1975 Evinrude 70hp (70573b)

woofer519

Cadet
Joined
Jan 26, 2014
Messages
6
hey!
Just got a 15ft trihaul with a Evinrude 1975 70HP (70573B) motor on it and i have a few question.

they guy i got it from replaced the lower unit last year. Now i dont know a whole lot about boats (this is the first one ive ever owned, nore has anyone in my life ever owned one ) however, i can read. and im fairly confident that there should be water venting from the two holes at the back of the motor. now one would believe if someone replaced the lower unit, they would replace the impeller. however, when the motor runns its bone dry back there, just fumes.

is this normal?
or does the boat need to be in gear?

my common sence tells me the impeller is fried and needs replacing. submerged the unit fully and nothing. also ran the motor while checking to insure it didnt overheat.

now that being said. after running the motor in a drum, i get some serious black oil in the water. tank is mixed at 50-1. motor runns like a dream stars right up. if the lower unit was replaced and never ran, could it be grease or something?

thanks guys
 

woofer519

Cadet
Joined
Jan 26, 2014
Messages
6
Re: 1975 Evinrude 70hp (70573b)

also .PDF manual wouldnt hurt for when i rip this sucker apart (if it exists)
 

eavega

Lieutenant
Joined
Apr 29, 2008
Messages
1,377
Re: 1975 Evinrude 70hp (70573b)

Welcome to iBoats! If you suspect the impeller, drop the lower and inspect it. Its really a simple procedure on those motors. The only tricky part is getting the shift linkage off. Just requires a little bit of patience. My trick was to remove the airbox, then the shift linkage was more accessible. If you don't know the history of that impeller beyond the previous owner having replaced the entire lower unit, I would take the time to inspect it and probably just change it out for peace of mind. Oh, since I see you don't have the manual; if you attempt to drop the lower, don't forget to undo the bolt that is hidden under the trim tab.

You may also want to verify that the thermostat is functional. Testing procedure there is to put the t-stat into water 150 degrees, and verify that it opens up. Then cool the water to below 130 degrees and verify that it closes up again.

The water coming out of the exhaust relief ports is really more droplets or sputtering water than a stream. Your best bet if you are running it in a barrel or on muffs is to check the head temperature. If you can't hold your hand on the head for a three-count you are overheating.
Better yet, get a non-contact thermometer and make sure its running at under 160 degrees.

If you are getting an excessive amount of black oil in the barrel, you may want to do a compression and spark test. you may have a cylinder that is not firing and is just dumping unburnt fuel back into the exhaust. Confirm that you have good spark on all three cylinders with a spark tester set to jump a 7/16" air gap. Make sure you are getting a nice blue spark. also confirm your compression is adequate and within 10% on all three cylinders. Those motors will run just fine on two cylinders as long as they are not under load.

When you do get everything running well, you may want to switch from running in a barrel to running on muffs. that prop will kick up a lot of water when engaged, which will empty your barrel in short order.

Good luck.

Rgds


You definitely want to get an OEM service manual for this motor. They are fairly easy to work on, but you need the manual so that you don't miss any steps. Those materials are copyrighted, so they don't exist in any free format on the web (legally)
 

wrmdunker

Petty Officer 2nd Class
Joined
Sep 17, 2010
Messages
170
Re: 1975 Evinrude 70hp (70573b)

X2 on what eavega said. I have owed the '74 70 hp for 20 years. Rule of thumb is always replace the impeller on a new to you motor. I learned some lessons on cheap oil that doesn't burn well. It also likely will not burn well at low rpm in a tank. I now use Evinrude XD50. Low ash synthetic. Even then, some leakage from the hub is expected.

You may want to look at the Top Secret Files at the top of this forum for a de-carb on the motor.

Check eBay for used OEM manuals.
 
Last edited:
Top