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Discussion Starter · #1 ·
Now that I have my Ford 3910 running, wife says I need to sell my 52 8N. I bought it in 2017, put new valves & guides in it, pertronix ignition + another $1500 in parts. It has run flawlessly since restoration - 16 hours on the new proofmeter. Had it sold, and it's getting revenge by suddenly developing a "popcorn" sounding miss. I'm stumped.

Here's what I've done so far in the last two weeks:

Replaced Plugs

Replaced coil

Replaced wires with resistor type per pertronix recommendation.

Blew propane around intake manifold to check for leaks - no change.

Checked compression (perfect)

Checked valve clearance (it's correct at 15 thou)

Re-adjusted carb mixture.

Rocked distributor timing through it's range (still misses)
Misses at both high and low rpm. Misses when cold or warmed up.
Pics here: http://scott-inc.com/ford8n/

Signed, stumped.
 

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Retired Canadian
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Check for loose connections or mountings, paint that could make a connection or insulation, the coil, coil wire, rotor button, dirt under distributor cap or it cracked(try putting the old one back on)and plug wires that might be touching or loose.
 

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Discussion Starter · #5 ·
Thanks! Yep I thought of that. That's why I replaced the coil, wires, plugs, and went over the pertronix installation again, cleaning all and re-dressing wires. The cap has 16 hours on it , is clean, and looks brand new.
 

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Discussion Starter · #6 ·
Re-clamped and sealed the manifold / tailpipe joint, still have the popcorn miss.

I'm thinking that it must either be the pertronix ignition, or something weird with the valves ( even though valves & guides were replaced 16 engine hours ago). I guess that I should swap back to a points & cap ignition to see what happens. But I through away the baseplate and points thinking I wouldn't need them.
 

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Re-clamped and sealed the manifold / tailpipe joint, still have the popcorn miss.

I'm thinking that it must either be the pertronix ignition, or something weird with the valves ( even though valves & guides were replaced 16 engine hours ago). I guess that I should swap back to a points & cap ignition to see what happens. But I through away the baseplate and points thinking I wouldn't need them.
I can't figure out why this thing is printing like this...Whatever.
How would sealing the exhaust joint affect how the tractor works? I used a 'late 8n' like yours, logging for years with no exhaust pipe at all.
I'm suspicious of the pertronix system it's probably higher volts, make sure there isn't a spark jump. Have it running in the pitch dark and look good!
 

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Discussion Starter · #8 ·
I would not have thought that the exhaust would cause it either, but if you look in the comments of the video I linked to, where his 8N had the same miss as mine, he said fixing that joint cured it. So I tried it.

I did the "dollar bill" test at the exhaust, and there is no reversal to vacuum when it misses. If any thing, the pressure increases during the miss.

I once had an engine miss at mid rpm - was very hard to find that the last rebuilder had put in a valve spring that had smaller gauge wire, causing valve float, or bounce. But this miss is at all rpm, and it's not like "morning sickness" with a sticky valve - it happens at all temperatures. I'm leaning to an ignition problem. Partly because I don't want to pull the head again.
 

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You have a really nice looking place. Park the 8n on the front lawn, take the battery cables off and let the kids play on it. They will all turn into old tractor addicts and when you tell them they can't drive it because there is something wrong, maybe one of them has a Grandad who will come and fix it! I'm grasping at straws for advise.........
 

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Discussion Starter · #10 ·
Last night I ran it in pitch dark to see any possible high voltage arcing. Saw none. Although they're just 2 years old, my next try will be to replace the distributor cap and rotor. If that doesn't get it, I'm installing the standard points and capacitor in place of the pertronix.
 

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condenser ??.

why do you think your problem is electrical ?, when you have what you think is electrical, will be in fact a fuel problem, and when you think you have a fuel problem, it will be electrical and that is fact, they go hand in hand and you have to decipher which is which .

I haven't commented earlier because I am not familiar with your carby, the only updraught carbies I have dealt with, are an updraught on my father's 1934 B4 Ford tourer in the 40's and 50's and a TE20 and a MF35 tractor from the 60's till the 80's,-- but tell me does this have a high speed mixture screw beside having an idle mixture screw,-- I reckon your problem is fuel based, if your carby has the high speed mixture screw, have you tried running the engine on "high idle" and either screwing the mixture screw out or in about 1/4 turn either way? to see if this changes things.

Another thing is you are now into your winter, so you may have a little moisture in your fuel, have you thought of placing a tin or bowl under the float chamber drain plug and draining the fuel so you can check for moisture?.

a carby running lean will also carry on like yours is doing.
 

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Discussion Starter · #15 ·
Thanks, Fred,
Two years ago when I restored the tractor I bought a new carb, and used a glass spark plug to set mixture - blue or yellow burn. I had to change jets to have adjustability into the rich zone, but got it set properly - it is a MS carb, and has run very smooth. A week ago I swung the mixture screws one way or the other, at idle and high rpm. I could get it to really lose power, but nothing would cure the popcorn. 4? days ago I completely drained the tank through the carb drain, and put very fresh gas in. No luck. I have not changed points because there are none with the pertronix ignition, but have a baseplate, points and capacitor or order to eliminate that as an issue. If that's not it, I'm thinking it must be a weak valve spring - valves and guides were new 2 years ago, and now the new proofmeter shows 16 hours. What do you think?
 

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Well Chris !!, you seem to be covering all of the bases and I don't know what you have left to check, have you tried running the engine and removing a HT lead from a plug one at a time and progressing through all 4 plugs to see which cylinder/s is/are missing ?.

I suppose a valve spring could have weakened if made over the pond, but if they are genuine, I doubt this very much, and at 16 hours, and I guess you would have worked out that the miss/pop will be on the exhaust valve side of things.

Apart from the weak valve spring, the only other thing is the Petronix ignition kit, many years ago an Auto electrician told me "a electronic ignition will either work or it wont", and I have gone with that statement ever since, it is a shame you threw the old base plate away, but that is the way of things, I have done the same and wished I hadn't.

When you get your new base plate, points and condenser, hopefully this may fix your problem.
 

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Discussion Starter · #17 · (Edited)
Update:

Warmed up engine. It popcorn misses.
Killed engine.

removed #1 plug lead
restart and run about 1300 rpm
Does not popcorn miss.

Repeated for each of the four cylinders.
Does not popcorn miss.

Reconnected all plug wires. Run at 1300 rpm.
It popcorn misses.

I would not have believed it possible unless I witnessed it.


Someone suggested that it really is still missing but I can't notice it. I don't think so, as it runs pretty smoothly on just 3 of 4, and when standing behind the tractor listening to the exhaust, the popcorn miss is plainly audible or not.
 

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Discussion Starter · #19 ·
I don't know. On a tip from a youtube guy who has the same tractor and the same sounding miss, I did reseal and re-clamp the manifold to to muffler joint. No change. The exhaust is plain stock, and was new 16 engine hours ago.
 

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I don't know. On a tip from a youtube guy who has the same tractor and the same sounding miss, I did reseal and re-clamp the manifold to to muffler joint. No change. The exhaust is plain stock, and was new 16 engine hours ago.
Check the runout on the distributer shaft, there should be no side play, also look at the lobes where the points would run. Had an old 8n that the lobes were worn so bad that it would not open the points all the time or trigger the pertronics ignition
 
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