Misfire at idle, goes away under load but will start to sputter after a while

[lahren68 6]

This one has got me puzzled. I picked up a 1969 charger 12 from an auction last summer and had been tinkering on it to get it running and driving. Within the last couple weeks i got it running. The issue I'm running into with this machine is on first start it will have a miss at idle when theres no load on the engine.

Ive gone through the carb and adjusted it to kohler spec for inital startup. I replaced the spark plug and wire. Ive replaced the carb mating gaskets to the cylinder head. It still has a miss at idle. It will get progressively worse as the engine reaches operating temp.

If you give the engine throttle, it will run right up without any hesitation. If you release the park brake and begin driving the miss seems to go away for a while. After a few minutes under load the engine will develop a slight miss but will never die.

If you engage the parking brake and try to idle it down slowly it will have a terrible miss to the point of it just flat dieing. Ive found that when it starts acting up when at operating temp and you attempt to idle it down, if you give it just a little bit of choke it seems to smooth out and run better. Ive tried adjust the carb to give it a little more fuel. Doesnt seem to help. 

Ive pulled the cylinder head off and found the usual carbon deposits.

It didnt appear that the head gasket was blown. Acr looks to be working correctly. Cylinder wall looks fine. I checked valve lash and it was in spec. I pulled the valves out to inspect the seats. Exhaust seat is slightly pitted along with the valve face but nothing catastrophic. The intake seat looks uniform but the intake valve looks like it had been extremely hot at one point. This machine has the breakerless igntion on it. I have new valves coming and run a pattern on the intake to verify the seat is okay. 

[702854boy 506]

Try replacing the condenser, when those go bad the usually have the same symptoms you are talking about. And if that don't work out the old condenser on and replace the coil because those usually because the engine to act weird when hot. If coil works Put the new condenser on for ease mind

Edited 9 hours ago by 702854boy

[lahren68 6]

This machine has the breakerless ignition system. Is there a condenser im missing on it somewhere? I have a few cubs with points ignition but im not very familiar with the breakerless system

[953 nut 64,140]

48 minutes ago, lahren68 said:

if you give it just a little bit of choke it seems to smooth out and run better. Ive tried adjust the carb to give it a little more fuel. Doesnt seem to help. 

There are some very small passages in the carburetor that need to be cleaned for proper operation. Since the engine runs reasonably well you could try a heavy dose of Seafoam or Berryman B12 gas treatment to see if they will help clean these passages.

Be sure to use gas that has no ethanol in it, use this website to find a local source.   https://www.pure-gas.org/

[pfrederi 19,914]

Breakerless ignition does not use points or condenser.  It is pretty much a binary system..it works great or not at all.  You might test the coil when it is hot. You could check the air gap but I doubt that is an issue.  also put a spark plug tester on it...does the spark stay strong when it is acting up?

[lahren68 6]

The engine has only had 91 run in it since ive gotten it running. Been very cautious of running ethanol especially in these old engines. Ive ran 2 different carbs on this engine as well. The one ive went through and one new out of the box. Has the same symptoms with either carb installed. 

[lahren68 6]

22 minutes ago, pfrederi said:

Breakerless ignition does not use points or condenser.  It is pretty much a binary system..it works great or not at all.  You might test the coil when it is hot. You could check the air gap but I doubt that is an issue.  also put a spark plug tester on it...does the spark stay strong when it is acting up?

Once my headgasket show up ill reassemble it and check it with a spark tester after letting it run and report back

Edited 8 hours ago by lahren68

[pfrederi 19,914]

This type of tester is more accurate in diagnosing weak spark as opposed to the neon bulb type

 

 

[ri702bill 11,267]

45 minutes ago, 953 nut said:

Berryman B12 gas treatment

I am normally NOT a fan of any of those "Mechanic in a Can" quick fixes, but the B12 is the exception. 

[squonk 45,539]

1 hour ago, lahren68 said:

This one has got me puzzled. I picked up a 1969 charger 12 from an auction last summer and had been tinkering on it to get it running and driving. Within the last couple weeks i got it running. The issue I'm running into with this machine is on first start it will have a miss at idle when theres no load on the engine.

Ive gone through the carb and adjusted it to kohler spec for inital startup. I replaced the spark plug and wire. Ive replaced the carb mating gaskets to the cylinder head. It still has a miss at idle. It will get progressively worse as the engine reaches operating temp.

If you give the engine throttle, it will run right up without any hesitation. If you release the park brake and begin driving the miss seems to go away for a while. After a few minutes under load the engine will develop a slight miss but will never die.

If you engage the parking brake and try to idle it down slowly it will have a terrible miss to the point of it just flat dieing. Ive found that when it starts acting up when at operating temp and you attempt to idle it down, if you give it just a little bit of choke it seems to smooth out and run better. Ive tried adjust the carb to give it a little more fuel. Doesnt seem to help. 

Ive pulled the cylinder head off and found the usual carbon deposits.

It didnt appear that the head gasket was blown. Acr looks to be working correctly. Cylinder wall looks fine. I checked valve lash and it was in spec. I pulled the valves out to inspect the seats. Exhaust seat is slightly pitted along with the valve face but nothing catastrophic. The intake seat looks uniform but the intake valve looks like it had been extremely hot at one point. This machine has the breakerless igntion on it. I have new valves coming and run a pattern on the intake to verify the seat is okay. 

 

 

The blue color on the intake stem is concerning. I'm thinking you have intermittent gremlins in you ignition trigger throwing the timing off. Or there is something loose in the muffler not letting the exhaust gasses escape and overheating that valve and leaning out the mixture. 

[rmaynard 16,873]

Check for play in the throttle shaft. Air leaking in around shaft can cause poor idle.

[lahren68 6]

2 minutes ago, rmaynard said:

Check for play in the throttle shaft. Air leaking in around shaft can cause poor idle.

I turned down some aluminum roundstock in my lathe and bushed the throttle shaft thinking that could be the issue

[lahren68 6]

6 minutes ago, squonk said:

 

 

The blue color on the intake stem is concerning. I'm thinking you have intermittent gremlins in you ignition trigger throwing the timing off. Or there is something loose in the muffler not letting the exhaust gasses escape and overheating that valve and leaning out the mixture. 

Im leaning towards something leaning the mixture out. Whats puzzling me is the slight amount of choke at operating temp will make it run correctly but it will sput and sputter when the engine is cold if you use any choke after its running. Maybe im chasing another symptom and not the root cause with this one. 

Edited 7 hours ago by lahren68

[pfrederi 19,914]

Be very interesting to see how the spark holds up when it is acting up.  

[lahren68 6]

I guess one thing that i was just thinking, is there a way to verify ignition timing on these breakerless engines? If theres a potential of the timing being too advanced?

[pfrederi 19,914]

You cannot adjust the timing on a breakerless system. The trigger is only adjustable in and out from the flywheel and even the manual indicates that gap isn't critical.  also there is no way it can change based on heat...

[702854boy 506]

If it ends up being the breakerless being bad there is a way to convert over to the points and condenser style of ignition.

[squonk 45,539]

26 minutes ago, pfrederi said:

You cannot adjust the timing on a breakerless system. The trigger is only adjustable in and out from the flywheel and even the manual indicates that gap isn't critical.  also there is no way it can change based on heat...

I don't know the specifics of the internal circuitry of the trigger but when transistors and triacs are involved anything's possible to mess up the signal and possibly the spark timing by a few degrees. I've seen electronics to some wild stuff that made no sense.

[pfrederi 19,914]

22 minutes ago, 702854boy said:

If it ends up being the breakerless being bad there is a way to convert over to the points and condenser style of ignition.

  Yes it is easy to do. The hole is already in the block for the points push rod

 

If it is a problem with your breakerless coil  I have one you can have.

[lahren68 6]

1 minute ago, pfrederi said:

  Yes it is easy to do. The hole is already in the block for the points push rod

 

If it is a problem with your breakerless coil  I have one you can have.

I sure do appreciate it. I will keep in touch. I hope im not coming off as a dummy here im just trying to gather info. The common things to try/check havent netted many results one way or the other. Im probably overlooking something simple

[sqrlgtr 2,173]

Does adjusting the high speed and idle on carb have any effect on the way it runs? If not, the carb is still stopped up somewhere. Had a similar experience a while back, clean carb, try another carb, replace fuel line etc. Finally came to the conclusion that fuel pimp was full of trash and would stop carb up almost immediately when I got it back together  Rebuilt fuel pump and haven't had any more trouble out of it. Just something to think about and hope you get it running.

[rmaynard 16,873]

I'm sure you have tried this, but if you haven't, the main jet needle is hollow with several tiny holes in it. All of those little holes must be open and be able to pass air for the full length of the needle.

 

Edited 2 hours ago by rmaynard

[lahren68 6]

1 hour ago, rmaynard said:

I'm sure you have tried this, but if you haven't, the main jet needle is hollow with several tiny holes in it. All of those little holes must be open and be able to pass air for the full length of the needle.

 

I checked that when i first pulled the carb apart. I was able to push carb cleaner through all of the orifices. The engine is very responsive to the adjustments done to the carb. Thank you guys for all of your input. I really appreciate it 

[sqrlgtr 2,173]

I would lean toward coil or trigger then. Sounds like you have fuel system covered. Would be nice if you had a known good coil to swap out just to see. That would be my next move if it was me. Symptoms do sound like a condenser but seeing how it's a breakerless ignition system I will say with 100% certainty that's not your problem .

Edited 49 minutes ago by sqrlgtr

[Handy Don 15,950]

7 hours ago, pfrederi said:

does the spark stay strong when it is acting up?

YES! This is the key diagnostic. Bad spark--electrical. Good spark--fuel/air.