73' Evinrude 25 HP (25352A) Idle Adjustment

JJBoatr

Petty Officer 2nd Class
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May 3, 2011
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144
Hey all,

I recently rebuilt the carb on my 73' Rude 25hp. It runs how it always did, unstable idle, slight burble when you punch the throttle to get on plain. Is there a "zero" starting point for setting idle mixture and syncing the carb to the timing advance? I've chased this erratic idle up and down with rich and lean settings but can't find the sweat spot. Unfortunately, I do not currently have access to a quality manual for this motor. Thanks for the help.

Josh B.
 

lindy46

Captain
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Nov 27, 2008
Messages
3,886
How did the tip of the slow speed needle look? Did you remove the welch plug on top of the carb and clean/inspect underneath? That is the idle circuit. The orifice where the needle comes through should not be chipped or cracked and the tip of the needle should not be worn and should fit perfectly through the orifice. As far as timing, there is a mark (or 2) on the cam - when the follower roller hits the mark, the carb butterfly should just start to open. Adjust it by either loosening the cam, or by adjusting the carb linkage rod.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Those motors never did idle real well, especially if you have ever had an older model and know how well those ran. It is due to the early attempt at environmental protection. One cylinder runs richer than the other one at idle, making fine adjustment nigh impossible. The "4-stroking" when you punch it is because the crankcase drains were modified / partially eliminated. ​Get it as good as you can and live with it.
 

JJBoatr

Petty Officer 2nd Class
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May 3, 2011
Messages
144
I did remove the welch plug on top and the idle passage looked good and I cleaned it out as well. The slow speed needle looked very good, smooth with no pitting or even varnish from being in there for who knows how long. Both very good points to inspect with this sort of problem so thanks for suggesting them. I noticed another thing while I was running this thing in the barrel yesterday. There is a small clear line that runs from the base of the intake manifold (below carb) back to the top cylinder port cover on the starboard side. The fuel pump is tapped into the cylinder port cover below this line. Anyway, I bring this line up because I noticed that it does not seal tightly on its barb fitting creating a small vacuum leak! I am picking up some line today hoping for a tighter fit but the bard is very small and it will be difficult to find line that small. This line looks as though it is a scavenge of some sort??? Can anyone lend an explanation to it's function?

F_R,

I had a feeling that the adjustability of this carb might be the culprit...I guess I should've just left it alone. Leave it to the EPA to make an engine run like sh*t. Still hoping I can make it run better than it was.
 

flyingscott

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That line scavenges oil from the bottom of the crankcase for the top bearing. 1.5 turns out to get you started for the low speed jet screw. What did you set the float at I set them just a hair beyond level and i let it drop so it hits the bottom of the float bowl.
 
G

Guest

Guest
I have to disagree with FR about the idle on the 73 25s. I have the 73 Johnson 25 and it idles down like a sewing machine. Those 22 cu. in. twins are some of the best idling engines OMC ever made, and I have owned several over the past 30+ years to know. (18's, 20's and 25's)

Here are some things to look at:

Back the needle all the way out if the carb and inspect the tapered end (This seats against the bushing in the carb that regulates mixture . If there is a groove worn in the taper, the needle needs to be replaced. IN addition, you will also need to replace the brass bushing it seats against in the carburetor. This is screwed into the casting and is designed to be removed and replaced when needed. Over the course of the motors life, if anyone cranked down hard on the needle, they galled the taper on the needle and ruined the seat.

Next, the fuel pump is located on the left side of the engine (facing the powerhead). There is a black plastic cap with a nipple on it (fuel inlet) that is retained by a knurled screw that keeps the assembly sandwiched together. Overtime, these black caps can get brittle and crack and can cause an air leak, which can result in a lousy idle. They too, are still available and can be replaced inexpensively.

Check your fuel system from the tank to the motor, you could be pulling air somewhere at the connections on the tank or the at the motor. Does your fuel bulb stay firm when the engine is running?

The idle for these engines are not hard to fix if you know where to look.
 

JJBoatr

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CaptEric,

I hadn't even considered looking at the fuel pump because I rebuilt it 2 years ago, I'll take a look. I had the boat out this weekend after getting the oil scavenge line to seal tightly on its barb. Idle is stable and smooth for about 20 seconds but then idle will slowly drop to a stall. It also has a shudder just off idle while under way. Advance the throttle just a bit more and the shudder goes away and it runs smooth as butter again. There were no lean coughs but several attempts were made to adjust these discrepancies out with the low speed adjustment. I'll inspect the entire fuel system and give results by this coming weekend.

My brothers 62 Johnson 18hp does not have the oil scavenge line and runs just fine. Being the same power head: why couldn't I get rid of this line? I feel like this thing is messing with the intake pulse that is drawing fuel through the reeds and may be the source of the burble mentioned above when going from idle to WOT. Any thoughts?
 

Effer

Cadet
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May 24, 2005
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6
" Idle is stable and smooth for about 20 seconds but then idle will slowly drop to a stall. It also has a shudder just off idle while under way. Advance the throttle just a bit more and the shudder goes away and it runs smooth as butter again."

I have a '75 25 hp that does the same thing and noticed if I push on the linkage arm (between the carb and timing plate) towards the timing plate and hold it there it purrs..... but if you don't hold it there it jumps back and forth causing an irregular idle. What would be causing that? Thnx.
 

JJBoatr

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May 3, 2011
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I haven't tried that but it sounds like your problem could be a worn or improperly tensioned throttle spring that is allowing your throttle to fluctuate from the vibration. That is just a guess though. There may be some others on here that could help.
 

lindy46

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Nov 27, 2008
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See my post #2 above. It tells how to time the carb to the magneto.
 

JJBoatr

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May 3, 2011
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My cam has 2 marks close together. I set it so its starting to open the throttle on the first mark. Is this correct?
 

lindy46

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Try it and see how it does. Spec is it should start to open on the second mark. I try to set it in the middle.
 

JJBoatr

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Thanks. I'll do that and provide feedback after I get it out this weekend.
 

JJBoatr

Petty Officer 2nd Class
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May 3, 2011
Messages
144
Well I shamefully admit that my problem had nothing to do with the carb. It was the points! Cleaned and re-gapped them and now the motor is running like new. I only had to make a small low speed adjustment to make it run smooth at idle while under way. Thanks for all of the effort applied to my question in this thread.
 
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