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12.5hp B&S engine rebuild, engine won't start


Hondal's Avatar
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Join Date: May 2007
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07-11-07, 07:07 PM   #1  
12.5hp B&S engine rebuild, engine won't start

Hey I just finshed rebuilding my 12.5 B&S model 282707-0116. It is bored .020 over. I did get it all back together and after finding out the coil was on backwards I got spark and had it Running. I can't figure out where to hook the black wire that comes from the coil so to shut it off I had to leave the wire out and touch it to ground to short it out. Now I tried starting it again and it bubbled and was a little louder and now won't start. I checked spark and the plug was getting wet. I checked the compression and I was only getting 60psi so I am not sure what gives. I took the head off and the valves and piston looked good and everything was moving ok. I am thinking maybe the rings moved when I installed them and the gaps may now be lined up and causing blow by? Anyone have some ideas to try before I start to tear the engine down again. Note I did not re-build the carb and gas might be of issue so I will try that this week. If it ran smooth before what gives now.

 
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07-12-07, 02:42 AM   #2  
Your on the right path on the possible carburetor issue and as for the 60 PSI on the compression check, I'd be more inclined to think you have a lack of valve clearance issue causing the low compression reading. The intake valve clearance will be .006" and exhaust will be .010".

 
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07-12-07, 01:29 PM   #3  
Do I have to tear down the engine to adjust the valve clearance? I had the machine shop adjust the valves and I am not sure if they did a good job. To note when I pull the head off the was some fule leaking out of the intake valve and exhaust so you might be right the valves may not be full closing.

Can you explain to me how to properly adjust the valves.

Thanks for all your help

 
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07-12-07, 02:21 PM   #4  
You won't need to tear the engine down to check the valves. You will need to remove the carburetor to gain access to the breather.
Take that off, and you will see where the lifters and the valves meet. Turn the engine by hand to top dead center, so the valves are fully closed (compression stroke) and then use a feeler gauge to check between the tappets (lifters) and valve stems. If they are out of clearance the valve should be removed and the stem *carefully* ground to get it within specs. Remember youíre dealing with thousandths of an inch here, so donít go crazy with a bench grinder.

 
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07-12-07, 08:18 PM   #5  
Now do I need a special tool to remove the valves? DO I grind down the end of the actual valve end? I tore the engine down and put it back together without a manual so I am researching all things to get this engine rebuilt. I have down many a two stroke but 4 strokes are new to me.

Thanks

 
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07-12-07, 11:12 PM   #6  
The bubbling you described, combined with the fact about the fuel running out the intake valve leads me to believe you have a carburetor problem, not an engine problem. It was running before, and the valves don't "adjust" on that engine, so they should still be good enough for it to run now. To "adjust" them, you have to grind the stem precisely to obtain the correct clearance.

The 60 psi is probably because all that fuel is washing out the cylinder.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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07-13-07, 02:49 AM   #7  
As for the black wire (kill wire) from the coil, it should attach to a junction block in the vicinity of the speed control bracket (governor area) along with one other wire that goes out to the remainder of the electrical circuit (safety switches, start switch, etc.).

 
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07-15-07, 07:41 AM   #8  
Finally Figured it all out and it's running now.

What it was is the carb had rust on the float bowel flooding the carb and must have sent gas into the combustion chamber and I sheared the flywheel key. The way I checked it poured some gas into the carb and it would backfire through the carb. I replaced the flywheel key and took the carb and cleaned it. While I was in there I inspected the valve clearance it was good, .005 on the intake and .011 on the exhaust, close enough. The engine wire is hooked up now correctly it all works great. Thanks for everyones response.

Now onto my other issues I need to adjust or replace the drive belts. But I think that one is easy.

 
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07-16-07, 12:00 AM   #9  
Glad you got it fixed! Thanks for the update!


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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07-16-07, 02:09 AM   #10  
Not to take the wind out of your sails, but, drive belt replacement is not necessarily easy, especially if you have an MTD veri-drive transmission. Glad the engine service was easy though!

 
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