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Al

Head Gaskets.

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Al
Hi, Just went to Kohler update school and got a few new inputs on head gasket failures. Problems with units that have normal compression and pass leak down tests, but have the crankcase vacuum drop under load. These units will blow oil out the breather and into the carb under heavy load. We have fixed a ton of Briggs 28 series and the earlier Vangard V twins for this problem as well as some Kohler Commands. Kohler has been doing a bunch of research on this issue. School says to ask owner how he shuts the engine off if it is a newer series engine with a carb solenoid. Not shutting off at wide open can cause several problems that never dawned on me before. I always thought the only issue was after fire and the rick of blowing a muffler. I have seen the gasket failures, never tied to the why. I will try to explain this. Let's take a big V-twin. It has been working and is thoroughly heated through. Now we idle it down for a minute to "cool" it down. In reality we don't. When we cut the speed in half we get 1/4th the cooling air, as a centrifugal fan operates on a square law function. If you double the speed you get 4 times the air. Double that speed and you again increase the air volume by 4 or 16 times the original volume. This is how turbo chargers are able to be so effective. So if we idle it down for a minute often a lot of temperatures go up because the heat in the oil and the mass of the internal parts is trying to seek the lower temperature. (outside air) We have just slowed the fan down and reduced our transfer mechanism, so temperatures inside the head etc. may rise, instead of cooling down. Next, at idle the carb is nearly closed and the idle circuit is in full operation providing fuel to the air. The main jet is shut off with the solenoid, but at idle it is out of the circuit anyway. No effect. Spark off, engine on wind down, air and fuel from idle circuit flow into the muffler unburned. Muffler still hot, after about 3 seconds Bang. ( BSS Brown seat syndrome) Some times this may crack a piece of muffle baffle, or even break a piece off, or while it is red hot inside bend it. This can cause some restriction in the muffler. What can a restricted muffler cause. Overheating in the restricted cylinder. This increased heat can cause increased expansion of the aluminum and increased stress on the head bolts and increased compression of the head gasket. On cool down reduced clamping of the gasket. Continue the cycle and eventually the gasket fails usually with a fine hair line crack that may barely be visible to the naked eye. Next we look at the same situation where we actually get a backfire in the engine on wind down, (ignition caused by a hot spot in the combustion chamber often called dieseling). This always causes the engine to kick back and spin backwards, blowing out the carb. What has just happened. We have an undesired thermal event when the piston is coming up and it is suddenly is forced back down. Pressures in the chamber get unbelievably high and the gasket is stressed. Another part is also really stressed. The energy it takes to stop and instantly reverse the flywheel may partially (or slightly) shear the flywheel key. Next time the engine is used the timing may be a little late and this raises the cylinder temperatures significantly. Now we are back to the thermal stress on the gaskets and hardware. Now lets look at scenario number two. Engine with a carb. solenoid. Key is shut off wide open. Solenoid blocks all fuel to the main jet. Engine speed drops slightly and the governor pulls the carb. wide open. The throttle plate allows large amounts of air to flow, but the idle bleeds only provide fuel when the throttle is closed and the opening at the edge of the throttle plate provides enough pressure differential to draw fuel. With throttle plate held open by the governor, pure cool air without fuel is drawn into the cylinder and pumped through and into the muffler which cools the muffler. The fan is still running at high speed and provides a large amount of cooling air on wind down. As the engine goes down through the idle rpm range, the idle circuit in the carb. is effectively out of the picture because the throttle plate is still open. The result, quicker heat dumping from the inside of the engine, no after fire and no extra stress on the head gaskets. All of the newer engines operate on very lean mixtures to meet EPA emissions requirements. One of the features of the TH engine was with no head gasket, the thermal barrier between the head and cylinder was gone as they were one piece. This helped with emissions and completely did away with head gasket failures. It just had too many other problems. We are seeing in the small engine field what the car makers have been doing for several years, using torque to yield head bolts. These bolts are torqued to a point the bolt actually yields and starts to stretch, a point just below failure. These bolts cannot be used once they have been installed. This is why you now get new head bolts with a head gasket on the new OHV engines. This is one step to addressing gasket failure. Improper shut down is not the only cause of head gaskets, but I never connected it to be any factor. Happy Holidays. Al Eden

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B-16_IC
Well, when you explain it that way!:D Thanks for the info Al, it all sounds clear to me. Nice job on the explaination! I will be remembering this when I put the 18hp twin in my garage to use mowing.

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Jason
Hi Interesting. Will start shutting down the Kohler Command in my 1920 at full throttle now. Never knew about this before. What years of Command Engines are known to have the head gaskets blow? Is there a Serial break or anyway to know if my Command is affected. Might have to pull off the head gasket and replace it, as I have been shutting it down at idle speed. I got around 1,121hrs on my command in my 1920, unsure how many hrs are on my Command in my 1914H. Interesting update. Take Care Jason B

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Al
Hi, I hope I didn't imply that this was an automatic sentence to gasket failure. About 5 or six years ago, Kohler had some 25 hp gasket failures, and the problem was corrected with a change in the gasket shape that left some gasket in the push rod area. This reduced the tendency for gasket migration in this area where the gasket was narrow and the bolt clamping was distributed on both sides of the push rod ares. It was a relatively simple fix. As there were some gasket failures that seemed to follow customer complaints of "backfiring" more research was done. Enough to learn that improper shut down could also contribute to gasket failure in some of the ways I previously mentioned. Explaining this to us in school, was an effort to encourage us to look beyond "just a blown head gasket" for the cause. As part of "interviewing" the customer about symptoms concerning his problems, gathering information about shut down is an are we should be questioning for background info when a unit comes into the shop. It is also for us to explain the importance of proper shut down (which is contrary to long held beliefs)with new families of engines when we sell and deliver equipment. I also forgot to add the damage that can result from using these engines at less than 90% of recommended RPM. This results in extreme internal heating and also contributes to gasket and valve seat failures. Another issue that was addressed was to have torque wrenches calibrated (recommended once a year) due to the critical torque needed on the new TTY torque to yield bolts. Also don't use ft lb torque wrenches when inch pounds are called out. Example a spec of 240 inch pounds is equal to 10 ft lbs. If you use a 100 ft lb wrench at 10 ft lbs. you are in a very inaccurate portion of the scale. Torque wrenches should normally be used in the upper 2/3s of the scale because the bottom 2/3 of the range is much less accurate. Happy Holidays, Al Eden

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MikeES
Al, great unfo. But the pre solenoid carbs, especially with the Kohler twins usually backfire when shut down at higher rpms. I have always had to idle the engine for 10 to 15 secs. to prevent backfire. Should all these air cooled engines shut down with open throttle??

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Al
Hi, I don't think so. I would say that it would be better if they idled for a minute or 2 though. However they don't run as lean as the new EPA engines and the combustion chamber temperatures should be a little lower also. Plus if you shut them down at WOT (wide open throttle) you are still dumping all of that fuel into the muffler. This is what makes this such a difficult issue to be on the right train on. Confusing huh? Al Eden

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Jason
Hi Thanks for the info. Any gasket problems with the Command 20hp or was it just isolated to the 25hp version? In any case I will probably shut down my 1920 at full throttle. Yeah I always run my Command at 90%. I only slow her down to engage the mower deck or snow blower and then throttle her back up. Sometimes when I pickup leafs and shes got the track vac on the back, I have to stop and rake the leafs into the vacum system by hand. I try not to idle her to long to prevent over heating, same with the 14hp Command in my 1614H. Take Care Jason B

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olcowhand
Hey Al, I've heard this of the newer engines. I only have the older K series singles which I must idle for a minute or so or I WILL get a very loud backfire in the muffler. One of my tractors has a 20hp Onan & strangely enough it never has backfires & has no after-fire solenoid either. Wonder why the Onan doesn't act the same? Maybe the Onan runs a bit cooler? It doesn't seem to run as hot. BTW Al, I'm the dairy farmer from perr.

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Al
Hi, Idling without a load for a while should be fine. It is when it has been running under a load and all of the internal temperatures are up and then just slowing it down for a few seconds and shutting it off is what can be detrimental. A longer idle period will help dissipate the heat also. Also, Simplicity recommends in service schools to have the engine wide open when engaging the electric clutch. The first thing we have to do with a new tractor is take in and with the engine wide open start and stop the mower 10 times. This burnishes the clutch faces in. When you engage the mower at low speed, you will note this causes the clutch to "Grab" and it jerks the belts and the mower. Continuously engaging at low speeds causes the clutch surfaces to get rough and "key" together causing violent grabbing. When engaging at full throttle, the clutch slips and burnishes the surface, eliminating the "keying" effect and the mower will start smoothly and the belts will last much longer as the shock load on the belt cords are reduced. Try it, you will see when you do this the mower starts smoothly without all of the jumping and jerking. Seems there is no justice, the things they said in the early 80s when the first Sun Stars had the electronic module with the rpm sensor that prevented high speed engagement only lasted a year or so and them the new module came out and this was gone and new rules. You just can't win. If it is this tough with electric clutches, its no wonder NASA can't get the fuel gauges reliable. Happy Holidays and Merry Christmas to all. Al Eden

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andrewk
This is or could be an issue with all Command twins, or even singles if conditions are right. This could even be an issue on non-kohler products, as the principles of operation are the same on the other makes, as are the EPA requirements, and the equipment used to meet them. The key is, as Al mentioned, that head gasket failure is not an automatic consequence of improper shutdown, but that the way these engines are designed, you are doing more harm than good by letting it idle, then shut it down than if you shut it down at WOT. Jason, I believe you are doing more harm than good by slowing your engine down to engage the electric clutch. In fact, I believe Simplicity has a service bulletin out about this very subject. Al, can you comment on that? I run equipment at WOT, all the time. The engine is governed, and designed to run there, and it is most efficient there too. Only time I run them at less than WOT is during warm up, and when I set the idle speed, or get off to stretch. While older equipment may have benefited from running at less than WOT, I have seen nothing but problems arise from those that run the new stuff at less than WOT. But, like was mentioned by Mike, older pre-emmissions engines will backfire if shutdown too fast. More stress is endured this way, because as the throttle plate is pulled open, lots of fuel is getting pumped through that main jet, into that nice hot muffler. My reccomendation as a rule of thumb would be if it has no solenoid, idle it down, and if it has a solenoid, shut it off at WOT. Al, thanks for taking the time to share some of the valuable information we learned at school this year- It always makes for some good discussion. Andy

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Jason
Hi See, guys around here told me to idle down before engaging the PTO. I will quite doing it, now that I know it will cause more harm then good. Will help the tractor last a long time. Never knew this about the Command engines until now. With the 14hp Command, I never throttle down to start the deck, so I will do the same with the 1920. Take Care Jason B

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