I have a 31GD LT on a 18 ft aluminum boat. I have a carb spacer, bps muffler and K&N filter.

Saturday I was running 20.5 consistently by myself but only turning 3300 rpms. Motor ran good at all speeds, though mostly I was running wide open. My fear is when I add a 2nd and 3rd person, by rpms are gonna drop down to 3100 and my speeds to 16-17mph.

Sunday I decided to try to gain a couple rpms by messing with the throttle spring. I took a piece of wire and wrapped it around the throttle spring so there wouldn't be so much stretch. I cranked it up on the trailer and could see a huge difference in how quick the throttle was responding.

I took it to the river and was running 3400 rpms and 22.8 on the gps and then realized the river was swollen and the current was the reason for my speed. Turned around and was running 3390-3400rpms and 20.5 upriver. Got into a backwater slough and it was running 21.5.

Problem is when I slowed down to an idle it was running rough. A couple times it shut off and the only way I could get the motor to start again was to pump up the fuel bulb. Not sure why it was doing that. My motor has always required me to start it with the choke wide open, but even this would not get it started. I had to pump up the bulb and close the choke to get it to fire.

Got it back home and cleaned out the carb and made sure the plugs were gapped right. Plugs seemed a little dry. Pumped the fuel bulb to fill the carb and started it up and it seemed to be running better. Let it run for a couple minutes around 2500 rpms. Moved it down to idle and it was spittin and poppin. Went to give it gas and it shut off. Cranked it back up and after a couple pops from the intake it fired back up again. No improvement.

My guess is by limiting the play in the throttle spring, maybe I'm messing up the fuel delivery to the carb making it do these things.

I have 204 hours on the motor and have confidence in it starting, but I'm starting to wonder if my means to gain 1-2mph is having an effect on the way it is idling.

Also can't understand why I'm only turning 3300-3400 by myself. I have a rebuilt prop and it does have a good bit of cup on it. I've been tempted to get the angle grinder out and knock some cup off of it. When I bought the prop, it had "35" written on it. I'm beginning to think its too much prop for my heavy boat.

What do the gear heads think?

By the 'throttle' spring I assume you are talking about the governor spring. It has nothing to do with your fuel delivery. The governor spring only controls how much pressure is on your governor arm at full throttle.

You may have a tank of bad gas which can cause rough running.

Yeah, the governor spring it is.

Might be a bad tank of gas. I just put 93 octane into an empty tank saturday and only have 2 days of running it. But its running through a fuel water seperator and a fuel filter.

Maybe I need to stay away from the Raceway station.

your motors fuel pump is designed to pull fuel at max 36inches and 12 inch lift-I put low pressure elec fuel pump between fuel line bulb and motor and turn it on when motor is running-this solved my problem.bought pump at local auto parts store. if needed also put regulator between pump and motor so not to put more than 4 lbs of pressure on needle seat

Try changing the fuel water seperator. If you run more than one fuel filter, that can actually restrict fuel flow too much. Too many filters is not good.

I am no "gearhead", but your motor seems to be starving for fuel from what you have described. I had the same problem after my mods, I added the Mr. Gasket 2-4lb fuel pump, and solved this issue completely. I actually picked up around 150 RPM's by doing this. The Mr. Gasket is push fuel pump, so has to be mounted very close to the Fuel Tank. For me it is mounted underneath the extended deck in the bow with about 1 foot line from the tank to the fuel pump, and then I ran the fuel line down the side of the boat into a bulb, and then into a Racor water separator, into a quick connect, into the motor. I only have about 15 hrs on the fuel pump, but so far so good. I have it wired into the key switch. I would recommend an additional switch to turn it off, which I have not added yet. Not necessary, but I am going to add one.

My first thaught is do you have the gas tank breather open? (that will make it do this exact thing)
second: you may have a stopped up fuel filter or bad gas.