2015 GTR with carb, cam, heads, and gutted stocker. 40 hours on mods, and all parts came from boat Dr. After I did mods I started F/A screw on the suggested 1.75 turns. Motor had a small stall to it when accelerating from idle, so I adjusted the screw to 2.5 turns where it ran the best. Ran great for a while there, but the stall came back. I readjusted to 1.5 turns. Again it ran well set there until this past weekend when the stall came back. Adjusted today to .5 turns and the stall is gone. What would be causing these symptoms? Thanks for any tips in advance.

Temperature? Fuel difference? The accelerator pump should be solving any off idle stall. Make sure it's working, is pointed at the needle, and is spraying fuel when it should (refer to mikuni hsr tuning manual for instructions on that, it's real clear).

Sounds like the accelerator pump isn't coming in soon enough. Maybe a slight adjustment is in order

Thanks for the suggestions guys. I know absolutely nothing about carbs, and pretty sure I lost my manual. I'll have to get my hands on another one

Just Google mikuni hsr. On the mikuni site, you can dload the tuning manual

I found a pdf manual. If it is an accelerator pump problem, why is the F/A screw helping the problem?

Because they both are affecting the fuel air ratio off idle. The accel pump is a band aid that solves stumble off idle due to the velocity of air being too slow moving through the carb from idle. Once you get the engine rolling, the velocity of the volume of air through the carb grabs enough fuel from the jets so the accel pump isn't needed. But it takes a moment or few when you open the throttle for that air to get moving to create the negative pressure, in order to grab the fuel it needs. The accel pump gives it that fuel, and is mechanically operated, so if it's not adjusted correctly or pointed correctly it won't provide the fuel upon throttle opening that the engine needs. Adjusting the air fuel screw changes the amount of fuel for a given volume of air in the same circuit that the accel pump comes in (pilot circuit)

Check your boot for cracks...

Ok fellas, I did a little adjusting this morning. I started by moving the F/A screw back to 1.75 turns out. The gap between the accelerator pump shaft and the plastic cam was probably a 1/4" or better. It was not contacting the shaft until half throttle. I decreased the gap until the stall was gone. The plastic cam now contacts the accelerator pump shaft immediately when I pull the throttle. I greatly appreciate the help and hope this solves my problem.

It should. If it idles fine and slows idle when going either direction, the af screw is right. The stumble off idle when applying throttle is because of the ability of the carb to be efficient in its flow of air as the engine needs it, and how much fuel it grabs in the process of giving the engine more air. That's what the accel pump corrects.

After the adjusting I did on the 7th the boat was not run again until the 11th. Got to the ramp and started warming motor. It ran TERRIBLE. Didn't know that I was going to make it to the hole, after first light I turned F/A screw back to .5 turns. Ran like a top then. So I assumed I needed to go ahead and rejet carb. Called Boat Dr. about problem and was told that with fuel like it is these days that he was seeing this problem more and more. He said run it at .5 turns and don't worry about it. All of that leads me to this, when I was warming the motor on the morning of the 11th the starboard side would shoot a blue flame every couple seconds while it was idling. Now that the motor is running better, it will still randomly shoot a blue flame on the same side. Is the flame caused by fueling being too rich or too lean, or do I have a problem elsewhere?

Good info- thanks for the update.

Could be either, but most likely its burning off excess. Note that the stock carb is jetted differently for the two different cylinders. On a horizontal they are 138/144

Interesting...I'm going to take a look at this when I get home...my motor has always been a little sluggish off idle. If I slam her open she'll just stall. I have to ease her up to about 1/4 throttle then I can just give her hell. It's a pain in the ass when I'm in the soupy shit and I just want that thing to roar open.

I've never worked on a mud motor but have worked on dirt bikes a lot, and usually when your air screw is that far in you are lean on your pilot jet. Screw should be from 1 to 2 turns out. I usually start the motor and warm it up, then give it a little throttle just to bring it to a high idle and hold it there, at the same time adjust the air screw until I get the highest idle without it breaking up while still holding the throttle in the same spot. If you are less then 1 turn out the pilot is to lean if your farther then 2 turns out your pilot is rich