View Full Version : B&S 12.5 I/C won't start
06-06-2007, 10:28 PM
Model 289707 Type 0154 01 Code 9411214E
I replaced the connecting rod on this motor. Although I've never been inside this type of engine, everything seemed pretty straightforward. When I reassembled the motor, I made sure to line up the dots on the timing gears, get the governor back in the right place, etc. When I first started it, it ran well (a little white smoke) for about 30 seconds, then quit. It has not started since. Searching this forum, I found some other things to check:
Spark: It has spark
Flywheel: The key is in place and the crank / flywheel keyways are lined up
Fuel: I've tried pouring a little fuel down the carburetor and putting a little down the spark plug hole. Nothing.
I observed a couple other things that seemed a bit odd. When the timing dots are aligned, the piston is about an inch down in the cylinder. I would have expected it to be at TDC. When the motor is cranking, every 5-10 seconds, it will 'catch', then sneeze back through the carburetor and blow some vapor out. Thinking the valves might be out of time, I removed the muffler / pipe and cranked the engine while putting my hand over the exhaust port. Every few revolutions my fingers would get sucked against the port like it was opening the exhaust valve on the intake stroke.
I'm stumped. I pulled the motor back apart and everything seemed to be in the right place. Any help would be appreciated.
06-06-2007, 11:53 PM
Sounds like you have some valves that are not closing fully, Worn valve faces/seats worn to the point the stem to tappet Clarence is too small.
Could also be worn valve guides.
Did you have the valves out ?
If not, That could be the next place to go.
06-07-2007, 12:25 AM
Have you tried a new plug? If the valves are correct as to Restrorob's suggestions you may need to check the timing marks again. Did you compare the new and old rod for length. I don't know how or if a different rod will hook up - but I thought I would throw it out there.......
06-09-2007, 05:44 PM
I pulled the valves and cleaned some crud off one of them. The other one was pretty clean. I also cleaned the valve seats. I did not see any wear marks or damage on either the valves or seats. I also installed a new spark plug and confirmed the ignition system is sending fire to the plug. I double-checked the timimg marks before reassembling it. Sucker still won't start, and has the same weird things happening during starting. It 'hangs' every few seconds, then spits vapor back out the carburetor and also tries to suck my fingers in when I put them over the exhaust port. That seems like it would come from the exhaust valve not closing all the way, but I would expect to feel air blowing out on the compression stroke if that was the case. Any other suggestions would be greatly appreciated.
My next step may be to replace the motor. If I go that route, how do I find one that will bolt in the same location, or should I just plan on redrilling the motor mount bolt holes in my mower frame and just get a similar size motor? The mower is about 13 years old.
06-09-2007, 07:00 PM
If the valve lash is correct then it must be the mechanical valve timing or lazy (stuck) exhaust valve. Take it apart one more time and take a digital picture of the timing marks and post it. Do you have a service manual for this engine?
06-09-2007, 07:52 PM
If the valve lash is correct
That's just it Khouse, There is no mention of checking valve clearance.
We know this needs to be done to either fix the problem or eliminate that being the problem.
Jeff, If you don't know; The piston must be positioned 1/4" down the cylinder past top dead center and a feeler gauge inserted between the valve stem and tappet. Intake should be .005 to .007 exhaust should be .009 to .011, That is with the valve springs removed and holding the head of the valve in place with your thumb while checking.
If they are too tight you must grind a little material off the valve stem, But go SLOW and don't take too much off and exceed the max limit and grind them as flat as possible.
If you did check the clearance what did you get ?
06-09-2007, 10:54 PM
This is something I've never understood, they all talk about TDC and how important it is,
no doubt, but WHICH TOP part of the stroke is it?
On a 4 cycle engine, these are the four things that happen:
Intake, compression, ignition, exhaust (and this repeats itself).
And that goes DOWN UP DOWN UP (in the above order).
Notice the two UP's...
So, are there not TWO instances where the piston will be at TDC:
Once at the very end of the compression stroke and right before ignition,
and again at the very end of the exhaust stroke and right before intake.
So, could it be you got the wrong tdc, or is this not important?
Wouldn't it affect which valve opens when?
Would this explain why your fuel gets shot back out the carb and the exhaust is sucking?
If this happens to be the solution, would someone care to educate repair manual makers over this, and get these fools to explain this crap to the layman once and for all, rather than just saying "Well it has to be tdc huhuhu" or is it just assumed that if you're this smart then you're smart enough?
Because above explanation aside, I don't get it because how do I know?
As of now and for many years, if a repair involves TDC, I don't touch it.
06-09-2007, 11:22 PM
Well EXCUSE me Mr. Topsites, I tend to leave a word out once in a while. Must be Ole age catching up or something. :cry:
ALL valves are adjusted on COMPRESSION stroke !
This is how my sentence should have read; The piston must be positioned 1/4" down the cylinder past top dead center compression stroke.
Sorry for rocking your boat !
06-09-2007, 11:39 PM
A picture of the timing gears is attached, with the dots aligned. I assume the 'dot' on the cam gear is the little dimple and not the big hole about a third of the way around the gear.
I don't have a service manual. I've never had one of these apart this far until a few days ago. On a positive note, I've gotten really good at removing the motor from the mower and getting it apart. I can do it in 5-7 minutes now.
As for the valve lash, I did not check it when I had the valves out, but since the the motor was running fine until I replaced the connecting rod, I don't have any reason to believe valve length is an issue. I bought the mower new and have never had any trouble with the motor until now. Also, I inspected the end of the valve and both ends of the tappet when I had the valves out and there was no noticeable wear. There was also no evidence the valves had receded into the head. Since it was such a pain to get the valve spring retainers back in (I don't have a valve spring compressor), I'm trying to avoid pulling them back out. If it turns out I really need to check the lash to rule that out, I will go by the auto parts store and get one of their loaner valve spring compressors tomorrow.
I've always understood TDC to be on the compression stroke when both valves are closed.
Thanks for all the help.
06-10-2007, 12:03 AM
The timing marks are fine, Why did you replace the rod ?
06-10-2007, 01:22 AM
It doesn't take much wear to add up to .005" The valves may look good with no wear but a feeler guage will tell the story.
06-10-2007, 02:16 PM
"So, could it be you got the wrong tdc, or is this not important?
Wouldn't it affect which valve opens when?"
IF you are aligning timing marks on the cam correctly, you can't have a "wrong" TDC! The mark on the crank gear is always going to be in the same place when the piston is at the top of the stroke.
Anybody with a modicum of knowledge that has worked on any kind of 4 stroke engine knows that TDC is inferred to be at the compression/power stroke. To ASSume the opposite would be simply stupid.
06-10-2007, 05:09 PM
The rod siezed on the crankshaft journal, I think due to a low oil level. Yeah, I know, it's a pretty dumb reason and there's really no excuse, but I've learned my lesson. Surprisingly, there was no apparent damage to the crankshaft or camshaft surfaces. I've since pulled the piston to see if there were any broken or damaged rings. Everything looked OK and there are no gouges on the cylinder wall. The valves (again) are next. I wish I'd done a compression check before now while the engine was together, just to have that data point. That's number two on the list.
06-10-2007, 05:13 PM
So, When the rod seized did it break and lock the engine up ? My questions are leading to something else to check before closing it back up.
06-10-2007, 05:42 PM
I'm working on the same engine with the same issue with broken connecting rod. It's from my daughter’s lawn mower that she stored in a lean-to outside. Over the winter mice thought it was a great place to build a home under the cooling baffles around the cylinder. The cylinder had no airflow around it. The surprising thing is there seems to be no real bad damage other then the connecting rod. I have to order a new one.
If you get your motor back together be sure to squirt a little oil down the spark plug hole before trying to start. If the compression is somewhat low the oil will help increase it enough to allow it to starting. Normally it is not a good idea to put gas directly into the spark plug hole, as it will wash the oil off the cylinder killing the compression. If after trying to start the engine and if it fails, pull the spark plug and see if it is wet with fuel or not. If it is dry you may not be getting fuel to the cylinder, if it is wet you maybe loosing spark.
Hope you get it going after all the work,
06-10-2007, 06:49 PM
The rod didn't break. Just before the motor quit, the mower started to shudder like when you get wet grass built up under the deck, putting the motor under too much load. I disengaged the blades, then the motor just slowly came to a stop (3-5 seconds). It stopped with the piston at the top of the stroke.
06-10-2007, 08:13 PM
The rod didn't break
Well, What I was thinking will not apply then. Have you checked valve clearance yet ?
06-14-2007, 10:09 PM
Before I took the checked the valve clearance, I made one more thorough check of everything else. I discovered the crankshaft / flywheel key had sheared and the ignition timing was off by about 60 degrees. You may be saying "I thought he checked the key and it hadn't sheared". Well, I did check it and at the time it was in one piece. I suspect that one of the times when the engine was cranking at a pretty good speed and then kicked back, it was enough for the key to shear due to the inertia of the flywheel. I'm not sure why it kicked back originally if the timing was correct, but once I replaced the key and reassembled everything, it started and ran just fine.
Thanks for all the help. I know a whole lot more about these engines than I did a couple weeks ago.
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