OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Can I run briggs & Stratton motor without blade?

Hi,

I have a mower here with a briggs & stratton Quantum 50 motor, it has a vibration that I am not completly happy with, I was thinking maybe the blade has been bent? It's just a 1 piece bar and the blades are not removable.

So what I was thinking is removing the blade and starting the motor up and see if the vibration goes away? Will it cause any damage to the motor without the blade present?

I read something that it could damage it but I have a feeling it is to do with the 2 stroke motors.

Thanks

Hi victa,i think you could take the blade(bar) off and run the engine.the fly wheel for the magneto etc takes all the enertia energy for the engine to keep ticking over for your test,once you have sorted your vibration problem put your blade bar back on and tension up the nut/bolt to correct tourque.some one else might have other ideas/info...ken

With bar-type blades, I generally balance them by hanging them on a nail driven into a tree beside the shed.
After sharpening (particularly if I've had to grind a bit off the blade as a result of someone using it for a laser-leveller) I hang the blade on the nail to see if one side is consistently heavier than the other. Judicious use of the grinder on the offending side until the blade balances is usually enough to prevent any unwanted vibration.
I have never had any problem running a B&S without the blades (just watch the revs - they can climb really fast without the inertia of the blade to ease them up a bit).

Briggs and Stratton command that their aluminium-flywheel mower engines are never run without the blade plate fitted. The main issue is a safety problem, which I'll explain in tedious detail in the hope that someone will find the information useful. Small B&S engines traditionally have fixed ignition timing. They would give maximum power with a timing of around 30 degrees before TDC, but the most advance they can actually use, is determined by when it begins to kick back when starting. What they have done, is set a timing (about 6 degrees before TDC, from memory) which is at the limit: the most they could use without risk of kick-back, provided the engine either has the cast iron flywheel, or has the blade plate attached. Trust me, you will not enjoy a decent kick-back: if it doesn't break or dislocate your fingers on the pull cord, it at least feels as if it has.

So grumpy you are pretty much saying it is a safety issue for the person and it shouldn't cause any issues to the engine itself running without the blade plate?

If you can get it to run, it will act like a hairy goat without the blade plate and jump and vibrate worse.

As Joe has said (from experience) the engine will run badly without sufficient crankshaft inertia. If I recall correctly, as well as the safety issue B&S is also concerned that after a short time it will shear the aluminium flywheel-to-crankshaft key, due to the strong torsional oscillations of the crankshaft-flywheel system.

Thanks for clearing that up, definatly won't be doing that then.

Can a better type of blade plate be fitted, i've never really liked the single piece design.

Something like this is what i'd like, but not sure if it would fit? http://cgi.ebay.com.au/Disk-4-BLADE...pt=AU_Lawnmowers&hash=item4aa884686e

That blade plate will only fit a blade-plate boss that has three attaching screws, with whatever size spigot and whatever pitch circle diameter for the screws, Honda uses on those models. You haven't told us what make and model of mower you have, so we just can't tell. It would be best if you tell us that and attach a picture of your blade-plate boss plus telling us the crankshaft extension (i.e. blade plate spigot) diameter and the pitch circle diameter of the attaching bolt holes.

If it is a commonplace make and model, most likely someone here will be able to tell just from a picture, which blade plate will fit. Usually the lawnmower manufacturer specifies (and maybe installs) the required blade plate boss on the crankshaft extension. Briggs and Stratton will do what the mower manufacturer requires. Victas tend to have a tapped hole in the end of the crankshaft, with no other attaching screws (in my opinion a dismally poor, even stupid, piece of engineering) while Rover and some other manufacturers tended to use three or four screws tapped into the blade plate boss.

It is a Masport Combo Mulch & Catch mower if that helps. I will try and get a photo of the mower blade setup tomorrow sometime.

If need be I will purchase a standard replacement blade, but if possible I think i'd rather have a round plate with replaceable blades on there.

Not only are the blades replaceable if you have a blade-plate design; they also swing back when they hit something, which makes engine damage (such as flywheel key-shearing) less likely, and may reduce the severity of some types of injury to people, though the injuries can still be very nasty.

Generally, B&S engines have just a very few standard crankshaft-extension sizes. The main variable is the number, size, and pitch circle diameter of the blade plate attaching screw holes in the attaching boss, which is pressed onto the crankshaft extension. This means the boss can be removed, and perhaps replaced by one that suits a particular blade plate. Anyway, the first step is to report the standard dimensions (crank diameter, number of screw holes, size of screws, and pitch circle diameter for the screw holes).

Ok, I just took a photo of the blade setup.

The crank diameter is 25mm and there is a steel boss with keyway that sits on the shaft. The boss has two locating lugs to position the blade. A single bolt takes care of securing the blade & boss to the the crank.

Well, it seems you need a blade plate that takes a one inch crankshaft extension, and has either two or four attaching bolts. The most important dimension is the exact distance between the two attaching bolts (that is, the pitch circle diameter of the attaching bolts). However you also want a blade plate that has the correct size holes for the attaching bolts, if you don't want to have to drill them out.

Fitting a blade disc is feasible with a new disc and blades from a mower with the same cutting width and the blade boss.

The problem I see with doing this is the mulching function will probably no longer work well at all. While they are not the best the blade bars are not that bad, they are thick enough you can sharpen them many times and long lasting.

As to your vibration I would be checking to see if maybe the crankshaft is slightly bent, to cause noticeable vibrations you should be able to visibly see it by comparing the height of the blade as compared the the mower deck on one side, turn the engine over 180 degrees (SPARKPLUG LEAD OFF) and comparing the other side of the blade to the same spot on the mower deck.

Thanks for the advice, as I mainly use this mower with the mulch plug fitted, I guess I will have to stick with the blade bar.

I will check the crankshaft tomorrow, hopefully it's not bent.