, near the idle mixture screw, below the accel pump. This port is very easy to overlook, as it is very well hidden once the carb is bolted down on the intake. I spent at least 3 minutes looking for it, and I knew approx where it was...... It's that well hidden from view.

 

And that's exactly why I posted it here. It just doesn't make sense to me either. What I don't understand is how it ran on all 8 with one carb on there, started missing on #2,5 with the same carb installed, and now still runs on only 6 cylinders with the original carb back on there. I think I found that vacuum port you're talking about, and it appears to be plugged. I've replaced the carb gasket as well as the one between the spacer and manifold itself.

Again, I have a hard time swallowing that 2 cylinders went down at the same time...

-MK

 

At this point, I would do 2 things. 1st, check the points for condition and proper adjustment. It sounds like the dropped cyls started right about the same time the mechanic messed with the points.
2nd, get a can of spray carb cleaner, and go on a hunt for vacuum leaks. If there is a leak somewhere, and this stuff is sprayed near it, it will get sucked in and the engine will respond rather quickly.

If those fail to turn up anything, I guess it could be possible that a rag or paper towel (ect) could have gotten into the intake runner that feeds 2 & 5, restricting the flow. This *could* explain the oddball compression readings.......
Pulling the carb and having a good look around inside the runner could shed some light on the situation.

 

Is the stock intake manifold on these trucks dual plane??

Thanks for all the responses. You guys have affirmed my gut feeling and I'm not sold that two cylinders died at the same time. I am intent on some further investigation before writing the old lump off.

-MK

 

On a carbed V8 yes. The I6 was single plane but it also had a single bbl carb.

 

I would check points plugs and wires. if all that is good and still running bad redo the compression check. with those numbers on the compression check I would redo it even if i was the one that got them.

 

I'm not at all surprised in 120 when off and 20 when running, those two cylinders are still air pumps and with throttle closed to idle, they're sucking hard.

You don't do a compression test when the engine is running because you're doing it with the carb NOT wide open.

You do compression tests with engine ignition off, all 8 plugs pulled out, turning it over with a good battery, with throttle plates held wide open and you do all in turn and compare readings. That way the "air pump" can pump it's maximum. If within 10% or so and all readings are above a minimum threashold, the motor passes.

You can use a remote starter button aligator clipped on the starter solinoid on the fender across the big wire from battery and smaller wire from ign sw "s". Some do it simply with a wrench or pliars across the two terminals instead. Safest way is have a helper turn it over with the ign switch but do unplug electronic ign boxes or Dura Spark first or unhook + wire to coil first.

If some are low or if all are way low, you put a teaspoon of light oil in each as you retest .... if readings improve you have leaking piston rings.
If no improvement, a valve of cyl head gasket is leaking.

You might also remove valve covers and with a start button and ign off, turn engine over and watch all 16 rockers for full travel. Any that aren't moving as much bear closer inspection, could be adjustment, broke rocker, bent push rod, bad lifter, or wiped cam lobe.

 

The only thing that makes sense with what he is describing would be for the intake valves not to be opening. With the engine running during a compression test the exhaust flow would keep it from sucking air in during the intake cycle.

 

Even as the intake valves do open, the throttle valve at the base of the carburetor is still nearly shut. Just the very slight crack of the plates to allow idle.

He never said he tested all 8 and that the other 6 were higher, he just talks about testing 2. I doubt any of them would test much higher with the engine running at idle. That's why a compression test isn't done that way.

If he had opened the throttle a little, that 20 would have gone to 40 and if he had had someone jerk it wide open, it would have shot way up there. To do a compression test you have to have something in there to compress and an engine at idle with 20 inches of idle vacum ain't letting it in.