High RPM
 

Hello folks,

New issue with my sx19 4.3l 190hp mercruiser. Until this weekend, WOT trimmed up would get me to 5k RPMs. Running with 19 pitch prop. Had it out this weekend and noticed I was hitting 5k RPMs at only 3/4 throttle trimmed all the way down. Thoughts on what might cause the change?

Thanks in advance for your thoughts,
LifeIsGood

Re: High RPM
 

Why , pray tell .............. are you trying to run trimmed in all the way at 5000 RPM to start with?:shakehead:

Re: High RPM
 

Did you hit anything with the prop? Spun hub or messed up blades on the prop could cause that sort of change.

Re: High RPM
 

Blockp

LAst time out, I did rough up the blades ever so slightly on a sand bar but did not believe that would have such a big impact on the rpms. I have since filed the small burrs and will test it out soon but was looking for other possibilities. How does one determine if the hub is spun?

Btw, prop is 21 pitch, original post was incorrect.

@ColoradoDiablo, do not run at 5k rpm often, only when testing top speed.

LifeIsGood

Re: High RPM
 

I wouldn't think small burrs on the blades would have that much effect... unless you buried it hard and took some cupping out of blades or something like that.?

Re: High RPM
 

You mentioned a 'spun hub'. How do you determine if that is the problem?

Re: High RPM
 

Have a look at this 'sticky' in the Prop Talk forum. I doubt a spun hub is the problem though as it usually causes a lack of acceleration and high rpms.

Something i would check is the tach itself and then the wiring connections. Pull each connection, clean it and reinstall. On the back side of the tach is a small dial showing "2-4-6-8". Give the dial a spin a few times and reset it to 6.

As Larry notes.... running at 5k rpms with trim all the way in??? And a 21" pitch prop hitting 5000 rpms is a bit unusual as its generally a 'cruising' prop on these motors. A 19" will definitely hit that high.

Re: High RPM
 

thanks mikeyt. Good sticky post on props. I'll remove mine and look for signs of damage to the hub.

regarding: "As Larry notes.... running at 5k rpms with trim all the way in??? And a 21" pitch prop hitting 5000 rpms is a bit unusual as its generally a 'cruising' prop on these motors. A 19" will definitely hit that high.

Guess I need to get my terminology down....I assume trim up/out and down/in are the same. Prior to this issue, when running at top speed I would hit ~48mph (perhaps not that high) @ 5k RPM WOT with trim up/out. Now I am hitting 5k RPMs at 3/4 throttle with trim down/in.

Will have it out again Saturday and see if cleaning up the blades helped (doubtful).

LifeIsGood

Re: High RPM
 

I have been considering the purchase of a 19 pitch aqnyway as I prefer hole shot over top speed (for slalom skiing). Any suggestions on best Make/model/Retailer for props. I have purchased Michigan Wheel before...I think form Overtons.

14.25x19

LifeIsGood

Re: High RPM
 

Personally i run a Michigan Wheel Apollo SS 20" 4 blade. Gives excellent low-mid range acceleration. I bought mine thru Iboats.com

I think that what Larry was questioning is why would you run with the outdrive trimmed down/in at full speed? I might do it for high speed cornering purposes but never on a straight run.