Push mower RPMs too high with new blade installed

[runflat]

I have an older Murray 22” push mower model 96112005900 with a Briggs 5 HP. The mower is still in very good shape—regular oil changes etc., and over the years I’ve resharpened the blade several times so now it’s time for a new one. I took the blade to Home Depot and was shown a universal blade that looked almost like an exact match except of course both cutting edges had a lot more metal than the old worn-out blade. I installed the new blade and started the engine but when the engine started it ran up to what I would describe as really-high RPM speed and the carb speed/lever control has no effect. I do not have a tach but I thought there’s no way this is right and it must be something with the throttle springs or linkage or gov. I usually blow off and WD40 the linkage once or twice a year so nothing was gummed-up and the springs and everything seems fine. The odd part is when I put the old blade back on the engine RPM stabilizes at its normal level and the lever/speed control works normally. I can’t run the mower with the new blade as the RPMs are so high I’m afraid it’s literally unsafe even though side-by-side both blades look virtually identical.
Any Ideas why I’m getting this RPM difference with a new blade installed?

 

[Bertrrr]

Has to be a different weight in the blades

 

[7394]

I agree.

 

[runflat]

I will weigh them the first chance I get. I'm not 100% sure how the gov works differently when different weight blades are installed, but are you saying that assuming there no other issues like weak springs, etc., you could expect a mower to run at a higher RPM if slightly heavier blade was installed?

 

[Bertrrr]

No I'd imagine the lighter weight blades would not put enough strain on the machine and possibly let it run away,
the governor should react and limit your rpms either way in my thinking. Just check the weight and go forward from there, make sure all your linkage is correct and free. I've never seen one run away like you're describing unless the governor was at fault.
Go back the figure what else is different besides the blade.
There is a device to check rpms on a small gas engine but I've never used or had a need for it, you can usually tell if it's too high. - It will throw a rod or drop a valve if it's too high an rpm so beware.

 

[MParr]

Sounds like the weight of the new blade has the governor kicking in.
Order or borrow a cheap tachometer. The better ones have an Alligator clip for attaching to the plug wire. Adjust your low idle speed, wide open throttle speed, and Governor as detailed in the engine service manual.
You should probably stay with the OEM blade or aftermarket direct replacement.