help diagnose this

Brian

Brian

Well-known member
Friends 39 Mercury is fitted with engine I built him; C69A block bored 3 5/16", iron C7RA heads, stock 32 steel camshaft, single two barrel carb, nothing particularly special. He has run this vehicle on a fairly regular basis, until a while back the engine started to run ragged, he limped home and called me. I rebuilt him another distributor [stock 39 variety], all timed up with new points, rotor, condensor, subsequently fitted and made no difference.
The engine fires up and runs fairly well when cold, but as it warms up it begins to miss fire, run rough, backfire through the exhaust, getting worse and worse the warmer it gets, until it won't even idle.
Compression test reveals 125 psi all cylinders give or take a coupla pound, so all good there. In the process of trying to diagnose this problem, we have tried 3 different rebuilt dissies, 3 different carbs, new coil, condensors, renewed the dissy terminal side plates with NOS, double checked HT wiring is indeed in correct order, bypassed original wiring with jumper direct from battery to coil, thinking the heat riser in intake may have blown out, have replaced intake, thinking [press on] cam gear may have slipped on cam, have removed front cover and checked- all good, and whilst in there drilled and tapped and fitted a scotch key to preclude this possibility from ever occuring.
I have not tried a vaccum gauge on it, this vehicle is a bit of a drive away from my residence, so I cannot devote a lot of time to it, but, really I'm stumped as to what is wrong!
Any of you wise old sages out there care to venture where to look next?
Basically, engine fires up and runs when cold, then progressively gets worse and worse [sorta makes one think straight away of condensor/dissy trouble], but is not! All suggestions much appreciated, thanks, Brian
 
Sure sounds like coil or condenser.Being new does not make it good.Possible replacement coil or condenser is also bad?Just a thought.Had the same problem and had 2 bad coil that failed with heat.
 
First get a vacuum reading both hot and cold. Could it be one or more valves hanging up as the engine warms up? Just a guess. The gas that we have here can cause strange things to happen . Is your gas laced with ethanol?
 
Just a guess here.....Non-vented gas cap?It happened to me once with a lawnmower and a gas can.I got the caps switched.
 
Try running on fresh fuel. The fuel today sucks, I don't know why but it seams to go bad quickly and the ethanol makes the engines run hotter. I've seen valves break of and lodge in pistons on a newly done head because the builder didn't leave enough clearance in the guide. Go figure. JMHO.
 
Have you tried jumping the resistor when it's acting up badly ? Had issues with them before .
 
I had the same problem and I had my coil rebuilt by Skip Haney in Punta Gorda, Florida. I installed it and the first time I hit the starter button it fired up and has run as smooth as glass for the last year !

Mike
 
Brian said:
...runs fairly well when cold, but as it warms up it begins to miss fire... getting worse and worse the warmer it gets, until it won't even idle...new coil, condensors, ...bypassed original wiring with jumper direct from battery to coil...engine fires up and runs when cold, then progressively gets worse and worse...Brian
Click to expand...


Brian, your coil can't handle the voltage you're sending it with the bypass hotwire. Voltage higher than 4 volts heats the coil, and breaks down the internal insulation, delivering less and less spark until it quits altogether. Cool it down and it's good to go again. You have to run 6 volts through the resistor on the firewall above the steering column, which reduces voltage down to 3 1/2 to 4 volts, exactly what your coil is happy with.

Do this simple test: The nest time the engine fails, immediately check the spark. Coil failure under hot conditions will deliver either no spark or very weak orange spark.

If indeed the problem persists while using the resistor, then perhaps the problem is the source of the "NEW" coils you have been using. NEW or NOS coils are trash. You need an original coil rebuilt using modern materials. Take one of your coils and send it to Skip Haney in Florida for the rebuild service.

Skip Haney www.fordcollector.com
Remanufactured coils and high volume modified water pumps for flatheads
 
Sorry boys, I must elaborate; we are using conventional coils and the adaptor plates that fit directly on top of distributor, tried three of each, same result. One more thing to add; I noticed the car is wired 6 volt Neg ground, however, the generator is polarised correctly and charges, likewise the pos terminal on coil is fed from ign switch, neg to the coil, so that is all good.
 
Also, even with engine running rough, I have removed the HT wire that goes into top of distributor, [from the coil], and holding it about 1/2" away from the hole I'm getting a good strong continuous blue spark, does not seem to have any irregularity. This suggests to me there is nothing to wrong with coil or condensor.
 
So... if we've got a good fat regular spark getting as far as the rotor, if this problem is indeed spark related, we've got the spark going other places than the appropriate plug. We've tried three different distributors, with new points, condensors and rotors in different combinations, also new side terminal plates, new HT leads, and double checked that each lea is wired to the correct plug.
I agree, the symptoms sound like ignition related, but I'm pretty sure we can take the entire ignition system outta the equation.
 
Brian, have you tried checking spark at the individual plugs? You may determine an iregularity with the fewer hits per minute. Sure wish I'd figured the answer for you in my previous post.
 
Make sure the HT spring on the adaptor plate is making good contact to the dist. I've seen some that were too short.
 
I read thru this thread a couple of times and I see where you replaced the points, condenser, rotor... never mention the CAP. Nobody else has mentioned it either. Could have a crack(s).

Sorry for buttin' in, I usually hang out at the Late V8 Forum. I don't know much about the early V-8's.
 
I would get and old chevy,mopar 6 volt coil dirty better.No resister needed. neg to points.
Do you have after market coils. also check and make sure the dist bearing are not worn out and loose.
 
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