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Small Air Cooled Gasoline Engines

A little Briggs diagnosis needed.


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  #1  
Old 05-03-2010, 09:05:10 PM
Banjo Banjo is offline
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Default A little Briggs diagnosis needed.

Howdy all,
Today I got one of my new-old tillers out and put it to good use in some friends gardens. This is an older Agway with a B&S 3.5 HP vertical shaft motor. This thing kills me. I got it a couple of weeks ago out of the weeds at a friends house. The spark plug looks like crud, as does the plug wire and connector. I had to blow a bunck of crud outta the fins, remove a big wasp nest from under the blower cover, free up the governor blade pivot, and lubricate the recoil mechanism on the crank. but after all that and some fresh gas, this thing fires on the first pull every time and runs remarkably well.
Here's the problem. After it gets warmed up real good, it boggs down and stalls very easily. Blows smoke when it does. I can't tell if its fuel or oil, but I think it's oil. Let it cool down and its ready for another round till its hot again.
What's most likely the cause here? Valve clearances, rings, worn cylinder? Could it be the plug and wire increasing resistance/ overheating the coil.Or am I totally off base.
Thanks for any ideas. If nothing else, just writing stuff down helped me think.
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  #2  
Old 05-04-2010, 07:46:24 AM
K D Redd K D Redd is offline
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Default Re: A little Briggs diag needed.

What color is the smoke? Black smoke is to much fuel, Rich or in racer talk you are running FAT. Blue smoke is oil. Be sure you have the engine full of oil. The correct oil level is when the oil in the crankcase is fill to the TOP of the fill hole.

Kent
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Old 05-04-2010, 02:54:30 PM
makoman1860 makoman1860 is offline
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Default Re: A little Briggs diag needed.

Since you had nests and clogs everywhere else. A place to look is a clogged breather. They have been known to gum up after sitting. Could be a lot of things, for the price of a head gasket you can pull the head and look at the vitals of the power cylinder. Tillers tend to ingest a lot of dust, and tend to wear out quickly, so honestly I would start there.
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Old 05-04-2010, 09:13:31 PM
m l addison m l addison is offline
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Default Re: A little Briggs diag needed.

in the shop we do leakdown test, it checks rings and vavle probs.
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Old 05-04-2010, 09:45:31 PM
Banjo Banjo is offline
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Default Re: A little Briggs diag needed.

Thanks guys. Some good ideas. I have a leakdown tester. never gave that one a thought. Engine has oil. possibly even a touch overfull. I checked it when it bogged down and stalled the first time, and it was right at the fill plug level. It only briefly blows smoke when it happens. Looks like oil smoke, but I'm not 100% sure. Sometimes it can "clear" itself and pick back up, but once it's at the temp where it does it, it will keep bogging down under load till it finally stalls out. Letting it cool off eliminates any issues till it heats up again. It doesn't seem like a carb issue to me. It really feels more internal mechanical. But I thought I'd run it past you guys, and see if anyone else had encountered the same thing.
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Old 05-06-2010, 07:30:46 PM
BHartley BHartley is offline
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Default Re: A little Briggs diag needed.

You said you had to clean a bunch of junk out of the fins, did you get it all out? Sounds like it might be overheated. When it gets hot does it feel like it might be ready to sieze up? What weight oil is in it? If it is running really rich it could be "washing" the oil off the cylinder walls and causing the piston and rings to drag in the cylinder as it heats up.
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Old 05-06-2010, 09:56:48 PM
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Brian Lynch Brian Lynch is offline
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Default Re: A little Briggs diag needed.

Kinda sounds like a sticky exhaust valve to me. Pull the head and remove the ex valve. Clean the stem and head. Run a .25 caliber pistol bore brush thru the valve guide to remove carbon deposits. While your'e in there, check the valve clearances against specs. Try it. Hard to diagnose digitally....but really, what have you got to lose? The cost of a head gasket & a few minutes of time. Hope this helps! Keep us posted on your progress...
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Old 05-07-2010, 11:36:24 AM
BHartley BHartley is offline
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Default Re: A little Briggs diag needed.

Brian, great idea. I hadn't thought of the sticky valve scenario. It does make sense.
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Old 05-08-2010, 07:51:33 AM
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Default Re: A little Briggs diag needed.

Checking back to see if you have made any progress with your engine. Any luck? -Brian
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Old 05-15-2010, 09:28:07 PM
Farmboy1 Farmboy1 is offline
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Default Re: A little Briggs diagnosis needed.

a head gasket wont just destroy itself when you take it off and will generally seal up as good or almost as good if you are careful, so explore a bit a clean up , look around ect... It sounds like from what you have said it could very easily be a combination of 2-3 thing debris in the fins + too heavy oil + too lean fuel mixture, not too rich since you say the somke is not dark.
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Old 05-15-2010, 10:04:56 PM
Sky
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Default Re: A little Briggs diagnosis needed.

This reminds me of a troybilt with a horizontal tecumseh i worked on one time. Guy complained about how it would run fine, but suddenly bog down and blow smoke. It was oil. I thought it was a clogged crank case breather so i soaked it and tryed the tiller again. Same thing. So then i baught a new crank case breather and installed it and tryed the tiller again. Slight improvment but...it did it again. This particular engine had the crank case breather hose factory routed to the air filter system (emissions thing) and come to find out was loading the air way with oil residue which would then fog the carburator and make the engine bog and blow smoke. Finding that out, i unplugged the breather tube from the air filter assembly, plugged the hole and just let the pipe coming from the crank case breather-breath externally like most engines do. End of trouble. Now im not sure how YOUR engine is designed to be and if it was designed with emmisions in mind but this happened to be a simular problem of mine one day.
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Old 05-15-2010, 10:56:24 PM
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Default Re: A little Briggs diagnosis needed.

I have an 8hp Briggs on a mower. When I fill the oil to the top of the filler hole, every once in a while it will suck up a slug of oil making it smoke like crazy just about killing the engine, then it will clear itself and recover. I found that if I run it with the oil level a little below the threads on the fill opening it hardly ever does it.
I did not think to check the breather, that might be my problem also.
Steve
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Old 05-16-2010, 09:16:39 AM
Sean B. Sean B. is offline
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Default Re: A little Briggs diagnosis needed.

To much piston ring leakage (blowby) will cause a build up in crankcase pressure after running a few moments, then it will force oil out of the breather, into the carb like sky mentioned. If the engine has been really hot, the tension could be lost in the rings.
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Old 05-16-2010, 09:37:27 AM
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Default Re: A little Briggs diagnosis needed.

I forgot to mention my breather is not vented to the carb. it is external. I dont know how it is sucking the oil in? Any ideas Sky?
Steve
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Old 05-16-2010, 10:31:17 AM
Sky
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Default Re: A little Briggs diagnosis needed.

You wont have that problem then if it's not directed into the breather. I was just indicating that one day i had bizzare trouble like that and found that some great engineers idea of emissions was also fogging the carb making the engine bog and blow a cloud of smoke. It is possible to still have crank case breather trouble but it wont be the same trouble i had as you say the breather tube isnt connected to the air filter system.
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Old 05-16-2010, 05:36:11 PM
Sean B. Sean B. is offline
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Default Re: A little Briggs diagnosis needed.

Sky, it was not the "engineers idea of emissions" breather design that was causing your problem. If it was, then ALL the engines with that setup would have the same issue. It was probally a worn cylinder/and or rings. But your bypass deal is a good way to keep a worn out engine going, sometimes we need to do things like that!
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Old 05-16-2010, 07:20:44 PM
Sky
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Default Re: A little Briggs diagnosis needed.

It was a healthy engine yet.
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Old 05-16-2010, 09:16:32 PM
Banjo Banjo is offline
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Default Re: A little Briggs diagnosis needed.

Update for you guys interested. I haven't had time to tinker with the tiller yet. I managed to work it through all my friends and relatives gardens the way it was. When I get some time this summer I'm gonna do most of what you guys suggested. New plug and wire, pop the head and check valves and whatnot, make sure crank breather is ok, new air filter, and check all else while I'm there.
Oil level is ok, if anything the oil is a bit thin.but it never gets "tight" when it boggs down. It will pull ok, but just not want to fire till it cools. I tried to remove the rotten out old muffler, and managed to leave the threads in the exhaust port, so the next order of business is to extract the rest of that and put a new one on.
I may not get a chance to work this thing hard enough to make the problem happen again till next year, but in the meantime I'll get it freshened up and looking better than it does.
If I get any progress made soon I'll definitely let yo know what I found.
For what it is, it's a darn good tiller.
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Old 05-16-2010, 09:38:01 PM
Sean B. Sean B. is offline
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Default Re: A little Briggs diagnosis needed.

Quote:
It was a healthy engine yet
Well, then I guess it didnt need repair then did it? LOL
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  #20  
Old 05-16-2010, 11:23:00 PM
Sky
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Default Re: A little Briggs diagnosis needed.

Obveously it did becouse of the symptom it had which was bypassed but it was healthy as in internally healthy. It should have been, being it was a fairly new industrial commercial engine.
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