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Hi,

I recently picked up a Cobra (M51 SPB) mulching mower with the intention of tidying it up and replacing my current Mountfield SP454.

The Cobra has the Briggs and Stratton 650 exi easy-start engine (https://www.briggsandstratton.com/e...engines/lawn-mower-engines/650exi-series.html) and it's relatively new with the date stamp of 2016. It starts excellently...pulls over really nicely. However it doesn't rev that high....it feels like it's running at 70-80% of the normal speed when on full throttle. It's still able to mulch grass but it's clearly not running as it should.

To date I've done the following: I've cleaned down the deck with a damp towel, removed and cleaned the carb throughly, cleaned the spark plug (not too sooty/pitted), drained and replaced fuel with fresh fuel and still the same. I haven't re-examined the governer springs since cleaning the carb but I was thinking of looking at the springs next unless anyone else would recommend looking elsewhere for the issue.

Any help would be greatly appreciated. Thanks.

The governor spring adjustment could of been bumped causing the slower speed, or it could be that your new mower has a longer blade therefore the engines will turn at a slower speed to maintain what is said to be a safe blade tip speed. Without having a tachometer it would be difficult to make sure the speed is correct for your blade length.

ILENGINE said:

The governor spring adjustment could of been bumped causing the slower speed, or it could be that your new mower has a longer blade therefore the engines will turn at a slower speed to maintain what is said to be a safe blade tip speed. Without having a tachometer it would be difficult to make sure the speed is correct for your blade length.

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Thanks for the reply. A good point about blade length...I'd totally overlooked that. The Mountfield is 44cm or 18" and the Cobra is 50cm or 20"....would that account for a noticely slower sounding engine? or is this all in my head? :laughing:

lord lucan said:

Thanks for the reply. A good point about blade length...I'd totally overlooked that. The Mountfield is 44cm or 18" and the Cobra is 50cm or 20"....would that account for a noticely slower sounding engine? or is this all in my head? :laughing:

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Yes it could then there are other things like noise restrictions, a few hundred less rpm makes a massive difference to the amount of exhaust noise.
On top of that a better muffler that reduces the sound will make you think the mower is running slower.

I fit a few new mufflers on old 2 stroke Victas and the customers regularly tell me the mower is down on power & speed when I set them with a tacho so the speeds are identical.

bertsmobile1 said:

Yes it could then there are other things like noise restrictions, a few hundred less rpm makes a massive difference to the amount of exhaust noise.
On top of that a better muffler that reduces the sound will make you think the mower is running slower.

I fit a few new mufflers on old 2 stroke Victas and the customers regularly tell me the mower is down on power & speed when I set them with a tacho so the speeds are identical.

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Thanks very much for the reply. I see...another aspect I hadn't considered.

In terms of age and specification: I do believe the Cobra to be newer and of a higher specification engine. The link I posted above does mention it has a new generation muffler so perhaps it could be that. I don't have access to a tacho to check it but in the quick test I gave the mower it was able to cut the grass and drive the rear wheel without any noticeable difficulty. I'm starting to think my ears have fooled me on this....

lord lucan said:

Thanks very much for the reply. I see...another aspect I hadn't considered.

In terms of age and specification: I do believe the Cobra to be newer and of a higher specification engine. The link I posted above does mention it has a new generation muffler so perhaps it could be that. I don't have access to a tacho to check it but in the quick test I gave the mower it was able to cut the grass and drive the rear wheel without any noticeable difficulty. I'm starting to think my ears have fooled me on this....

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Hondas always sound like they are going slow as well but it is just a better muffler.