Almost there...Please Help!

[IRBFishin]

All-

I don't post often but read on here a whole lot. I have a 1978 70 HP Johnson on a 15' Tri-Hull bass boat. I picked it up for nothing and have worked on it off and on for two years to get it tip-top (restoration). I've got it to where I have a great hole shot, good speed, great idle, etc.

This should be enough but there's one thing...wakeboarding. I can't seem to get the umph to pull up a skier/wakeboarder. It seems to drag as soon as I put rope on it. We're not big guys (160-180) and this 70hp should not struggle like this.

I set the timing per J.Reeves, new fuel pump, new fuel lines/bulb (gets nice and hard), new spark plugs (QL77JC4), link and sync right to specs (this was the last thing which got me the good hole shot), rebuilt carbs, fresh gas (several tanks).

Can anyone give me any last tips to improve this little snag. Should I be adjusting the plastic screws on the throttle cables? Should I turn the timing screw in a bit and try it? Out a bit? Move the timing base adjuster a bit more?

I will say it seems to slightly start to bog (without a skier) when I jam full throttle but immediately grabs and takes off. Oh, please don't tell me its the carbs...I never want to see the inside of those again. Any tips? Thanks.

 

[ezeke]

Re: Almost there...Please Help!

Get a propeller with a lower pitch than what you are using.

And, make sure that you can reach 5800 RPM at WOT with your average load.

 

[IRBFishin]

Re: Almost there...Please Help!

Get a propeller with a lower pitch than what you are using..

Should have mentioned this, Replaced 19 prop with a 15 recently. Noticable decrease in top speed.

And, make sure that you can reach 5800 RPM at WOT with your average load.

In my restoration I didn't replace the broken tach yet. Need to do that ($$$). If I need to increase/decrease the RMP's can I do so with the WOT stop screw, by turning it in or out? I hear horror stories about too high RPM, whats the max I should run at? 5800? Thanks a bunch.

 

[dwco5051]

Re: Almost there...Please Help!

Got to agree with ezeke. My daughter had a similiar set up back in the seventies with a 70 Johnson. Best all around performance was with the motor one notch up (No power trim) By lowering it one notch it would take me up on one ski and I weighed about 185 back then, was in much better shape, and skiing 3 or 4 days a week. Up where the boat performed best it just couldn't quite do it without dragging me 50 feet or so fighting the water. We knew a different prop would help for pulling someone on skis but money was a little harder to come by back then. Plus she only weighed about 100 pounds so it wasn't a major problem. Usually the girls went in her boat and us bigger fellows would use my bass boat or my 17 foot IO. That 70 needs all the help it can get. And it is always good to have a spare prop anyway.

 

[ezeke]

Re: Almost there...Please Help!

Should have mentioned this, Replaced 19 prop with a 15 recently. Noticable decrease in top speed."

You can't have everything, but if you want to pull things, lower the pitch. That will give you more thrust and less top end.

WOT on your engine with an average load should reach 5800 RPM so that you don't lug the engine and shorten the life.

What you are doing now is like driving up a mountain in high gear with a loaded truck.

 

[wilde1j]

Re: Almost there...Please Help!

In my restoration I didn't replace the broken tach yet. Need to do that ($$$). If I need to increase/decrease the RMP's can I do so with the WOT stop screw, by turning it in or out? I hear horror stories about too high RPM, whats the max I should run at? 5800? Thanks a bunch.

You don't change the RPM by screwing with the timing, which is what you seem to be saying. There is no magic "WOT RPM screw". You set the timing once, get it right and leave it alone. Any improvements are in the setup (kickout, motor mounting height, prop pitch, etc.).

 

[IRBFishin]

Re: Almost there...Please Help!

You don't change the RPM by screwing with the timing, which is what you seem to be saying. There is no magic "WOT RPM screw". You set the timing once, get it right and leave it alone. Any improvements are in the setup (kickout, motor mounting height, prop pitch, etc.).

I apologize if I'm being dense here. When I hook up my tach and at WOT I'm at 7K RMP's, shouldn't I turn the throttle stop screw in so the throttle stops at the 5800? What then is that upper screw for if RPM's can't be messed with?

Not to hijack my own thread, but I'm working on the tach thing now and I've found three wires coming from the control box ("command center"). One is hot when I turn on the key, positive for the tach. One is grey and actually leads back to the motor, signal for tach. The last wire enters where the first one does but is not hot when I turn on the key. Can someone tell me what this last wire would be for? Thanks again.

 

[wilde1j]

Re: Almost there...Please Help!

Are you saying your in the water RPM is 7000 @ WOT? If so, you need a bunch more prop pitch or your tach is whacked. I would put the 19" pitch prop back on or never let the RPM exceed 5800 for decent motor life, if you're actually going up to 7000 RPM.

Tach typically needs three leads:
switched 12 VDC - purple
ground - black
pulse sense - grey
I've shown current colors, don't know for your motor.

I don't have a shop manual for your motor, so I'd only be guessing about the upper screw. After timing is set properly, idle RPM is the only thing to mess with, and I don't know which it is on your motor. As EZEKE has stated, prop for 5800 RPM @ WOT with a normal load. In any case, top RPM is never adjusted with timing/throttle linkage.

 

[ezeke]

Re: Almost there...Please Help!

I apologize if I'm being dense here. When I hook up my tach and at WOT I'm at 7K RMP's, shouldn't I turn the throttle stop screw in so the throttle stops at the 5800? What then is that upper screw for if RPM's can't be messed with?

Not to hijack my own thread, but I'm working on the tach thing now and I've found three wires coming from the control box ("command center"). One is hot when I turn on the key, positive for the tach. One is grey and actually leads back to the motor, signal for tach. The last wire enters where the first one does but is not hot when I turn on the key. Can someone tell me what this last wire would be for? Thanks again.

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