K321 backfires.. Bronco 14

[daveoman1966 3,752]

My 1972 Bronco 14 with K321 motor--- breakerless ingnition---  BACKFIRES occassionally....but keeps running.

Recently, it quit running on me---midway of mowing, of course.  I had to push it to my garage and have not tried to start it. 

What causes the BACKFIRE.... 

In 2016, I had a local shop LAP the valves and have not had any issues since then...until now. 

I have a KOHLER carb rebuild kit but not sure that is the BACKFIRE source problem.

ps... The engine smokes very little when running, even at 3/4 throttle.  The KILL WIRE is and has NOT been working for 2 years and I have to FLOOD it out (choke it) to stop the motor.     

HELP WANTED---  

    

Edited October 25, 2023 by daveoman1966
notes

[kpinnc 12,064]

I would try a different muffler to start with. Most backfires come from detonation after the exhaust valve. Carbon buildup in the exhaust will get hot enough to cook off any unburned fuel. 

 

If your kill wire doesn't work, and you're choking the engine to shut it off- that may be the biggest contributing factor. 

[Mike'sHorseBarn 2,998]

23 minutes ago, kpinnc said:

If your kill wire doesn't work, and you're choking the engine to shut it off- that may be the biggest contributing factor. 

 

I would agree with this. You may want to check your valve lash again, and if that picture of the valves is from 2016 you may want to check them again. Breaklerless ignition is good at burning valves. I have an electro 12 that needs a good valve job done to it.

[WHX?? 48,820]

Flooding the engine to shut it down is never a good thing but guilty as charged on occasion.  Fix the kill and see what shakes.  

Timing issues? 

6 hours ago, kpinnc said:

Carbon buildup in the exhaust will get hot enough to cook off any unburned fuel. 

 

True KP... isn't that why they came up with the solenoid under the bowl? That was usually  just on shut down. 

[daveoman1966 3,752]

Just a bit more info:  When mowing, I have the CHOKE at about 10% or so. Less than that it stalls out.

Always needs full CHOKE to start ---3/4 THROTTLE.  I've added SEAFOAM on two tries, to no avail. 

QUESTION:  Would a STUCK ex valve cause the BACKFIRING---even when running?

The muffler I have on it has been there for YEARS...  (spin-on, like this one on my C-160Auto) as I'm not in a position to drop a Benjamin + for a new one.  NOTE----The C-160 turns off witth key swithch....NEVER backfires.

 

[daveoman1966 3,752]

13 minutes ago, WHX?? said:

Flooding the engine to shut it down is never a good thing but guilty as charged on occasion.  Fix the kill and see what shakes.  

Timing issues? 

True KP... isn't that why they came up with the solenoid under the bowl? That was usually  just on shut down. 

BUT BUT BUT---- Even when motor is running???  

[WHX?? 48,820]

13 minutes ago, daveoman1966 said:

BUT BUT BUT---- Even when motor is running???  

That's what is throwing a monkey wrench in a diagnosis Dave. Just might be a valve hanging up?

Your choking procedure might indicate a carb cleaning? 

[kpinnc 12,064]

31 minutes ago, WHX?? said:

True KP... isn't that why they came up with the solenoid under the bowl? That was usually  just on shut down. 

 

That was part of it, yes. And also to keep fuel from seeping into the crankcase when the tank is mounted higher than the engine. 

[RED-Z06 2,221]

If you have to choke it to keep it running, it needs more fuel in the tune, a lean engine is a hot engine.  Turn the main needle out 1/4 turn..its a big swing but, looking for obvious results 

[daveoman1966 3,752]

I take ALL input in a positive light and, after a board meeting --to include Mr Coors-- will give feedback in a few days of either SUCCESS ...or ...FUBAR.  Thanx.    

 

[Tuneup 1,433]

Yeah, what RED said - you should be able to enrichen the main to mow on full throttle without adding choke. If you have to add choke sometimes and not other times, too much wear on the throttle shaft? As long as she didn't burn the exhaust valve from running too hot, points to the carb for me but I had always learned 'ignition first' and that electronic iggy is a big unknown.

[953 nut 55,253]

21 hours ago, daveoman1966 said:

breakerless ingnition---  BACKFIRES occasionally....but keeps running.

.  The KILL WIRE is and has NOT been working for 2 years

I would begin by repairing the known problems and looking for related issues. It could have a fuel issue but I'm thinking  the backfire is ignition related. Most intermittent problems are a poor connection or insulation breaking down. 

Your ignition system has a trigger device under the flywheel shroud that is connected to the module by a thin wire that has been subject to heat, vibration and other things that could cause the insulation to become compromised. Not a dead short but an intermittent momentary short to ground. Remove the fan shroud and carefully inspect the wire from the module.

Connect a new kill wire to the "M" terminal of your ignition switch, even if this doesn't cure the problem it should be done.

Replace the spark plug wire, it could have a poor connection causing the problem.

If the engine runs well and has good power I doubt that you have a valve problem.

The choke being needed has been addressed by @RED-Z06 and @Tuneup, follow thier advice.

[Rick3478 428]

Passing shot - worn out throttle shaft can let in extra air, making it lean, and occasional sputtering as it bounces around.

 

[Gasaholic 224]

Very first thing I do for backfire is , replace the spark plug with a clean, properly gapped plug and see if the problem repeats.  Quite often there gets to be slight carbon fouling in the plug that causes an intermittent short of the spark, unburned fuel in the exhaust goes "boom"   Depends where backfire comes from also - You'd want to identify if it is backfiring out exhaust or backfiring through intake. If intake, with breakerless, I would be checking the wire terminals for metal fatigue (If they do not fit nice and snug, and have a little wiggle to them when running, there can be intermittent loss of contact, causing a misfire or premature trigger) 

[kpinnc 12,064]

If you haven't done so already, I would and dump and clean your sediment bowl too. What @RED-Z06 suggested is a good idea. Adjust the carb a little richer and see what happens. 

 

And just a thought: if your ehaust valve was stuck, I doubt you'd have enough compression to start the engine.

[daveoman1966 3,752]

57 minutes ago, kpinnc said:

If you haven't done so already, I would and dump and clean your sediment bowl too. What @RED-Z06 suggested is a good idea. Adjust the carb a little richer and see what happens. 

 

And just a thought: if your ehaust valve was stuck, I doubt you'd have enough compression to start the engine.

MAYBE---  that's why I had to push it into my garage.  It wouldn't start, so your stuck exhaust valve theory may be spot-on. (Hope not)   

[953 nut 55,253]

53 minutes ago, daveoman1966 said:

MAYBE---  that's why I had to push it into my garage.  It wouldn't start, so your stuck exhaust valve theory may be spot-on. (Hope not)   

When you were attempting to restart the engine did it turn over more freely as if there was little or no compression?

[kpinnc 12,064]

23 minutes ago, 953 nut said:

When you were attempting to restart the engine did it turn over more freely as if there was little or no compression?

 

Good point. It would freewheel as if the plug was removed.

 

One easy way to verify would be to remove the carb, and clean it. Not necessarily a soak, but at least drop and clean the bowl, remove the high/ low mix needles and spray everything out with carb cleaner. Reinstall the needles just until they gently bottom out, and back them back 1.5 turns. You can fine adjust them after the engine warms up on the next run.

 

While the carb is off, check your valve clearances under the breather. A stuck exhaust valve would certainly show up there. 

Edited October 26, 2023 by kpinnc

[WHX?? 48,820]

Not too far off topic but is it possible for a valve to hang up "occasionally" ... say a broken valve spring or carbon build up? 

[squonk 41,120]

45 minutes ago, WHX?? said:

Not too far off topic but is it possible for a valve to hang up "occasionally" ... say a broken valve spring or carbon build up? 

Exhaust valve sticking is common enough.

[RED-Z06 2,221]

53 minutes ago, WHX?? said:

Not too far off topic but is it possible for a valve to hang up "occasionally" ... say a broken valve spring or carbon build up? 

 Its not terribly common in older worn flatheads because the springs were fairly heavy, valve stems large, and the valve in iron block design kept things cool.  But its possible if there's varnish on the stem, or severe carbon build up.