Spark Plug Specs for PT 425 w/ Robin EH72

tracdoc · Oct 17, 2005

I got a surprise today, and I thought I'd share it here, since I could find no prior references to PT Robin engine spark plug specs on the forum.

Basically, I pulled my spark plugs today to check them, partly because the manual says to do it every 50 hr, and partly because I am still experiencing that annoying "bang" (a single exhaust manifold detonation, aka backfire) at shutdown. Well, the plugs looked fine. However, I noted that the plug ID was NOT what is specified in my manual, nor in the current on-line EH72 manual at the Robin website {HERE}

The plugs I found in my engine: NGK BPR4EY
The plug specified in manual: NGK BPR6ES

The 4EY is a 2 heat ranges hotter plug than the 6ES. The heat range/ID can be found HERE by clicking the hyperlink at the bottom left (Mfr Numbering System).

I called Jack at Robin, and he told me that the company had made a changeover awhile back from the 6ES plug to the 4EY on the V-twins, and that the 4EY plug is the currently recommended plug for the EH72.

All I can say is Geez Louise, wouldn't it be good to publish this information somewhere, like in the website manual, perhaps? Anyway, thought I would add this potentially important factoid to the burgeoning forum-based "PT master service manual"

Would anyone consider going to a colder plug to eliminate the shutdown backfire? Jack suggested possibly a sticking carb solenoid, but that didn't seem to be an issue with mine.

Barryh · Oct 18, 2005

This will be interesting to watch as mine does the same thing after it has been running for a long time. I idle down for a few minutes then when I go to shut down, sounds like gun fire on the mountain. I have not really had the time to check my engine out thoroughly as of yet.

I have been too involved with other things here lately. Nice to have this forum and you boys to keep me up on the PT's. Hopefully after I get more time under my belt I will be able to contribute in some small way to this forum. /forums/images/graemlins/ooo.gif

NJBill · Oct 18, 2005

I have no problems wwith Champion N11YC.
Remember the is R is mainly to stop interfering with your neighbors radio.
Bill

MuncyBob · Oct 18, 2005

Are you saying that after switching to the Champion plugs you don't get the backfire any longer???? The only remedy I've had for this was to idle at lowest throttle for at least a minute(as previously posted). /forums/images/graemlins/confused.gif

JackRobin · Oct 18, 2005

( ( All I can say is Geez Louise, wouldn't it be good to publish this information somewhere, like in the website manual, perhaps? Anyway, thought I would add this potentially important factoid to the burgeoning forum-based "PT master service manual"

Would anyone consider going to a colder plug to eliminate the shutdown backfire? Jack suggested possibly a sticking carb solenoid, but that didn't seem to be an issue with mine.)

This information about the spark plug change was published on the Service Bulletin which was distributed to the Service Centers by our distributors. I'll check with the Publications Dept. why the manual at our website was not updated.

Also, DO NOT change this plug, unless you want to have problems starting this engine in the winter. I still think the backfire is a sign of diesling, most likely your solenoid is sticking and not stopping the fuel delivery to the cylinder in the time of an engine shut down.
If you're sure that this is not it, please be kind enough and throw some ideas on what do you think this could be caused by?

Regards,

Jack Gawecki
Robin America, Inc.
Technical Support Dept.

tracdoc · Oct 18, 2005

The Champion N11YC is 1 heat range cooler than the currently recommended NGK BPR4EY as per the cross- reference table {HERE} . Click on the thumbnail pic to enlarge.

I echo Farmall's question on whether or not shut-down backfire is seen with this plug. Thanks

Phils · Oct 18, 2005

My 422 has had that same backfire problem since I received it. Occasionally it'll run-on (diesel) for a few revolutions before stopping after it's turned off, but most of the time it's that single "bang". Rarely it shuts off like any other machine.

Since Jack is aware of this thread maybe together we can find an answer. I would think that a cooler plug could be the correction as fuel is igniting after the ignition circuit is turned off. A hot plug could cause that.

Last weekend I finally replaced the stock fuel hose and wondered if it would have any effect on this shutdown problem, but it didn't. My power has increased dramatically, tho, due to not having that clear fuel line kinking at the filter inlet and also having picked the pieces of sealer out of the fuel tank elbow (as mentioned by others here in the forum). I had thought that maybe it was running lean, therefore hotter, and maybe that was causing the "pop". So dang, I'd have been right here with an appropriate answer but it still did it after the new fuel line and filter were installed.

I can't believe it's the fuel shut-off solenoid though. When that is deenergized the motor is still revolving, and fuel mist and air are still in the manifold. Something is igniting it and since it happens "out of the box", carbon buildup can be eliminated as the ignition source. That puts it back to plugs.

Depending on where this thread goes, I'm voting for a cooler plug too. I don't have weather even near freezing, so winter starting won't be a factor for me.

Phil

Bird · Oct 18, 2005

This may have absolutely nothing to do with your problem, but mention of the fuel solenoid makes me wonder if it does. Now being an old timer, I grew up thinking it was best to shut off engines when they're at idle. And then I bought the Toro ZTR with a 14hp Briggs engine and both the Toro and the Briggs manuals say to turn off the key with the throttle wide open. That works fine, but if you move that throttle down to idle and turn off the key, you get one terrific bang that makes you wonder why it didn't blow the muffler off. I know absolutely nothing, beyond what I've read on TBN, about Robin engines; never owned or used one, never seen a manual for one, but I was wondering whether they're the same as this Briggs in that regard.

KentT · Oct 18, 2005

Bird, I have no experience with Robins either, but some of the newest Kohlers also call for shutting them down at full throttle... That RPM clears the last remaining fuel/air mix so quickly it is not ignited in the hot muffler -- the cause of most backfires on shutdown. These new clear-air requirements have small engines running so lean that the exhausts get VERY hot... so when that final bit of fuel/air hits them (having been expelled from the engine without firing), it explodes in the muffler.

Might be worth a try...

tracdoc · Oct 18, 2005

( I still think the backfire is a sign of diesling, most likely your solenoid is sticking and not stopping the fuel delivery to the cylinder in the time of an engine shut down )

Jack,

I quite agree that the shutdown solo backfire is an abbreviated form of dieseling or "running over." In my case, I inspected the carb solenoid as you suggested. After shuttting off the fuel supply, I removed the air cleaner and the air cleaner platen. I then removed the solenoid. On manual testing, the solenoid plunger had a smooth, unrestricted action with no sticking nor hesitation whatsoever. Although it looked clean, nonetheless, I sprayed it with carb cleaner, and re-installed it. I used the PT to apply fertilizer to 3 acres, and when I shut down, BANG! a single loud report originating in the exhaust manifold or muffler.

Based on comments here, I would guess that there are quite a number of PT owners who experience this problem, and only a fraction are reporting it in this forum. It is a characteristic shutdown event, that is, there is no prolonged dieseling, just a single explosive bang. It would be hard for me to reason that it would be attributed to a sticking solenoid in all these cases, unless there was some bizarre electrical malady with an entire batch of solenoids /forums/images/graemlins/confused.gif

Other possible causes of the problem:
* use of low octane fuel in an engine requiring high octane--resolved with use of 93 octane fuel + lead substitute
* hot spots in the combustion chamber
[*]*spark plug in too hot a range: ??return to an NGK 6 series plug
[*]*carbon build-up in combustion chamber
[*]* un-faired or sharp edges, e.g., casting flash in cylinder heads or valve seats
*spark timing retarded too far, resulting in additional heat as opposed to propulsion
* Air-fuel ratio out of optimal range.

I would expect that there are other potential causal culprits. In any event, I recommend that we investigate this as a systematic problem since it appears to affect more than just the "occasional" engine. If it affects only PTs, and not Robin EH72's in other applications, perhaps the "funky" PT exhaust manifold/muffler design may be at fault, developing hot spots causing detonation in that location. Just a thought as to a possible cause. What do the gurus at Subaru think, Jack?