Author Topic: B&S Engine Question  (Read 3188 times)

Offline kckndrgn

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B&S Engine Question
« on: February 20, 2017, 05:18:53 PM »
Afternoon all, once I again I am looking for input from the TSP brain trust  ;D

My sister gave me her old lawn mower since it quit working last year and she has a replacment already.  Not that I need another lawn mower, but I figured it was just the carb was jacked from the ethonal fuel.  Well, I'm sure it was that, and it appears to be more.

First think I did was pull the carb and inspect.  I noticed there was more water in the gas tank than gas, great.  Drained the gas, cleaned the carb the best i could as I wanted to see if it would run, before spending too much or any money on repairing.

After assembly, I tried to pull the starter rope, nope. not moving.    Started rotating the via the  blade (plug removed, no gas in system so no worries about it starting ;) 

I could go back and forth almost 2 full turns before the motion was stuck.  Found out that the intake valve was stuck.  I'm guessing due to the ethonal in the fuel causing water to get in the system.
Well, go the valve moving freely and then I noticed that on compression stroke the drive shaft "jumps" the fly wheel moved up about a 1/16" to 1/8"  then drops back down.  Can't say that I've ever seen this before.  Happens in the same spot spinning the motor in forward or reverse.

here's a quick vid where I hope you can see what I'm talking about.
https://youtu.be/X_kOQApNTIs

So, considering that if I get this running I'll probably just sell it, does this look like a tear down is required?  Any idea of what would be wrong?  Plastic gears sheared?  Timing still seems to be ok.  Getting a gasket kit and a diaphram for the carb will cost me about $20.00.

Offline ColdSteel

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Re: B&S Engine Question
« Reply #1 on: February 20, 2017, 07:07:19 PM »
Without spinning the engine how far can you move the crankshaft (flywheel) up and down? Also does the blade move up and down when the flywheel does? It would seem to me that it has a ton of wear on it.

Offline kckndrgn

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Re: B&S Engine Question
« Reply #2 on: February 20, 2017, 07:10:18 PM »
Without spinning the engine how far can you move the crankshaft (flywheel) up and down? Also does the blade move up and down when the flywheel does? It would seem to me that it has a ton of wear on it.

About the same vertical movement 1/16th to 1/8th inch. yes the flywheel and blade move together. 

 Knowing my sister the oil was never changed, but looking at the cylinder wall it still looks good.

Offline ColdSteel

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Re: B&S Engine Question
« Reply #3 on: February 20, 2017, 07:29:09 PM »
I would say that there is a spot on the cam gear that makes the crankshaft do that. You may take the engine apart and find something simple but I can't imagine the time and parts being worth it. Push lawnmowers can be bought pretty cheap in running condition.

Offline machinisttx

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Re: B&S Engine Question
« Reply #4 on: March 01, 2017, 07:25:10 PM »
You might as well tear it down completely and have a look. I most likely wouldn't try fixing it at all after watching the video. It's usually the exhaust valve that sticks, so that's unusual too. I can take a guess at what the problem is though.

The crank is normally retained in position by bearings or bushings, thrust washers, and the crankcase halves being bolted together. Some engines don't have the thrust washers, and some may not need them due to manufacturing tolerances stacking up just right or due to design. The thrust washers would be located between the shoulders of the crank and the case half.

What I'm getting at is that I think either the thrust washers are worn/disintegrated, or the locating surfaces of the crank/crankcase are worn far beyond acceptable limits. I think the counterweights are rubbing on the inside of the case, and that's what is causing the crank to move. I could be wrong though, and I'd be interested in seeing the inside of that engine anyway.

I can't see the cam having any effect at all like that on the crank, as it's only point of contact with the crank is the spur gear that drives it. Strange things happen though. *shrug*

Offline kckndrgn

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Re: B&S Engine Question
« Reply #5 on: March 01, 2017, 07:37:04 PM »
You might as well tear it down completely and have a look. I most likely wouldn't try fixing it at all after watching the video. It's usually the exhaust valve that sticks, so that's unusual too. I can take a guess at what the problem is though.

The crank is normally retained in position by bearings or bushings, thrust washers, and the crankcase halves being bolted together. Some engines don't have the thrust washers, and some may not need them due to manufacturing tolerances stacking up just right or due to design. The thrust washers would be located between the shoulders of the crank and the case half.

What I'm getting at is that I think either the thrust washers are worn/disintegrated, or the locating surfaces of the crank/crankcase are worn far beyond acceptable limits. I think the counterweights are rubbing on the inside of the case, and that's what is causing the crank to move. I could be wrong though, and I'd be interested in seeing the inside of that engine anyway.

I can't see the cam having any effect at all like that on the crank, as it's only point of contact with the crank is the spur gear that drives it. Strange things happen though. *shrug*

Thanks, yeah, i did tear it down before ordering any parts, just to "see in side"  (not my first tear down).  2 things noted, it didn't look like the oil had ever been changed (shocked there   ::))  as the sludge on the bottom was thick.  On the cam shaft gear there was one tooth that had maybe 1/16 of an inch chipped off of it.  I didn't see any bearings, just seals at the top and bottom of the crank.
Pulled both valves out, and after cleaning the gunk off the intake valve it worked just fine.  After a through inspection I found nothing wrong, no broken parts, nadda.  I reassembled the engine and rotated the crank, no more jumping.  Nearest I could figure is the part of the tooth that chipped off was lodged somewhere and causing the crank to shift.  After splitting the engine the chip came out (never found it).
So I ordered the gasket kit ($12.00) and put the engine back together last night.  During re-assembly I lapped the valves to make sure they had a good mating surface.
Engine cranked on the first pull, but would only run when pressing on the primer bulb.  So, time to clean the carb, well, clean it better anyway.

Once it's running this will be my backup spare mower, or I may sell it for what I have in it. 

For me it was just nice to sit in the garage and putz around.

Offline Carl

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Re: B&S Engine Question
« Reply #6 on: March 01, 2017, 10:04:36 PM »
I also note that some slack can be expected when the piston is not the motivation of the engine though your video did look strange as it was at a predictable point of motion. You may be correct on something displaced though. I have rebuilt or repaired a few one cyl engines ,but don't recall any that had that 'action'

Offline machinisttx

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Re: B&S Engine Question
« Reply #7 on: March 02, 2017, 07:57:54 PM »
The bearings(or bushings, depending on the engine) are pressed into the crankcase halves, closest to the inside of the case. Oil seals would be pressed in on top(on the outside of the case) of the bearing/bushings so that oil can splash lube them. I have never seen an engine with that much endplay in the crankshaft. Service manuals will specify what the acceptable amount is, and it's always been just a few thousandths in the manuals I've seen.

Congrats on fixing it. Now it's not "junk" anymore.  ;D