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AC720...backfiring like crazy!!!


Itsmechuck

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Itsmechuck

Was mowing with my 1978 AC720, when all of a sudden it started backfiring like crazy!  Just continuous, loud backfiring.  I barely got it back to the workshop before it died.  Does this sound like it's jumped time?  I tried to move the distributer box, but it seemed to be tight.  I took the lid off it to double check, and I was able to tighten the allen screws just a little.  I don't know if they were loose enough that the box could have moved.  I could also tighten the three screws a little that hold the points in.  What do you all think?

I bought this tractor new 43 years ago and have really enjoyed it.  I've had to tinker with it down through the years, but for the most part, it's been pretty dependable.  I can remember timing it years ago with what is now my 43 year old timing light.  But I really don't remember the process for that now.  I'm 72 years old now and don't always remember everything as good as I used to.  Me and my old tractor have slowed down a little bit in our old age.  Ha.   Can you guys give me some advice?  I sure would appreciate it.  Will try to put up a couple of pictures.

Thanks guys.

Chuck

614.jpg

Points.jpg

Name plate.jpeg

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rbstuartjr

I don’t know much about those Onan engines, but there could be a couple of reasons for the backfiring. The condenser could be going bad which would cause erratic spark. Also backfiring could also mean the carb could be getting plugged and running really lean until she finally quit. I would check for fuel then for spark. The timing on the kohler engines is done with the points, not sure about the onan.

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Fine looking machine! Congratulations on being a man that has owned one of these since new!  As for the backfiring, I agree with the above sentiments.  In reading your past posts, I see that the engine was rebuilt in the 80's & you have added an electric fuel pump & replaced the points & the condenser once before.  I had a similar situation with one of mine last summer & mine turned out to be a slowly dying coil.  I had removed the points & had it converted to electronic ignition so i know the points weren't  the issue.  I rebuilt my mechanical fuel pump & then ultrasonic cleaned my carburetor & then rotated my fuel tank full of Crossman BB's in an electric cement mixer for 6 hours (spotless inside when done).  Last thing I did was try a different coil & the problem instantly disappeared.  

I sure hope you are able to get the problem rectified in a timely manner.

Edited by ShaunE
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Talntedmrgreen

An upgrade to electronic ignition for your machine would be a great improvement, but I do feel the timing slipped, and with a light it is a very quick and easy process to reset them.  The trouble comes when they consistently lose time, at which point the points box simply will not tighten and hold position.  Hopefully that isn’t the case for yours.  
 

I am not in a position to show the manual section describing the points adjustment, but it is simple.  A little adjustment goes a long way though. The light can be connected to either side of the motor, as the cckb fires both plus simultaneously. 

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Itsmechuck

Thanks everyone for all the good information.

I hope to get a chance to work on this in the next couple of days.  Will let you know if I make any progress.

Thanks again...

Chuck

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On 5/15/2021 at 8:08 PM, ShaunE said:

then rotated my fuel tank full of Crossman BB's in an electric cement mixer for 6 hours (spotless inside when done).  

ROFLMAO!!! Only because I did the same thing for a Gold Wing fuel tank!
Wrapped it up in carpet to protect it, changed the angle on the cement mixer to get all the nooks & crannies, and then reversed the tank in the mixer and ran it again.

https://www.facebook.com/gary.hickman.54/videos/1241322479284308

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10 hours ago, Hick said:

ROFLMAO!!! Only because I did the same thing for a Gold Wing fuel tank!
Wrapped it up in carpet to protect it, changed the angle on the cement mixer to get all the nooks & crannies, and then reversed the tank in the mixer and ran it again.

If the tank wouldn't have had a brazed in pickup tube inside, I would have put it in a paint can shaker & had the job done in a few minutes but I was afraid the pickup tube would get damaged.

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