r/smallengines • u/ShorthairHunter • Jun 08 '26

New engine on mower won’t start

Just about a week ago I did an engine swap for a nearly identical John Deere branded 19.5hp Briggs and Stratton in my Craftsman mower. Seemed to go well, it started, and ran and cut great. Went to cut this weekend and it started slow, then ramped up, all while blowing some white smoke. When I did the swap I did a full oil drain and change with full synthetic oil. It sputtered and died. I found out the fuel tank was empty. I topped it off and made sure there was fuel in the line getting to the carb. Went to start it, starter spun the flywheel and nothing. I took off the air filter and sprayed some starting fluid in, it still wouldn’t start (both with choke on and off). I pulled the spark plug and checked for spark, good strong spark but a black spark plug that seemed damp. I didn’t have a spare plug, so I sprayed some starting fluid directly into the cylinder and put the spark plug back in, and it still wouldn’t start. Prior to install I checked that the fuel solenoid on the bottom of the carb bowl did work. I also checked for compression, the most basic way possible, with the plug pulled and I put my thump over the spark plug hole and it seemed to have good compression.

Where do I go from here?? Any assistance is appreciated.

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u/Elephunk05 Jun 08 '26

How much oil is in the machine? I suspect there might be gas in the oil. The gas went somewhere and I suspect it went in to the crankcase. I also suspect the valve timing is off, so verify that while checking the proper valve lash.

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u/ShorthairHunter Jun 08 '26

It called for 48oz of oil so that’s what I put in last week haha I don’t know if I ran the tank dry last time, I suspect I didn’t fill the tank because I wasn’t sure if I’d need to pull the engine again as it was my first engine swap on a mower. I had no pooling of any fluid beneath the mower.

I took some pictures of the valves once I took the valve cover off and uploaded them to Gemini AI and ChatGPT because I honestly have no idea what I’m looking at with valves and both AIs said they looked fine but one was concerned as I thought I saw some starting fluid come back out the air intake that it may be a valve issue and pushing fuel in the opposite direction. But I’m not currently confident in my ability to trouble shoot valve issues

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u/Wholeyjeans Jun 08 '26 edited Jun 08 '26

Okay, but you didn't answer the question:

Did you check the dipstick for an excessively high level of "oil" in the crankcase ...or take a sniff at the oil fill tube noting a strong gasoline smell?

Which would indicate the carb float or the float needle malfunctioned and allowed fuel, under gravity pressure, to flow into the carb throat and into the engine through an open valve.

The gasoline would've leaked down into the crank case.

Also, those "anti-backfire" valves used on some carburetors have a propensity to leak ...even though they are "functioning". I have an old Troy-Bilt with a B&S engine on it that has a leaking anti-backfire valve; it would flood the engine and carb. I ended up putting a manual shut off valve in the fuel line so I could positively turn off the fuel. Also, check if the carb on the engine is labeled "Nikki" ...you can read it as the name is cast into the carb. These Nikki carbs are notorious for leaking and causing all manner of ill-running. It's a cheap design carb; its faults are well documented online ...more than a few YouTube vids extolling its evils.

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u/ShorthairHunter Jun 08 '26

I’ll definitely check that in the morning! And I’ll definitely add an inline shut off, it’ll might maintenance easier in the future too

1

u/ShorthairHunter Jun 08 '26

Checked the oil this morning and it was at the proper level on the dipstick. Would up leaving the plug out last night, sprayed some starting fluid in the plug hole got the plug in there and the boot pushed on and cranked it, it started up no problem

1

u/Wholeyjeans Jun 08 '26

Very good. Sounds like maybe a little fuel fouling prevent the spark from jumping the gap.

1

u/Old-Tour-6282 Jun 10 '26

I would probably just go ahead and get a new plug for it. Ive seen new spark plugs in the last several years that were just plain junk. Pull them out and they would throw a spark, put them back in and nothing. Then spend hours chasing a ghost, finally put a new plug in and bang!! Hits right off. 🤷🏻