Who knew a Briggs 3.5 would run without a Idle Mix Valve???

[Jeff T]

About a month ago I bought a used gas edger with a 3.5 Briggs. Took the seller a few pulls but it started. Once running it searched and searched. OK, needs a diaphragm. Pulled the carb, cleaned it, replaced the diaphragm. I'm no expert at these, but it started much easier, but still searched. I kept looking at the parts diagram. Tried it today and still ran like crap. Pulled it off and started looking at all the holes in the unit. Suddenly, there it was, a threaded hole with no idle mix valve.

Time to order some parts!! Or am I wrong??

 

[ILENGINE]

I don't think that threaded hole is for a mix screw. the mix screw or in this case could be an air bleed screw is that brass straight slotted screw below the welch plug.

 

[Jeff T]

Doing more searching online, I believe you're right... That being said, I cannot get this engine to run right. Disassembled and cleaned the carb, new diaphragm and it still searches. Any suggestions???

 

[Rivets]

Did you replace the carb to tank gasket? This setup likes to leak there. To test, start the engine and spray carb cleaner all the way around this mating area. Any changes in speed mean the gasket is leaking.

 

[Jeff T]

Did that test and it was fine.

 

[Rivets]

When you replaced the diaphragm, did you remember to install both the spring and protective cup under the diaphragm with the cup against the diaphragm. I’m also assuming you made sure that both pickup tubes were clear? Does the surging change when you close the choke slightly? Have you check the mixture screw, to make sure it is in good condition? What happens when you open or close the mixture screw? Sorry to ask all these questions, but I’m not standing next to you because on social distancing.

 

[Jeff T]

When you replaced the diaphragm, did you remember to install both the spring and protective cup under the diaphragm with the cup against the diaphragm. YES, all parts in a tub, none left, installed cup to gasket. I’m also assuming you made sure that both pickup tubes were clear? YES! Does the surging change when you close the choke slightly? NO, limited working window. Choke open all the way and it dies. Have you check the mixture screw, to make sure it is in good condition? No expert, but can see nothing visibly wrong with it, all three holes cleaned. What happens when you open or close the mixture screw? Appears to have little effect one the engine. All the way in to almost three full turns out. Sorry to ask all these questions, but I’m not standing next to you because on social distancing.

Lastly, when I engaged the blade and started to edge it immediately died. Something is off just doesn't appear to have power. New gas, fresh oil, new air filter. Only old item left is the spark plug.

 

[Romore]

Spark plug won't cause that

 

[bertsmobile1]

I you can turn the mixture screw more than a 1/2 turn in either direction and it does not make a drastic difference to the engine then the passage way is not clean or the needle is defective .
Old school carbs have coarse adjustments which makes them a lot easier to work on.
New ones will have an operating range of several turns because the threads are finner but old ones will have an operating range of 1 turn or less .
Is the needle new ?
If so get the magnifier & torch out and have a good look at the seat

 

[Jeff T]

Update: Removed the carb and disassembled again. Carb cleaner and a small wire through every orifice. Reassembled and no different. When running on high there is the occasional backfire.

This carb has a jet that is screwed all the way in, see first photo. There is no idle mix valve and spring.