OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines B&S 3.5 quantum govenor speed question

I have a Quantum motor here that revs too high. (well it does compared to a 3.5 classic) I was wondering where i should start looking.

What model?? It is not the early one with hte fuel tap issues. It has no tap, oil filler is opposite the fuel filler in the cover and the pull start is on top of the crank.


I can take a picture if need be.


Thanks, Bob.

I think Joe Carroll is the Quantum guy, I've never seen one. Was that the first model Chinese Briggs? The maximum engine speed is set by the governor. It was supposed to be 3,500 rpm until about 1980, and 3,000 after that, on other Briggs vertical crankshaft mower engines. As far as I know the Quantum used a centrifugal governor inside the crankcase like a Honda or old Tecumseh, instead of an air vane like other small Briggs engines. However the spring will still be external. As a starting point, inspect the spring (it will be close to the throttle butterfly) and see whether it has stretched (it should be close-coiled: if it isn't, its stretched and ruined) and the plate the spring mounts on. Those plates were a bit flimsy even when the engines were made in the US out of steel, and seem to have become worse when they began being made in China out of cheese. People sometimes crash the mower into a tree or bush and bend the spring anchor plate, thus reducing the spring tension and raising the governed speed.

The picture sounds like a good idea. The area around the governor spring and throttle butterfly, plus the spring's fixed anchor point, is what we need.

Thanks Grumpy.
Heres the pictures.

The motor.

Linkages

From the front.


My best guess would be that the Red screw is mixture.

The yellow one at the top is a Torx head. Is this where you set the tension for the govenor spring??
The blue one undereath i had a fiddle with and it does not much if you wind it out and makes it rev harder if you screw it in.

Cheers, Bob.

I hope we'll hear from Joe, who has experience with these. If you took the pictures with the speed control set to more than minimum, the blue screw seems to be the idle speed. It acts as a stop for the throttle butterfly when you set the speed to minimum. Idle speed for B&S engines is usually 1,750 rpm. The yellow screw seems to be limiting the movement of the speed control, which means it sets the maximum governed speed - probably 3,000 rpm. In theory punters should not mess with it, in case they decide to increase it, and the aluminium connecting rod lets go. You are probably right about the red one being the mixture screw - I'd need to see it from right angles to the current picture to take a guess on that one.

I bought it from a guy that was fixing mowers, i cleaned him out on Victa catchers and give him a lobster for this and a power torque.

Good running motor he said. Yeah it runs great, too great.

I would have wound the yellow screw out but it looks to be nearly all the way out. Screwing it in looks to me to be untensioning the spring and im thinking thats the wrong way. Im not sure how it works and thats the best i could come up with from the brief look i had at it before.


if iget a chance i will have a peek at the earlier Quantum i took off this mower and see what i can make out of its linkages. Its got the top cover off so i can see the vane.


Any thoughts or advice would be greatly appreciated.


Bob.

The governor spring is the one above the yellow screw. Just move the speed control from slow to fast and you'll see the moving link that holds the end of the governor spring, move toward the screw. At max speed, the link will hit the screw. It looks to me as if were you to screw it inward, you'll make the link hit the screw sooner, with the governor spring stretched less. That will reduce the maximum engine speed. I think some loon has backed off that screw. Just reset it to 3,000 rpm and all is well except that the loon has probably stretched your connecting rod all to hell, and it will let go sooner or later. B&S connecting rods seem to usually break because someone let the engine run out of oil, but an aluminium connecting rod that has run for a long time at excessive speed has had its life shortened.

Mixture screw, wound it right in and back out one and a half turns.


The yellow screw i wound right out and then set the idle with it running by ear.


The blue screw for the govenor i have screwed right in and i still think it might be a touch high.


Im near enough to happy with it.


Cheers, Bob.

I still think the blue screw should be idle speed and the yellow screw (with the oddball screw-slot) should be maximum governed speed. The idle speed screw is just about always the one that comes into direct contact with the throttle butterfly lever. The max governed speed screw (if there is one - traditional Briggs engines don't have one) is the one that comes into contact with a lever that moves directly with the speed control. So, I think in your last post you got the colours backwards. Apart from that, what you have done sounds right. I'd measure the speeds with a stroboscope, and if necessary bend the speed control lever or fit a longer yellow screw to get the right maximum governed speed, but if it's not far off, it doesn't matter much. You can probably sweeten up the running a bit with a finer adjustment of the mixture screw. Just set the engine at max governed speed, and screw the mixture screw clockwise until the engine gets just slightly rough, then anticlockwise until it gets slightly rough, then put it halfway between those two settings. On a Briggs, the setting should then be stable for years.

Yeah thanks grumpy.

Your right, i had the screw colours mixed up.

I will tackle the mixture today.
I just took a punt, most bikes are about that and so was this one, i only touched it to see if it was something silly like just off seated or wound right out.

On to the next one. It might be more up your alley, will start a new post.

Cheers, Bob.

Thanks for clearing that up Bob, it is helpful to others who may refer to this thread in the future - it is a potentially useful topic. Incidentally, B&S recommend starting at 1.5 turns when tuning most of their carburetors.

I will close this thread.