15hp johnson fuei pump

Have another question. What makes the fuel pump work on a 15hp Johnson?
just the vacuum from the fuel flow?? Thanks.

Crankcase pulses make the pump work. If you take it off you see the hole behind it.

Thanks flyingscott. I just replaced the fuel pump. Hope it takes care of the problem. Runs great at low speed but acts like its runs out of fuel at high speed. Like to go over everything I have done with you if this new pump does not solve problem.Thanks again.

Help
This 1979 15hp Johnson still runs out of fuel at high speed. Have cleaned the carb replaced the fuel pump fuel lines and both connecters and the fitting on the tank. Can the bulb cause this?? Out of ideas.

Check inside your fuel tank for leaks.------And post results of a compression test.

Personally, what I would do is temporarily T a fuel pressure gauge into the line between the fuel pump and carburetor to monitor the pressure as it runs and see if it actually is starving for gas. You have already spent more than the cost of a gauge on all the parts you bought. There is an el-cheapo way also. See if it will keep running if you pump the primer bulb.

When you did the carb how did you set the float level.

Had similar issues with my '76 omc15. After many months of searching for the high speed float level drop, 3 fuel pumps with one being a mikuni single pumper I discovered the economy primer bulb had a set of real stiff check springs. Took out the springs and guess what...problem solved.

Thanks again guys. I am on a family emergency right now. But when I can I will try all suggestion and get back to everyone.
Just for the heck of it, I think I will replace the bulb first of all.

OK, On our family emergency we passed through Shasta Lake City 1 mile from Shasta Dam. Lots of outboard boat shops in the area. We pulled in to one who specializes in old Johnson motors. He said they check the fuel pressure with a vacuum gauge, must be 3 to 5 inches of vacuum.But like ones on the forum he told me to go out on the lake warm the motor up then turn it on when it starts to run out of fuel pump the primer bulb.I did this it planed the boat ran full power stronger than ever, Then the bulb went completely flat like it was sucked dry and the motor died.But did start up first pull and run fine at slow speed.One would think there is nothing wrong with motor or carb overhaul.Or the new fuel pump.I doubt now even the old one was bad. That leaves only the fuel bulb , and the fuel tank which is a plastic after market tank.I see no way to pull the pickup tube out or check it.I will replace the bulb. I have done lots of mech.work but none on boat motors. WHAT am I missing here? thanks.

loosen the cap on the tank when it happens with the ball going flat sounds like a vent issue especially with a cheap plastic tank. Also make sure the arrow on the ball is pointing towards the motor.

Yep, I will try that next.Must be something simple after going to all the work.But cleaning the carb really did the trick for trolling. The fuel cap has a vent screw on it, And I open it when running, could be restricted w/something I guess. Will loosen the cap like you say and find out. thanks again.

Probably the vent. But if not, think about it for a minute. Something obviously is preventing the fuel flow from the gas in the tank to the primer bulb. That is why the bulb is sucking flat. Whatever is preventing the flow probably is a vacuum in the tank (vent). But it could be something with the hose between the tank and vent. Including the pick-up tube in the tank.