'89 J70 runs fine wide open...for a few minutes, then not so much

Lord Tywin

Petty Officer 3rd Class
Joined
May 16, 2017
Messages
80
If I run this around 70% throttle it will run all day. If I push it to full throttle it runs fine for about 5 minutes or so. Then it acts like it drops a cylinder. It will "fast idle" back home or if I turn it off for ~5 minutes it goes back to normal. One thought was some trash in a carb but I rebuilt all 3 and not only were they clean already, it still does this. I have an actual service manual and can do almost anything needed, I just don't know even where to start diagnosing it. Some kind of ignition problem?

Any ideas welcomed!
 

flyingscott

Fleet Admiral
Joined
Apr 8, 2014
Messages
8,095
Try pumping the primer ball when that happens. If that has no effect try pushing the key in when it happens.
 

Lord Tywin

Petty Officer 3rd Class
Joined
May 16, 2017
Messages
80
Will test with another tank if I can find one. Mine is steel. I will also try pumping the bulb/pushing the key and see what the effect is. Thanks!
 

fireman57

Captain
Joined
Aug 24, 2004
Messages
3,811
Make sure the tank vent is clear. Sounds like it is not venting properly. You can also loosen the cap
 

Lord Tywin

Petty Officer 3rd Class
Joined
May 16, 2017
Messages
80
Will check that as well...seems most of the opinions is fuel starvation. I will investigate that fully, thanks!
 

airman1094

Petty Officer 2nd Class
Joined
Mar 30, 2013
Messages
108
What kind of fuel line do you have? Also you said steel tank. I have an old steel tank on my I/O. I rebuilt the carb 4 years ago roughly. I never took it out after that. Ran fine up to 3/4 throttle on muffs but I wouldnt push it any further with no load on engine. Took it to the lake. Ran fine up to half throttle but would act fuel starved at any higher. Checked the carb fuel filter and it was plugged with sediment from the tank...in short order. So tank cleaning is in order but for now I added an extra in line fuel filter that's easier to spot crud buildup...solved my problem. Seems it would allow enough fuel to get through for up to 1/2 throttle but was restricting enough fuel for more throttle. Hope that helps... would like to hear if new tank fixed it.
 

JerEazy

Petty Officer 1st Class
Joined
Jul 26, 2017
Messages
234
its a stretch - but my concern is it going into limp mode/slow mode/stop running me mode. It would explain the 5 minutes of time off and then running normal again. The engine cools down or whatever issue is causing it returns to normal. Fuel starvation doesn't really solve itself while the boat is sitting with the engine off.

Do what Racer posted - run on a different tank and make sure it's vented properly. But test and make sure your alarms all work too (just in case).
 

Lord Tywin

Petty Officer 3rd Class
Joined
May 16, 2017
Messages
80
So we took off the VRO pump and put a new fuel pump on and started mixing the gas. I have MUCH less smoke now as the VRO pump was not working right. Didn't change anything. SLOW mode and overheating/bad sensor may be the right path. I bought a new coil to try and see if a new one fixes the issue.
Odd thing is that the water coming out isnt even warm. It's lake temp and the engine feel cool to the touch. If that's the case it could be a faulty sensor. Will keep digging. Tried it with a different tank but it made no difference.
 

aspaziani

Recruit
Joined
Jun 19, 2012
Messages
4
i have a similar issue. 2001 70hp johnson, rebuilt the carbs earlier this year, new plugs, new gas line with bulb (tank to motor), new gas. idles great, i took it out and it ran great ranging from idle to WOT. after about 5 minutes at WOT it suddenly dropped down to about 75% throttle, still ran but just not as strong. I dropped it down to idle and it died, finally got it started back up after some priming and it would only just idle in gear, if i gave it any gas it would die. Was able to limb back to the dock very slowly.
 

Lord Tywin

Petty Officer 3rd Class
Joined
May 16, 2017
Messages
80
we finally figured it out thanks to the help here. It was the overheat sensor. We ran it WOT until it did it, unplugged the sensor and it stopped doing this. Was able to run it WOT with no problem. Hooked up sensor and the issue returned. Unplug, quick off and on switch and it ran fine. Now going to replace the sensor and hopefully call it a day, errr...summer!
 
Top