FR691v loss of power/backfire

[Nam11b]

I have a toro ss5000 with a Kawasaki FR691v that started popping and surging under load last season. Seems to idle fine and will drive okay, but with the blades engaged or on a slope it will start popping (I assume backfiring) and the power will noticeably drop. Power will come back for a few seconds and then will drop again.
I changed both plugs, air filter, fuel filter, oil, and fuel with no change. I pulled one plug at a time and found cylinder one would run solo, but not cylinder two. Removed and replaced both coil packs without any change. Removed and replaced the carb and now both cylinders run independently, but the mower still is popping and losing power. Verified both valves were within spec (approximately .005in). Running out of ideas, any words of wisdom?

 

[sgkent]

I read some threads on that motor with similar problems. In one a welch plug on top of the carb was missing. On another it was a carbon build up on the intake valve causing it to not fully close. On the third it was the idle fuel mixture was too lean and that also affected the high speed operation. Beyond that I don't know. You might throw a timing light on it and see if it flutters under load indicating ignition issues. Or maybe do a compression test / leakdown to see if both cylinders are similar. do adjust the idle mixture to optimal. With most gasoline engines if you hit the throttle quickly and the mixture is right there is very little stumble as the engine speeds up. If the engine stumbles it can be too lean or too rich. I am used to adjusting carb mixture pre sniffers by cupping my hands near the carb to see if that speeds the engine up or slows it as I move my hands inwards. That richens the mixture but when you are dead on there is very little change until the hands are close to the carb. There are some folks here who service these mowers for a living so when they come along they will know more than I.

 

[bertsmobile1]

Classic head gasket leak
Do a leak down test both sides

 

[Nam11b]

Classic head gasket leak
Do a leak down test both sides

Thanks Bert. That was my fear. Per the service manual, 64psi is the minimum. What is the allowable or expected bleed down?

 

[bertsmobile1]

64 psi is the minimum cylinder pressure for the engine to fire
How much you get will be drastically affected by the valve lash
Leak downs are % bypassed and in reality anything more than 10% ( through ring gaps ) is bad
Don't be surprised if the bad cylinder has a pushrod detached from the rocker or badly bent
All 4 of them are identical , move the same amount & should sit in the same position when closed
If you adjust them do not over tighten the lock nut there is a reason why it is 1/3 thickness .

 

[StarTech]

Thanks Bert. That was my fear. Per the service manual, 64psi is the minimum. What is the allowable or expected bleed down?

Don't confused compression test with leak down test. They are two different tests.

I have seen engines that pass a compression test but fail a leak down test. I also use a modified leak down test on OHV engines so I can test the mid and lower cylinder conditions.

 

[Nam11b]

You nailed it Bert. Bought a tester and finally got around to testing the motor. Compression was good, but I was blowing air out of the dipstick during the leak down test. Thanks for the assist

 

[bertsmobile1]

Head gasket is blown between the cylinder & push rod tube
Easy fix if nothing breaks when you try to undo them .

 

[Nam11b]

Head gasket is blown between the cylinder & push rod tube
Easy fix if nothing breaks when you try to undo them .

Replaced this weekend and runs like a champ. I didn’t see any obvious damage or leakage to the gasket, but it definitely fixed the issue. Thanks for the help!

 

[bertsmobile1]

Glad we guessed it right .
And thanks for coming back it is a big help to others who are too timid to post but read the archives .