45 Hp classic fifty question.

[Spillman1979]

Hello all, I have an 87 45hp classic fifty and I was wondering what RPMS I should be turning at full throttle? I have it on a 17ft bass tracker and have just installed a 13 pitch quicksilver prop on it. I am only turning 4k at full throttle trimmed out. not sure as to what my MPH is cause I have to fix that, but not sure how to yet. Any help would be greatly appreciated. thank you

 

[CharlieB]

4200 to 4600 at WOT is recommended. Your WOT RPM can vary with the differences in load on the boat. Full tanks, full coolers, a couple three people and gear, the greater load sets the boat deeper ini the water, increasing the load on the motor and reducing WOT RPM.

And of course the same in reverse for a lightly loaded boat, the motor will turn higher RPM at WOT.

You are very close already. You need to verify that your tune up is perfected. Good fuel, clean spark plugs, clean carb, ignition timing exactly as spelled out in the service manual, and know how light/heavy your boat is loaded. Only then can you properly determine how well suited your prop selection is.

 

[Spillman1979]

thank you

 

[jimmbo]

I thought the WOT rpm range for that engine was 5000 - 5500?

 

[CharlieB]

You may be correct. Many of the smaller motors with a smaller stroke do have a higher RPM range.

In general most all marine motors, both 2 stroke and 4 stroke, are running in the mid to high 4 thousands for increased torque and longevity.

When in doubt we should defer to the service manual. There are far too many motors to rely on memory alone. Knowing where to find the information can be far easier than attempting to remember so much.

 

[CharlieB]

Somewhere I have a factory 'cheat sheet' listing a great number of motors and the spec ranges for each series. It was a very handy thing I had posted on the wall of the shop. It should be packed away in a box of manuals somewhere in the barn. I should dig it out and stash it inside my CDI Troubleshooting Guide that I keep stashed in my tool bag on the shelf next to my big toolbox. Then it would be far more convenient to get to. As I do tell people, "When all else fails, READ the directions!"

 

[flyingscott]

You may be correct. Many of the smaller motors with a smaller stroke do have a higher RPM range.

In general most all marine motors, both 2 stroke and 4 stroke, are running in the mid to high 4 thousands for increased torque and longevity.

When in doubt we should defer to the service manual. There are far too many motors to rely on memory alone. Knowing where to find the information can be far easier than attempting to remember so much.

Most of what is said here would be true for an I/O boat and outboards made before 1970 as a rule of thumb. The 60s and earlier outboards a lot of those had sub 5000 RPM range some were higher, also most I/Os run in the sub 5000 range. In the 70s the RPM ranges started getting higher usually between 5000-6000 rpm engine size did not matter,some were lower of course . Later model motors especially run in the 5000-6000+ range including 4 strks the size of the motor really doesn't matter. If you prop your motor for outside your RPM range you can damage the motor. I wholeheartedly agree with getting the service manual. I also Agree with jimmbo about the 5000-5500 rpm range. This is just a general explanation there are exceptions of course.

 

[Chinewalker]

If you're only spinning 4000 RPM at wide open, you're either lugging the motor, or there's something wrong with it mechanically. I will assume, for now, that the motor is otherwise running well and you're lugging...

Lugging is akin to driving uphill, towing a trailer, while still in 5th gear. Motor builds up heat, pre-detonation can occur, and all in all, not good for the motor. Let it spin - drop down in pitch enough to get your RPM up into the 5000-5500 range, preferably the higher end of that range. That is akin to dropping down to 3rd gear while towing that load up the hill. Motor spins higher, but is much happier doing so.

 

[Spillman1979]

Guys, thanks for all your help. I very been trying to figure this motor out, as it was having issues with taking the gas but I think I've got that figured out with cleaning of the carbs. This last outing Everthing ran well just only 4k RPMS at WOT. I have good compression 120 on all four cylinders, new plugs, and great spark. so kind of confused as where to turn.

 

[jimmbo]

There is a possibility your tach is not set correctly.

 

[dfrants]

I once owned that same set up, if its the aluminum bass tracker bass boat. I remember it would turn a 13" prop 5400-5500 and go 32-33 mph. it had trim and tilt.​make sure your firing on all 4 cylinders

 

[Spillman1979]

I have checked all cylinders and they are sparking, I have not checked timing but engine is not missing at idle or anything.

 

[Spillman1979]

anything else I could look at?

 

[flyingscott]

Your tach may be wrong what is your gps speed

 

[Spillman1979]

can i do that with my phone, not sure how to do it?

 

[CharlieB]

Ignition timing is CRITICAL to performance. More than a couple of degrees off either way can cause overheating leading to engine damage.

Before sizing a prop always ensure everything is per spec. You have already cleaned carbs, got good fuel. verified spark. Now verify timing. And check your boat weight if possible. You may be surprised to find out how many boats are over-weight from waterlogged floatation hidden under the floors. Reducing performance and causing owners to suspect motor problems. Just check and make sure you are not one of 'those guys'.