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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 26278
Experience:  have worked on and around most engine models for 35 yrs./Polaris ATV expert
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I have a 17.5 Briggs and Stratton riding lawn mower that will

Customer Question

I have a 17.5 Briggs and Stratton riding lawn mower that will not idle even with the butterfly closed. It runs about haft throttle and higher with no problems. When I engage the blade the motor slows a bit but giving it a little throttle, which opens the butterfly and I can cut the grass, ride it around with no problems. rebuirl card, new float valve and seat, spark plug gapped correctly, new o rings on maniflod and standard gasket where applies. Checked coil gap, cleaned and blew out carb, new fuel lines, filter. I inherited this machine and it has had a few different self proclaimed mechanics work on it so I have been looking at everything. Even cleaned out the crankcase breather. Starts easy, no smoke.
Submitted: 3 years ago.
Category: Small Engine
Expert:  Richard replied 3 years ago.
Hi there – my name is Richard and I’m happy to help with your question today.

Let me calrify please.

Are you saying the engine will not idle ? Are you saying the engine will not idle while cutting the grass at slow speeds and not giving it full throttle ?

I also need the model and serail number of the mower. Usually located under the seat, also the model number is on the same tag as the serial number !

Thanks Richard !
Customer: replied 3 years ago.

model: 31707, type: 0230E1 ,code: 0211082

The throttle butterfly is closed there is a gap between the adjustment screw and the lever. Starts easy, runs about half throttle when the butterfly is closed. can be Accelerated to full throttle without hesitation. I am used to working on engines but this has me stumped. Rebuilt carb....If I force the butterfly closed the engine will slow down a bit but opening up a little and the engine speed increases. What I was saying about cutting grass is that when I put a load on the engine such as when I engaged the blades the motor slowed a little bit so I open the throttle a little bit..increasing rpms and I was able to cut grass with the machine. I was thinking in respect to an air leak and a lean mixture....

Expert:  Richard replied 3 years ago.
Hey Bruce, Thanks for getting back with me !

I think is what your saying is that you rebuilt the carburetor, then now your having the following touble.
The engine idles high and you can close the butter fly and the rpms decrease, "only" if you close it manually. So sounds like you have played around with the idle screw and now have backed the idle screw out to where it dont even touch the throttle lever to open the throttle. This could be one of sevral issues. Throttle linkage not installed correctly or misadjusted or the governor isnt adjusted properly. There could also be a worn spring. I would look at the service manual and make sure you have the linkage on correctly and the governor is adjusted correctly. The throttle cable should be adjusted correctly as well. If you have done all these things, just let me know. A lean condition would be indicated when you pulled the sparkplug and it would be white on the sparkplug tip. Let me know if this isnt the case !

If your satisfied with my answer, please leave a high rating. If your not satisfied with my answer, please let me know. If your not satisfied I can opt out and at that point another expert will answer your question if their answer is different from mine !

Thanks Richard !
Customer: replied 3 years ago.


no the condition existed before. The linkage is correct and set according to the service manual for the engine. Including the governor setting. Everything has been done since. Check the float for seepage inside since it was plastic and clear it was easy.Seat is set to the same depth. There's no setting.

Expert:  Richard replied 3 years ago.
So to clarify, the engine is idling high. Correct ?

Is what you have is a high idle ? Correct ?

Thanks Richard !
Customer: replied 3 years ago.

yes I guess you could it that...about half throttle.



Expert:  Richard replied 3 years ago.
Thanks Bruce ! Let me ask you this then.

You said you can move the throttle by hand, right ?

Moving the throttle by hand, how far can you move it to where it will not move anymore. By this I mean, " moving it towards idle " ?

Thanks Richard !
Customer: replied 3 years ago.

the butterfly is stopped by the sides of the venturi. I checked to see if it was centered correctly and it is. What I can do is jam it by twisting the butterfly shaft and that action will reduce the engines rpms a little but not to idle. Letting it go so it returns tits normal resting spot against the sides of the the motor dramatically increases rpms. But the butterfly is closed. There is no opening visible. I understand gas fuel mixtures.I just cannot find where the intake could create such a high vacuum situation to pull the fuel out with the butterfly closed. Looking for vacuum leaks I sprayed some carb cleaner around the carb and intake but it didnt reduce or increase rpms. I disconnected the breather tube from the carb and that didnt change anything. The only things I haven't done is checked the the key on the magneto and the valve clearance settings. I was thinking about replacing the float anyway thinking it may getting a little gas inside and running a little richer mixture than it should be.

Expert:  Richard replied 3 years ago.
Thanks for getting back with me !

I am all out of ideas at this point ! So instead of rating my answer, I will opt out, another expert will respond if there answer is different than mine. If I do think of anything else I will let you know !

Thanks Richard !
Expert:  Curtis B. replied 3 years ago.
Welcome to JUST ANSWER small engines, let me try to help you !! I think you are all over the problem. The butterfly is not fitting the housing when it is closed. Leakage around the butterfly and you get idle that is too high. Have you loosened the butterfly on the throttle shaft, worked the butterfly to seat good against the housing and then tighten the screws. I have seen this before and if the butterfly does not close completely, idle can not be adjusted. If the butterfly can not be adjusted, you will need a new carburetor.
Customer: replied 3 years ago.

yes, I already tried that. I checked with the service manual to see if it was installed correctly and it is. The problem defys logic. I checked the mag and set the gap using brass feeler gauges according to the service manual.It is as if the engine was built to run at half throttle as the norm.

Expert:  Curtis B. replied 3 years ago.
Problem is in the carburetor . It has nothing to do with the magneto . Butterfly can't be closing.
Customer: replied 3 years ago.

I will have see if I can measure the rpms. Meanwhile I will remove the intake ,cjeck yhe carb with a magnifying glass and look for cracks,warpage ect.




Expert:  Curtis B. replied 3 years ago.
For engine to idle too fast, gas has to be drawn into the cylinder, and the only way it gets there is through the carburetor. Let me know what you find. Thanks for the reply.