OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Sprint 375 Revs Highly

Hi all, got an issue with a relatively late model sprint 375 B&S motor. It starts great but the throttle seems to be at max rpm at all times and moving the throttle cable does not seem to do anything until I pull it all the way back to turn motor off..

what could be the problem here? ant help would be great!

If the speed does not vary when you move the speed control, the governor is not working properly. Please post a picture of the governor plate, where the speed control cable connects to the speed control lever at the engine end. Most likely something has become bent or dislodged near there. The speed control lever stretches the governor spring, and the air vane has to overcome that spring to limit the engine speed. If the vane is stuck or its link to the throttle butterfly is jammed, for example, the throttle may be open all the time. Do not run the engine until this is sorted out, or you may find part of the connecting rod comes out to join the larger world outside the crankcase.

thanks Grumpy. Will take photo tonight and post it.

The newer mowers do not have a great variation in RPM between full noise and idle.

So that would lead me to ask, is it acutally revving too high?? More than 3000-3500rpm.


The mixture on those is set, so running it with the air cleaner off will mess with things.

it only runs at one very fast speed. rpm does not get adjusted via lever at all..

luvpsi,

Maybe you should fit the idle spring which is missing in your picture.

thanks bruce! do you have a picture of a motor with the idle spring fitted?

Lack of idle spring will not cause it to rev high, the main spring where it attaches to the throttle wheel is way out of shape, bend it back toward the mower some.

So whats the problem with????
Do you understand how the govenor works???
I assume you do not Because if you did you would be able to work out what the problem is caused by.

*flywheel spins making wind.

*Vane is connected to throttle linkage. (thats the black bit, the two tabs, one is flat out, the other is not idle, it is closed. As Bruce said, theres meant to be another srping, thats a govenring spring for idle speed but i dont see that stopping the motor from revving lower then full song.)

*Spring pulls vane toward wind opening throttle.

*Wind pushes on vane closing throttle.


So as you load the motor up, the wind blown onto the vane is reduced to less than the govenor spring tension so the spring wins and that opens throttle. Then as the RPM builds the wind is then blowing more pressure than the spring and that closes the throttle.

As above its the same for the spring for idle.

The shape of the 'cam' on those throttle linkages where the spring hooks on does not see it travel through much of an arc, and arc thats relative in distance to where the other end of the spring is attached on the link between vane and throttle shaft.

Here is what the two-spring system should look like - Briggs call it "governed idle" (it saved them the cost of having an idle speed screw and a tapped bell-crank on the throttle butterfly). Note that in your picture, the throttle butterfly is wide open. If you gently push the air vane lever (the white plastic bit the left end of the governor spring connects to) towards the left, the throttle should close. When the speed control is set to minimum, it should be just about closed with the engine stopped. If it isn't, find out why. Is the bottom end of the speed control Bowden cable properly adjusted and anchored?

I suggest you read right through this thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=33135&page=1

In that instance the problem was a bent link between the air vane and the throttle butterfly, but yours does not look bent. I doubt the missing idle spring is the problem, since Briggs built them for decades without the second spring, but by all means fit the spring since the Sprint engines did have them.

just an update on this.. i played with the governor spring a bit and eventualy the throttle started working (i was able to adjust the rpm)... however it was struggling to idle well due to the black plastic tirangle thing (dont know the name sorry :)) moving left to right.. I am assuming it was doing this due to the lack of idle (anti surge) spring?

It would be nice to know where the jamming of the linkage was happening, because it may come back since you probably haven't changed what was causing it.

To get a stable idle - mainly, to keep the engine from stopping when it is idling - you must have either a "normal" idle stop screw in the bell crank on the carburetor's butterly valve, or a secondary governor spring to give you a governed idle. Your carburetor does not have an idle stop screw, or anywhere to put one, so you must fit an idle spring as shown in the picture I posted earlier. Otherwise there is simply no mechanism there to keep the engine from stopping if you put the speed control on the handlebar in the idle position. The throttle butterfly will meander about at excessively low speed until you happen to get a flameout.

thanks Grumpy. I have ordered a new set of springs which I will fit soon and hopefully be able to solve my issue.

I will update back with my results.

Refitted new springs today and while fixing the idle issue, the engine was still revving high. I took the main cover off and found lots of grass and other rubbish behind the white plastic butterfly. once this was cleaned the mower started running beautifully

The white plastic butterfly is the governor vane. Air pressure from the cooling fan on the vane, limits the engine's speed: the vane pushes in the opposite direction from the way the governor spring pulls.

The obstructed vane explains why you suddenly had a temporary improvement when you fiddled with the linkage at the carburetor end: you must have reshuffled the stack of grass, but the fan would push it back into its jammed position quite soon afterward.

If you have the vane moving smoothly, and the right springs, the only other thing you should do is set the minimum and maximum speeds precisely by bending the brackets where the two springs terminate, but unless you have a tachometer this is impractical.

Thanks for coming back with the final outcome - I'll now close this thread. If anyone needs it re-opened, just send a PM to a moderator.