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AC720 limited power and backfiring

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healinggreenfarms View Drop Down
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Joined: 21 Jun 2021
Location: Raleigh, NC
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Post Options Post Options   Thanks (0) Thanks(0)   Quote healinggreenfarms Quote  Post ReplyReply Direct Link To This Post Topic: AC720 limited power and backfiring
    Posted: 21 Jun 2021 at 7:46am
Hello all!

I tried searching the forum and didn't see exactly what I'm experiencing, although perhaps I don't know the correct lingo.

Reba is a '78 720.  Relatively new transmission.  Working great and then in the middle of a job lost *almost* all power to the wheels.  Front and rear PTO working fine.  If I jostled the gear selector I could get her to move a little.  We limped back to the shelter, BARELY making it up a little hill.  Same problem after I let her rest for a few days.  

Now, when ever she is shut down she shudders then backfires.

Any thoughts?

Thanks!
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Acdiesel View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Acdiesel Quote  Post ReplyReply Direct Link To This Post Posted: 21 Jun 2021 at 11:13am
so, the engine starts and runs at an idle and if you try to give it any throttle it looses power and back-fires??

check the breaker points gap- 
also a failed condenser will give you the same issue.
D19 diesel,D17 diesel SER.3 D14 NF D14 WF, D15 SER.2,
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I'm a pharmacist (farm assist) with a PHD (post hole digger)
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AC720Man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AC720Man Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jun 2021 at 8:52am
Points and timing is a good place to start but before you do that, see if your getting adequate fuel through the clear fuel filter if you have one. Fuel pump may be going bad and if enough fuel isn’t getting to the carb it will cause the same issue. Mine did it last weekend and I am now deciding if I’m going to rebuild the pump or put an electric pump on it. I like staying factory but the fuel pumps are problematic. Have also had a bad fuel cap do the same thing, but if it sits for a half hour it would run again just fine for a few minutes until it began creating a vacuum and start sucking the tank in from the sides and die. The vent goes bad in the cap, no cleaning I did resolved it. So I replaced both of mine with a cap that has a fuel gauge which makes it nice to get a quick visual of the level of fuel in the tank.
1968 B-208, 1976 720 (2 of them)Danco brush hog, single bottom plow,52" snow thrower, belly mower,rear tine tiller, rear blade, front blade, 57"sickle bar,1983 917 hydro, 1968 7hp sno-bee, 1968 190XTD
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farmtoybuilder View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farmtoybuilder Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jun 2021 at 1:37pm
Stick a Kubota 3 Cylinder diesel in it! :) Onan motors in them were a pain!

5 different TT-10's,5 TT-18's Terra Tigers,B-10,2 B-207's,B-110,2 B-112's,HB-112,B-210,B-212,HB212,2 Scamp's & Homilite T-10. Still hunting NICE HB-112 & anything Terra Tiger & Trailers for them.   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PaulB Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jun 2021 at 8:40pm
ONANs are known for having valve seats come loose: check compression.
If it was fun to pull in LOW gear, I could have a John Deere. If you can't make it GO... make it SHINY
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AC720Man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AC720Man Quote  Post ReplyReply Direct Link To This Post Posted: 29 Jun 2021 at 9:15pm
After reading this again, is the engine running ok, except for the backfire, is this a hydro problem? Does the engine run fine but as you move the hydro lever forward is the tractor just not moving? Need some more info
1968 B-208, 1976 720 (2 of them)Danco brush hog, single bottom plow,52" snow thrower, belly mower,rear tine tiller, rear blade, front blade, 57"sickle bar,1983 917 hydro, 1968 7hp sno-bee, 1968 190XTD
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AC720Man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AC720Man Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jul 2021 at 6:44am
Update?
1968 B-208, 1976 720 (2 of them)Danco brush hog, single bottom plow,52" snow thrower, belly mower,rear tine tiller, rear blade, front blade, 57"sickle bar,1983 917 hydro, 1968 7hp sno-bee, 1968 190XTD
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AC720Man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AC720Man Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jul 2021 at 7:55pm
My 720 acted up again yesterday, back firing and started loosing power. Mixture screw on the bottom of the bowl backed out again. Need to put some pipe tape on the threads this time. Apparently the packing isn’t keeping it tight.
1968 B-208, 1976 720 (2 of them)Danco brush hog, single bottom plow,52" snow thrower, belly mower,rear tine tiller, rear blade, front blade, 57"sickle bar,1983 917 hydro, 1968 7hp sno-bee, 1968 190XTD
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dee_veloper Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jul 2021 at 8:33pm
I know of 3 things which cause backfiring

1) A timing problem
2) Sticking valves that aren't closing
3) excess fuel detonating in a hot exhaust.

Check the valves to make sure they are opening and fully closing

I don't know what type of ignition system you have but if points, check they are in good condition and gapped correctly.  If you have an ignition module it may need to be replaced

If you have a carb. then check the fuel air mixture setting to make sure you aren't running rich
Don't confuse my personality with my attitude.
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talntedmrgreen View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote talntedmrgreen Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2021 at 8:16am
9 times out of 10, the backfiring ont hese is timing.  The points box loosens, and moves...the issue can come, go, worsen, cure itself, then come back, as the points box shifts around.  It's easy to press on the points box left or right, and see if the condition goes away or worsens, and then you have your answer.  Timing is really pretty easy to set on these.
Patterson B1*B1/B1Forklift*B10*BigTen/B1Loader*B110*B208*B206*616*Homelite T16H/T8/T7/RM7E*Landlord/Loader/Trencher*Broadmoor717*Serf525*2110*7790*Legacy XL*Twin 20*Squire9*Squire6*JCPenny 494
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talntedmrgreen View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote talntedmrgreen Quote  Post ReplyReply Direct Link To This Post Posted: 30 Aug 2021 at 12:52pm
I just combatted this, this morning, on a 4041 I brought home last week.  A minor points tweak solved the backfire and a carb adjustment put the fuel mixture where it needed to be for full power.  Brought this thing home, after dragging it out of the spot it sat for over 30 years.  The last oil change date was '89.  Runs like a top!
Patterson B1*B1/B1Forklift*B10*BigTen/B1Loader*B110*B208*B206*616*Homelite T16H/T8/T7/RM7E*Landlord/Loader/Trencher*Broadmoor717*Serf525*2110*7790*Legacy XL*Twin 20*Squire9*Squire6*JCPenny 494
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AC-DVB Quote  Post ReplyReply Direct Link To This Post Posted: 11 Sep 2021 at 6:40pm
Thats an awesome find, keep them alive!!
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