RPM seems low

[Cphil]

Motor is a Vmax 150 carbureted on an Xpress X70 with a manual jackate. I have tried different engine heights and props. It came with a 25P prop and I’ve tried a 24 4blade, a 23 3 blade and the 25 3 blade it came with. It seems no matter which prop my motor will not go above 5100-5200 RPM. Compression is good and motor runs great otherwise. I’m off Thursday and I’m going to make sure my blades are opening all the way with the throttle open and check some other things. New spark plugs also. Any suggestions on what else to check?

[CJ1]

Your tach, double check that it is accurate. Once you verify the throttle plates are opening all the way check your lift pumps just to be sure. If it is not running lean stick a 23 trophy on it. That was the best running prop on 2 different 150's that I had. CJ

[Cphil]

I did replace both fuel pumps, water separator, fuel lines/filters, etc just for piece of mind when I got the boat recently

[dannyman]

Clean carbs. Check that reed valves are still intact.(I’d install carbon or fiberglass reeds at this time as well). Hook up timing light and confirm throttle advance is set correctly. If you have good compression, fuel or timing is off. I’d lean towards fuel. Run the motor with the air box off, if you get fuel spitting out of one of the carbs you’ve got a bad reed. Don’t run balls out until you get her figured out. My 97 ProV 150 runs just over 6000 with a 23m prop. Around 5800-5900 with a 25m. That’s on an 18ft xpress. I went through this a couple years back and I had ate a reed. Good luck and let us know what you find.

[bassinman215]

Have you done any work to the carbs?

I had this issue after chasing down an issue on my carbed VMAX. My main issue was that I'd accelerate on plane and then lose speed. The fix was the anti-siphon valve in the fuel tank.

In my trobleshooting, I cleaned the carbs and replaced the fuel pumps. The boat stopped losing speed after replacing the anti siphon valve, but I couldn't get full rpms.

What ended up fixing the full rpm issue was a link and sync, per the manual's instructions. Try that if you've done work or had work done in the past.

What clued me in is that my carbs weren't opening fully after I cleaned them. Mind you, I marked everything with a marker prior to cleaning to prevent this very issue.

Please post your solution if you've solved this.

[Cphil]

Thanks for the suggestions guys. I will either report back here or start a new thread when I figure this out. Now that winter is here it may be a while before I get her out and run her and start troubleshooting again. Good advice as always. Thank you