Briggs Model 91202?? Mine Bike Exhaust

#1
Hey everyone!!
So building a mini bike with my son and this place has been the upmost of helpful with information during this build. Have run into another question.

does anyone have experience with this motor? I have one and it didn’t come with any exhaust on it. Does anyone have any information on a head pipe for this bike or an exhaust? I got a flathead pipe but it’s too big.
Any help is greatly appreciated! Thanks
 

Harquebus

Well-Known Member
#2
I think that's a 3.5 horsepower.

If the exhaust port is threaded, you know what to do... 1/2" threaded iron pipe and a Briggs #89966 hot dog muffler.
EDIT--otherwise, something custom made welded together...
 
#3
I think that's a 3.5 horsepower.

If the exhaust port is threaded, you know what to do... 1/2" threaded iron pipe and a Briggs #89966 hot dog muffler.
EDIT--otherwise, something custom made welded together...
it is threaded. But has flange holes as well. A piece of 1/2” threaded iron will thread into there??
 

Harquebus

Well-Known Member
#4
it is threaded. But has flange holes as well. A piece of 1/2” threaded iron will thread into there??
I think that's the size. Plus the aforementioned "hot dog" muffler will screw right in.
I'm not really sure now. All I have is a few 3 HP Briggsies, not a 3.5. :D

For a flange exhaust pipe, you could find a large steel washer that has a 1/2" hole in it but also covers the flange holes. Drill the holes for the flange screws, then weld a pipe onto the washer of the desired length.

I like that hot dog muffler though. Flows well enough but tones down that braaap braaap of the raw exhaust.
 
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#6
So I got the hot dog muffler on there and all is well with that. However, upon firing it, it'll just stumble for a couple of second and the goes wide open. The throttle is of no effect. Just runs wide open until I pull the plug wire. I've strongly considered ditching the onboard gas tank and carb combo, but can't seem to find an intake for it. Any idea why it would just run wide open? or have an idea where I can find the right intake to put an aftermarket carb on it?
 

Harquebus

Well-Known Member
#7
Yay! Great success.

Any idea why it would just run wide open? or have an idea where I can find the right intake to put an aftermarket carb on it?
Uh, could it be that the idle screw is turned in all the way {thereby keeping the throttle butterfly open}?
Do you have a throttle return spring hooked up or is the throttle just swinging in the breeze like a barn door?

Make sure everything is operating the way it's supposed to.

As for a custom intake, it can be made the same way I instructed you on making a custom exhaust header. Tubing, sheet metal for flanges, a drill and a decent welder. There might be forum members who can fab one up for you.
 
#8
Yay! Great success.



Uh, could it be that the idle screw is turned in all the way {thereby keeping the throttle butterfly open}?
Do you have a throttle return spring hooked up or is the throttle just swinging in the breeze like a barn door?

Make sure everything is operating the way it's supposed to.

As for a custom intake, it can be made the same way I instructed you on making a custom exhaust header. Tubing, sheet metal for flanges, a drill and a decent welder. There might be forum members who can fab one up for you.

This is great information! Everything appears to be operating as its supposed to. I'll check the idle screw and make sure its not closed all the way.

As for someone that can fab up the intake, I'd be open to someone being able to fab one. Or I may try to tackle it myself. I'm determined to get this Briggs running right lol, I dont' want to take the easy way and just go get a predator. This is more of a challenge and more entertaining.
 

Doc1976

Active Member
#9
So I got the hot dog muffler on there and all is well with that. However, upon firing it, it'll just stumble for a couple of second and the goes wide open. The throttle is of no effect. Just runs wide open until I pull the plug wire. I've strongly considered ditching the onboard gas tank and carb combo, but can't seem to find an intake for it. Any idea why it would just run wide open? or have an idea where I can find the right intake to put an aftermarket carb on it?
How bout a couple pics? It's most likely going to be a carb linkage or governor issue.
 

Harquebus

Well-Known Member
#10
Pics are good like Doc1976 said. A lot can be determined from a few simple photos.

You can also create a longer pipe with the muffler on the end by using 1/2" NPT pipe from the hardware store and a coupler on the end to accept the muffler. Or use a 90 or 45 degree elbow coupler and just the muffler.

..I'm determined to get this Briggs running right lol, I dont' want to take the easy way and just go get a predator. This is more of a challenge and more entertaining.
You got that right. These old flathead engines just go with the minibikes better than anything newer. And for the most part, most of the engines you'll find on C-List or Failbook Marketplace just need a tuneup and minor attention to get running again if they're DOA. Totally rewarding for a side project.
 
#11
Not a bad idea, may check that option out once I get it to run decent-ish.
Here are a couple pictures I just took. Nothing has been removed or changed, this is how I got it. Let me know if you see anything missing that would cause my issues, or others for that matter. Thanks!!
 

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Harquebus

Well-Known Member
#12
It appears the idle screw is turned way out {fast}. It's the screw mounted on the horizontal-rotating throttle plate. Turn counter-clockwise on that screw to lower the idle speed.

Since your engine is setup with the tank mounted controls {choke and throttle} the waters are muddied a bit. How is your throttle cable hooked up?
 
#13
It appears the idle screw is turned way out {fast}. It's the screw mounted on the horizontal-rotating throttle plate. Turn counter-clockwise on that screw to lower the idle speed.

Since your engine is setup with the tank mounted controls {choke and throttle} the waters are muddied a bit. How is your throttle cable hooked up?
As of right now the throttle cable isn’t hooked up correctly, I’ve just been trying to use the mounted throttle arm on the tank to run it, or let it run wide open rather. Lol it seems I’m missing a spring on the back side of that rotator plate for the butterfly. But from what I can tell that spring would hold the throttle open.

if I get time today I’ll turn the idle screw in a couple of turns and see if it has any effect.
 

Harquebus

Well-Known Member
#14
As of right now the throttle cable isn’t hooked up correctly, I’ve just been trying to use the mounted throttle arm on the tank to run it, or let it run wide open rather. Lol it seems I’m missing a spring on the back side of that rotator plate for the butterfly. But from what I can tell that spring would hold the throttle open.

if I get time today I’ll turn the idle screw in a couple of turns and see if it has any effect.

Well, it's all useless if you don't have a throttle return spring to close the throttle. It will and should run wide open without one, because the vacuum running through the carburetor throat will swing the throttle plate open.

I'm a bit mystified myself on how to run a throttle cable on these engines and still retain the governor function. It's easier {perhaps} on the larger 4 and 5 HP engines which have an internal gear driven governor but yours has an "air vane" governor that acts upon on the air flow generated by the flywheel/blower.

If you want to eliminate the governor {or bypass it}, I can advise a throttle solution.
 
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