MTD Bending Pushrods

[Jagmandave]

A co-worker gave me his 10 year old MTD with a 14.5 hp Briggs single, because he was frustrated because he kept having to replace bent intake pushrods. When I took it apart I found both a bent pushrod and a broken rocker adjusting nut. I figured out that since he'd been asking questions about this for the last 3 seasons, he didn't understand how the little allen screw in the nut was used to tighten it after the adjustment, and when he replaced the pushrod he wasn't getting the adjustment right.
As I was standing in line at the AES store to buy a pushrod without so much curve in it, another fellow in line asked if it was off a B&S. He said he knew what was causing it to bend, since he had a number of similar B&S engines in his lawncare business and was always having to replace them. He says it's because they shut the engine off at wide open throttle, instead of throttling back and then shutting off the switch. The next day I asked my co-worker what his shut down procedure was and sure enough, that's what he did too.

Anyone else ever heard of this?

I've used it several times without problems so far, and there's no mention of this procedure in the owners manual as far as I can find. I figured it was bending because he didn't lock the adjuster and it kept coming loose on him..............
Also, anyone know the real reason why these engines have one steel pushrod (on the exhaust) and one aluminum? My theory is that because of the expansion rate of the aluminum block and head, using a steel pushrod that wouldn't expand as far allows the exhaust valve a little more clearance as the engine heats up. The aluminum is used on the intake so that it will maintain the same clearance as it heats up.

Your thoughts, TBN braintrust?

 

[Bird]

This topic has come up before, but I don't know where to other threads are, so I'll repeat myself.

All my life, I've let engines idle briefly before shutting them off, but late last summer I bought a new Toro ZTR with a 14hp Briggs engine, and both the Toro manual and the separate Briggs manual say to move the throttle to the "Fast" position, then turn off the key. I don't understand it, but so far that works just fine. If you move the throttle to the idle position, then when you turn off the key, you get a terrific backfire that makes you wonder why it didn't blow something off the engine.

And my daughter has a Lawnboy walk behind mower with a Tecumseh engine and it doesn't even have a throttle; runs the same speed from start to finish.

So that seems to be the opposite of what you've been told, but I would expect there to be some differences between the new engines and one that's 10 years old. Of course I don't know that there are differences, or if so, what they are.

 

[Moon]

I saw the other thread where you mentioned that Bird, so the last time I mowed I tried it. Normally, I let the engine run at a slow idle to cool down a bit (like the manual suggests). When I would turn it off, I'd get a small backfire. This time I let it cool down a bit, but put the throttle all the way open and then shut down. No backfire. So, it worked when I tried it. Just not sure which is best for the engine overall. I did not notice any instructions like this in the manual ...but I could have overlooked them.
Thanks for the tip.

Moon of Ohio

 

[Jagmandave]

Well, it seemed a bit strange to me as well, I couldn't think of any reason why having the throttle open or closed would matter to the push rods, especially as the engines don't turn that high of RPM, even at WOT. I guess the proof is in the tasting as they say, I think the incorrect adjustment of the clearance was more to blame than engine speed. We'll see.....

Wonder if anyone else has heard or disproved this idea?

Back in the day, we used to blip the throttle on our cars before shutting them down, but that was more because I liked the way it sounded than any real reason! /forums/images/graemlins/cool.gif

 

[DaMadman]

</font><font color="blue" class="small">(

Back in the day, we used to blip the throttle on our cars before shutting them down, but that was more because I liked the way it sounded than any real reason! /forums/images/graemlins/cool.gif )</font>

Hahaha I still do that a lot in my truck. It has Duals on it and sounds "too Kewl" when you rev it up and then shut it down. /forums/images/graemlins/grin.gif

 

[GerardC]

As Bird said, I too go to full throttle before turning the key to off on my B&S 16.5 HP OHV engine. If I don't, I get that wonderful backfire.

Also my push mower with a B&S doesn't have a throttle. Runs wide open all the time. It's several years old now.

P.S. The title should be B&S bending push rods, not MTD. /forums/images/graemlins/wink.gif

 

[Jagmandave]

P.S. The title should be B&S bending push rods, not MTD.


Good point!

It also leaks oil, smokes pretty good on startup, and in general I think it's just pretty much worn out, although it seems to run with pretty good power and starts easily...........
So far, I've run it about 5 times, throttling back on shutdown, with no adverse effects.

Oh, and no answers on the steel vs aluminum pushrod question?

 

[slammer]

You get the backfire from unburned fuel. On most newer engines you will notice a wire attached to the bowl of the carb? This is attached to a little switch that automatically cut's the fuel as soon as the engine starts shutting down. This keeps raw fuel from pooling in the muffler and igniting on shut down. If you don't have this feature or the little valve is stuck open you can prevent the backfire by shutting down at at a higher RPM. This works because your burning fuel much more efficently and it never has a chance to pool in the muffler.

The little switch will also prevent your mower from running if your altenator screws up because it will not open back up unless it get's at least 9 volts.

FWIW, if you have the switch on your carb and your getting a backfire at shutdown you need to replace it as eventually whatever has jammed it open (trash?) is gonna let it shut and if it sticks shut your out of business till you fix it. However the little plunger can be replaced with just a cap and you will never have to worry about it.

As for the original topic...shutdown from high RPM IMO is not an issue with any engine as long as it is naturally aspirated. Turbo's and such are a differant story altogethor. Turbos take time to spool down for a safe shut down. 2 to 5 minutes will usually do it.