Engine rpms drop when releasing brake

[puttinalong]

Hello, it's been a while and I have tried to search the forum but I can't find anyone with this particular problem...I have a garden tractor GT6000 Sears 917.288613. When I start the mower from sitting and I have the brake pedal depressed it idles fine, but when I release the brake you can hear the rpms drop kind of like a slight "miss or sputter" but the engine comes back up to speed. That used to be all that it did but now when I try to engage the blades, the PTO switch will kill the engine until it has been running for a few minutes. Once I can finally get the mower going and cutting grass, I can cut the tractor off and depressing the brake pedal will still result in a very small "sputter" but nothing like when it is cold. As for the PTO switch, the blades come on like it should when the mower has run for a while and is warmed up.

*All electrical connections check tight...I took off the interlock switch and cleaned terminals and checked it and seems fine (after running the mower)...can the temperature affect the switches like this???? Thanks for any input.

[RoyM]

I would look for a fuel delivery problem. As the engine is placed under load the governor opens the throttle, it sounds like not enough fuel is being delivered to compensate.

[puttinalong]

I will check that today and report back.

[Luffydog]

Like Roy said sounds like it is running on one cyclinder at times then other times it gets enough fuel to run the blades. When u have it running pull one plug wire off and see if it dies.

[puttinalong]

Well this is what I have found out after a few frustrating hours...

1- Both cylinders are firing
2- Fuel is getting to carb and carb is clean
3- This is an electric clutch so when I have the belt off of the electric clutch and engage the PTO, there is no change in rpms
4- Despite #3, there is still a small "sputter" when disengaging the brake and releasing the interlock switch. If I gradually let out on the brake the engine sputters but comes back up to speed, but if I just abruptly release the pedal, the engine cuts out (this is during NO LOAD on the PTO).


The only thing I can think of is (and electrical components throw me for a loop), the interlock switch must have a ghost in it. You think just replace the switch and go from there???

[KE4AVB]

Okay if you're suspect the brake clutch switch disconnect it jumper only the two white wires together. This will bypass the switch so you can start it. It will not disable either the reverse or seat switches. If the seat switch connector is loose it can move and cause this problem as it has a built-in shorting strip to prevent operators form simply disconnecting it.

I suspect you have a twin barrel Nikki carburetor and they can wear a lot causing air leaks around the throttle and choke shafts. The idle jets can also get plugged with things like a grain of sand and cause problems. I had two here this year that had something like a grain of sand in the idle jet on one side. Can be very hard to get out to due to size of the jet.

[puttinalong]

KE4AVB....the tractor has 70 hours on it. I not thinking it is carb/fuel related as I took that apart today and it was clean. What has me really suspecting the interlock switch (but again it is showing that it has continuity) is releasing the brake pedal and disengaging that switch that is when I get feedback...it was doing that a little last year but not to this extreme and engaging the blades never killed the engine when it was cold like it does now...just don't understand why after it has run for ten minutes or longer, none of these symptoms are present...I will bypass the switch like you suggest tomorrow and see where I am then with it. Thank you for your time. I will report back after trying the bypass.

[puttinalong]

Okay if you're suspect the brake clutch switch disconnect it jumper only the two white wires together. This will bypass the switch so you can start it. It will not disable either the reverse or seat switches. If the seat switch connector is loose it can move and cause this problem as it has a built-in shorting strip to prevent operators form simply disconnecting it.

I suspect you have a twin barrel Nikki carburetor and they can wear a lot causing air leaks around the throttle and choke shafts. The idle jets can also get plugged with things like a grain of sand and cause problems. I had two here this year that had something like a grain of sand in the idle jet on one side. Can be very hard to get out to due to size of the jet.

Okay, I couldn't wait until tomorrow so I went out this evening and jumped the two white wires together and the mower started with no problem but it would start with no problem before , the problem came when releasing the brake pedal while it is still idling...the engine would slow and pick back up to speed. Because I bypassed the switch like this and had no problem with it idling, does that mean the interlock switch is quirky enough to be bad? I tried engaging blades and it cut out twice when doing so, but I didn't have to run the mower no where near as long to be able to engage the deck this time. This is one weird mower...Anyone want a cheap garden tractor? I am ready to get rid of this thing...

[KE4AVB]

Very possible the switch is bad for one thing. They do go bad like this as the plastic slides wear and dirt gets in.

As carburetor being worn believe or not I got one customer that has only 100 hrs of his Briggs and I had to replace the throttle shaft this Spring and the choke shaft needs it too but I was more interest resolving the no return to idle problem first. I didn't get to figure out why its throttle linkage are hangs at idle overnight. The customer wanted his mower back fairly quickly so he just leave throttle at half on at shut down so he has not notice any more hanging.

[OverKnight]

Could there be a problem with the mower deck or any of the belt connections between the clutch and the deck? Normally if spindle or idler wheel bearings are bad, they'll make a lot of noise, but I'm wondering if there's something that causing more than usual power to be needed to start the blades moving. Check if it's excessively difficult to turn the blades by hand.