3 cylinder Model A

nkaminar said:
As you said, it could be a sticky valve. The original poster said: "I only put in new rings and did a valve job." He reported that the compression was good, 70 psi. Might be interesting to do a compression check on that one cylinder while the engine was running on the other 3. I don't know if anyone has ever done that.
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Yeah, I’d pull all 4 plugs, have someone turn it over with the starter and carefully with a leather glove feel if theres any compression. Or use an auto compression gauge tool to be safe. I bet the exhaust valve is sticky.
 
Well I wrote a detailed post then the site logged me out and lost all my words of wisdom.

Second try. Been cold here, 25F, thanks to our Muppet govt our energy prices are highest in G7 so I can’t afford to heat my shop: four times what you pay in USA.

Managed to do compression test today, left the tester in position and en I change the motor. Sure enough compression dropped off so somethings sticking. I think when it improved on changing the coil lead that was just a coincidence but it send me off down a rabbit hole into the electrics.

Conversation with my mate John: So it pulls well? Yes once you’re up to speed, just slight sense of imbalance. Why not drive it, see if it clears?

I’ll think I’ll do that. Got detailing to do first, then I’ll put a few miles on, do the valve job if I have to.
 
Thank you for the feedback. It is always good to know what a problem was. Nice that you found it.

You might want to try some additive in the oil and/or fuel. Many people use Magic Mystery Oil. Your local auto parts store may have some suggestions. Yes, driving it will usually clear it up. Many times the valve stem to guide clearance is too tight and running the engine will loosen that up.

You may have been the first person to do a compression check on a running engine.
 
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Further to the compression test: I screwed the tester into no1, turned it over and got 70 psi. Started motor, hit the pressure release button on the dial, reading dropped to about zero and wobbled there. This was at a fasi idle, cold motor. When I stopped the motor, on the last half turn the pressure flicked up to about 60. I assume the guide is a bit tight so the valve closes slowly. Fine at cranking speed, at running speed the lifter is coming back up before the valve can close.

We don't have MMO here but a home grown product Redex. I've ordered some and will run around and see what happens
 
To see what is actually going on, whether it is a sticky valve or a broken valve spring, you can make a valve chamber cover out of plastic. Polycarbonate is the best. This was done to investigate the oil consumption on the Burtz engine and the modification to that engine to fix the problem. To get a good seal use a thick sheet of polycarbonate, such as 1/2 inch.

While you have the valve cover off you can look for a broken valve spring.
 
If you remove intake and exhaust manifolds it is then possible to use "redex" on top of valve guide to lubricate valve guide and valve stem without removing the head.
 
Check the point gap on all four of the lobes of the distributor cam. Repro cams can have different lifts and profiles on each of the lobes, and it may be that one of the lobes is low enough that the points aren't opening when that lobe swings by, but the other three lobes do open the points. You'll have no spark at that one cylinder where the points don't open.
 
The problem was determined to be a sticky valve that was lowering the compression in that one cylinder when the engine was running. The ignition system is fine. See post #46.
 
Jim/GA said:
Run the engine in the darkest place you can at night and look around the engine for stray sparks running to ground that should be going to #1 plug.

Look inside distributor body in strong light for carbon trails inside the body. Wipe out inside the body with a solvent, like isopropyl alcohol, to remove any grease film.

A 1 inch spark at any plug is a very hot (high voltage) spark that can then find a ground through other paths that are less resistance than through the spark plug. Are you using a super high output coil?
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A "super high output coil" is not likely the problem! By design, a coil only produces enough voltage to jump the plug gap. When the points are closed a magnetic field builds up in the secondary windings of the coil, fed by the primary windings. When the points open, this primary circuit is "broken" (thus the term "breaker points"). This induces the secondary windings to produce a high voltage which is looking for a path to ground. As soon as this induced voltage is high enough to jump the plug gap, the plug fires. If it takes only 5,000 volts to fire the plug, that's all the coil produces even if it is a Super Duper "40,000 volt" coil. A cracked or dirty cap or fouled plug can drain this secondary voltage away before it builds up enough to fire across the plug gap. Sometimes holding the plug wire an inch or so away from the fouled plug allows enough voltage to build up to fire the plug.
 
40 Deluxe, the OP narrowed it down to a valve that won’t operate properly under running conditions. Possible broken valve spring or sticking valve in the guide.
 
AKJurnee said:
40 Deluxe, the OP narrowed it down to a valve that won’t operate properly under running conditions. Possible broken valve spring or sticking valve in the guide.
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Yeah, I read that after i posted, but i also wanted to dispel the "super hot coil" myth.
 
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