18 hp vanguard running rough

[Theotherside]

My wife has a CC with an 18 hp vanguard big twin. I bought the cart for her a couple years ago. Up until now it has run great. She put bad gas in it once I drained and refilled with rec-90 which is what I normally put in it. Well it started running rough and I drained the fuel put fresh gas with sea foam in it. I bought new spark plugs Raisman RC12YC and put a new vacuum fuel pump on it. It really didn’t help it .
I just pulled the plugs to look at. They had mayb an hour of runtime and were really black. It’s hard to start and is backfiring a lot . She really hates this and is threatening to go buy a new cart if I don’t get this one fixed. Her and the grand kids love driving it around town. Other than the occasional backfire it’s been a good ride for her.

I ordered some ngk BCPR5ES I like ngk plugs and figured I’d try this. Also I noticed the extension from the muffler to the back of the cart has fallen off. I’m wondering if this could be causing all the backfire out the exhaust. I don’t see any adjustments on the carburetor.
Sorry for the long ramble. I’ve worked on cars my entire life I’m not real good with these lawnmower engines. I could use some help here I don’t want to spend 10 grand on a new cart for her but you know if momma ain’t happy nobody’s happy.
I’ll also ad it doesn’t sit for long periods up until now. This has been going on for a couple months now. I’ve been tinkering around with it and am contemplating taking it somewhere if I can’t get it figured out.

[BIKERMIKEYBLACK]

Pull the carb off and go through it clean it out that would be my first step

[Theotherside]

Thanks that’s what I was thinking also

[slonomo]

Pay special attention to the pilot jets. I always remove ALL the jets and clean them and run wire through them. I use small copper wire, like strands of speaker wire or battery cable strands. Over time fuel can leave a waxy buildup called varnish that needs to be cleared from the holes on the jets. The pilot jets are the smallest of them all and are responsible for starting and idling. They are usually the most problematic.

[Theotherside]

My brother is a Yamaha outboard tech I will probably pull it off and let him take it to work and soak it . Does anybody know the part number for a gasket kit for the carb ?

[Theotherside]

I got everything sorted out. Cleaned the fuel tank and fresh rec90 cleaned the carb ( it was really clean already) new NGK spark plugs. It’s running pretty good now I need something to hook on to the muffler to get the exhaust out from under cart. It’s 1 1/4” anybody have a suggestion?

[Theotherside]

Well she took it out for a ride and while it is running very good her words it’s still backfiring when she lets off the gas. 😡😡 do you think a new muffler would fix this ? I’m about ready for a motor swap. I don’t want to buy a new cart but that’s where we’re heading if I don’t get this fixed.
Any help is greatly appreciated.

[slonomo]

Check valve lash and reset.

[Theotherside]

Yeah I’m gonna have to find someone for that. I’m not familiar with this on one of these engines. I may YouTube it.

[Theotherside]

Watched a YouTube video. Seems pretty simple so I will check them tomorrow and report back. We have had this cart 2 1/2 years and it didn’t back fire near as bad as it does now. Valves haven’t been adjusted since we have had it.