Need help with a 312-8.

[rrbennett 1]

Have 312-8 purchased new in 1990.  2 years ago was having problems with  engine missing, backfiring, and would just die while mowing.  Then would not start at all.  Fuel pump was bad so replaced that.  Afterwards engine started fine at about half throttle, and seems to run ok.  If I throttle up quickly, the engine dies, slowly it runs but misses a little.  Also engaging pto does the same, slowly ok, fast engine dies.  Also if I throttle up slow and engage pto slow and try to use, it will run for maybe 5 ,minutes then will lose power and die.  I got so frustrated I just parked it 2 years ago in the barn and bought another mower.  It has been shed kept its entire life, and maintained  regularly.  Yesterday I charged the battery I had left in it, put in some fresh gas, and it started easily, and ran just as it did when I parked it.  I can't believe there is much wrong with it, but I am NO engine mechanic, and am lost.  If anyone has some suggestions please respond in layman's terms this NON-mechanic will understand.  

 

Thanks for any advice you may have.

 

Ron

 

 

[N3PUY 1,031]

If it backfired thru the carb that indicates a "lean" condition.  (dirt in carb)   If it backfires out the exhaust that indicates an electrical problem. (low voltage or dirty points)

 

Does it run better with a little choke?  If yes try opening the small mixture screw ¼ turn.   Or take it out and spray into hole with carb cleaner.  

[rrbennett 1]

Thanks. I'll check those things out.

[KC9KAS 4,744]

@rrbennett 

[953 nut 60,264]

  Have you been using ethanol free gas? If not the rubber fuel lines have probably deteriorated and should be replaced along with a new filter. You can find stations that carry non-ethanol gas at buyrealgas.com. Once the line and filter have been replaced the magic potion that has worked well for lots of us is "Seafoam", two of their products used in combination  can work wonders on neglected fuel systems. You will want to add Seafoan Fuel treatment to the gas according to label instructions. Next you will want to use Seafoam spray carburetor cleaner. With the engine running at half throttle or faster and air filter removed, spray it into the carb. air inlet until  it is about to stall, allow it to pick back up and repeat. It will smoke quite a bit, but after a few shots it should run a lot better, has worked for me.

[can whlvr 994]

that machine has no points,its a module ignition,its pretty rare that anything goes wrong with the electrical on a magnum,i would bet its fuel related,like stated,put new lines on and fuel filter,take carb off and clean if you you tube it theres good info on how to do this,you will need a can of spray carb cleaner,and if you can find on a carb kit for the magnum engine,i like to have one on hand for the bowl gasket at minimum,might need a float needle too,its not real hard to do just need some time and be patient,she will run good again

forgot to mention look in the gas tank at the spot where the fuel line goes in,it needs to be cleaned sometimes too

Edited March 14, 2016 by can whlvr

[roadapples 6,983]

good luck and  

[grinchsr 647]

I would remove the gas tank and clean it out and also remove the fuel shut off from the tank and give the strainer area a good cleaning and blow it out good with compressed air.  I had this problem once.

[rrbennett 1]

Thanks to everyone who responded with advice.  I replaced the spark plug, returned the carb  to factory settings, and now runs 90% better.  I guess that when I had the fuel pump issue 2 years ago, and adjusted the carb, I fowled the plug, causing all this headache. All I know is it's much improved.  Thanks to all.

[gwest_ca-(File Mod) 11,512]

Factory carb settings should allow any engine to run. Since every engine is different you have to fine tune it from there. Once you start to dial it in you may find that after more hours you can improve on it even more. Marvel Mystery oil or Seafoam in the fuel often makes an improvement given time. 

If you have a tachometer adjust to maximum rpm for the given throttle plate position (idle or full throttle) and then make it 1/8 turn richer. If it stumbles on acceleration give the low speed another 1/8 turn richer.

 

Garry

[JC 1965 1,532]