I'm new - this is my first post. Just wanted to share this information as it took me a long time to fix this issue with my Toro mower.
Symptoms - nearly impossibly to start cold. Sluggish starter - slowing down (fighting a closed exhaust valve). I set up a car battery on a trickle charger that I would then hook jumper cables to my mower battery.
Anyway starting this mower kept getting worse. I would throw a tarp over the engine and even run a space heater thinking it was just not going to start if it was below about 70 degrees.
Finally I started finding out about the ACR system. I went through the valves, looked at the valve guide, set and reset the lash, figured out the little set screw that goes in the rocker nut was never installed from the factory. Didn't matter.
I pulled the flywheel and opened the engine compartment and finally found the problem.
The mechanism that activates the ACR feature was broken it two pieces. This is the "weight" that rotates a little shaft attached that lives in the exhaust cam gear. By 'in', I mean the flat part of this part sits on the top of the gear, it's round section passes through the gear, and the half round section extends down into a slot machined into the cam. This part adds an extra bump to the cam surface while the engine cranks. This leaves the exhaust valve open longer and provides the compression relief. Automatic Compression Relief - and this part is the key to the whole thing working right.
So I pulled gear and found only the top half of the part in-tact. The bottom of the part - the half-round section had broken off and was down in the bottom of the crank case. I found it with a couple of bamboo skewers after fishing around for a while.
The fix - less than ten bucks on ebay. I picked up a set of gaskets, the little set screws, a new exhaust gasket off another site.
Put everything back together and she started right up. It's rainy and cold today - there's no way this machine would have started on a day like today...
The fracture on the part looked like it started at the reduction in material - right where the half-round section starts. The fracture was brittle - it started as a crack and worked back and forth over time. This explains the gradual onset of the problem - maybe. Part of that theory doesn't make sense to me but the part did not fail suddenly. The fracture slowly progressed across the half round section. Once it broke, I don't see how the broken section could have stayed in the cam slot but maybe. If it managed to ride in there for a while it could explain why it would suddenly crank like normal sometimes - but only after the oil was warmed up. Still - I don't think this explains everything.
My mower - 8 year old Toro Z4200 purchased new at Home Dept - Courage 19 SV590. You can find the manuals on line in PDF form, the parts websites have excellent drawings. I found this solution by going in after it. I didn't know what I was going to find, and everything I read on-line pointed to fixes that didn't work in my case.
Hope this helps someone else with the same engine (or similar). I ran with this problem for several years until it finally got bad enough to keep the engine from starting except in rare moments.
Symptoms - nearly impossibly to start cold. Sluggish starter - slowing down (fighting a closed exhaust valve). I set up a car battery on a trickle charger that I would then hook jumper cables to my mower battery.
Anyway starting this mower kept getting worse. I would throw a tarp over the engine and even run a space heater thinking it was just not going to start if it was below about 70 degrees.
Finally I started finding out about the ACR system. I went through the valves, looked at the valve guide, set and reset the lash, figured out the little set screw that goes in the rocker nut was never installed from the factory. Didn't matter.
I pulled the flywheel and opened the engine compartment and finally found the problem.
The mechanism that activates the ACR feature was broken it two pieces. This is the "weight" that rotates a little shaft attached that lives in the exhaust cam gear. By 'in', I mean the flat part of this part sits on the top of the gear, it's round section passes through the gear, and the half round section extends down into a slot machined into the cam. This part adds an extra bump to the cam surface while the engine cranks. This leaves the exhaust valve open longer and provides the compression relief. Automatic Compression Relief - and this part is the key to the whole thing working right.
So I pulled gear and found only the top half of the part in-tact. The bottom of the part - the half-round section had broken off and was down in the bottom of the crank case. I found it with a couple of bamboo skewers after fishing around for a while.
The fix - less than ten bucks on ebay. I picked up a set of gaskets, the little set screws, a new exhaust gasket off another site.
Put everything back together and she started right up. It's rainy and cold today - there's no way this machine would have started on a day like today...
The fracture on the part looked like it started at the reduction in material - right where the half-round section starts. The fracture was brittle - it started as a crack and worked back and forth over time. This explains the gradual onset of the problem - maybe. Part of that theory doesn't make sense to me but the part did not fail suddenly. The fracture slowly progressed across the half round section. Once it broke, I don't see how the broken section could have stayed in the cam slot but maybe. If it managed to ride in there for a while it could explain why it would suddenly crank like normal sometimes - but only after the oil was warmed up. Still - I don't think this explains everything.
My mower - 8 year old Toro Z4200 purchased new at Home Dept - Courage 19 SV590. You can find the manuals on line in PDF form, the parts websites have excellent drawings. I found this solution by going in after it. I didn't know what I was going to find, and everything I read on-line pointed to fixes that didn't work in my case.
Hope this helps someone else with the same engine (or similar). I ran with this problem for several years until it finally got bad enough to keep the engine from starting except in rare moments.