Not again........

[wm_simpleton]

Last year you folks were nice enough to coach me through understanding my 918 (with a Kohler CH18S) has to be turned off at half throttle to prevent a back fire that had probably caused it to blow a head gasket. (I had a lot of soot build up on the right side of my tractor as you face the front) Tonight, I finished mowing my yard and got out the blower to clean the dust off it. Then hopped on and went to my neighbors to return the deck I had borrowed (mine has a pulley problem and I am waiting on parts). Once at the neighbors, turning it off at half throttle produced one of the loudest back fires I have heard yet. We removed the deck, and I started it back up and it ran terrible all the way home. Once in my own garage, I noticed soot starting to build up on the hood already. We are talking about maybe a 2 minute drive, not across town.....sm02 Even after the first head gasket replacement last fall. The spark plug on that side still had issues with getting really gummed up. Very black oily build up. So I am wondering if the head on that side has issues? This is depressing........

[DMedal]

quote:

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Originally posted by wm_simpleton This is depressing........

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I'm sure! I have a question for the experts. As I recall the term backfire defined to me way back when, it was a backwards explosion out the carb due to hot carbon, really off timing.. not sure what else. Backfire was why you kept an extinguisher handy and didn't lean over the carb at all when playing with choke to start. It wasn't possible to have a backfire out the exhaust, but I don't pretend to know what the right term is for when we flood the exhaust with rich fuel/air on shutdown and blow the muffler off the tractor. In either case, intake or exhaust, it happens with a valve open, meaning low cylinder head pressures I would think, compared to a compression firing. So how, using normal gas and an otherwise running engine, do you generate enough head pressure to blow a (good) head gasket or blow a plug out of the threads with a "backfire" or any other sort of detonation with an open valve? or closed valve for that matter! Geez Don

[rsnik]

With a diesel engine the higher compression ratio, say 20:1, raises the air pressure in the cylinder to about 600 psi as opposed to only around the 90-100 psi I see when I do a compression check on my cast iron Briggs engines. With a diesel, the extremely high psi upon compression raises the air temp to around 1000 degrees which is well beyond the combustion temperature of diesel; the diesel is injected at this moment and the fuel burns without the need of a spark plug. I keep a little Mastercool #52227 infrared heat sensor in my pocket when using the tractor. They are cheap and small, but very effective. If you don't already know your engine is hot you can shoot any area on the engine and see how hot it is. If the engine is real hot the combination of 100 psi and a cylinder temp of 350+ can reach the relatively low combustion temperature of gasoline, which is 495 degrees. Say you have shut off your hot engine. The piston may not be even halfway up the cylinder before the residual gas mix ignites, driving the piston back before reaching TDC and reopening the exhaust valve. Hence, the backfire comes out the exhaust. I don't think shutting down the engine at half throttle is the answer if the engine is running hot or has carbon or other caused hot spots in the cylinder which can ignite residual fuel/air mix. Running the engine at half throttle for even a short while I have found reduces the head temp as measured with the Mastercool gun. The first thing is to reduce the engine head temperature and then starve the cylinder of fuel. A good way of doing this is to shut of the inline fuel valve after running the engine at half throttle for a while to cool the cylinder and then starve the engine of fuel, ensuring that no residual fuel is present when the engine stops. The key on the flywheel is the only timing adjustment available on the cast iron and other L Head Briggs engines. It is possible to order a staggered keyway in increments up to 5 degrees to retard the timing if backfiring occurs as a result of operation as opposed to hot shut down. You risk breaking a rod but retarding the timing is also a performance mod and allows for adjusting the governor for much higher horse power at much higher higher rpms at the expense of low end torque.

[mbrook]

Backfire, come on it was more like an cannon going off. My chicken of a chocolate lab came running to my side with his tail between his legs. Seriously though, your looking for some advice and suggestions as to what is causing the problem. Last year you had a similar issue with it "backfiring" and damaging the head gasket. After replacing the head gaskets, plugs and cleaning the heads, it seemed to run fine, until a few weeks ago, when we noticed the right side wasn't firing and the plug was completely gummed up. After replacing the plug it ran good again (1-2 hours), until this last use when it "backfired" and blow the head gasket. What would cause the spark plug to foul out/ gum up, load the cylinder with fuel and "backfire". Would it be running too rich? Why wouldn't it affect the other side? Would it be a bad valve? Why would it build good compression and run normally? It seems to me that there has to be a cause, but where do you start to look, before bringing it to the shop? As for myself, I have exhusted my limited knowledge with the twin cylinders, but really want to help fix the problem.

[dentwizz]

Might be a long shot, but try running a hotter spark plug. If the heat retention of the plug is too low it will be more likely to foul.

[MPH]

My first question is '''does it burn oil? Is the plug fouling from oil or rich gas mix? Most auto repair manuals have a chart in them showing and explaining fouled plugs and likely cause. One way to cool down your engine before shut down is drive back to the shop at less then WOT.

[MPH]

Where can one buy this Mastercool # 52227? Sounds like a "must" for the shop.

[Willy]

A backfire through the exhaust is caused by an excess of fuel which happens usually when you decelerate. A backfire through the carb is usually do to a lean mixture. When you shut it down at full or 3/4 throttle gas is still pulled into the hot combustion chamber which will either cool the cylinder or ignite in the engine or a hot muffler. The reason for it running rough after backfiring may be from knocking carbon loose and getting under a valve or fouling a plug. I set my idle down as slow as I can with it still running when I'm done for the day I let it idle for about 30 seconds then shut it down, I seldom get a backfire.

[DMedal]

Thanks, Willy - I got the idea now. It doesn't happen to me except when I goof and turn the switch back on before it is dead. But I let it cool down and approach the parking spot going put-put. Will it help Joel if he shuts off fuel to shut down, put the valve where you can reach it from the seat? -Don

[andrewk]

What needs to be remembered in this case is that the engine in question is a newer Command, which has a fuel shutoff solenoid. The bang that comes from shutting down at idle is usually called "afterfire". What happens is that when you shut down at idle, the high vacuum of the throttle plate being closed draws fuel in from the idle circuit, and pumps it into the hot muffler. This then reaches flash point about the time you are swinging your leg over the steering wheel. Some people call this "brown pants syndrome":D If you shut the engine down at half throttle, when the engine is using the high speed circuit, fuel will not be delivered to the cylinder, since the fuel shutoff solenoid shuts off the high speed circuit. In the older engines that most people here have, Willy's procedure is textbook.

[andrewk]

quote:

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One way to cool down your engine before shut down is drive back to the shop at less then WOT.

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I don't agree. The flywheel provides the cooling air for the engine, and running it at lower RPM after it has reached normal operating temperature will make it get hotter, not cooler. If the flywheel fins move 1 cfm of air at WOT, then they will move 1/4 cfm at half-throttle, and 1/16 at quarter-throttle, etc. While running the engine faster will produce more BTU's, it will still run cooler.

[callwill]

I don't believe cooling effectiveness of the flywheel is going to be a linear progression. It is likely to be more effective at certain speeds. The other issue not apparenty considered by this model is the heat produced by the engine @ different rpms. I would belive that running slower w/ no load would produce less heat, so a cool down @ low speed, no load should help-ie,idle it a bit.

[ZippoVarga]

Does any one out there have an infared thermometer? This can all be quelled with a little test. As for the backfire and oily build up. My "guess" is that there are clogged fins under the shroud causing that side to over heat. Just another idea in the long list of possibilities.

[andrewk]

I've got one. When I get to work Monday, I'll do a quick test on a mower I am working on.

[RayS]

My operators manual on my Prestige states to shut it off at half throttle to WOT. My engine is equipped with a shutdown solenoid as stated in the picture below from the engine manual. I ignored what it stated and blew a head gasket. The engine in mine is a Kohler Command 20hp. Took it to the dealer that I bought it from was told that it was caused by not shutting it down as stated in the manual. It ended up using a 1/2 a quart of oil mowing my yard each time and fouling the plug on the side that the head gasket blew on. It would bakfire 3/4 of the time that I would shut it down at idle and eventually ended up costing $385.00 to have the head gaskets replaced. Now it has been 3 years since I had the work done, I shut it down as stated in the manual and it hasn`t backfired since. My dealer explained it to me the same way as stated by andrewk above has and so far no problems.

[gregc]

Hey Ray, would you mind posting your pics 640 pixels wide so folks with eyes as old as mine can read the type without straining so much? ;)sm06

[RayS]

I would have but the picture would have been over 100kb and would have been to Large to upload!

[gregc]

quote:

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Originally posted by RayS
I would have but the picture would have been over 100kb and would have been to Large to upload!

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Sorry Ray. I guess it must be because of the imaging software that you're using.

[wm_simpleton]

Thanks for the input folks. I decided this was a job for a professional. I can't afford to replace my tractor right now, and I needed to mow my lawn. Dealer said the head gaskets were in tact and actually complimented our replacement work (Kudo's to mbrook, he was the brains and I was the grunt labor on that job). A compression leak down test was done on that cylinder and the head gasket was not leaking, but compression still faded. So the diagnosis was the rings had to be replaced, even though there were no signs of adverse wear. Since we could tell during the head gasket replacement last summer that it had been done before. My guess is that the rings had been done as well, with possibly less than desirable parts. We will never know for sure. But I picked it up today, and mowed the lawn this evening. It ran better that is ever has. Very relieving...... :D