Spark Plug Specs for PT 425 w/ Robin EH72

toolz_not_toyz · Oct 28, 2005

( It may mot be what is recommended but I throttle back some before I shut down, but it still backfires. I wonder if when you cut the machine off if somehow the fuell pump would cut off first and still have a delayed power cutoff to the spark plugs to allow it to burn the excess fuel off while it is in the combustion chamber wouldn't solve the problem. )

You're doing it backwards. You need to open the throttle wide and kill the ignition. The solenoid cuts off the fuel supply immediately but the momentum of the engine continues, which causes compression, and when combined with the heat in the engine the remaining fuel is burned properly instead of igniting in the exhaust (which causes the backfire).

toy · Oct 28, 2005

No as my understanting of it ,and mine does this also , when you cut it off whether at full throttle or at half full throttle it backfires. My thoughts was that when you cut off the key switch it not only cuts off the fuel pump but it also cuts off the power tothe spark plugs simultaniously, thus the momentum of the engine siphons some gas after the pump is denergized and when it goes into the cylinder it don't have any spark to ignite it so on the exhaust stroke it pushes the gas out into the exhaust manifold which is hot and the conditions is right that it ignites it in the exhaust manifold.

J_J · Oct 28, 2005

The Kohler on my 25hp doesn't have an electrical fuel pump,
and you are right about some fuel is probably left in the fuel bowl that could be siphoned out at shut down.

In my post above, # 759899. I mentioned a test about cutting power to the fuel shut off solenoid to see if the engine would shut down. A solution might be to put a switch in the solenoid line, and cut off the fuel solenoid first and then cut the ign. This will cause the engine to cut off due to fuel starvation, and burn off any fuel left in the carb. If you do have an electric fuel pump, a switch in line would do the same thing. You could even tie the fuel solenoid wire and electric fuel pump wire to one switch.

duane · Oct 28, 2005

The carb solenoid plunger closes off the main jet, so theoretically it cannot pull fuel from the carb bowl. With the main butterfly open at higher throttle, it should not be able to pull fuel through the idle transition ports. The run on and backfire results indicate some amount of fuel is getting to the cylinders/muffler.

Gravy · Oct 28, 2005

Duane,

I'd say you've nailed it. Your explanation makes perfect sense. Shutting down at closed throttle would definitely create enough inlet vacuum to do it.

I can't think of a practical carb modification to prevent this, so any solution would have to be from the other direction - the exhaust.

The exhaust manifold on my 425 has the two round header pipes welded at a right angle to the square "log" type crossover pipe. I suspect that this creates localized extreme hot spots on the flat wall of the crossover. These would be ideal igniters for any fuel coming through, especially at the lower velocity at idle. I have noticed that allowing the engine to idle for a minute or so generally prevents the backfire, probably because the manifold cools down.

I wonder if replacing the exhaust system with the curved factory manifold and side mount muffler (page 44 in the Robin parts book) might make a difference. I suspect it would be quieter and flow better. I'm not ready to spend the time and $$$ /forums/images/graemlins/blush.gif to find out just yet.

Gravy

tracdoc · Oct 28, 2005

( The run on and backfire results indicate some amount of fuel is getting to the cylinders/muffler. )

( thus the momentum of the engine siphons some gas after the pump is denergized and when it goes into the cylinder it don't have any spark to ignite it so on the exhaust stroke it pushes the gas out into the exhaust manifold which is hot and the conditions is right that it ignites it in the exhaust manifold )

I agree on both points raised. Indeed, I can nearly always smell at least a whiff of "raw gas" when the "bang" is a small one. I was reasoning that big bang = complete burning of the gas in the exhaust system, while "little bang" = incomplete combustion.

Where does that leave us? How common is the phenomenon in the Kohler engines? I recall when I visited FOURTEEN to see a PT up close, his PT (w/ one of the last Kohlers) banged pretty loudly.

RegL · Oct 28, 2005

" Where does that leave us? How common is the phenomenon in the Kohler engines? I recall when I visited FOURTEEN to see a PT up close, his PT (w/ one of the last Kohlers) banged pretty loudly. "

I have a 25hp kohler in a Scag cougar and have never had a problem with engine backfiring. It's a common engine I think and I've never herd of a problem except with the PT. Must be the exhaust design. Did PT have to design their own exhaust to make it fit in the engine compartment?

JackRobin · Oct 28, 2005

( 

Jack,

Some of us have engines that are still under warranty. Would this presumed retrofit fall under that umbrella? I don't think that Power Trac would be particularly receptive to inquiries from customers regarding engine difficulties, especially since they specifically state that any and all engine issues during the warranty period should be addressed to the engine manufacturer.

How do you suggest we proceed with this in a manner that is fair to all? )

You'll need to take this up with Power Trac, as I said in the earlier post, we do not encounter this problem on any other application but PT, so it leads us to believe it's not a warranty issue because frankly, there's nothing wrong with the engine but possibly with the application.

As you know, warranty is a defect in material or workmanship, this issue unfortuantely is not it.

Regards,

Jack Gawecki
Robin America, Inc.
Technical Support Dept.

KentT · Oct 28, 2005

( Where does that leave us? How common is the phenomenon in the Kohler engines? I recall when I visited FOURTEEN to see a PT up close, his PT (w/ one of the last Kohlers) banged pretty loudly. )

My Kohler will backfire if the engine is real hot (prolonged use) and you don't shut it down at 1/2 throttle or above. If the engine is relatively cool, you can get by with shutting it down at lower speeds without it backfiring...

I had to go research this one myself in the Owner's Manual, as JJ describes, because on all my old B&S single-cylinders, you idle them down for a minute or so, then shut them down. Shutting the Kohler off at 1/2 throttle or more seemed totally foreign to me. Old habits are hard to break. Now, basically I just shut the engine off whereever the throttle is sitting -- since it's usually operated above 1/2 throttle.

MossRoad · Oct 28, 2005

Our 25HP Kohler never backfires... it does run-on occasionally.

We have a 8.5HP Briggs that ALWAYS pops after shutdown. It is so reliable, that the kids know it will pop. I point my trigger finger at them and shoot! The engine has been doing this for 11 years, at least. The fuel is igniting in the hot muffler.

I guess I missed the point of why this is bad. What did folks do before fuel cutoff solenoids? Just asking... /forums/images/graemlins/crazy.gif