the saga continues... PT-425 engine problem at 10 hours?

[dcdtn]

Specifically, check inside the elbow at the bottom of the fuel tank. MANY of those here have ended up with chunks of sealant in there left over from tank manufacture.

The backfiring and dieseling (running after shutdown) is normal. The exhaust manifold is insanely hot because the exhaust header and muffler are poorly designed. If you do a little searching you will find that quite a few of us have got rid of the stock header and muffler and replaced it with the Robin header and muffler, which seems to help.

These little tractors have problems with vapor lock, although it seems that it is probably cool enough out (YMMV) that this shouldn't be as much of an issue. The fuel pump is mounted on the same side of the tractor as the muffler, which seems to contribute to this...easy enough to relocate the pump.... nothing magical about that location. The pump is driven by pulsating pressure through a 1/4" line plumbed off the crankcase. You can put the pump anywhere you want.

I wish I'd have paid more attention to my new gas tank instead of assuming it was clean.

I may look into relocating gas pump. I was hoping the heat wouldn't be as much of an issue on these newer designs with muffler outside of hood.

 

[Kent]

Specifically, check inside the elbow at the bottom of the fuel tank. MANY of those here have ended up with chunks of sealant in there left over from tank manufacture.

The backfiring and dieseling (running after shutdown) is normal. The exhaust manifold is insanely hot because the exhaust header and muffler are poorly designed. If you do a little searching you will find that quite a few of us have got rid of the stock header and muffler and replaced it with the Robin header and muffler, which seems to help.

These little tractors have problems with vapor lock, although it seems that it is probably cool enough out (YMMV) that this shouldn't be as much of an issue. The fuel pump is mounted on the same side of the tractor as the muffler, which seems to contribute to this...easy enough to relocate the pump.... nothing magical about that location. The pump is driven by pulsating pressure through a 1/4" line plumbed off the crankcase. You can put the pump anywhere you want.

I also think vapor lock is a likely villain here. Before you do anything else, drain your tank and fill it with fresh 93 octane gas from a "TOP TIER gasoline" station. Treat the gas with Startron to deal with the ethanol and some kind of octane booster like Klotz Octane Booster to up the octane higher than 93 to lower the boiling point of the gas. If that doesn't help, then do the next least-invasive change, like moving the fuel pump and rerouting gas lines to cooler areas (and wrapping them end-to-end with a heat shield sleeve like Lava Tube Heat Sleeve). Then let us know if any of these things help!

I find Notes from the field: Ethanol, Vapor Lock and other Gas Woes - Military TraderMilitary Trader helpful. Think of the Robin engine as an antique car engine trapped in a heavy metal heat chamber.

Next steps are drastic, replacing the exhaust manifold and muffler with a genuine Subaru model and putting in a side fan. Or just waiting for Winter temps.

On a side note, I upgraded to the 28 hp fuel injection Robin engine, which has an electric fuel pump. This Summer I had a case of boiling fuel (can see it bubbling in the fuel filter) while bush hogging on a 85 degree day. Solution so far is to keep the gas tank full. You see, unused fuel is returned to the tank from the EFI throttle body, and when running low on fuel, the temperature of the gas in the tank keeps raising.

 

[MuncyBob]

Hope you get it figured out soon. I will also add that you should do the Robin exhaust mod sooner than later. Recently had to have valve job done due to heat issues. After the muffler mod(it really isn't that hard and I have some Walker flex pipe if you need it) I no longer have run on after shut down and temps have been drastically reduced inside the tub.

 

[dcdtn]

Hope you get it figured out soon. I will also add that you should do the Robin exhaust mod sooner than later. Recently had to have valve job done due to heat issues. After the muffler mod(it really isn't that hard and I have some Walker flex pipe if you need it) I no longer have run on after shut down and temps have been drastically reduced inside the tub.

I'll have to read up on the Robin muffler mod. Was your original muffler inside the hood or does the Robin muffler just work much better than the PT muffler? And thanks for flex pipe offer. I may take you up on that.

 

[elderlyphart]

My PT425 had same symptoms you describe. A bunch of times! Dirty fuel system. A fuel filter coming apart faked me out.
Anyhow same fix every time.
Pull the top off the carb. May have to remove one thru bolt to get the top off. Take the slo jet out. (part # 210-5 in the diagram) Get a strong magnifying glass and look at the hole. I betcha a six pack of my suds of choice you will find blockage of some sort. Blow it out and blow out the hole it came out of. Keep your face and eyes out of the way when you do that. Don't ask my how I know to not do that. Blowing out the hole should blow out the main jet in the process. Have a look down in the float bowl for any crud that came out or was already there. Mop it out.
As for that pile of gorilla crap they call a muffler. I pulled the whole mess out of there and made up a duel exhaust out of u-bends and some cheap shorty craigslist mufflers. Works slick and no heating problems anymore unless I lug it at slow engine speed. Sounds like an old BSA spitfire race motorcycle. Close to a Triumph but no quite the same. Way the heck better than that hugely restrictive way to small pipe crap they put on there. You listening Terry?
Also electric fuel pump etc. All good advice others have posted.

One more thing. If the accelerator diaphragm (part # 210-253) ruptures it will drain the fuel bowl through the vacumn line right into the cylinders and on into the oil. Without a shutoff all your gas is in the oil. Do not buy the expensive replacement from Suburu. Fifty some bucks or more. Go to the Harley Davidson dealer and get one that fits the big oinker hog motorcycles for way cheaper. It's the same part! Buy one for the Suburu off ebay and you will probably find its a hog part complete with the unnecessary extra gismo for hog carbs.
Edit this - the way to check for a ruptured diaphragm is - run engine to have carb full of gas. Shut off and pull the vacuum line (part# 210-267). If gas runs out the diaphragm is ruptured. Easy check.
(I spent my life wrenching on all kinds of greasy junk.)

I go back to being a lurker now...

 

[dcdtn]

My PT425 had same symptoms you describe. A bunch of times! Dirty fuel system. A fuel filter coming apart faked me out.
Anyhow same fix every time.
Pull the top off the carb. May have to remove one thru bolt to get the top off. Take the slo jet out. (part # 210-5 in the diagram) Get a strong magnifying glass and look at the hole. I betcha a six pack of my suds of choice you will find blockage of some sort. Blow it out and blow out the hole it came out of. Keep your face and eyes out of the way when you do that. Don't ask my how I know to not do that. Blowing out the hole should blow out the main jet in the process. Have a look down in the float bowl for any crud that came out or was already there. Mop it out.
As for that pile of gorilla crap they call a muffler. I pulled the whole mess out of there and made up a duel exhaust out of u-bends and some cheap shorty craigslist mufflers. Works slick and no heating problems anymore unless I lug it at slow engine speed. Sounds like an old BSA spitfire race motorcycle. Close to a Triumph but no quite the same. Way the heck better than that hugely restrictive way to small pipe crap they put on there. You listening Terry?
Also electric fuel pump etc. All good advice others have posted.

One more thing. If the accelerator diaphragm (part # 210-253) ruptures it will drain the fuel bowl through the vacumn line right into the cylinders and on into the oil. Without a shutoff all your gas is in the oil. Do not buy the expensive replacement from Suburu. Fifty some bucks or more. Go to the Harley Davidson dealer and get one that fits the big oinker hog motorcycles for way cheaper. It's the same part! Buy one for the Suburu off ebay and you will probably find its a hog part complete with the unnecessary extra gismo for hog carbs.
Edit this - the way to check for a ruptured diaphragm is - run engine to have carb full of gas. Shut off and pull the vacuum line (part# 210-267). If gas runs out the diaphragm is ruptured. Easy check.
(I spent my life wrenching on all kinds of greasy junk.)

I go back to being a lurker now...
View attachment 482003

Thanks so much for the info. I plan to start tackling the poor running issue this weekend. I've never had fuel or even engine issues before on my lawn mowers, so I've never had to work on them. Ironically here I am with a brand new machine faced with possible need for taking it apart, so it's admittedtly a little intimidating.

 

[dcdtn]

So far I've replaced the fuel filter. It was pretty nasty (see photo). Don't know if the trash was already in the new tank or if I wasn't careful enough when transferring gas from old tank to new.

That didn't work so I took KentT's suggestion and blew air through fuel line from in front of filter back towards tank.

That also didn't work so only thing I know to do is take apart carb. Not really looking forward to that.

 

[elderlyphart]

Slow and easy on unfamiliar stuff. Pay attention and you will be fine. I probably left out several little steps. If something confuses you to much stop and ask. We all been there.
Have to remove the float and needle to get at any crud in the float bowl. Be careful to not drop the needle down the carb throat. Don't bend the tab the needle sits in. (That is how you adjust fuel level in the carb- I'm sure it is adjusted correctly as is)
Be careful when you lift the top of the carb off. Kinda swing it around to accommodate the linkage rods so you don't bend them. Don't get all ham fisted.
Might pull the fuel line off the carb and make sure gas will flow to be sure any blockage from the tank is gone for sure. Or just observe flow into float bowl with float depressed. Whatever works.
When you get it fixed you can stick your fingers in your suspenders and strut around like some kind of big time nascar wrench.
All part of the fun. Sometimes I write things down as I go on unfamiliar stuff to keep myself out of trouble.

 

[dcdtn]

I finally figured out that my carb is not the one posted on Subaru Robin site or the diagram here. It is configured similarly to the EH90/EH99. Only correct diagram I found was on jackssmallengines:

 

[elderlyphart]

Well now, big help I am.
Seen that carb before but not familiar with it at all.
Maybe Subura decided its a better carb than that overly sensitive thin I'm talking about.
Looks like a guy could drop that float bowl and have a look.