One last stupid question>max safe RPM's for 70hp

[calm like a bomb]

What is the normal,safe maximum RPM that a 1988 70hp should turn?<br /> I do not intend to run at max level but would like to know how much upper limit I have.I run either a 19 or 21 pitch.The 21 for the last few years.It's slower on takeoff but better top end.I'm trying to remember if I normally turn about 4700 rpm wide open now.That sounds close....<br /> Thanks to all of the posters on here!!

 

[ob]

Re: One last stupid question>max safe RPM's for 70hp

Prop it to turn 6000 WOT with a light load .

 

[calm like a bomb]

Re: One last stupid question>max safe RPM's for 70hp

ob,<br /> Would the 21 pitch prop cause it lose that many rpm's vs. the 19 pitch prop? Both under normal loads.<br /> I notice from looking at the linkage on the carbs that there is a ton of room left before the carbs open fully when the engine is NOT running,if moved by hand.I'm not looking to blow the old girl up but wondered how much performance is restricted by this.It is the black roller that rides on the small cam plate.When the throttle is set wide open (engine not running) the roller can still go a mile before the carb butterflys open fully.The cam plate has a small line machined in it that the roller lines up with perfectly when it is in neutral.It looks like it still retains the factory setting based on this.Engine idles and runs fine so I probably need to leave well enough alone.I was just curious as to how many RPM I am not utilizing.

 

[ob]

Re: One last stupid question>max safe RPM's for 70hp

Adjust the carb linkage where the center of the linkage roller is dead center with the scribe line of the throttle cam and "just touching" when the controls are in neutral.Use the throttle cable trunion and/or the roller lock screw for this adjustment setting.Make sure you spin the prop when shiifting when engine isn't running.Those carb flaps should be opening fully horizontal at wot setting,but not past.Setting the linkage roller should insure this.Also each carb flap has its own lock screw.All three should also fully close when controls are in neutral.<br /><br />21" prop is going to be too steep on a 70 hp engine unless its driving a rig under 800 lbs.19" maybe ,but final wot RPM test will dictate the final choice in prop pitch.<br /><br />What boat is this 70 pushing?Also insure that the engine is mounted to the transom where the lower unit anti-ventilation plate is running on the same plane as the boats bottom when trimmed level.Too deep equates to excessive exhaust back pressure and drag.Too high equates to exfcessive prop ventilation and prop blowout in turning maneuvers.Again only a watrer test can confirm final placement , as each hull and setup is unique.

 

[calm like a bomb]

Re: One last stupid question>max safe RPM's for 70hp

ob,<br /> It's on a 17' center console.I believe the carbs are set correctly now.Pretty much as you described.<br /> The height of the engine on the transom is correct,as I run a small Hyrofoil that just glides over the water surface with very minimal drag.Trim tab adjustment is also correct as the boat does not torque to either side at WOT.<br /> I'll put the 19 pitch back on it and see if the RPM's don't go back up.The 21 is OK but is sloooww on takeoff. Thanks Again

 

[ddaigle]

Re: One last stupid question>max safe RPM's for 70hp

Man, I can't believe you can even turn a 21 pitch. I run a 15 as do my friends on much lighter boats than yours. I turn about 6000 wide open and have a great holeshot. Top speed is about 34 mph. What kind of speed are you getting? You might look into a 17 pitch for a good all around prop.

 

[Javelin 353]

Re: One last stupid question>max safe RPM's for 70hp

I have a 1990 70 hp on a Javelin bass boat can't remember the prop pitch off hand but it will turn right at 6000 with lite load 5500 with heavy load top speed @40 mph Hope this helps Jay

 

[calm like a bomb]

Re: One last stupid question>max safe RPM's for 70hp

ddaigle,<br /> I get about 37 or 38 mph max if I am by myself with the boat trimmed out right.<br /> It just takes it a little bit to get there.Takeoff is pretty slow.I'm gonna go back to my 19 pitch and use the 21 for a spare.

 

[DumbBass]

Re: One last stupid question>max safe RPM's for 70hp

Calm, I run a 15 pitch on my '89 60HP and turn 500-5600 RPM. 21 pitch seems like alot!

 

[calm like a bomb]

Re: One last stupid question>max safe RPM's for 70hp

Went fishing today and ran 5000 RPM @ 39mph with the 21 pitch prop.<br /> Looks like ya'll are right..I need to go back to my 19 or a 17 if I had one.