757, Half power

DT86 · Mar 27, 2012

A friend down the road called yesterday and says his 757, The zero turn Deere not the Boeing jet... is only operating at half it's usual power. And dies when the blades are turned on. (25 hp v-twin Kawi, 1000+ hrs)

After a quick inspection we determined it was only firing on one cylinder. We installed 2 new plugs and fuel filter. Still one dead cylinder. Its getting gas and spark. Can't figure this one out. Here is a pic of the new plugs, the difference is obvious even after less than 10 mins run time. Plug on the left is from non firing cylinder, it is discolored from being soaked in unburnt gas. The other has a burnt look due to combustion.

I guess the next things on my list are a compression test and new plug wires??
Help!!! Anyone?

Zork · Mar 27, 2012

Improper valve seating should cause popping from either intake or exhaust. and fuel is an indicator there is enough compression/vacuum to draw the fuel into the cylinder -unless it is injected. Is it fuel injected?

Before you do a compression check can you safely remove the wire from the dead cylinder while the engine is running and modulate the wire at different distances from the plug and see if the plug fires and the cyl recovers? This will indicate several things from bad plug (doesn't fire under load) to ignition wire coil issues.

DT86 · Mar 27, 2012

Zork said:
Improper valve seating should cause popping from either intake or exhaust. and fuel is an indicator there is enough compression/vacuum to draw the fuel into the cylinder -unless it is injected. Is it fuel injected?

I looked up the valve clearance and found that it should be .004" -.006" on both intake and exhaust. As far as the seat, with 1,000+ hours its possible.

No it is not injected, it has a twin barrel carb.

Another thing I found out today is that this engine has had new rings. I just don't know when.

This is driving me :confused2:

DT86 · Mar 28, 2012

Anyone?

Zork · Mar 28, 2012

Did you try my diagnostic proceedure with the plug wire???

DT86 · Mar 28, 2012

I think I did what you mean. If not help me understand.

It has a strong spark at varying distances from the plug to the wire. But now what?

Keep in mind i swapped sides with the old plugs and it made no difference, then got new ones.

Also the compression tested good. Someone else mentioned a bent push rod.

Zork · Mar 29, 2012

This is not the case for a mechanical malfunction. Sometimes when plugs or wires begin to fail they appear OK or test OK but under load they don't work properly and removing the wire from such a plug (or dead cylinder) while the engine is running and modulating the distance of the gap between the wire and the plug causes the dead cylinder to fire indicating either the plug (usually) or the wire.

A bent push rod should be easy enough to check with a visual confirmation that the valves are moving and the lash is correct.

DT86 · Mar 29, 2012

That is kind of what I thought. I think I'll recheck the valves and have him get some plug wires. Thanks.

Yellowfever440 · Mar 29, 2012

Well, the bad news it is a mechanical problem, more bad news you will prolly have to replace one of the heads. If the plastic shroud around the fan has never been removed and the grass blown out around the jugs then I would tell you it has over heated one of the heads and the valve guide has seized to the valve and the aluminum head expanded and the guide started moving in the head. Come on machines that were in the earlier serial number. Take the valve cover off and turn the motor over and make sure all the valves move the same, check the jugs to make sure the grass isn't built up, if it is it prolly over heated the head.