1977 Johnson Outboard 70 hp bogging down while towing tube

[hatboat]

Hello all, my 70 hp johnson outboard it working great when I'm not pulling anything. However, when I was pulling a tube the other day it bogged right down. I took the tube off and was able to go top speed. I was pulling a 80 pound child so not a big load but it still bogged down. What would cause this to happen? I have a 17 foot bowrider so wondering why this would happen. Any insight would be appreciated. Thanks

 

[fireman57]

Has it happened again? Could have just had a piece of crud in a carb that cleared. Lots of things could have happened but if it happens all the time then someone will have an idea of what to look for.

 

[oldboat1]

Just need to tow lighter kids.... If it happens when trying to accelerate (like a hole shot), it could be some water in the tank -- change in angle might put the water at the tank intake. But could be lots of things, as suggested.

 

[flyingscott]

What pitch prop do you have on it and what are your RPMs wide open. I had that motor and I underpropped it for towing tubes.

 

[emdsapmgr]

You are asking too much from your powerhead. Remember, back in 1977 the 70 hp engine only had 49 cubic inches. You just don't get any torque out of the small powerhead like that. Your best bet is to run two props on the boat. The normal one you use now, and a second prop for pulling skiers/tubers. The second prop should be at least one size lower in pitch. It can be your pulling prop. I use two different props on my boat all the time. Makes it easier on the engine and the skiiers.

 

[hatboat]

Thanks everyone for the feedback. I'll try a lower pitch prop to see how that works. Additionally, I'll put the kids on a diet asap .

 

[hatboat]

One more thing, if my carbs are dirty would that impact the motor going with fewer people in the boat? While towing a tube and having 5 people (2 adults, 3 children) the boat bogged down. When My son and I went on a ride, no problem. Would dirty carbs impact that when I have more people or tow a tube? Thanks

 

[emdsapmgr]

If you can get the engine to run well/normally at WOT with one other person in the boat- it's unlikely a carb problem. At this point, I'd check the compression. Weak compression would definitely change the way the boat runs under load.

 

[David Young]

I agree with emdsapmgr, check your compression. If you don't have a compression gauge, you can rent one for free (after paying a deposit and returning it) from most of your local auto parts store.

 

[flyingscott]

17' bowrider is a heavy boat what pitch prop are you running now a lot of those motors have 17" pitch props on them.I set my 79 70 hp on a 16' sylvan aluminum up with a 15" pitch prop and ventilated it. . You have a lot of weight in the boat and what kind of tube are you pulling? If you are pulling a cheap tube with an open hole in the middle forget about it. Seems like you are asking a lot of that motor.

 

[hatboat]

Thanks for the feedback! I now have a 17" prop on it. The tube has a closed bottom, but you're right there is a lot of weight when towing it. The last time I checked the compression it was 135, 132, 128 respectively. When I had the 5 people, it bogged down completely and wouldn't get off plane...as soon as I got rid of 3 people (no I didn't throw them in the drink) it planed easily and quickly. Thoughts?

 

[hatboat]

Also, I've got a battery that doesn't hold a charge for too long. Would a weak battery cause bogging down?

 

[oldboat1]

No offense intended, but think you've been using that phrase "bogging down" for over a year, and still not clear what you mean -- and you are still struggling to correct it. To me it means the hull wallows -- rpms stay constant, or nearly constant, but hull speed slows. It's like driving a carbureted car in the mountains -- pedal to the metal, runs smooth, but no accleration. Not missing, stalling, etc. In your context, it means to me the motor is just underpowered. Repropping could make some difference, but probably not worth the effort. (Exception would be if the motor is lugging -- would go to a lower pitch, as suggested.) Venting can sometimes work miracles (or getting a vented prop), but don't think venting would do it for you either.

So, what is this "bogging down?"

 

[hatboat]

No offense taken and thanks for responding. By bogging down I mean, when I hit the throttle wide open, the boat moves but does open up. Just seems to create a lot wake and go slow. Other times, I hit the throttle and it planes and goes quickly. So basically, when I want to go fast it doesn't go fast at all. Hope this clears up things . Thanks again.

 

[fireman57]

5 people and pulling a tube. No way that engine can do all that. I have a 1976 70hp on a 16ft boat and it will run 30 with three people in the boat but I can't pull an adult skier and get over 23mph or so. Planing takes time too unless I have them move to the front until I get on plane.Tubes have a ton of drag. Repropping will help but you will lose some top end. I like the suggestion of two props. When you find one that will pull the tube with a 3 or 4 people on board put it on. When just cruising change to your other. Takes all of 5 minutes. Keep an extra castle nut and cotter pin handy though.

 

[hatboat]

Thanks, I agree with that, but I meant to say that I had 5 people on the boat as one scenario and pulling the tube as another, not doing both at the same time.

 

[oldboat1]

Yeah, think it's just underpowered for some uses. It sounds like you continue to get your money's worth out of it though, and maybe switching out props would help some.

Might check out the charging system (battery voltage 13-14v when running). I'm pretty sure it's only a 6amp system, so might be a little stressed if you have a lot of electronics -- want to keep up cranking speed for dependable starting. Maybe the battery is just past it's time.

 

[hatboat]

Thanks for the feedback. I think I'll try a new battery and get realistic about what the boat can do . Thanks again!

 

[flyingscott]

Look into the venting of the prop dealer can help you with that really helps with holeshot. There is a service bulletin for doing that for that motor.

 

[hatboat]

Ok, will do. Thanks for that. I'll get that done.