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blowback through carburator


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11-14-04, 03:26 PM   #1  
denniso
blowback through carburator

Have a 12 horse B&S lawnmower motor, model 283707, type 0129 02, code 9211114A. Previous owner ran it out of oil and it sat unused for several years. Had a friend rebuild it (He usually knows what he's doing). Went to fire it up and it wouldn't start and blew lots of gas back through the carb. As the starter turns over it blows as forcefully through the carb as it does through the muffler. Builder says cam is keyed to only go in one way and piston was at top dead center and both valves were closed when he assembled it. Is this a timing problem. carb problem, or something else altogether? Help!

 
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11-14-04, 04:31 PM   #2  
denniso
I can't say from experience regarding that particular motor but I've done enough of the others that one thing is common and that is valve timing. To accomplish this, when assembling the motor, the crank and cam have notches in them. They have to match in order for the valve to be in the correct position. On your motor, if you pull the head and look at the valves when the piston is at top dead center after the exhaust stroke, and you move the piston back and forth, you will see either the intake and exhaust valve starting to open. This is commonly referred to as "dancing". If you don't see this then it's time to look at the actual valve timing.
That would be the first thing I'd look into.

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11-14-04, 09:45 PM   #3  
It does sound like a valve timing problem. You could take the head off and visually inspect the valves to see if they are doing their thing, or you could simply remove the spark plug. With the spark plug removed, you can put your finger over the plug hole. Turn the flywheel and feel for suction then compression. If you don't feel either that would confirm a valve timing problem. With the air cleaner off you might even be able to detect a stong flow of air OUT of the carb. That's not a good thing, and if you felt that, I'd feel confident that I'd find the valve cam gear dot (or notch) not aligned with it's mate on the crank gear. I've almost forgot to align them a couple of times, so I could understand how something like that could happen.

 
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11-15-04, 11:35 PM   #4  
Agreed. The cam can go in any way you want. It's not "keyed" to go in one way only. It needs to be lined up with the crankshaft. The marks lining up is all that's needed. If he did it looking at the piston and valves and not the marks, it could be off, could be right, who knows until it's checked. The only way to time a cam by watching the piston and valves is when you do it the way snowman said.


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11-21-04, 04:24 PM   #5  
denniso
b & s valve timing

I asked earlier about blowback through the carb. You guys confirmed what I thought about the cam being installed wrong. I do have compression but i have very strong blowback through the carb after. Took the pan off but not quite sure where to go from here. The crank is keyed and the cam gear has a hole through it and a small round mark (for timing?). I haven't disassembled it any further. There is a gear on the crank that slides on to the key. Not sure what holds the cam gear in but didn't want to go pulling things apart until I knew how they were supposed to go back together. If there is a corresponding timing mark on the crank - where do I look for it? What should align with what to have correct timing? Is there any need to disassemble the head? Even if I never get this thing running again I'm having a good time trying to figure out what makes it tick!

 
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11-21-04, 11:58 PM   #6  
There is a dot between gear teeth on the camshaft, and a dot on one gear tooth on the crankshaft gear. Line them up and the cam is timed.


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11-22-04, 11:03 AM   #7  
denniso
Thanks, Cheese. Timing was about 90 degrees off. Is there anything else I need to check before putting the pan back on? Valve adjustment etc.? Is there a specific order that the pan bolts are tightened? Torqued to 160?

 
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11-23-04, 12:36 AM   #8  
Make sure the governor contacts the lever as you put the pan back on, make sure the key is in the crankshaft and holding the gear in place, make sure the gasket is good, and it wouldn't hurt to replace the seal while the pan is off. Don't worry about any order on the pan bolts. 160 inch lbs is fine, but I usually just tighten them good. It wouldn't hurt to clean the bolts and use some BLUE locktite on the threads sparingly.


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11-23-04, 03:07 PM   #9  
denniso
Thanks again, Cheese. Got it put back together and it runs great. God bless You!

 
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11-24-04, 01:03 AM   #10  
Great!

Glad you got it! There's nothing like fixing it yourself, huh?

Take care!


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