Low compression

[Tony Wheeler]

Yes I noticed that too Lynn, seems rather a coincidence. The mathematical probability is: 4/8 x 3/7 x 2/6 x 1/5 = 1/70. In other words, in a motor with 4 deficient pots, the odds of them all being centre pots is 1 in 70. It's quite suspicious but I can't think what it might indicate, unless perhaps BOTH head gaskets are leaking between adjacent pots, which is quite some distance in this case.

Of course, the effect of this 1 in 70 chance is that all four weak pots fire consecutively, followed by all four strong pots, followed by all four weak pots, etc. In terms of crankshaft acceleration / deceleration it's the worst possible combination.

[Malcolm Towrie]

Originally Posted by Tony Wheeler

Of course, the effect of this 1 in 70 chance is that all four weak pots fire consecutively, followed by all four strong pots, followed by all four weak pots, etc. In terms of crankshaft acceleration / deceleration it's the worst possible combination.

Good point! Further evidence of the massive moment of inertia of the rotating assemblies in these older engines which allows acceptable running even with such large differences between cylinders. I came across a Ferret last year that had the ignition timing set 90 degrees BTDC. Yet it started and ran. The only way I figured that could happen is the rotating assembly was damn near unstoppable once it got moving so it just drove through the period when combustion was trying to reverse its direction.

I wondered if this engine had ran very rich in its past, choke stuck closed-type rich, with liquid fuel trickling along the floors of the dual plane intake manifold and preferentially dropping down into the first holes it came to, the cylinders nearest the carb. But I suspect the gas wash would still have affected the corner cylinders to some extent, and my wet/dry compression testing didn't support bore/ring wear, so that scenario seems unlikely.

[Lynn Eades]

Malcolm, I assume that the MkI* dizzy is the pre 42 divers helmet type.
If you had a crab dizzy it would be the leads that come off the cap from 11.00 o'clock around to 5.00 o'clock.
Another coincidence is that the compressions on the low cylinders are the same on each bank 30,30, and on the other bank 50 and 50. Very unusual.
Calculate that one Tony.

The design of the manifold would let the petrol do that if the engine sat level in the carrier.
What was the situation with the fuel pump (i know you had another thread about it, but wasn't really following it)
These engines don't bend valves, blokes trying to remove valves can bend them.
If your pump was over riding the needle valve I guess it might have washed oil from the valve stems,while it was trying to stay running. (before it was parked up)
As Tony said, It probably would have fixed its self while running.(this time around)
Does this engine run the original carb, or is it running a 94?
If the choke was on, it probably would have flooded, and stalled.

I think the answer is to run it up, get it hot, re do the compression test, dry, then wet. Open the throttle once, and lock it open for the duration of the test. write down what you get, and if it is still the same as before, lift that inlet manifold, and check the clearances.
Then you'll know what has to be done.

[Tony Wheeler]

Originally Posted by Lynn Eades

Another coincidence is that the compressions on the low cylinders are the same on each bank 30,30, and on the other bank 50 and 50. Very unusual.

Another very astute observation on your part Lynn, which escaped me completely! Strangely it seems to point even more strongly to head gaskets as mentioned: "perhaps BOTH head gaskets are leaking between adjacent pots..."

I don't know the likelihood of that, as it's quite some distance and would normally involve water jackets, but it would certainly explain equal compression in adjacent pots, because they're both leaking through the same passage (in the opposite direction).

Of course, even it were possible, there would need to be some explanation for it occurring on BOTH sides. I wonder if it could occur in a rebuilt motor if the heads weren't torqued (or re-torqued) properly. Might be worth warming it up and re-torquing heads before next compression test, taking special note of the central stud (which is first in the tightening sequence anyway).

Again, it's a longshot (extreme longshot...?) but very easy to do and always worthwhile anyway.

[Keith Webb]

Just came across this Ford film featuring valves. Won't necessarily fix your problems but may at least be a distraction! https://www.youtube.com/watch?v=-x9HIZuTmsE

[Malcolm Towrie]

Originally Posted by Lynn Eades

Malcolm, I assume that the MkI* dizzy is the pre 42 divers helmet type.

What was the situation with the fuel pump (i know you had another thread about it, but wasn't really following it)
If your pump was over riding the needle valve I guess it might have washed oil from the valve stems,while it was trying to stay running. (before it was parked up)
Does this engine run the original carb, or is it running a 94?
If the choke was on, it probably would have flooded, and stalled.

I volunteer at a military museum in Oshawa, Ontario and there are always lots of things needing fixed so my focus has swung away from this carrier for a while. However, I will check the torque on the head nuts cos that's easy to do.

To answer your questions, yes, it has the divers helmet dizzy.

The fuel pump wasn't pulling up fuel from the tank when I tried to start it to do the testing. I had to rig up a temporary supply.

Re carb, I can't remember. The owner buys restored vehicles from the UK, so it might be a British carrier, so a Solex? I'll check.

I meant choke stuck partly closed rather than fully closed. It was quite common with the automatic chokes on cars over here. I've seen it cause severe wear right at the top of the bores, where the compression ring needs all the help it can get.

Malcolm

[Malcolm Towrie]

I checked the head torques tonight. With the wrench set to 55 ft-lb, I was able to get some movement on maybe 1/3rd of the nuts. But hot compression test results were exactly the same.

I also did a wet test on the bad cylinders and saw very little difference, a few psi higher. In fact I was surprised by how small the difference was. I squirted 4 good shots of oil towards the back of the cylinder, so I think that should have got to the rings?

After the tests, just out of curiosity, I started the engine again to check timing with a timing light since I had marked TDC on the crank pulley earlier. The light showed only occasional spark on #1, occasional no-spark periods on #2, and steady spark on 3, 4, 5, and 6. The engine ran out of gas before I got to 7 and 8. I got some more gas, and it wouldn't start hot. (This is the first time I have tried to start it hot.)

I'm beginning to think this engine doesn't run as well as reported.

Malcolm

PS: the carrier has a Zenith carb with a governor.

[Phil Waterman]

Hi Malcolm

What RPM was the engine running during your spark test? What type of timing light pickup? Induction or direct connection to spark plug?

Running engines in on the test stand I have seen the same apparent intermittent spark using an induction pick up. Seems to smooth out at around 1000 RPM, never have tracked down the actual cause of the intermittent nature of indicated spark. Wonder if it is just slightly lower voltage at idle.

Because of the phantom problems with stored vehicles, I try not to get to excited about rough running, hints of missing, etc. generally I wait till I've run through a tank full fresh gas before doing tune-ups etc.

Please keep us posted on your mechanical detective efforts.

Cheers Phil

[Malcolm Towrie]

Originally Posted by Phil Waterman

Hi Malcolm

What RPM was the engine running during your spark test? What type of timing light pickup? Induction or direct connection to spark plug?

Running engines in on the test stand I have seen the same apparent intermittent spark using an induction pick up. Seems to smooth out at around 1000 RPM, never have tracked down the actual cause of the intermittent nature of indicated spark. Wonder if it is just slightly lower voltage at idle.

Because of the phantom problems with stored vehicles, I try not to get to excited about rough running, hints of missing, etc. generally I wait till I've run through a tank full fresh gas before doing tune-ups etc.

Please keep us posted on your mechanical detective efforts.

Cheers Phil

Phil,
It was an inductive light. Engine wasn't idling because I can't get it to idle down (I forgot to mention that), so throttle screw was holding rpm over 1000. I was planning to try another light last night for confirmation but that's when I couldn't get it to start. I'll try it today. It starts OK cold.

Malcolm

[Tony Wheeler]

Originally Posted by Malcolm Towrie

I also did a wet test on the bad cylinders and saw very little difference, a few psi higher. In fact I was surprised by how small the difference was.

That's great news Malcolm. It means all pistons/rings/bores are in excellent condition. Once you've fixed the leaky valves (or head gaskets) you can expect at least 95 psi all round, which would point to the motor having been rebuilt. If that's the case you'd expect the valves to have been reground already, so it may be something simple like head gaskets or valve clearances. Fingers crossed! You'll know more when you pull the intake manifold off.

[Malcolm Towrie]

Originally Posted by Tony Wheeler

That's great news Malcolm. It means all pistons/rings/bores are in excellent condition. Once you've fixed the leaky valves (or head gaskets) you can expect at least 95 psi all round, which would point to the motor having been rebuilt. If that's the case you'd expect the valves to have been reground already, so it may be something simple like head gaskets or valve clearances. Fingers crossed! You'll know more when you pull the intake manifold off.

What are the chances this engine has adjustable lifters? Is that a "mandatory" thing to install on a rebuild?

Malcolm