Mower loses RPM's like a switch was flipped

Mr. G

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Craftsman 26 HP Pro V twin- Model #SV735S
The problem has gotten worse since the beginning of the season. The mower starts fine but at some point it loses RPM's, not much, but enough to slightly bog down the blades. It then smells like the exhaust is running rich. It' s random on when this happens and how long it lasts. Eventually, like someone hits a switch, it instantly revs back to full RPM. Now it runs with loss of RPM more than it runs at full RPM.
At the beginning of the season I changed the oil and filter, fuel filter, air filter, the plugs, and cleaned the deck.
The only thing I see is when I remove the air filter the carb has two holes about the same size, one looks like it has a nice finished edge and the others looks like it has a melted edge? Is there supposed to be two holes there?
Any help is greatly appreciated.
Thanks,
Mike
 

Rivets

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I would start by cleaning the carb and replacing the float needle and seat.
 

Mr. G

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Thanks for the reply Rivets.
I'm a novice in small engines; where can I get more info on the carb for more details.
I would like to see if the other hole in the carb is normal.
 

Mr. G

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Thanks, the manual is great, but it still doesn't show me the view I need to see if there is one hole or two in the top intake. I sprayed some carb cleaner in there with it running, a few short bursts. It seemed to run fine but that was only for about 15 minutes or so. The problem is random and I doubt that a simple dose of carb cleaner will solve my problem.
I'm going to cut the grass again soon so I'll see how it acts again and get specific about the symptoms.
 

Rivets

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Remember, I can't see what you are talking about with two holes. I can only assume that you mean the air vent and bypass openings when you are looking into the carb from the air cleaner side. Back to my original statement, time to clean the carb. If you don't feel comfortable doing it, you'll need to find someone who does.
 

Mr. G

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AH-HA, so there are two openings (maybe). Anyway not important now. I'll see what I can do about cleaning the carb. I have cleaned a carb on an old Honda motorcross bike; I guess it will be similar. Do you have any tips or suggestions... products to use?
Thanks for the help!
 

Rivets

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Best way to do it is to take it to a shop which has an ultrasonic cleaner. Most of your top repair shops should have them. You will be able to get your rebuild kit there also. With limited experience, doing this yourself could end up costing you big bucks for a new carb.
 

Mr. G

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Before I had read your last post I watched some you tube videos and decided to take apart my carb. It was very clean. The mower has about 200 hours on it. I got the float needle out, it was shiny and clean, rubber tip still looked brand new. The float seemed to be working fine, no gas in the float. The fuel bowl was shiny clean. I was spraying carb cleaner on everything and using compressed air to blow out the jets? or anything that looked like it moved gas or air. I unscrewed something from the bottom of two jets? just under the float, sprayed cleaner and air through those; put everything back together and within 30 seconds of starting the mower it lost RPM's all at once , like someone flipped a switch, not much RPM's but enough for the engine to change sounds and the exhaust to smell rich. The blades still turn, just slightly slower.
When the mower did this, I shut off the blades and pulled the lever to the slowest setting, idle, within 5 seconds you can hear the engine kick back up, I then would push the lever back to full and kick the blades on and it would run fine for about 10-15 minutes, then lose those RPM's again at some point. I was able, everytime, to get it come back up to full song by going to idle with the blades off until I hear it kick back in and then going back to full and turn the blades back on.
What else in the carb should I be looking for?
Do those symptoms tell anyone where else I should look?
Thanks for the help
 

Rivets

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Tells me that you have two choices, either remove the carb and have it ultrasonically cleaned and rebuilt or replace the carb.
 
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