Old Evinrude 70 idling far too fast

dpround · 17 November 2014, 10:43

I have an old Evinrude 70 that was bought as "hopefully OK". I finally got it on the boat and tested it yesterday and it ran nicely, but idled far too fast. I am pretty pleased it ran at all and stunned how easily it started, but not sure what to do about the idle. I can't find an idle adjust screw on the carbs. Manually closing the throttle linkage so no chance of friction, definitely fully closed, etc doesn't help. The engine manual I have suggested either fuel pressure or air leakage problems. It seems to be running nicely, so would guess fuel/air mix is OK, which may mean neither excess air or fuel?

When I disconnected the fuel line to drain the carbs it ran as normal, sped up a bit as fuel running out (getting lean) and then ran at a nice idle to a while, before speeding up a bit and stopping! The engine is an inline 3 with one carb/cylinder. Carbs are very basic. Anyone got any ideas?

Many thanks

David

sbunker · 17 November 2014, 11:36

What age is the engine? If you are not sure then let me know the model number and I can look it up.

dpround · 17 November 2014, 12:12

I've managed to leave the bit of paper behind with the model and serial number of it... I'll post it later. It is probably 1980s

All the best

David

m chappelow · 19 November 2014, 07:14

Did you run it on muffs or in water ?

dpround · 19 November 2014, 09:55

I ran it with muffs. I am glad that I didn't take it to the water or I would have been tempted to put it in gear, which would probably not have been a good idea at that RPM!

Thinking about it there *must* be a way of adjusting the idle speed. It was getting dark and so perhaps I missed it. I'll have another go this weekend. Working hypothesis is that it was idling badly, so the previous owner increased the idle speed while messing with it. I have dry stored it for 12 months so may have fixed some types of fault. The carbs are really crude, single barrel, fixed idle jet and single main jet which means the idle must be controlled by a stop on the main throttle butterfly. I'll have a look in better light. What with scouts committee meetings and scouts, I haven't had time to look at it yet since the weekend.

All the best

David

m chappelow · 20 November 2014, 03:07

Try it in water or in a large barrel without the muffs ,, once the exhaust is deep
underwater back pressure will cause the revs to drop slightly.
I have seen loads of people setting up engines on muffs in the yard getting the tickover just right only for them to dip the boat in the water then engine revs are then too low .

9D280 · 20 November 2014, 09:03

Quote:

Originally Posted by m chappelow

Try it in water or in a large barrel without the muffs ,, once the exhaust is deep
underwater back pressure will cause the revs to drop slightly.
I have seen loads of people setting up engines on muffs in the yard getting the tickover just right only for them to dip the boat in the water then engine revs are then too low .

...or stall!

dpround · 24 November 2014, 14:40

I figured out my problem when I looked at it in daylight. I had the cable length for the throttle wrong. The carbs were completely closed, but if you continue closing the throttle, the linkage then changes the timing which reduces the RPM to a sensible level. I have never seen anything like this before, but have no real experience with magneto engines. Thanks for all the help.

David