Riding Mower Engine Overheating

[Rick'sToro8-32]

Thanks very much for all of the posts! :smile:

If I am hearing things correctly it seems as though the consensus is that my mower is konking out after about 40 minutes of run time because of a timing issue.

It occurrs to me that this mower has never had a valve job. It think poorly seated valves can mess up the timing. I wonder if this could be the 'cause' of the problem.

Also, in the back of my mind, I think I remember something about a vapor lock in the gas line that creates a situation like mine from a mower when I used to work at a local cemetery. It seems like we had a mower that would run until it heated up and then konk out and then run again when it cooled down. I think I remember the mechanic there saying it was a vapor lock or something like. Does anyone know anything about this?

That is the 'symptom' of my mower: simply put, it runs great until it heats up then it konks out--and when it cools down it runs great again until it heats up and konks out again.

 

[gainestruk]

I have seen fuel get too hot and bubble in filter, you could add some line and route it away from engine and also move filter closer to the tank.
Is this a flat head Briggs ?
If so pull head off and clean carbon and look at valves.

 

[Mad Mackie]

Carbon buildup on the piston top and combustion chamber will get hotter as the engine is running. It will get hot enough that it will start burning the incoming air/fuel mixture before the spark plug fires. This is caller detonation, and when it occurs, a more rapid than normal burning of the still compressing air/fuel mixture occurs to the extent of hammering the piston and slowing it down while still in the compression stroke. Detonation will cause power loss and overheating to the point of engine shutdown. The incoming air/fuel mixture with the fuel in an atomized state, when exposed to the higher than normal temperature, vaporizes and no longer remains in an air/fuel atomized mixture which is necessary for an internal combustion engine to continue running.

 

[Rick'sToro8-32]

Carbon buildup on the piston top and combustion chamber will get hotter as the engine is running. It will get hot enough that it will start burning the incoming air/fuel mixture before the spark plug fires. This is caller detonation, and when it occurs, a more rapid than normal burning of the still compressing air/fuel mixture occurs to the extent of hammering the piston and slowing it down while still in the compression stroke. Detonation will cause power loss and overheating to the point of engine shutdown. The incoming air/fuel mixture with the fuel in an atomized state, when exposed to the higher than normal temperature, vaporizes and no longer remains in an air/fuel atomized mixture which is necessary for an internal combustion engine to continue running.

Thanks very much! This sounds right to me!

I have to work the next five days 12 hours per day, but now I am very anxious to pick up a new head gasket and pop the head off next week.

It is true that after things heat up, I do start to loose power before the engine finally shuts down. I'll bet this is exactly the root cause of my problem.

I'll report back next week after I work on the mower.

Thanks again very much Mad Mackie!! :thumbsup:

 

[motoman]

So detonation occurs in lowly air cooled tractor engines?? No one I have read in this forum has ever suggested this. Are these engines so loud you cannot hear the detonation? Have never seen a tractor piston melted in the classic detonation manner. Never heard of a knock sensor on these engines. Just saying...

 

[Mad Mackie]

So detonation occurs in lowly air cooled tractor engines?? No one I have read in this forum has ever suggested this. Are these engines so loud you cannot hear the detonation? Have never seen a tractor piston melted in the classic detonation manner. Never heard of a knock sensor on these engines. Just saying...

Why does every small engine manufacturer recommend removal of the cylinder head(s) at specified hours of operation to remove carbon buildup?
I have never heard actual detonation occurring in a small engine, however I have seen the resulting internal damage. During operation when an unexpected deterioration of performance happens, detonation sometimes is the cause. Many small engines don't get run long enough for the carbon buildup to get hot enough to cause a problem, but some do. An engine decarb is more involved than other routine engine maintenance and as result doesn't get done very often.
Carbon buildup is more of a concern with L head engines where the piston at TDC comes very close to the cylinder head. On some engines, Onan twins in particular, when the piston makes contact with the hard carbon buildup, the engine performance has already deteriorated and now serious internal engine damage starts. Overheating and broken connecting rods are the nasty results, sometimes the broken connecting rod swings around and pokes holes in the crankcase destroying the engine.
I don't know of any small engines that have a knock sensor, the ability to change spark timing would be needed all of which would increase the cost of the engine substantially. Engines with DFI or EFI will already have a computer to control fuel management and would be easier to incorporate a knock sensor and the ability to vary spark timing. Retired now although I still do limited equipment servicing, I stay away from DFI and EFI equipped engines.
All small aircooled engines be they 2 stroke or 4 stroke consume oil during operation, some of the consumed oil ends up in the combustion chamber as carbon buildup which in time becomes a hard crystalline appearing substance requiring mechanical removal.
The unburned incoming air/fuel mixture contributes somewhat to engine cooling, but when it starts burning before the spark plug fires, then it's contribution to engine cooling is greatly reduced which results in increased combustion chamber temperatures and decreased engine performance.
Having been a military flight engineer on large piston engine powered aircraft, I have experienced detonation, not a pleasant experience as it frequently results in engine failure at the least opportune times like takeoffs at high ambient temperatures.

 

[motoman]

Well, I believe the inherent design of the low rpm, slow moving air cooled lawnmower engine is to operate in a high heat condition. A condition not far from engine damage. Especially for many owners who see these machines as kitchen appliances. I repeat you cannot know how hot the engine is without measurement . Apparently earlier machines had oil coolers and warning devices on them which have been discountinued (IMO) for lack of attention by owners and perhaps by marketing priorities (buy another). Yes, I am saying this from my sample of ONE machine and therefore cannot offer conclusive evidence. Keep 'em running as cool as possible -it ain't easy.

 

[KrashnKraka]

A person can not tell how hot something is. It requires a gauge which is not expensive. If you can obtain expected engine temperatures from some source (not likely) you can use an infra red gun or a heat probe furnished with a $50 DVM to take readings around the engine...cylinder fins, head. Or you can buy some temperature crayons at a welding supply house. But the best for AC engines is an oil temp gauge because as the oil goes so goes the engine. I start my fan on my oil cooler When oil temp climbs to 280F- which is very often when mowing here in summer. And using the craftsman 3 bin baggers will quickly push the oil temp to 300F as the damp grass weight increases. The ignition modules on the Intek I have sits on thick cast iron seats which helps keep the modules below failure range of 180F-240F. But I would imagine heat is more of factor on lawnmowers with smaller metal mass.(almost done)

You can easily find and mark true TDC and put a ref mark on your machine. Then hook up an inductive pickup style auto timing light. I did this . You may have to work in a darkened area to see the weak strobe the ignition module puts out. Mine was a faint reddish flash. But it is probably easier to check flywheel key first. Finding TDC is probably not worth the effort if you can avoid it .

Sound info in that post.
All being equal the engine may require oil cooler help. I have a new engine
I am experimenting with in such a way - better than 40Celcius ambients on
summer days.

 

[motoman]

Here it only takes about 85F ambient and full grass hoppers on a slight incline to drive oil temp to 280-300F. Even with the oil cooler and fan on it will slowly climb to 280F. In the shade with the fan running and hood up,idling I can pull 50F of heat out of the oil in several minutes rest. (Craftsman 48 with Intek 24 V)

 

[Rick'sToro8-32]

I picked up a head gasket the other day.

Is anyone else agreeing that my 30 year old mower could be carboned up in the head/piston area and this could be the problem?