Carb swap or ?

[GeauxLSU]

OK, picked up a sweet deal on a Honda CRF100f this weekend for my son.
When we went to look at it, the bike would not really idle and eventually wouldn't even start. The owner was a former motocross racer so has some knowledge about engines (this was a bike for his son). Since he literally had it priced for half what many are listed for, I had to grab it.
It's an '05 but cosmetically is in the best shape of any I've seen. He swapped plug and air filter and it's running but not perfectly. It does two weird things I can't figure out.
1) Intermittently but fairly often the rpms will 'lag' coming down. Meaning if you give it gas while idling or adjust the idle up it frequently (but not always) takes a long time for them to come back down or they may just 'stick'. When you load the engine, they drop back down as normal. Is that a throttle cable sticking (doesn't feel like it all and again it may do it when just sitting idling) or something internal in carb?
2) The owner told me he often ran in it with the choke in the 'half' position as in his view it ran lean. It will, more often than not, have the popping/crackling exhaust especially under harder accerlation followed by deceleration.
And yes, when asked about ethanol, he was clueless (which was really odd). I'm pretty sure this bike has been sitting, too much in recent months.
Carbs are cheap (thankfully) and I "think" a swap would be easy but since I have about zero mechanical inclination, I'd figure I'd ask first if it might be something else.
To be clear, the bike runs OK (hard to tell if it's running hot or not. I did do an immediate oil change (which was clearly overdue). It goes WAY faster than I thought it would/should but it's obviously not perfect.
To make matters worse... now I want a dirt bike.

 

[Ken Ford]

Could be carbs, but I'd check for vacuume leaks and bad rubbers from carb to head.
Also most racers modify the air box, which will cause drivability problems if not jetted right.

 

[GeauxLSU]

Could be carbs, but I'd check for vacuume leaks and bad rubbers from carb to head.
Also most racers modify the air box, which will cause drivability problems if not jetted right.

It's all stock. To be clear this was never a race bike (he was just a racer, using other bikes). Sounds like it was used to just terrorize the cul de sac. (And rear tread supports that unfortunately).
Rubber looks and feels good (new actually). I have no clue how to check for vac leaks. The original air filter was in really bad shape but as mentioned he changed it.

 

[GeauxLSU]

Could be carbs, but I'd check for vacuume leaks and bad rubbers from carb to head.
Also most racers modify the air box, which will cause drivability problems if not jetted right.

And forgot to mention, he mentioned that he thought the 'ignition timing' need to be adjusted. Would that cause the popping/backfiring?

 

[BenStark]

To check for vacuum leaks get a can of starter fluid and spray around vacuum hose joints and gaskets. If there is a leak the engine will rev up.

I don't know much about dirt bikes but the typical suck squish bang blow always applies to 4 stroke motors.

 

[Ken Ford]

Have you swapped the plug yet?

 

[GeauxLSU]

Have you swapped the plug yet?

He did (plug and air filter).

 

[StuP]

Leak somewhere is very likely, torn diaphragm's possible, dirty jet circuits won't help. Probably electronic ignition, so shouldn't need adjusting. Whatever you do, it'll need the cabs synched afterwards. I think I've go a neat little electronic balancer somewhere, which makes it simple and is plenty accurate, if you want to use it.

 

[Zeake]

If it has had ethanol fuel in it the filter inside the tank could be disintegrating

 

[albatros43]

Change the flux capacitor. Sorry, I'm not very mechanical. That's all I got...