Rough Running 756

Bill VA · Oct 27, 2018

Looking for wider audience - posted the same on RP too...

Been wanting to post a video with sound and symptoms of my otherwise trusty 756 gasser faltering at high rpms. ?First thought was gas flow. ?Screen from the tank to the carb is clear.

Background - after carb rebuild and thorough tune-up, the tractor ran like a champ. ?Mowed one field with my Krone disc mower conditioner - no problems. ?Couple weeks later, started mowing another field and about half way through the tractor started sputtering and trying to stall. ?Drop back to idle, and the engine recovers. ?Put it under load - after a bit, it repeats.

I've been using the tractor for light duty chores and good. ?Recently, the problem has become worse. ?Probably time to do something...

Any ideas?

Thanks!
756 Rough Running...

Janicholson · Oct 27, 2018

The symptoms point me to a distributor bushing issue. AS they wear, the point gap becomes erratic, and leads to changes in timing between cylinders, and at higher speeds. The timing can be off more than 10 degrees if the wear on the shaft is only .008 or so. The shaft must not wiggle fro side to side. Also check the function of the centrifugal advance (under the breaker plate. Jim

Bill VA · Oct 27, 2018

Thanks! So is the remedy for the bushings a replacement bushing or would retrofitting with an electronic ignition make this a non-issue?

ss55 · Oct 27, 2018

If the distributor shaft bushings are loose, an electronic ignition will be just as erratic as a points ignition, BTDT.

the tractor vet · Oct 28, 2018

well Bill i hate to tell ya what is going on as the experts will tell you i am wrong . The problem is the GAS not the dist. bushings it['s the gas . Your 756 runs a C291 and ing timing is 18 degrees BTDC at rated full throttle RPMS . The last of our gas powered 706's both with the C291 are now DEAD due to melted pistons and scored liners and they were doing just what you were doing at the time , Mowing hay . One was on a John Deere 1219 hay bine in heavy first crop and the other was hooked to a Kuhn disc bine . The new load of what they call 93 is really 90 octane spiked up with AL-KI-HOL at the rack . 90 is the highest coming down the pipe now and then blended at the rack . I am not rebuilding them anymore it is a waist of my time . If i do anything it will be a trasplant to something else .

Mike Van · Oct 28, 2018

The 2 guys that answered have diagnosed a lot of problems, but i'm not sure either is right this time - I watched your video, when the engine isn't breaking up & missing, it sounds pretty good. Seems if the internals were burnt up from bad fuel, it would run crappy all the time. Bad compression doesn't come & go as a rule. Also, a loose distributor shaft is loose, right? It can't tighten up & loosen up by itself. I would look to all you did in the 'tune up' first, a bad electrical connection, loose wire? Or something floating around in the gas tank? Hose collapsing? One or more cylinders are cutting out, I don't think it's huge, you just haven't found it yet. Let us know when you do -

Rex Simpkins · Oct 28, 2018

Check the easy stuff first new points from China are not worth a damn I don't care who you buy them from. See if you can find a set of NOS IH points if not find a set of old(new) Echlin points from the 60's or earlier or use petronix, check your carb again clean it then clean it again and check your fuel tank for trash

dwag · Oct 28, 2018

Remove sediment bowl assy and make sure not plugging? Dist advance stuck? Bad condenser? Worked w/too hot spk plgs? Intake manifold loose/bad gskt?

Bill in IL · Oct 28, 2018

Pull the plug out of the bottom of the carb bowl and be sure you have good steady gas flow for a minute or two or three even. It will run fast for the first 15 seconds As the bowl empties but you need to be sure gas is getting replenished quickly enough to maintain power. Its not uncommon to get a speck of dirt in the float needle and slow the flow even after being filtered at the sedinent bowl.

mattofvinings · Oct 28, 2018

Amongst the other checks I'd run my fingers along the spark plug wires with engine OFF. Check for fraying or chafing especially where wires cross each other. If any wire is compromised the spark can end up finding another path of lesser resistance to ground. At higher speeds and higher loads, more energy is required to jump the gap in the combustion chamber, and if there's "another way out" the electrical energy will choose it, causing misses. I've heard it called "stealing spark" but its just a path of less resistance.

mattofvinings · Oct 28, 2018

Sorry, forgot to add, you can compensate for a lean condition with the choke, bring it in slowly and see if the additional fuel from additional vacuum helps the situation. If that helps restore smoother operation then I would spend my time working thru the fuel system and carb.

dwag · Oct 30, 2018

Hook timing light to coil wire and watch for steady flash when eng is "acting up".

Rough Running 756

Bill VA

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Janicholson

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Bill VA

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ss55

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the tractor vet

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Mike Van

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Rex Simpkins

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dwag

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Bill in IL

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mattofvinings

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mattofvinings

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dwag

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