I’d like to know where to start looking for a cause for the hesitation, before I have to take it back to the shop.

The XR6 Black Max starts and runs great, idles a little fast 700-900 RPM, wobbles slightly when idling. I use premium gas from new service station with Sea-Foam additive. Tope out around 55-59 mph depending on load. I had gas leak at the ty-straps, the ty-straps where replaced but not he hoses. The primer bulb is firm, but not hard after running, can still squeeze it a little. No signs of gas leaks when bulb is pumped. Runs really sweet up to about 45 mph/4000 rpm to two-third throttle. Then every time you go for full throttle it will hesitation, if you do if fast it will die. Service manual say it could be one of the following: restricted main jet, water in fuel, excessively lean fuel mixture, incorrect synchronization, or a faulty acceleration pump (do not think the150 XR6 has an acceleration pump).

The shop that did the 100 hr maintenance last spring installed cab-kits and cleaned the carbs twice. They could not get it to idle properly so they set the idle a little high. The sea-foam and time cleaned the engine out after a month or so and it really runs nice except the hesitation at high speeds.

What are you thoughts as to what is causing the problem?

Mine would do the same thing. It ending up being the fuel hoses. even though they were rated for ethanol they degraded and swelled on the inside for a lack of better term. The ECU will shut down motor with the even slightest of lack of fuel delivery. It does this to protect the immersed fuel pump. I now change the on-board water separator about every 50 hours.

How often does Merc recommend changing out the fuel lines?

What setting do you tension the Cable Tie clamps?

I'll say it's a rich fuel mixture........Tom

every 5-7 years fuel lines need replaced