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B&S 21 HP won't start after replacing connecting rod


Bob Walters's Avatar
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08-18-17, 12:10 PM   #1  
B&S 21 HP won't start after replacing connecting rod

I have a B&S 21 HP engine (331877) on 46" Craftsman lawn tractor, on which I replaced the broken connecting rod. Now that it is back together I can't get it started. Has spark and spark plug is wet so seems to be getting fuel. I watched a video on how to replace governor springs to make sure it was connected correctly. I did not have the governor link spring ends in the holes on the governor lever and throttle arm. If you would, please help me troubleshoot this to get it started. Thank you.

 
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08-18-17, 12:19 PM   #2  
Spark? Or no spark?

Did you get the camshaft timing marks aligned correctly? Did you lube the rod journals well as you reassembled it? Checked your valve clearances?

 
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08-18-17, 09:55 PM   #3  
And the flywheel key sometimes shears when a rod breaks... one more thing to check.


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08-20-17, 10:23 AM   #4  
Yes, the timing marks are aligned. Although I believe the first time I assembled the engine the camshaft was 180 degrees out of synch. But I'm pretty sure it's correct now. I did not lube the rod journals. However, I did check and adjust the valve clearance.

 
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08-20-17, 12:51 PM   #5  
180 degrees out of time is in time on the camshaft. It times up in 2 places (timing marks aligned and timing mark 180 degrees out).

Did you check the flywheel key?


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08-22-17, 11:13 AM   #6  
I don't know what I was thinking when I typed camshaft was 180 degrees out of synch. Just was not thinking...
Flywheel key was sheared but I replaced that. I'm thinking that I might have to take it apart and check the timing marks again but I'm pretty sure they were set correctly. I just can't figure out what else it could be. Do you have any other suggestions?

 
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08-22-17, 01:21 PM   #7  
I think cheese is saying that when you open it up, make sure that your spark is timed to occur on the compression stroke, when both valves are closed. Your crank makes 2 revolutions for every 1 camshaft revolution. So you could have the marks correct but the crank on the wrong stroke.

 
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08-22-17, 08:11 PM   #8  
No, it doesn't matter which stroke it's on, as long as the cam mark lines up with or is exactly 180 degrees opposite of the crank mark, it is in time. The spark plug fires on both strokes but only makes power on one. If you think the marks are lined up, they probably are. It's not something that just bumps out of whack while you're putting it together. Did you have any oil or grease on the flywheel or tapered part of the crankshaft when you put the flywheel back on? Are your valves set properly? If the plugs are wet, it may well be flooded... could it have been flooding and thinning the oil with gas, possibly leading to the engine blowing?


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08-23-17, 08:16 AM   #9  
I replaced the rings when I had the engine apart. So they should be sealing fine. Could I really be getting that much blow-by to impact compression?
Here is another thought. Although it does not seem like it would matter. When I replaced the connecting rod, could I have put the cap on the wrong side? It seemed like there was not space to get the bolts for the cap in place if I had turned the rod and piston around. Would rotating the connecting rod 180 degrees matter?

 
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08-23-17, 08:42 AM   #10  
Oh yeah, forgot that part about the spark.

Seems I recall that when replacing rings, they needed to be lubed quite well, and the gaps on each set of rings were initially to be staggered 120 degrees away from each other. The oil in the cylinder often fouled the plug until it was started and broke in.

Have you tested compression?

 
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08-23-17, 11:06 AM   #11  
Thank you. I don't have a compression tester, although there certainly seems like there is adequate compression.

 
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08-23-17, 10:48 PM   #12  
You did you hone the cylinders before replacing the rings, right?


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08-24-17, 12:40 PM   #13  
Did not hone the cylinder.
When I try to start it the engine will sometimes backfire. What does that indicate?

 
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08-24-17, 03:29 PM   #14  
When I try to start it the engine will sometimes backfire. What does that indicate?
Checking compression would be a good first place to start.
Honing the cylinder along with new rings provides a surface for the cylinder wall and the rings to wear together forming a seal.

One sign of possible low compression would be spark is present, fuel introduced....wet spark plugs.


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08-30-17, 08:10 AM   #15  
Compression is 100 psi. I did a quick check on line and found that engine will run on 80 psi.
I have been thinking about taking the engine apart to ensure that the timing marks are aligned. Although I thought for sure that I had them lined up.
What do you think about the compression and what about taking the engine apart to check the timing marks? What else can I check?

 
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08-30-17, 10:03 AM   #16  
100 psi works. It really sounds like you have a timing problem. If the flywheel key is good it's probably with the valve timing = either in the cam or valve clearance.

 
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