Briggs 10.5 runs rough/won't start with air cleaner fitted

[Maxrevs]

Briggs 10.5 fitted to a customer's 1998 CastelGarden F72 ride-on, motor was running fine but now runs rough and is near impossible to start with air filter fitted.
Model 28B707, Type 0146-01, Code 970318ZB
Carb is Walbro LMT 5-4993 (no fuel cut-off solenoid)

Mower came in for battery not charging and a service (tune-up). I'd removed fuel tank temporarily to get better access to wiring. I was testing alternator & regulator output (engine running normally with air filter), when after about half-a-minute the engine spluttered and died. Doh - I'd forgotten to reconnect the fuel hose.

After reconnecting the fuel hose the engine would not start. Then I started to smell petrol (gasoline) and noticed fuel running out of the carb. I assumed some debris in the float bowl had got sucked up and jammed the float needle open.

Stripped and cleaned carb, refitted to engine with air filter in position, turned fuel back on and no leaks. Problem solved, I thought. No. Engine would not start at all. Tested spark, put new plug in. Nothing doing apart from the occasional pfft.

Intending to re-check carb, I removed air filter and for some reason I just decided to try starting it - started first time and ran sweetly!!! Held air filter in position and engine dies, lift it up and engine runs.

Stripped and cleaned carb again, using new air filter, if I lower it into position slowly with engine running at max speed, I can just about keep it running but it's really struggling - not running evenly, occasional misfire - lift out air filter and it runs sweetly.

Is there an airflow restriction somewhere? If so, I can't find it.

Could there be something else going on that's not carb related?

My customer wants his mower back and I'm feeling guilty & stressed because the darn thing was running OK (apart from not charging) when he brought it in.

I would be SO GRATEFUL if any of you more experienced guys could offer some suggestions to help me out.

Thanks.

PS, I'll post some pics so you can see what I'm looking at.

 

[ILENGINE]

I can't tell for sure but it looks like there is a jet missing that screws into the side of the part that fits down into the bowl.

 

[Maxrevs]

Thanks for your response - I think I know the one you're referring to - is it the one shown in pic #9 for carbs with the anti-afterfire solenoid?

http://outdoorpowerinfo.com/repairs/briggs_walbro_lmt_carb.asp

My carb doesn't have that solenoid so I don't think that applies here.

Any other thoughts?

 

[mechanic mark]

https://www.briggsandstratton.com/us/en/support/manuals/results?NTT=28b707-0146* engine manual & parts, if you decide to purchase carburetor kit make sure numbers are correct as several kits are listed.

Amazon.com : Briggs & Stratton 698171 Carburetor Replacement for Model 697594 : Lawn And Garden Tool Replacement Parts : Patio, Lawn & Garden

Genuine Briggs Stratton 799728 Carburetor Replaces 498027 498231 499161 | eBay

 

[ILENGINE]

something that I have seen once or twice. Pull the dipstick and check for a gas smell in the oil. I have seen gas mixed with the oil, and the oil level will be right on the money, but when the air cleaner is installed will pull gas fumes through the breather system, and it will act like the choke is on. but with the air cleaner off, it will vent the fumes directly into the air, instead of being drawn into the engine.

 

[Maxrevs]

Thanks ILENGINE, that was it - fuel in the oil. I'm so grateful for your inspired suggestion - a stroke of genius on your part. :thumbsup::cloud9: And thanks to mechanic mark for the helpful info.

 

[mwrman]

Just a thought. Have you checked the flywheel key?