Mystery missfire .
 

My 31 CCPU decided it would idle on 3 cylinders . Found no.4 was not firing . Checked the plug it was oily so had not been firing . Changed the plug but no change . Did a cold compression check 1,2,and 3 all show 52 PSI . No 4 shows 60 PSI . I thought higher compression due to oily cylinder . I then got to thinking it has spark it must have gas as it shares the inlet tract with no 3 which runs fine . I pulled the leads off 1,2and 3 and see if no 4 would fire ,after full choke briefly no 4 fired up and idled DONKA DONKA DONKA with a bit of throttle . I put the leads back on the other cylinders and it idled on all four as it should . Gave it another try today but it is back idling on 3 .Although I did expect this, I am now a puzzled 81 year old . There has to be a logical explanation but right now it escapes me . Could be bad gasket. I will investigate further and let you know if I solve the mystery .

John in its been raining nonstop for three days and gusts to 60 MPH and I should be in my snowbird trailer in sunny Chandler AZ DRAT !!! And I just paid my ground rent till April 2021 !!! Still in Suffolk County England thanks to Covid 19 .

Re: Mystery missfire .
 

Maybe you have American plugs, they don't work well in the land of eternal fog! I'd check the manifold to block bolts, maybe even squirt a little oil or something around the port. Interesting problem, keep us posted.

Re: Mystery missfire .
 

I checked the manifold bolts all are tight tight tight ( oilfield talk for tight ) Changed the distributer body . I will try a squirt of oil not done that yet .

John in same place same weather .

Re: Mystery missfire .
 

Check the distributor cap and the #4 position on the inside of the cap

Re: Mystery missfire .
 

Re: Mystery missfire .
 

I will try the hottest plug I have where the electrode is furthest into the cylinder . I have tried other bodies but I will pull one off a known good running car . The engine uses very little oil and no blue smoke from the exhaust .

John in same place same weather .

Re: Mystery missfire .
 

Sticking valve?

Re: Mystery missfire .
 

I would not start taking things apart thinking you have a major problem.

Was it running right and than just started giving you a problem?

If it were mine. I would give it a complete tune up, complete. Than some Marvel Mystery Oil in the gas. I would also change the oil. Putting in 20w-50 HD.

Now after doing all that, if I still had the same problem, I would start being concerned.

Model A’s engines can be completely worn out and still run well. I once had one that smoked so bad, you could not always see the car behind you after coming down a hill. But it still sounded great and was running on all cylinders. Little compression and very little power. Was still fun to drive. Just replaced rings, nothing else. They take a licking and keep on ticking. Show me a car made today that will still be around in 90 years.

Enjoy.

Re: Mystery missfire .
 

As its idling, lift the #4 spark plug strap enough to watch the spark. Make sure its steady and hot. If the cylinder fires with the strap lifted then you likely have a plug issue.

Re: Mystery missfire .
 

Change the order of the spark plugs and see if the miss fire follows #4 plug.

Re: Mystery missfire .
 

Any resolution yet?

Re: Mystery missfire .
 

No resolution as yet I have done the trick of holding no 4 lead off to induce a hotter spark but no change and I have swapped plugs about and tried different plugs .A previous post suggests a sticky valve . Got to thinking that the valve seats at cranking speed hence good compression but does not at idling speed . Next trick will be to do a compression test with the engine running on 1,2 and 3 hopefully low compression revealed if good compression its got me beat !!! I will let you know .So far swapped carbs ,distributer new gaskets on manifold plugs etc etc .

John in same place same weather .

Re: Mystery missfire .
 

My thought is that the problem is heat sensitive. For example, a thin valve head / recessed valve will warp and leak when hot, but it will pass a compression check at ambient temperature. I wish you the best of luck finding the issue.

Re: Mystery missfire .
 

But did you have spark?. if the spark jumped from the lead and you have swapped the plugs around and the problem persists at #4 then the problem is not an ignition one. As you stated, #3 and #4 share the same intake runners so it can't be a fuel problem with #3 firing fine so that only leaves valves/rings. One thing you can try is remove the vacuum line from the intake manifold and add a fitting with a hose with a small funnel. Ideally you would put a ball valve in the line somewhere so you can stop a vacuum leak while you start the car. Then, with the engine running on high idle, pour MMO in the funnel and open the valve. You can use the ball valve to regulate the flow so you don't stall the engine but with that small of a line it likely wont. That will be sure to free up any sticky valve and, like most things on a car you only need three hands to achieve this ( one holding the funnel, one pouring the MMO and one to work the valve). Also do NOT do this inside as in very short order you will not be able to see through the smoke in a garage.

Re: Mystery missfire .
 

With the engine heated up I would pressurize the cylinder looking for a intake or exhaust valve leaking air.

Re: Mystery missfire .
 

Yes I did have a hefty spark jumping the gap . Anyway folks it is back idling happily on 4 . What was wrong is still a mystery to me. I did another cold compression test and got 55,55,60 and 62 on no 4 . The distributer originally in the car has a Nu Rex top plate which worked fine . I swapped it for a near new Nu Rex without the pigtail setup the miss remained . I then took the distributer out of my 28 phaeton which runs fine in that car but the miss remained .This has the original points setup . I then fitted a good tillotson off the phaeton but no change . I then changed the manifold gaskets they were blowing on no 1 exhaust but no change so no ill store there . I thought maybe an air leak on induction . I built up another distributer from scratch to try that . I kept with the original points setup (" Century" Made in Japan) and fitted it and Hey Presto !!! back idling on four sweet as a nut . So the mystery is not solved in my mind at least . Maybe I ought to reinstate the original components and see what happens but as old English sayings go "Quit when you are ahead" and "Let sleeping dogs lie" Tomorrow I will fire it up to see if the miss has returned but I wont bother to fix it if it does . I gave it a fair shot i think. If it ever stops raining I will go for a blatt around the countryside I will soon know if I am missing a quarter of my horsepower .

John in same place same weather .

Re: Mystery missfire .
 

I think RAY 64 has put me on the right track . The ONLY logical explanation is a sticky valve most likely the exhaust . At cranking speed the valve seats in time for good compression but at idle speed the sticking valve does not get time to seat . The fact I "cured" it by the last distributer swap was a coincidence . My starting and stopping heat cycling etc etc has freed it enough . The proof of the pudding will be an extended road test . Could be many engines have lazy valves but revolutions stay low enough that it does not show up . I will get some "sea foam" or similar and give it a dose. Thank you all very much for your suggestions .

John in sunny breezy morning Suffolk County England .

Re: Mystery missfire .
 

Typically, the exhaust valves run clean because of the exhaust gas heat. It is usually an intake valve that fails to seat because of deposits. However, exhaust valves are more prone to warping, pitting and burning than the intakes.

Re: Mystery missfire .
 

I started up a '74 ford 1 ton a month or so ago that had sit for several years. We drained the fuel tank and added fresh gas, primed the carb and started it up. It started (under protest) and would run but only over 1000RPM. It would not idle at all. I had my son hold it at around 2000-2500RPM while I poured MMO down it's throat. I would pour until it was about to stall then let it catch it's breath and do it again. Used 250ml all together. When the smoke cleared (and there was a LOT of smoke) it ran perfect and idled smoothly.

Re: Mystery missfire .
 

Ditto