Lets see who can figure this out

[SeaSlugs]

Going out on a limb here but here it goes:

Car is stock besides a higher compression head (winfield head) and its 12V-ground

what happened was headgasket was replaced, and top 2 waterneck nuts were not torqued the same as the rest and blew the headgasket out of the front of the motor. Caught it in time and no overheating. no water in oil but oil was changed anyways.

What it was doing was he went to start it started and idled OK, got out on the road and had a misfire on one cyl. More he drove it it fixed itself and didnt have anymore trouble and normal power.

Saturday he went on a 140mile round trip tour with the club and misfired most of the way.

Today he drove it and got worse.

Checking obvious stuff but nothing was really jumping out as a culprit.

Had a bunch of spare known good parts that have been swapped out with no change:
entire distributor, cap, rotor that work great on a test engine
3 different carbs that ran great on a test engine
swapped coil out
has great fuel flow with fresh gas out of the line for the carb
set of 4 new NGK plugs (14mm style)
getting decent blue/white spark from each wire to head
timing has been checked and rechecked twice
~90PSI on all cyl dry.
added a jumper wire from headstud to frame rail


applied power directly off starter switch lug to pos side of the coil to eliminate dashboard wiring issues with no change. (coil is a proper 12V coil with polarity correct)

it seems cyl 4 and possibly 3 are misfiring or not firing at all.

If the engine is cold, it starts and idles good on all cyl, when you press the accelerator it falls on its face and bogs down. the longer its idling (hotter it gets) it seems to get worse and worse to the point it barely idles and misfires while idling and eventually stalls and wont stay running after that until it cools down.

What the leading "idea" of whats going on is sticking valves that stick open when its hot but once it cools down or in the time it takes to remove all spark-plugs and cap/rotor to do a compression test it cools enough to allow the valves to move.

No weird noises from engine or colors coming out of the tailpipe.

We did check for manifold cracks/leaks by spraying with WD40 while it was running and trying the propane test with no change in performance, the exhaust manifold does have several small leaks.

new plugs (all 4) are black and sooty from it running while misfiring.


Any ideas? Pretty stumped here...

[1crosscut]

Did you replace the condenser?

[Gary WA]

Try using a different heat range plug? Trouble in the ignition system. Or Head gasket?

[Mitch//pa]

some ideas

run it with an inline spark tester and see if the spark from the coil gets erratic or weak when it misfires.. it should have a steady rythmetic bright flash.

you bypassed the pos side of the coil try doing the neg and eliminate the switch and armored cable

richen it with carb cleaner through the vacuum wiper port, if its lean the miss should subside

do a leak down test

[1955cj5]

I'm thinking vacuum leak...cracked intake manifold?

[Rowdy]

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Exhaust leak at the clamp, then carb sucking in the oxygen depleted hot air. Just a thought. Rod

[SeaSlugs]

Plugs were same make and heat range as they were when it was running good, we changed an entire distributor that ran great on a test engine with no change on this engine, spark seemed steady when arcing against the head - cyl 4 had a good spark and consistent arc but grounding it out didn't change the engine performance...

we thought of a cracked intake manifold, couldnt see any cracks, sprayed liberally with WD-40 and stuck unlit propane torch all around intake while engine was running badly with no change in performance.

Also sprayed WD-40 and stuck propane torch in the inlet of the carb with no change in performance. - also checked around the vacuum port with no change.

Good idea on neg side of coil with armored cable - will have to try that, compression test seemed good 90 across the board. One thing weird is cyl 1&2 very first compression stroke went to 90 instantly and stayed there, 3 and 4 went to 85ish on first compression stroke then to 90 on 2nd stroke. Will have to do a leak-down test to see more.

[Rex_A_Lott]

Can you put your hand on the carburetor bowl? Just wondered if you have enough of an exhaust leak to boil the gas in the bowl.
It seems like you have covered the basics, I'm stumped too.
Have you tried more than one compression gauge? My first guess was head gasket, but you seem to have eliminated that.
Maybe run the valves. If the exhaust valve gets hot enough to lose the clearance it might be holding it open when its hot and then as it cools it will seal off again.
Good Luck

[SeaSlugs]

Yes you can touch almost everything as it starts misfiring after about 2-4 minutes of running. after about 5-7 it can barely run.

yea we were thinking some valve issue also.

[daveymc29]

Loose connection in the main power feed or a faulty ground? I fought that symptom for years and it was a loose wire within the connector to my alternator. Crimped the clip and all the missing went away. Watch the sparks within the distributor with a clear cap or one with only the center and the clips remaining, so you can see what is happening within the body while the car is cold and see if it changes as the engine warms up.

[quickchange]

You say a good flow of fuel with PIPE off carb BUT if pushed into far against the filter can restict flow. Because you have changed out dissy then weak points spring that allows point bounce ruled out & that usually happens at speed , Other than that pos valve clearance too tight , Derek Praying for dry salt in August,

[colin1928]

Check your fuel Quality ???

[daveymc29]

I cured some valve sticking by putting Marvels Mystery oil in the offending cylinder and letting it sit over night. Next morning I put the plugs in and drove to coffee with friends. I then added four oz. of the Marvels Mystery oil to all my fill ups. No more sticking valves from a gasohol crap. It is too dry to give any lubrication to the valves from above as the old gasolines did with lead and no alcohol. Just that this worked for me, maybe not for others.

[SeaSlugs]

Hmm its possible, i forgot he does have an alternator on this car, i do know i checked for tightness but could be corroded in the crimp (have had that on modern cars before)

We have a very very very simple 1 speed carburetor hooked up with its own fuel resivour so that eliminates everything from the carb back to the tank. (no fuel line connected to it) Runs exactly the same as a zenith (misfiring wont accel, and eventually wont idle and dies) but it runs beautifully on a test engine so we know its good also.

I've heard good things about marvel mystery oil freeing up stuck stuff - may give it a shot. We've had Ethanol in our gas since the early 80's here and dont have fuel issues like you guys that just recently got it so i doubt thats the cause.

Fuel runs great in 3 other model A's out on the road (from same station) and test engine in the shop (same station same day same red gascan)

When it dies it kind of behaves like its running out of gas, starts off with a misfire then just slowly gets worse to the point it wont idle then it wont even stay running more than a few seconds until you let it cool. intake is still cool enough to touch with your hand so its not vapor locking or anything and has same issues with multiple fuel sources/carbs so its not vacuum locking due to a clogged vent (although it behaves that way)

Thanks for the suggestions!

[larrys40]

When you have symptoms like this you have to step back and logically test and troubleshoot.... Sounds like you have thrown a lot of things from 3 carbs, coil, etc at it. Sometimes in the process things can happen and get installed that sometimes make it worse. I'm about troubleshooting logically and test... don't guess. Make sure the health of the basics of the engine ( aside from the distributor and electrics) are good. I would do a leakdown and ensure that valves are sealing.... if there is an issue I would pull the manifolds first and take a look to see if the valves are moving freeling.. soak with kroil or good penetrent if needed. Retest leakdown if necessary.

Test condenser - use good known or A&L if need be. Use tested coil 12-18KV output.
make sure you have a good functioning primary circuit from the ignition switch/popout, to the distributor points to ground. check voltage with a voltmeter and look for low voltage, and poor continuity with resistance and/or low voltage. Once verified ensure that high tension (secondary) side is putting out to the plugs.

Ensure that timing of engine with relation to TDC is correct. This would have more likely shown up in the leakdown but I have seen loose timing gears that rotated slightly on the cam dowels cause issues as well. Once the health of all respective aspects of the mechanics of the engine and electrics are verified it's down to the carburetor to deliver fuel at the proper amount.. and that shouldn't be too hard to verify.

Sun's old slogan was "Test, Don't Guess". It is so true and is the best methodology.
hopefully you will have it pinpointed quickly. Please keep us advised.
Larry Shepard

[pooch]

Throw a timing light in the coil wire and watch the flash .

[western77]

Check the compression when the engine is hot and see if you can find a sticky valve. File an angle on the end of the gas line. This will allow gas to flow even if the line is in too far in the carb.

[rocket1]

coil winding opening up slightly when hot affecting spark

[Kurt in NJ]

Are the plugs wet black, or dry black ----when looking at the plugs where the spark jumps between electrodes is it black, grey, or speckles of black and grey --if the spark is strong at the gap it will clean the carbon off and be grey across the face of the center electrode

[gweilbaker]

If he has the repro junction box, check that the heads of the terminal are not shorting to the fire wall.

If he has the fuse on the starter, check it, they can become faulty due to poor design and age.

Make sure there is a good ground path from the engine to the battery.

Also don't thread the armored cable too far into the distributor, it will short the bus bar to the housing.

Good luck.