1991 70hp johnson dies after 30seconds at full throttle

[Steveng]

Gentlemen please help!!! I will try not to make this too long. I purchase a used boat with a 70hp Johnson and am having the following problem(s).

I start the engine and let it warm up. The idle is a little low, but i can still get it into gear without stalling it. After cruising for a minute and going to full throttle, the engine comes up to speed, but after 20-30 seconds in starts to make a ticking sound that starts lightly and gets louder and then the engine completely dies and then it's a bear to start and will not run. I choke it and can get it to start, but it will not idle and I cannot get it into gear without it stalling. After a few minutes I can coax it into gear but it's hard to do. Even then its not running correct. I have done the following repairs and am at the end of my experience with 2 cycles.

1. Rebuilt carbs
2. Replace the ignition module
3. Replaced the fuel and fuel tank (had crack in pick up tube)
4. Changed plugs
5. Checked for fuel leaks ( none found )

I was wondering if it's the fuel/oil pump (expensive) making the ticking noise and then causing it to malfunction and die (until it cools down) or if there is something simple I am missing.

Please ask any questions you have and I will respond asap. I just don't want to keep throwing parts at it as this can get very expensive.

Thanks in advance to all that respond.

Signed,

A new boat owner,
Steve

 

[fisherman912]

Re: 1991 70hp johnson dies after 30seconds at full throttle

Based on your info posted, your carbs should be clean if they were done properly. I would check for the following:

Lean carb adjustment, restriction in the fuel system. Have you changed the fuel filter? Check the filter screen on the fuel pump. Is your primer bulb collapsing starving the engine of fuel ? Is your tank vented ?

Fuel pump may be weak.

 

[Steveng]

Re: 1991 70hp johnson dies after 30seconds at full throttle

The carbs should be ok. Haven't changed fuel filter, but easy to blow through. I'm not sure how to adjust lean carb adjustment other than the idle adjustment screw on the starboard side of the carb. I also don't know where the filter screen is located on the fuel pump. The primer bulb requires only a slight pump to get it back to firm. The vent on the gas tank is open. How do I test for a weak fuel pump?
Steve

 

[Steveng]

Re: 1991 70hp johnson dies after 30seconds at full throttle

I also forgot to mention that it is a VRO2 fuel/oil pump. Steve

 

[fisherman912]

Re: 1991 70hp johnson dies after 30seconds at full throttle

To test fuel pressure on VRO: Disconnect the fuel hose on the outlet side. Install a 5/16 tee connector and re connect the line. Attach a 0-15 psi guage to the tee leg. Start the engine, run at 800 rpm in gear and observe the guage. The guage should read no less than 3 psi. If no pressure or low pressure, replace the VRO pump or better yet, discard it and install a regular fuel pump and pre-mix your oil manually.

 

[kenmyfam]

Re: 1991 70hp johnson dies after 30seconds at full throttle

Have someone pump the primer bulb and see if it goes away.

 

[Steveng]

Re: 1991 70hp johnson dies after 30seconds at full throttle

I will try all that everyone has suggested. If problen goes away when someone is pumping the fuel bulb, what doews that indicate?
Steve

 

[patrick4266]

Re: 1991 70hp johnson dies after 30seconds at full throttle

It would indicate a bad fuel pump. If it were mine I would take out the vro pump, install a regular fuel pump and use pre mix.

 

[Steveng]

Re: 1991 70hp johnson dies after 30seconds at full throttle

I looked up the regular fuel pump and found it to be almost as expensive as the VRO2 pump. What would I gain in doing that. Are they prone to going out often? Also I was wondering, after teh engone cools down it fires right back up and runs good until I take on the water and once I come up top full throttle for 30 seconds, it dies and then its hard to get going again until it cools down. Could it be an electrical issue?
Steve

 

[marvxyz1]

Re: 1991 70hp johnson dies after 30seconds at full throttle

Sounds electrical to me

 

[kenmyfam]

Re: 1991 70hp johnson dies after 30seconds at full throttle

I looked up the regular fuel pump and found it to be almost as expensive as the VRO2 pump. What would I gain in doing that. Are they prone to going out often? Also I was wondering, after teh engone cools down it fires right back up and runs good until I take on the water and once I come up top full throttle for 30 seconds, it dies and then its hard to get going again until it cools down. Could it be an electrical issue?
Steve

Pre-mix pump should be available for under $100 OEM part as well.

 

[Sixmark]

Re: 1991 70hp johnson dies after 30seconds at full throttle

So let me guess, you run it, then it starts to lose power and making this "ticking" sound you have mentioned, then it dies.

When you attempt to restart, it turns over extremely slow....if at all, then after sitting and cooling off for awhile all seems fine

You are probably going to find that the motor is overheating, the ticking sound you hear is actually a pre-detonation in the cylinders.

The hard starting is because everything is extremely hot and attempting to weld itself together.

I would suggest checking for proper water flow , and proper operating temperature, overheat alarms ARE NOT failsafe so if you have been using that as a guide to proper operating temperature then don't.

The unfortunate side to all of this is that if you have gotten it hot enough, often enough, you may have done some damage to it.

Start with the basics, it doesn't cost anything to check for proper water flow and engine temp!!!

 

[Steveng]

Re: 1991 70hp johnson dies after 30seconds at full throttle

I just had the water pump replaced and there is a solid stream of water coming out of the pee hole. I have touched the motor and the heads do not seem to be too hot. But I guess that I could remove the side cover and do an inspection to see if there is a scale build up inside around what ever is under that cover. I'm thinking of disconnecting the oil injection and going with pre-mix. Could it be that the engine is noy getting enough oil and starts to get hot?
Steve

 

[fisherman912]

Re: 1991 70hp johnson dies after 30seconds at full throttle

If you think it is not getting enough oil, pre-mix some in your fuel tank before disconnecting the VRO pump. If it helps or the problem goes away, you know
what to do next.

 

[Steveng]

Re: 1991 70hp johnson dies after 30seconds at full throttle

To all that sent suggestions,
I may have found the problem from the comments made in regards to the VRO2 oil pump. I decided to disconnect the oil line from the motor and see if the line was plugged. Low and behold, instead of oil coming out, water came out. All in all it spit out apprx. 4 -6 oz. of water befopre it cleared to just oil. My therory is that as long as the boat was sitting level it sucked oil. When it went up on plane it threw the oil back far enough that the water hit the pick up tube and sent water into the carbs killing the spark and thus the engine. I have cleaned out the oil tank, put in new oil and pumped up the bulb on the oil tank. I will let everyone know how this turns out. I just hope that no major damage was done to the cylinders. Cross your fingers for me. Again thanks to all. What a great bunch of people and a terrific forum.
Steve

 

[dmoriarty51]

Re: 1991 70hp johnson dies after 30seconds at full throttle

sounds like what my 70 was doing, and it cost me 1500 to get rebuilt when it blew itself up....

vro was given a clean bill of health and i was sent on my way. with my 28 gallon tank full of premix to finish break in..

fast forward 3 weekends, that 28 gallons is mostly gone and i fill it with plain gas... guess what at higher speeds after about 10mins it did the same as you describe yours doing,

which is exactly what it did before it grenaded a piston....i got no alarms, no warnings no nothing, just dry ticking sounds that end in the motor dying and barely turning over and restarting.

i have now ditched vro, and it ran for over 2 hours at all speeds and never skipped a beat.

i wish i had done it right away when i bought it, and i would prolly have saved $1500