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MTD riding mower stumbles under small load


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08-12-15, 07:41 AM   #1  
MTD riding mower stumbles under small load

Lawnmower: MTD Yard Machine 42" Riding Lawnmower.

History: the lawnmower was bought new in 2010. It ran great. It was used maybe for 20 hours until 2012 then it sat in a shed due to a bad battery and the owners now knowing how to fix it. The owners are my inlaws. I took it out of the shed and started working on it. I've changed the oil, put a new drive belt, new air filter, new carb, new fuel lines, new fuel filter, new blades, new battery.

Problem: it will start perfectly. When it runs with the blades disengaged And not in gear it runs great. When it is running in its highest gear driving up a hill it will studder under that load. When i engage the blades and drive it studders worse. When mowing and it gets to mow grass it tries to stall. It seems to not have much power.

I have noticed that when I turn the carb fuel screw I can not flood the engine. The screw seems to not do much. It does a little but it seems the engine is running lean. The fuel cap vents well. I have tested it to see if it was making a vacuum. The fuel runs to the carb well and there are no air leaks on the carb.

I have noticed that it runs better at less than full throttle. If I put it at full throttle it revs higher but has much less power and tried to stall easier.

I have not changed the spark plug.

It doesn't seem to be a problem with the deck due to it happening also when just driving in high gear.

Im thinking maybe something wrong with the new carb? Why can't I adjust it to run rich?

 
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08-12-15, 07:45 AM   #2  
I forgot to say that I bought a new carb because it was having the same problem. With the new carb it runs better than with the old one but it still tries to stall under load.

Also I'm using 93octane fuel.

Could it be how the cable attaches to engine. Maybe it's out of adjustment?

 
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08-12-15, 08:56 AM   #3  
What does it have on it?
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08-15-15, 06:43 AM   #4  
What do you mean by that?

 
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08-15-15, 11:25 AM   #5  
Engine model and type numbers tell us what kind of engine you have so we can look it up to see what type of carb you have.

 
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08-15-15, 11:35 AM   #6  
Cheese can tell better, but 93 octane in a lawnmower?? I see valve and piston problems with that high an octane rating.

 
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08-15-15, 02:30 PM   #7  
Your lack of power could be a throttle cable adjustment, but the way you've described everything else it sounds like a fuel issue - especially with the "running better at less than full throttle." The mixture screw on the carb you have may just be an idle mixture adjustment and not for full throttle.

If you have a choke you might try closing it slightly to see if does anything. Also run it for a time and check the spark plug for light brown coloration. If the color is charred black or if the exhaust smoke is blackened suspect an overly rich mixture or a weak ignition system.

Also check the exhaust and intake for any blockage. With it sitting an insect may have sit up house keeping in it.

The higher octane shouldn't affect anything negatively but you wouldn't need anything over 87.

 
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08-15-15, 06:03 PM   #8  
Meaning engine. What engine does it have on it.. we are diagnosing without important info. Could be several manufacturers, several engines within each manufacturer, single or twin cylinder, etc...


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08-15-15, 07:03 PM   #9  
I remember back in the 60's putting 100 octane Amoco gas in my VW beetle to "clean" the engine. Sure did. Burned the entire exhaust system out. Too hot.

 
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08-15-15, 07:30 PM   #10  
Normally the high octane will burn slower since it's harder to ignite which is how it prevents pinging/knocking from preignition in higher compression engines.

The only theory (and just a theory) I would have on that one is a lower compression engine designed with different valve timing may allow a slower burning fuel out the exhaust during the cycle and end up with more unspent fuel in the exhaust system to burn there.

 
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08-16-15, 01:00 AM   #11  
Yeah, using higher octane is not required, recommended, and might actually be detrimental because of higher heat and less power produced.


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08-16-15, 06:07 AM   #12  
Never ran across the higher heat connected with the higher octane. 99 and 100 octane in the '60s and early '70s was a lot easier to come by. A company I worked for then ran 99 Octane in six 454 Chevy gas engines and a couple 460 Fords. We'd put about 125,000 miles a year on each of those and never had a heat problem with it.

There was always the lower octane gas we put in everything else.

 
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08-16-15, 06:28 AM   #13  
Different engines than a lawnmower. We had to run higher octane in the cars we built in the 60's due to compression.

 
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08-16-15, 11:26 AM   #14  
Yup, the compression was why it was used, I just don't don't where the higher temp would come from. Higher temperatures are connected to preignition which you're wanting to avoid with the higher octane.

 
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04-02-16, 08:58 PM   #15  
I'm sorry I never got back with y'all on this. It's been almost a year. The mower still runs best at less than full throttle. I ended up forgetting I posted this on here. Found it today. When I get home I will check the engine model info and I will try to close the choke a little to see if that helps.

I still don't know why turning the adjustment screw doesn't do much. I'm pretty sure it's not the idel screw and it is the fuel screw. That one has a cap over it and is not adjustable. The one I'm turning is the normal one like on most small engines I've worked on.

Also I will be checking the exhaust for blockage. That may be.

And I'm going to check the tube after the carb for leaks.

I'm pretty sure it's running lean. I need to make it more rich but can't because the screw won't let more fuel out.

 
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04-02-16, 09:02 PM   #16  
Also I've been using it for mowing since the new carb but I've never been happy with its power. Still stalls on medium grass and I can never run it full throttle.

 
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04-02-16, 09:41 PM   #17  
just with a quick look at the info, perhaps an intake leak, gasket somewhere. If you do have a true mixture adjustment, and it has no effect, then I imagine you have an intake leak downstream of the carb to intake.


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04-04-16, 12:18 PM   #18  
I think that may be true. Maybe the gasket/oring where it connects to the engine. I seem to remember having a slight issue seating it last year. I'll check it out when it's here.

Thanks

 
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04-27-16, 07:00 PM   #19  
Last weekend I took the carb off and put it back together and made sure there are no leaks. Just isn't right still. Same thing. I'll spend more time on it tomorrow and hope I can figure it out. Maybe the intake tube is cracked. Maybe the carb is bad. Maybe the fuel filter is bad. Maybe the spark plug is bad. Maybe I should post the darn engine info. Hopefully I will get around to posting it tomorrow.

 
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04-27-16, 08:45 PM   #20  
Yeah, there are too many possibilities depending on what you have. Everything else is guessing.


I just saw the added side discussion about the high octane gas. If you are running a high compression engine, you won't have heat problems using it because the higher octane slows the burn rate to help with preignition. If you use it in a lawnmower engine, it can still be burning after the exhaust valve opens and you lose power because of a slower burn and you get heat buildup because of a longer burn. Exhaust temps can get high. In a water cooled engine, this isn't usually a problem because the cooling system is capable of handling the added heat. A air cooled engine that already runs 150 degrees hotter than a water cooled engine may not be able to get rid of that much more heat on a hot day working hard. The higher the compression, the faster the burn happens so this isn't an issue in the higher compression engines.


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04-27-16, 10:14 PM   #21  
Yes the engine model would be a great help. Even though some are seasoned mechanics on here, you might have a spring broken or a linkage not hooked up right ~!~!

If we would have the mower in front of us we could probably fix it in a hour or so ~!~!

A picture would help too !!!

 
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07-12-17, 09:44 AM   #22  
I ended up taking the mower to a shop. He was useless and didn't fix it. Does the same thing. Sorry about forgetting about y'all. I'm at it again trying to fix it.

Mower info

Yard Machine by MTD
7 speed shift on the go 42" cut
17.5 hp Briggs and Stratton engine

Numbers from white sticker barcode
31C7070603E1
051018ZA70630

It still has no power to cut much grass.

I noticed it was not charging. It was charging 32v ac so I put a diode on it and now it's charging well.

When I engage the deck on high it has power. When I engage the deck sitting still on low the blades dont turn. It just sits there and gets quieter and lowers RPM. I'm thinking now that a few years ago I replaced the belt with one. I don't remember where I got the belt but I doubt it was a MTD belt. And I remember when putting the belt on it it was super tight on the tensioner. Like crazy tight. Much tighter than it should be. Could the belt be my issue!!!??? Could the belt be lugging down the engine and making it not have power?

Thanks so much for the help so far

 
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07-12-17, 10:23 AM   #23  
the belt that i replaced was the drive belt.

 
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07-12-17, 10:30 AM   #24  
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07-12-17, 11:12 AM   #25  
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Above is a picture of the belt that i had replaced...im not sure which belt this is. I think its called the drive belt...but maybe its called something else...its under the seat connects to the axle.

and above is a picture of the model number...finally found it.

 
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07-12-17, 12:13 PM   #26  
If it's engaging on high but not low, it sounds more like a belt problem. Make sure the pivot point for the blade brakes and the tensioner pulley arm are all freely moving and lubricated well, rust in those areas is usually the cause of your problem.


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03-25-18, 06:35 PM   #27  
Well guys...itís finally fixed!!! I started tinkering with it again this year once it warmed up. Went ahead and bought another new carb and it did the same thing. Then I took the deck off and checked the bearings and all were good. But like the below post the engage mechanism wasnít right. It would engage but the pulley brake on one side would keep braking. Turns out at some time I hooked a spring in the wrong spot. Also the engage mechanism was tight. I fixed that, put it back together and cranked it up. Engages the blades and it didnít stumble!!!! Engaged like new!!! Went and mowed my front yard with it and our other house yard down the street! Worked perfectly!!!

Key switch is bad so I have to pull the spark plug wire off to turn it off but it RUNS!!! New key switch will be in Friday

After all the money I spent and years of time...it was the darn deck set up badly.

I learned a lot though.

Persistence paid off!!!!!!!!!

Thanks everyone for the help over the years

 
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