520-H - Rough start/No Start. Sometimes backfiring

[TC10284 113]

Model 4120OE02 520-H with 60" deck. 

Used it a week ago to mow the front yard. It did pretty well although it slightly choked on thicker grass. 

When I was finished and idled it down, it slowly died. Let it sit for a few hours while I used another WH to mow the back yard. After I sprayed out the deck on the 520H, it cranked up and ran OK to put it up. Started it up again a few days later. Very rough start. I filled the tank last time I used it and got a hold of some bad gas. The exhaust was white smoke that also smelled like old/stale gas. So I let it run to try to burn it out. I had to keep it around 1500RPM or higher to keep it running. Finally got most of it burnt out after repeated plug cleanings when it died. The plugs were pretty black. Maybe a bad idea to try to burn it out...ugh. 

 

So today I drained the little bit of gas was left in the tank. Put fresh in. Cranked and it ran for maybe 20-30sec and cut off. Let it sit for 10min, tried again. Backfired and would not start. Let it sit for another 10min. Started again and it would run for 20-30sec and cut off. 

Both times it ran, it was again blowing out white smoke. Both times it died the exhaust got pretty heavy black. 

Replaced plugs with two new NGK TR5. Checked spark - good. Checked compression. Both cylinders were around 100-110 PSI. No change. 

 

I took off the carb and replaced with a spare from a P220g that I had cleaned months ago. When doing so I noticed that the rear cylinder had maybe 1/4" of gas in the intake valve. Also checked valve clearances - both sets were in good spec. Intake was at 0.005 and exhaust at 0.013. Turned the engine over by hand and could see both sets of intake and exhaust valves opening and closing fine (from looking at them in the intake and exhaust). Nothing stuck. 

I didn't have much time to test it since it was already dark, but the couple times I tried starting it, it didn't start, and I only heard what sounded like a bubble popping, as if it was eventually going to backfire(?). Tomorrow I'm going to try some more. 

 

Not sure what's going on with this problem. Any thoughts? 

Edited April 29, 2019 by TC10284

[ericj 1,579]

sounds like your bad gas fouled your carb and then didn't atomize the fuel causing it to run to rich/ flooding the motor. Check the oil to make sure you didn't dump gas back down past the rings. good luck

 

 

 

 

 

 

eric j

[TC10284 113]

6 minutes ago, ericj said:

sounds like your bad gas fouled your carb and then didn't atomize the fuel causing it to run to rich/ flooding the motor. Check the oil to make sure you didn't dump gas back down past the rings. good luck

 

 

 

 

 

 

eric j

Good call on checking the oil. I'll do that tomorrow. Needs to be changed anyway. 

[TC10284 113]

Update:

Tried again this morning - cranked and cranked. Nothing. 

 

Took a chance and replaced the plugs with new NGK TR5 plugs. Turned over and fired right up. Took a minute or so, but the white smoke finally cleared out!

Shut it off for maybe 15min, tried again, and it cranked right up!

 

Is there a way to save the fouled TR5 plugs since they were basically new? 

[lynnmor 7,824]

Why would you not use the recommended Champion plugs especially since you had a failure with the TR5's?

[TC10284 113]

10 hours ago, lynnmor said:

Why would you not use the recommended Champion plugs especially since you had a failure with the TR5's?

 

Because I had a harder time finding the recommended Champion plugs and the NGK TR5 cross referenced the Champion model, unless I was looking at the wrong Champion model. 

 

Champion is what came out of it. I've used NGK TR5 in other 416/520's and they've done well thus far. The only exception seems to be this run in with very bad gas. I would imagine that happening to any brand plug though.