Low compression

Malcolm Towrie · #1 08-03-16, 01:23

I checked spark again today with another induction timing light. Same findings, #1 barely sparking, 2 missing occasionally, rest OK.

I checked for spark on a couple of the "good" cylinders when trying to start hot today, and there was none. Like you, Lynn, I suspect coil, maybe condenser.

I also found the 6V terminal studs on the remote coil stripped so the wires weren't very secure. And the clamping screw on the dizzy was loose so the points drag had rotated the breaker plate as far CW as it could go (looking from rear to front). Based on an old dizzy I have at home, that's retarded about 5 degrees cam so 10 degrees crank assuming clamping screw is normally set around the middle. But these are side issues.

So the plan is to pull the dizzy, coil and wires and go over them. (The plugs checked OK on the tester at 140 psi.)

Malcolm

Malcolm Towrie · #2 12-03-16, 07:12

Another update regarding no sparks on the Canadian carrier when hot. I got the divers-helmet dizzy, remote coil, radio suppression doohicky, and wires off. I've got more questions than answers.

The remote coil says it's a "Standard, 6V, no external resistance required" coil. Lots of oil sloshing around inside. Primary side resistance is 1.4 ohms. I subjected the primary side to a steady 6.4 volts. Current was 4 A, dropping slightly as the coil heated up. I was hoping for a sudden drop in current as the temp rose, but it got up to 70C and was still flowing close to 4 A. So the primary coil is good, I guess.

I also measured the voltage between the secondary coil terminal and the -ve primary coil terminal during the above testing and it also stayed steady at 6.4 V. So the secondary coil didn't go open circuit as it heated up.

BUT the secondary coil resistance is 11 kilohms which seems high. I'm used to seeing around 7 kilohms. Is that a concern?

Also, the wires between the dizzy caps and the the plugs are solid core so their resistance is low as expected, <0.5 ohms, but the wire between the coil and the dizzy is carbon core, with a resistance of 11 kilohms. Is that a problem?

The capacitor is an oldie. It's a Ford piece stamped "6 45", so it may be 71 years old! It tests OK, but it's rated 1 microfarad (confirmed by DMM), which is 3-4 times more than it should be by any standards. I suspect someone used a noise suppression cap by mistake. I'll change it, but I don't think it explains no spark when hot, as I heated the cap with a heat gun while meggering at 500V and it didn't die.

The drivers side points, which initiate the spark, are badly "peaked", with the peak visibly holding the points apart. That can't be good. The points open time on a flathead is so short, I'm wondering if that is the problem?

I need to check if the original ballast resistor that flatheads had in series with the 6V coil is still in the circuit. Or did carriers not have this?

I hooked the radio suppression box doohickey up in series with the coil and passed 4A through it to see if it was reducing voltage to the ignition. It passed with flying colours. No voltage drop, no heating up of caps or inductors.

Sp I'm still wondering what is causing the no spark when hot. If I don't find anything, I guess I need to check voltage drop when cranking to confirm the ignition is getting decent voltage.

Any suggestions appreciated.

Malcolm

Mike K · #3 12-03-16, 10:34

Quote:

Originally Posted by Malcolm Towrie

Also, the wires between the dizzy caps and the the plugs are solid core so their resistance is low as expected, <0.5 ohms, but the wire between the coil and the dizzy is carbon core, with a resistance of 11 kilohms. Is that a problem?

Malcolm

Hi

I would get rid of that carbon core wire and replace it with a copper cored wire , same as you have for the spark plug leads . Mike

Malcolm Towrie · #4 17-03-16, 23:51

An update on the running problems: the distributor shaft was quite badly bent. It had 0.010" runout at the cam. Since the points gap is only 0.014-0.016" this seriously messed up the dwell and timing with big changes on a cylinder-to-cylinder basis as the shaft did a full rotation. The points being set too wide didn't help.

With a good, used shaft with 0.002" runout, and the points set at 0.015", dwell was right on the money and she's purring now.

Malcolm

rob love · #5 18-03-16, 00:09

Good for you.

When you say "distributor shaft" I take it you are talking about an 8BA engine with the distributor up top? Seems like it would take a lot of force to bend the shaft on either a crab or a divers helmet distributor mounted on the front of the engine.

Malcolm Towrie · #6 18-03-16, 02:29

Quote:

Originally Posted by rob love

When you say "distributor shaft" I take it you are talking about an 8BA engine with the distributor up top? Seems like it would take a lot of force to bend the shaft on either a crab or a divers helmet distributor mounted on the front of the engine.

No, this is a divers helmet distributor. And it doesn't take much force at all to bend it when all the parts are stripped off. I chucked the small end in a lathe and measured 0.060" runout at the journal on the other end. It only took a few taps with a soft hammer to get it straight to within 0.002". It's pretty spindly in the middle as you can see but I was surprised how easy it was to straighten. I cleaned the shaft up thinking maybe it was cracked but I couldn't see any.

Some rough handling would have bent it easily.

I bet the last person to set the points must have struggled.

Malcolm

DSC00055.JPG

Lynn Eades · #7 18-03-16, 02:34

I think somewhere back there Malcolm confirmed it was a divers helmet type.
How to bend a shaft in one of those, I don't know.
As it was the centre cylinders and one half of the firing order rotation, I think I was headed in the right direction as far as a misfire goes, but can't see really why it had matching low compressions, when comp test pointed toward valves.

Good news that it is sorted!

Malcolm Towrie · #8 18-03-16, 02:54

Lynn, I should have got into the ignition problem in a separate thread to avoid confusion as it was a totally separate problem. The low compression hasn't gone away!

That shaft wouldn't bend in service but throwing it in a box and then throwing other parts on top of it would bend it easily, based on how easily I straightened it.

Malcolm