1989 Yamaha 70HP "bogs" down at low RPM

TelSupply

Cadet
Joined
Nov 17, 2009
Messages
13
My 1989 Yamaha 70HP ETLF just started bogging down momentarily at low rpm. When I enter a "no wake" section and back of the throttle/power it is fine for about a 100 yards (a few minutes) then it just sounds like it gets no gas, it doesn't misfire or have any other obvious motor issue when it happens. If I back off the throttle to neutral and give it a slight rev, then return to forward I can go right back to the same rpm/speed for another few minutes before it happens again and again.

Things you should know:

1- it is inconsistent in the time it takes to act like this, could be a minute sometimes and 5 minutes another time.

2- i worked on it this week, cooling system had a gunk clog that i flushed out.

3- when working on the cooling system i found it DOES NOT have a thermostat in it. i have a brand new thermostat to put in, but do I need it? I have had the boat for 6 months and it has run fine without it. However, is it running cold? Thoughts here?

4- when wake boarding it now "appears" to be a bit sluggish for just a moment when i "hit it" to start and pull the kids out of the water, almost like it waits to build up some rpm, this was not the case a few weeks ago when it just jumped out.

5- at high rpm everything seems to be fine.

6- it's so old and was so cheap no gauges or alarms work at all... :)

You may be able to hear the symptom here if you turn your volume up:

http://www.youtube.com/watch?v=fcjPutCqxvk
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,880
Re: 1989 Yamaha 70HP "bogs" down at low RPM

Yes, the motor needs the thermostat to circulate water properly and to run properly.

If the motor has good compression and spark, you can check to see what the timing is doing to make sure that is not an issue.

Most often there are fuel related problems, so check your fuel pump for pin holes in the diaphragm.

If no problem found then pull and clean the carbs.
A manual is well worth the money to help you along on your maintenance and repairs
 

TelSupply

Cadet
Joined
Nov 17, 2009
Messages
13
Re: 1989 Yamaha 70HP "bogs" down at low RPM

I added a thermostat which probably was not my problem, and changed all 3 spark plugs. Initially there was LOTS of smoke, then it sooooo smooth. I plan to check them more regularly, particularly the bottom plug which looked fouled. I have a concern the cylinders may be getting too much oil.
 
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