# MTD mower + Briggs motor running bad



## jarl (Aug 27, 2009)

Or should I say not running 
First time around, so here are the details: MTD "080 series" push mower, with Briggs and Stratton motor, 10G902-0217-B1 03042255

Now: I just got this mower and know nothing about it's past. Previous owner gave it away instead of fixing the starting pulley... but apparently he fiddled with some other stuff before doing it, or there was something else wrong before he broke the starter.

I replaced the pulley/spring assembly (what's the right way of doing it, btw?) and started the mower, but it was running really badly (surging & black smoke every 10-20 seconds). After attempting to remove the carburator -I know... bad idea without a manual, but screws were not too tight and I assumed the previous owner had messed with it-, I tightened the screws and now the mower starts but dies after 10 seconds or so. The spark plug was fowled with dry soot, and the air filter looked Ok. When the motor runs, I can feel the mower vibrating but I can't tell if it's "too much" -all I know is that it's more than my older mower-.

From previous posts I'm assuming I need to replace the diaphragm, but I'd like to know if someone around here is familiar with these motor+mower combination. Since the previous owner took it apart I don't know if there's anything missing on the top of the engine -in particular, the pictures I have seen show two springs attached to the throttle, but this motor has only one-

Ok... so I guess my questions are:
- Is there something I should test before replacing the diaphragm?
- How much excentricity of the cranck is too much when I rotate the blade by hand? I f it's more than <fill the blank> is it worth to replace the crank on a cheap mower?
- Does anyone has a picture of the linkages/springs around the carburator area?

(Holy smoke... this thing turned up too long! Sorry about that!)
Thanks!


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## Al Bunzel (Apr 20, 2009)

Hi,

I'm not familiar with this motor, but these documents could be helpful.

Parts manual:
http://www.briggsandstratton.com/ma...docmanualdetails.aspx?showpdf=MS5549_B_LO.pdf

Operator's manual:
http://www.briggsandstratton.com/ma...manualdetails.aspx?showpdf=277038TRI_D_LO.pdf

Soot in spark plug:
I'm assuming the soot on the spark plug is black.
This could indicate that the mixture is too rich in fuel, or the choke is stuffed
or the air filter is dirty/blocked or a cold spark plug is being used.


Vibration:
That could be caused by a bent crank or the mower blades could be unbalanced due to damage.


Goes for 10 seconds:
* Does the fuel cap permit proper venting or is it stuffed?
* Is there water in the fuel or has the fuel gone stale?
* Is the intake manifold lose and the connections from the carby lose 
ie. is there an air leak around that area?
* with the missing spring, that could affect the operation of the governor.
* choke could be stuffed.
* muffler could be blocked.

Other things to check:
* Ensure you have the correct quantity and correct grade of oil and that it is not due for replacement.
* Ensure the cylinder head bolts are not lose.
* Ensure the cooling fins are not blocked and the blower fan is pushing enough air around the engine.
* Ensure blower housing and engine cowling are in tact.
* Inspect the air filter (which you have done already).

Let us know how you go so we can work out the next steps to check.

Kind Regards
AlBunzel


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## jarl (Aug 27, 2009)

Thank you for the prompt reply 
I have checked the manuals before, but the information in there is limited. In particular, the size of the spring is nowhere to be found...

The soot is, indeed, black. However, these particular engines don't have a choke (do they???). Also, the problem is still present if you run the motor without the filter, so I guess there's something else going on. The mixture is probably too rich, I agree, but there isn't any obvious way of adjusting anything on this motor  The manual just says "take it to the B&S service guy".

I looked at the fuel and there isn't anything obviously wrong with it (fuel/dirt/etc). The cap... I'll have to check it. 

Other than this problem the engine is clean and there isn't anything broke/bent that I can tell. The intake manifold doesn't seem to have screws so it's hard to tell if there are any gaps between the carb and the intake...

Regarding the vibration: I guess the previous owner hit something at some point, but I wonder if there's any measurement that could tell me if it is /isn't a big deal.

J


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## 30yearTech (Sep 14, 2006)

The illustration for your engine only indicated a single governor spring. The size and spring tension rate are specific to the model of engine, so if it needs replacement you will need to purchase the correct one from a Briggs dealer.

The carburetor diaphragm Briggs # 795083 is most likely the reason you are getting soot on the spark plug and causing the engine too run rough and surge. It's leaking around the pulse port and allowing fuel to by pass the metering circuit. This may also be the cause of the vibration. 

You can check to see if the shaft if bent by removing the spark plug, setting the mower on it's side with the carburetor facing up and simply pulling on the rope while observing the shaft to see if it "wobbles" any. If the shaft is straight and there is still some vibration, then it's likely caused by the mower blade, being bent or out of balance.


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## jarl (Aug 27, 2009)

Well... 
I changed the diaphragm and gasket, put together the whole thing again, and it's a no-go 
The mower still tries to run for a few seconds and then dies. I have to do a lot of tries before it starts, though. You can hear like the motor "tries" to start, but only once every several times it runs -for ~3 seconds or so-
Is there any chance for this to be an electrical problem?


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## jarl (Aug 27, 2009)

Well I went a little further to see what could be wrong...
I first checked this video: 



and when I did the same test on my engine (holding the dead man's lever depressed) I couldn't get a spark... not sure what it means, since the motor starts even if for just a few seconds, meaning there got to be a spark  What would be the correct way of testing the magneto?

Now... being quite pissed at the moment, I decided to check if there was something else... the attached image shows what I found  I guess that's enough to throw the timing off...

Of course there got to be a reason for the sheared key: the crankshaft is also bent... not too much but you can tell it's not 100% true.

So... I'm very close to decide this is enough to throw the mower to the garbage. Is it realistic to think that replacing the key would be enough to fix the timing and get this thing to work?


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## 30yearTech (Sep 14, 2006)

That video really does not pertain to your engine. That is a much older engine with points, they will spark at a much lower RPM if they are set up properly. The electronic ignition module on your engine needs around 600 RPM's to generate a spark. The color of the spark means nothing, the best way to test for a spark is with an inline gap type spark tester.

Even though the flywheel key is bit slightly, I don't think it's enough to keep your engine from starting. There could be a problem with engine compression, or have you checked to make sure the throttle plate is open when trying to start?


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## jarl (Aug 27, 2009)

Thank you for following up 30yeartech 
I'm glad the video doesn't apply in this case... but that means that if the ignition module needs 600 RPM that pretty much means I have no way to test it  Any trick here?

The throttle plate is moving freely and the spring keeps it in the "open" position by default, so I guess the motor is breathing correctly. I'm not sure how to test the compression, though -not without buying tools-

What baffles me is the following: When I first got the mower it ran for about 10 seconds or more, but surged and ran rich. It wasn't that hard to start either. Now it runs for 3 seconds (how much would the 3 squeezes of the bulb last?) and it's hard to start. What can cause such change? What would allow the motor to run only when the mixture is too rich?


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## 30yearTech (Sep 14, 2006)

You can't test your engine for spark, rocking the flywheel as in the video. You can test it with the recoil starter installed as pulling the rope will generate the necessary RPM's. A simple way to test the compression is to just pull the rope and watch the flywheel as the engine slows to a stop, the flywheel should either stop abruptly or bounce backwards, if it does this, then the compression should be sufficient for the engine to start and run. 

About 3 or 4 seconds of run time is about right for the engine to burn through the prime. If it starts off the prime but will not continue to run, then there is a problem in the metering circuit of the carburetor. You need to have another look at the carburetor.


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## jarl (Aug 27, 2009)

Well... It worked 
I removed the flywheel and re-aligned it, and replaced the oil again after spilling a large amount of it on my driveway while checking the blade 

The motor now runs ok (it feels a little underpowered, but that can be just an impression given my previous mower was much more powerful), but the whole mower vibrates quite a bit... meaning the crank is bent more than I thought. I don't see myself paying for a new crankshaft, though...

How difficult is it to replace a crankshaft on one of these engines? In particular, are there parts that must be changed once you open this thing?


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