Low compression

[Tony Wheeler]

Originally Posted by Malcolm Towrie

Your post and its annoyed tone puzzle me, Tony.

I think you've misread me Malcolm. It happens sometimes in this kind of forum. It's probably my lame attempts at humour. I assure you I'm not annoyed!

Originally Posted by Malcolm Towrie

You say the motor is running well, yet it has 2 cylinders with 30 psi compression.

Yes, that's what you reported Malcolm: "The carrier was running OK last summer and fall in parades and displays. It started and ran OK just before I did the compression test two days ago, even with horrible compression on 4 of 8 cylinders."

It's not uncommon to find minimal compression in several pots on these old flatheads. They'll still run happily for many years to come, and if restricted to parades they'll probably outlive the owner. Even in hard use they remain indestructible - my F15A road truck shows 40-70 psi in all pots and rattles alarmingly when cold, but after warm up I never hesitate to wring every last screaming rpm out of it through the gears. I'm sure Jacques if he's reading this will attest to its performance up steep mountain tracks - albeit at 4000 rpm! In practice, apart from a thirst for oil, it's a perfectly serviceable motor, and I enjoy not having to nurse it like a $5000 rebuild.

Of course, being accustomed to modern motors, it can be quite disconcerting to see readings of 30 psi, and the natural response is an overwhelming urge to pull the heads off and investigate. Perhaps that's why most owners avoid performing compression tests! However, we need to remind ourselves that compression tests are purely diagnostic, and pots showing 30-50 psi at cranking speed will still produce plenty of power.

Originally Posted by Malcolm Towrie

Yes, it starts and drives, but it is not a well-running motor IMO.

This seems at odds with your earlier report Malcolm: "The carrier was running OK last summer"....."but it is not a well-running motor IMO." I guess it comes down to one's definition of "motor running well". To my mind, if a motor is firing on all cylinders, and drives the vehicle satisfactorily, it's "running well". It may be down on power, but that's a different question. That's generally tolerable on MVs, thanks to plenty of gears and no need to keep up with traffic. Indeed, lack of power is arguably essential to the MV driving experience!

Of course, I fully understand your earlier point: "the owner likes his equipment to be in good condition". Particularly when: "This carrier is in very nice, restored condition and the engine was supposed to have been rebuilt." I too would be keen to address low compression issue in such a vehicle. I was merely urging forethought and proper diagnosis.

Originally Posted by Malcolm Towrie

the tapping the valves idea was suggested and it was easy to do, so why not?

I suspect there was some misunderstanding / miscommunication here Malcolm. It wasn't immediately clear from your initial post that YOUR motor was already running, and in Ron's discussion of stuck valves it wasn't immediately clear that HIS motor was not running! Again, it's inevitable with this kind of information exchange. As you say, no big deal in this case.

Originally Posted by Malcolm Towrie

So as a newbie to these engines, with the heads so temptingly accessible, it seemed a simple job to pull both the heads and intake manifold and check for ALL the various top-end related issues that could be causing this, AND fix them in the time we had available. All in one fell swoop.

Yes, it's always tempting to whip off those heads, but in practice it achieves very little. Unlike OHV heads you can't send them away for a valve job! Unfortunately the "quick fix" you were hoping for does not exist on flatheads, because the "top end" is actually in the block.

Originally Posted by Malcolm Towrie

Of course, now I know it isn't that simple to pull the heads, nor is it that simple to do a valve job on these engines, so we'll just run the old girl as-is until there's a space in her schedule to do the work.

I think that's basically what we were advocating Malcolm, as the time frame seemed unrealistic, and the diagnosis unconfirmed. As Lynn says: "if the engine has become tired you might end up with a big job on your hands." Another possibility yet to be ruled out is broken rings, which can also occur in a poorly rebuilt motor. If I were you I'd be keen to complete the wet and dry comp test, and if it all points to valves as we suspect, I'd be keen to remove the intake manifold and check clearances.

Anyway good luck with it Malcolm and keep us posted with any developments.

Cheers,
Tony

[Lynn Eades]

I myself am wondering what else can affect the compression of the four centre pots?

[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.