thinking it might be a prop issue

[bob johnson]

I mounted the 1989 Johnson 70 hp onto a boat I rebuilt for my son. Its a 180 SS starcraft....its about 18.5ft long has a good beam, maybe 6 ft...3/4" plywood floor carpeting, 2 seats...MIGHT be 1000 lbs im thinking.... the motor has great compression , but sat a while...seems to not want to fully rev.....the motor is slow to get up on plane....like its working hard...but it will eventually climb to 5200 rpm....( if I play with the trim to get the last 200 rpms).....the prop is a perfect condition 13-3/4" x 15 pitch prop...aluminum. wondering a few things.....one: the boat is kinda heavy for a 70 hp... the manual says the WOT rpms need to be between 5500 and 6000..... im also wondering if the motor isn't running well at WOT..but it seems like if its getting to 5200 all the cylinders have to be firing good...and that its just the weight of the boat....I have another parts 70 hp and it has a 19 pitch prop on it!!!!!( telling me the previous owner had a super small boat..or I have issues with mine)....from the looks of it, I might have to drop to a 11 pitch to get in the proper RPM range..... looking for a little advice.....is it feasible a bad running motor would go up to 5200 rpms?? if so then I might just be underpowered and need to get a lower pitched prop... as the only fix.....the motor stop pin is in the second hole up from the bottom and the cavitation plate is dead even with the bottom of the hull( the keel actually).....

 

[Sea Rider]

One should make a wot test as usually loaded once break end period has ended for new engines or when buying second hand combos with delivered engines. Go for a prop maximization towards middle or max wot rpm range, but max rpm is preferred... Going one less pitch will pull wot revs higher than with actual prop.

Happy Boating

 

[bob johnson]

One should make a wot test as usually loaded once break end period has ended for new engines or when buying second hand combos with delivered engines. Go for a prop maximization towards middle or max wot rpm range, but max rpm is preferred... Going one less pitch will pull wot revs higher than with actual prop.

Happy Boating

yeah, I need about 600 more rpms..... that's 2 sizes down..

bob

 

[Sea Rider]

The rule of thumb does not always apply, go for 2 less pitch prop and test on a wot run if dialed + 600 wot rpm. if OB over revs just throttle bit less or add more load if liking running full hammer down.. Before going for 2 less pitch prop OB must be healthy that?s developing full power, correct carb and timing synch along a tach in god working order.

Test with trim set to neutral once combo is on plane and with load properly distributed throughout the entire deck. 2 less pitch sizes will better hole shot.

Happy Boating

 

[bob johnson]

yeah that's the issue deciding if the OB is healthy....to me it seems like it bogs down when you go full throttle quickly and the motor takes its time getting on plane and going to 5200 rpm... so wondering ...if its JUST a prop range issue.....if its a high speed carb issue, or maybe its a combo!... I ran by hand a .063 drill through the high speed jets...I pulled one out and it was stamped 63....and apparently that's the hole size... so I know they are not clogged......its very possible the timing of the throttle plates is off....I know I could just throw an 11 pitch prop on there and see if the hole shot and rpms get up to a nicer range..... if I was in the right rpm range and the motor still bogs on a quick throttle...I can believe its a motor issue..... the shops are so far apart around here and ive never "borrowed" a prop.... to test..... which one I need then buy the right one.........bob

 

[Sea Rider]

Personall would discard current prop having issues with hub with a hub slip test, then go from there...

Happy Boating

 

[Sea Rider]

If it be my combo, would discard current prop having issues with hub with a hub slip test, then going from there if found OK.

Happy Boating

 

[jimmbo]

It sounds like the engine isn't making all the power it should be. I think a compression test and a check of timing and verifying the carbs are opening fully is needed.

 

[bob johnson]

Ok, i can install a new prop of the same pitch(different blade design thouhj) the compression is great...ive got good spark at idle.....dont know if linck and sync is good..

 

[jimmbo]

I was referring to max timing, and checking to see if the carbs have opened fully

 

[Texasmark]

I mounted the 1989 Johnson 70 hp onto a boat I rebuilt for my son. Its a 180 SS starcraft....its about 18.5ft long has a good beam, maybe 6 ft...3/4" plywood floor carpeting, 2 seats...MIGHT be 1000 lbs im thinking.... the motor has great compression , but sat a while...seems to not want to fully rev.....the motor is slow to get up on plane....like its working hard...but it will eventually climb to 5200 rpm....( if I play with the trim to get the last 200 rpms).....the prop is a perfect condition 13-3/4" x 15 pitch prop...aluminum. wondering a few things.....one: the boat is kinda heavy for a 70 hp... the manual says the WOT rpms need to be between 5500 and 6000..... im also wondering if the motor isn't running well at WOT..but it seems like if its getting to 5200 all the cylinders have to be firing good...and that its just the weight of the boat....I have another parts 70 hp and it has a 19 pitch prop on it!!!!!( telling me the previous owner had a super small boat..or I have issues with mine)....from the looks of it, I might have to drop to a 11 pitch to get in the proper RPM range..... looking for a little advice.....is it feasible a bad running motor would go up to 5200 rpms?? if so then I might just be underpowered and need to get a lower pitched prop... as the only fix.....the motor stop pin is in the second hole up from the bottom and the cavitation plate is dead even with the bottom of the hull( the keel actually).....

70 IS light for a boat of that size. 15 is what you would run on a 15' light weight, shallow V tri-hull for day cruising or waterskis. 5200 isn't all that bad other than telling you that you are over propped. Your slow acceleration is also talking to you about being over propped. Hole shot probably sucks. Tilt pin and Anti Vent positions are right on.

Why doesn't he get a suitable sized engine for that boat or get a boat to fit the engine?

 

[jimmbo]

Most 15-16 ft boats with the small(49 in) 70 ran 17" props. the later larger 70s had more torque and a 15" on a 18ft should be fine. It sounds like the motors is weak

 

[Sea Rider]

If running solo, it's OK for any OB to slight over rev over its max wot rpm range. If like running full hammer down when doing so throttle bit less or add more load to compensate over revving. Runnning an under powered OB for any given application that slight passes its max wot range will benefit its hole shot significantly up to medium loaded when at an altitude or sea level.

OTOH, if propping right to rev at middle range when adding extra load or passengers wot revs will decrease towards min lugging side along a very bad time taking hole shot.

Happy Boating

 

[bob johnson]

im buying a HIGH THTRUST 13-7/8" X 13 prop today.....getting raked over the coals $177 before tax.....ive bought plenty when I had more time...for $130-$155....

ill try that , plus I have a brand new 15 pitch prop, just in case the prop on now has been slipping in the hub..... I have Friday to test and then Im off Saturday to deliver the boat to my son....