# MTD yardman carb



## Dbrown57 (7 mo ago)

I just bought a new replacement carb for my MTD yardman lawn tractor. 420cc . Everything on the carbs looks identical EXCEPT for a small brass screw on the side (looks like an air-screw) the old carb has the fitting but it was blocked and sealed a the factory. Can I just remove this screw and block off the fitting ?


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## BigT (Sep 15, 2014)

Good Morning DBrown57, welcome to the forum.

Seems like there is always some small difference in replacement carburetors. It obviously has a purpose. Leave it, and see how it goes.


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## Dbrown57 (7 mo ago)

BigT said:


> Good Morning DBrown57, welcome to the forum.
> 
> Seems like there is always some small difference in replacement carburetors. It obviously has a purpose. Leave it, and see how it goes.


thanks, I'll leave it and if it doesn't work right I'll just turn the screw all the way in, which should close off the air. I'm guessing.


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## FredM (Nov 18, 2015)

It will most likely be an idle mixture screw, it is a bit hard to tell with how the photo was taken, if you screw it right in, then the engine most likely wont idle, one and a quarter to one and a half turns out should be the setting, run the engine first and see how the engine runs.


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## Dbrown57 (7 mo ago)

Ok, I've tried everything I can think of. No start. I've got spark and fuel to the plug. Maybe the spark is too weak ? Coil ? Possibly out of time ? I'm about to find a BFH .
Thanks everyone


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## FredM (Nov 18, 2015)

Dbrown57 said:


> Ok, I've tried everything I can think of. No start. I've got spark and fuel to the plug. Maybe the spark is too weak ? Coil ? Possibly out of time ? I'm about to find a BFH .
> Thanks everyone


We really have no information of what your engine problem is, could you advise what the engine is in your mower and the model number and we may save you from having to use the BFH.

Does the engine have reasonable compression?, is the spark plug oily or clean?, remove the shroud and remove the flywheel retaining nut and washer, you will see the keyway in the flywheel, is this lined up with the keyway in the crankshaft?, if not lined up, key has sheared and timing is out, is the spark blue in colour or yellow?, needs to be blue for a strong spark but I have started engines with a yellow spark.


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## Dbrown57 (7 mo ago)

Sorry for the lack of info. The spark is yellow. I'm going to check the key tomorrow morning. 
This is an MTD 420 cc year 2015. I'll get the model # Tomorrow as well. The plug is new, no oil on it. If the key is intact and in place I'll check compression. Then I don't know. I'm lost.
Thanks for your advice !


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## FredM (Nov 18, 2015)

Have you thought to remove the valve cover and check valve clearances?, is your engine the MTD 420CC Powermore model?.


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## Dbrown57 (7 mo ago)

I haven't checked valve clearance yet. Yes it's the 420cc model. I'm going at it in a couple hours, it's 5:20 am here now, just getting coffeed up.


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## Dbrown57 (7 mo ago)

So, I rechecked the spark without lights on and its bright white. Compression is about 40lbs. Seems awful low ???


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## Dbrown57 (7 mo ago)

Looks like the Woodruff key is busted off.


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## FredM (Nov 18, 2015)

Dbrown I would check the valve clearances to see if you have a closed clearance causing the low compression, and replace the woodruff key and then give her a go.


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## Dbrown57 (7 mo ago)

Ok I'm not sure how to check valve clearances. I've had the flywheel soaking in penetrant all day trying to get that off.
Thank you !


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## FredM (Nov 18, 2015)

You will have to remove the valve cover, you will need feeler gauges, not sure if you would use a ring spanner or allen key to adjust, or both, haven't seen this engine, so not sure of the valve specs and how to setup for adjustment, I would expect the engine to have an automatic valve lifter for easy starting, so I would assume that you would rotate the flywheel a couple of inches past TDC on compression stroke so the valve lifter is off cam and allows the valves to be seated for adjustment.

If you don't have a flywheel puller, then you replace the flywheel nut back onto the shaft thread and roll the nut down until the shaft thread is not quite flush with the flat of the nut, this is to protect the thread and nut thread when you get your BFH to "squarely" hit the nut and shaft, you will also need a tyre lever or large flat blade screw driver that you will place under the flywheel and onto a solid place on the engine to place leverage against the flywheel, if your engine has electric start, then be careful how far under the flywheel you place whatever lever you use, there is a charging stator under the flywheel if electric start.

Your best bet would be to either buy a puller or hire one to pull the flywheel.

I'll see if I can find the valve specs for you.


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## Dbrown57 (7 mo ago)

Thank you so much !


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## FredM (Nov 18, 2015)

Professional Power Equipment Technicians & Education Network (PPETEN) • View topic - MTD engine valve lash setting







ppeten.com





If you can open this site, scroll down to the second post, this gives a good description on the valve clearance settings and spanners to use, I would still roll the flywheel a couple of inches past TDC as I explained before.


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## Dbrown57 (7 mo ago)

Thanks Fred !


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## FredM (Nov 18, 2015)

Happy to help where I can, have a great day.


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