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Old 08-09-2008, 09:37 PM   #1
andmoon OP
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Whay does my lawn mower's idle rise and fall?

Riding mower. with Brigs 5xxcc single. The idle now rises and falls like it is running out of gas. I changed the air and fuel filter to no avail. My bikes are finally running sweeet so the freakin mower starts acting up?
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Old 08-09-2008, 10:35 PM   #2
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The carb needs cleaning or rebuilding, should be a 30 minute job if you can get to all the screws OK.

What happens is the idle circuit is messed up, so it will not idle but when the RPM's fall too much the governor opens up the throttle so it roars into life, which makes the governor move back to idle and the process repeats itself again, about 30-40 times a minute.
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Old 08-10-2008, 02:57 AM   #3
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How old is it?
If it has a wind vane type governer they tend to do that.
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Old 08-10-2008, 07:59 PM   #4
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Quote:
Originally Posted by andmoon
Riding mower. with Brigs 5xxcc single. The idle now rises and falls like it is running out of gas. I changed the air and fuel filter to no avail. My bikes are finally running sweeet so the freakin mower starts acting up?
i don't know anything about the larger Briggs, but at least on their smaller engines the carb is mounted directly to the gas tank, with a thin rubber-like diaphragm between them. If anyone used carb cleaner to clean this (like I did) it can make that diaphragm just a little too floppy and you'll get that rise and fall effect. Fortunately those diaphragms are relatively cheap.
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Old 08-11-2008, 05:46 AM   #5
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Quote:
Originally Posted by GreaseMonkey
The carb needs cleaning or rebuilding, should be a 30 minute job if you can get to all the screws OK.

What happens is the idle circuit is messed up, so it will not idle but when the RPM's fall too much the governor opens up the throttle so it roars into life, which makes the governor move back to idle and the process repeats itself again, about 30-40 times a minute.
Thanks,

Marvel mystery oil carb clean attempt "1.
Sea foam attempt # 2.

fook it, I'll take the fooker apart....#3.

Thanks!
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Old 08-11-2008, 05:47 AM   #6
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Quote:
Originally Posted by Inane Cathode
How old is it?
If it has a wind vane type governer they tend to do that.
1 year old...no clue as to opt. hours as i do not count.

1 oil change old.
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Old 08-11-2008, 08:29 AM   #7
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OK, here is a trick you can try-

Take the air cleaner off all the way to the carb body. Start the engine and rev it up at high throttle. When it is roaring away block the air off completely with your palm. Hold it there until the engine almost dies then remove it in time for the engine to catch and roar back to life again.

Do this about 5 times and see if that makes a difference.

What happens is by blocking it off you create a strong vacuum which pulls gas through a lot of air passages and hopefully sucks out any junk stuck in there. It doesn't work all the time but suprisingly often enough (enough times that I used to have a large rubber ball glued to a chopstick that I would use to block off individual carbs on a multi cylinder bike).

Now the bad news is with briggs motors your problems are usually caused by that flexible gasket as metioned by Yad, but since you mentioned it is only a year old you may get lucky this way.
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Old 08-11-2008, 12:34 PM   #8
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Quote:
Originally Posted by Yad
i don't know anything about the larger Briggs, but at least on their smaller engines the carb is mounted directly to the gas tank, with a thin rubber-like diaphragm between them. If anyone used carb cleaner to clean this (like I did) it can make that diaphragm just a little too floppy and you'll get that rise and fall effect. Fortunately those diaphragms are relatively cheap.
Yeah, and nowadays the engines are set to run so lean that they will react to just the slightest bit of something being out of spec.
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Old 08-12-2008, 01:15 PM   #9
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Quote:
Originally Posted by andmoon
Thanks,

Marvel mystery oil carb clean attempt "1.
Sea foam attempt # 2.

fook it, I'll take the fooker apart....#3.

Thanks!
Mine did a similar thing. I Sea foamed it and it ran great after it smoked like bitch for 10 minutes.

Matter of fact I'm due for a foaming again.
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Old 08-12-2008, 04:30 PM   #10
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Try it with the gas cap loose. Maybe there is a blockage in the cap vent.
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Old 08-12-2008, 10:09 PM   #11
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Quote:
Originally Posted by andmoon
1 year old...no clue as to opt. hours as i do not count.

1 oil change old.
Wow i was WAAAAAY off. Theres no way yours has a wind vane governer.
My only guess is that its running lean. Go fix that :P
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Old 08-13-2008, 02:59 PM   #12
Mista Vern
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Quote:
Originally Posted by andmoon
1 year old...no clue as to opt. hours as i do not count.

1 oil change old.



Quote:
Originally Posted by Inane Cathode
Wow i was WAAAAAY off. Theres no way yours has a wind vane governer.
My only guess is that its running lean. Go fix that :P
That's the problem - no way to adjust the new carbs. I curse the engineers/bean counters who devised such waste.
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Old 08-13-2008, 08:37 PM   #13
Inane Cathode
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Theres no adjustment at all? No blank caps over mixture screws or anything?
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Old 08-14-2008, 01:53 PM   #14
Mista Vern
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Quote:
Originally Posted by Inane Cathode
Theres no adjustment at all? No blank caps over mixture screws or anything?
Nada. I thought I was going nuts not finding them, but in fact there are none. I might try that Seafoam idea and see if that make a difference - the tiller runs fine under load but all the other times it cycles revs up and down.
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Old 08-14-2008, 06:15 PM   #15
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Quote:
Originally Posted by Mista Vern
Quote:
Originally Posted by andmoon
1 year old...no clue as to opt. hours as i do not count.

1 oil change old.





That's the problem - no way to adjust the new carbs. I curse the engineers/bean counters who devsised such waste.
EPA bastages forced this, but worse some small engines now have plastic camshafts too-
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