Why has my lawn tractor got such low power?

[ejenner]

A couple of years ago I got a Cub Cadet EFI V-Twin 679cc lawn tractor. I can use it for pulling a trailer around the garden for collecting logs as well as mowing the lawn so it does a couple of jobs.

In the USA where Cub Cadet also sell lawn tractors they have machines with a similar size engine that make something like 13hp. Which is good for a lawn tractor.

Mine isn't the same as the US model, it's actually made in Germany by MTD and just pretends to be a Cub Cadet. MTD make mowers for a number of brands sold here.

Question though is the engine can bog down and cut out if you try to shove it into some long grass and I just don't think it's really making the power that a v-twin with over 600cc should. I think of a 600cc motorcycle being capable of 150mph and then look at my lawnmower and I don't get how the two engines could be so different.

The lever for the throttle only moves about 1/4 of the way along and the linkage it connects to for the throttle body only moves a tiny bit before it hits a cast restriction which is there to block movement of the lever.

So has my tractor being neutered by the EU because it's the sort of thing they like to do? It has stickers on it saying it complies with EU regulations.

Can I just grind the notch off the throttle body so the lever moves properly? Anybody else had this conundrum?

[Davey]

Can you see how much the throttle butterfly is actually moving?

If your only getting quarter throttle then surely removing the stop would be a great start.

[ejenner]

The lever input is proportional to the throttle movement. i.e. it's not geared up or anything. I can't see the throttle plate... maybe I could have a look by taking off an intake pipe or something.

I just wonder whether or not they've tinkered with the fuel delivery. I suppose I could check the condition of the spark plugs. At the moment they're black and dusky as if it is running rich so maybe I could remove the throttle notch and check them after running it a bit faster.

There's no instrumentation on a thing like this so what would I do to make sure I'm not going to break it? (apart from leaving it alone!)

[Davey]

I imagine its well within safe limits from the factory. but depends on the particulars.

Is it carb or injection?

[ejenner]

It's electronic fuel injection.

[bmw2101]

are teh blades sharp? instead of cutting the grass it maybe wedging it between the blades and that would require a lot more power.

[dodgerover]

Can you see if the throttle butterfly is opening and closing fully?

[ejenner]

Feb 11, 2023 10:47:21 GMT bmw2101 said:

are teh blades sharp? instead of cutting the grass it maybe wedging it between the blades and that would require a lot more power.

The blades are about standard sharpness for what you'd see on any sort of normal lawnmower. I've replaced them once but they're cutting well at the moment so won't replace them this year.

[ejenner]

Feb 11, 2023 11:03:45 GMT dodgerover said:

Can you see if the throttle butterfly is opening and closing fully?

All you can see from the outside is the throttle cam with a rod connected to it. When you move the throttle lever it moves the cam about 1/8 of a turn. So I would guess it isn't opening it fully and that's how they've restricted it.

[dodgerover]

Can you see anything if you take the air filter off?

[ant260z]

Feb 10, 2023 10:51:02 GMT ejenner said:

There's no instrumentation on a thing like this so what would I do to make sure I'm not going to break it? (apart from leaving it alone!)

You could try one of these so you have an idea of the RPM at least  Digital Tachometer Hour Meter Timer

[Rich]

The main differences between this and motorcycle engines are that a lawn tractor engine is designed to be cheap and operate for all of its life in a very narrow, comparatively low RPM window. Push them outside of this and often enough you’ll find they will suffer with violent unexpected self-disassembly syndrome. They aren’t engineered the same as a motor vehicle engine and in most cases won’t run like one.

[ejenner]

Feb 11, 2023 14:24:43 GMT Rich said:

The main differences between this and motorcycle engines are that a lawn tractor engine is designed to be cheap and operate for all of its life in a very narrow, comparatively low RPM window. Push them outside of this and often enough you’ll find they will suffer with violent unexpected self-disassembly syndrome. They aren’t engineered the same as a motor vehicle engine and in most cases won’t run like one.

It probably would be unrealistic to assume it would perform as well as the engine from a sports bike. They do insane RPM on those things. I just get the feeling it has more in it than nanny state says I can have.

[LowStandards]

I expect it's restricted for rpm, it will make it's power low down anyways


Give it a service?


Make sure you're in the appropriate gear?


I've used an 8hp tractor to pull a disabled van about the yard, they have a hell of a lot of ooooomph

[ejenner]

I did a service on it on the weekend, changed oil and plugs.

I took a closer look at the throttle body and it seems large enough and the plate does open a full 90-degrees. If you look at the flat of the spindle that's going through the TB you can see it gets fully open.

[ejenner]

Feb 14, 2023 10:58:54 GMT LowStandards said:

I expect it's restricted for rpm, it will make it's power low down anyways


Give it a service?


Make sure you're in the appropriate gear?


I've used an 8hp tractor to pull a disabled van about the yard, they have a hell of a lot of ooooomph

Doesn't have gears, it's an 'automatic'.

I've not really got to the point of wishing to test how strong it might be as I don't want to break it. But I did use it for dragging a tree out of the garden last year and it had no problem with that.

[Frankenhealey]

Feb 11, 2023 14:24:43 GMT Rich said:

The main differences between this and motorcycle engines are that a lawn tractor engine is designed to be cheap and operate for all of its life in a very narrow, comparatively low RPM window. Push them outside of this and often enough you’ll find they will suffer with violent unexpected self-disassembly syndrome. They aren’t engineered the same as a motor vehicle engine and in most cases won’t run like one.

As rich says designed (or used to be) as a long life industrial engine. However long grass will bog anything down, it slows the blades down so they don't cut efficiently, makes the belt slip, that makes them less efficient again and then eventually the belt wears/burns out. Long grass should be cut in narrower strips than the deck width and on a high cut first with a finishing cut afterwards. Wet grass just complicates the process and if the deck is not discharging the grass then you've got even more problems.

My equipe will bog down on long grass with overoptimistic cutting heights

[dodgerover]

It's also probably governed to quite a low maximum rpm, you can normally tweak things to get a few more revs which can often make the difference between cutting well and bogging down. If you didn't buy it new it is possible somebody has messed with it previously and shut it down.

If you open the throttle hard whilst under load will it try to rev but then shut down to a lower rpm?

[nalesutol]

Lawn mowers are fitted with a governor that limits the RPM to around 3600rpm. This is because these engines aren't designed or built to rev much higher, as opposed to similar sized engines in bikes.

If you do a search on YouTube for adjusting lawn mower governors to get maximum rpm you will find loads. Even if you can't find one for your exact engine they are all fairly similar as to how they work.

How do I know? I have one fitted to my gokart and have been trying to find ways to speed it up. You can remove the governor but you risk the engine exploding :-)

Sharp blades make a hell of a difference, but long, wet grass will stop most hand propelled mowers and even ride-ons can struggle.

[ejenner]

Thanks for that. I didn't know about the general theory of governors being fitted to these kinds of engines. It would explain why the throttle looks large enough and the plate opening all the way but the output still seeming a little on the low side. Will be good to look at the YouTube vids on governors.

vis-a-vis can I hear it being governed when revving under load... I think potentially yes... but as we've established nearly all of them have governors then I guess that's the next thing to look at.

With it being EFI it is potentially controlled by an ECU and if that's the case then there's a possibility it's governed by software and that would be likely impossible to workaround without hacking the ECU or changing it entirely.