So I’ve got 2 years on my BB 351 and no regrets at all. I still have the stock CVK carburetor on it, stock header with a Two brothers slip on. My question is “How much of a difference will a bigger header make compared to installing a pumper carb? One thing at a time for me.

 

Found the answer on klxtrs sticky. Carburetor it is

 

I installed both, a FMF powerbomb header and a Keihin fcr35 pumper carb. Since the klx351 inhales and exhales more air I felt they both compliment one another. Immediate throttle response with the pumper carb and a freer reving engine with the larger diameter header. I took advantage of the added power and added three teeth to the rear sprocket looking to slip the clutch and having the front end pop over obstacles easier. Then the stock clutch springs & fiber plates couldn’t handle it but your clutch might be in better condition.

 

I went ahead and installed heavier clutch springs right after the BB351. Bill recommended not running synthetic oil in it due to the added torque. I put the new springs and the oil he recommended. So far so good. I love the 13/44 gear set up off-road but put the 14 back on for hwy use

 

We simply do not know how taxing 351cc is on the stock ports. We don't know if the ports (intake and/or exhaust) become a bottleneck to performance enhancement.

Full FMF exhaust on the 250 is verified to have the ability to create a whole new animal compared to the same FMF slipon + stock header. When fueled correctly, the power curve above 7000 rpm comes alive to such a degree that it has to be felt to be believed - the charted HP curve tells all - it's a hard pull all the way up until you accidentally hit the redline cutoff.
Point being, with a 250, the bottleneck IS the header, not the ports.
We do not know if the 351 can also achieve the same phenomenal upper RPM power enhancement from a full exhaust system.
Logically, getting rid of the stock header ( which muzzles a 250 ) would seem to be the smart thing to do.

There is no "verified correct" fueling setup for the TM36-68 pumper. You'll be using hand-me-down setups with unknown resulting fueling.

 

You may want to "enhance" your CVK setup before making any decisions on additional mods.