Yamaha 25 HP 2 Stroke Idle Problems - UGH!

baylazr

Recruit
Joined
Dec 19, 2014
Messages
4
I have a 1997 Yamaha 25 HP (MSHV) outboard. I've owned it a few years, but have never been able to get a satisfactory idle. In fact, it's just been a "cantankurous" SOB since I got it (probably the reason the previous owner sold it). Anyway, I've been through the carb (it only has one) several times, soaked it, poked it, sprayed it and blown it... I've also gone through and cleaned the fuel pump and filter, replaced spark plugs and endlessly tweaked the air mixture screw. It also has a clean tank and hose filled with fresh ethanol free marine fuel. Bottom line... I'm stumped and really sick of fighting this beast. Any suggestions?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
You have 4 threads now, yes the website still has some issues.

Have you checked compression?

Forgot :welcome: to iboats
 
Last edited:

baylazr

Recruit
Joined
Dec 19, 2014
Messages
4
Thanks AllDodge - yes, I struggled with the site not working earlier! No, I haven't checked compression - not exactly sure how to do that (buy a gauge and test it in the spark plug holes?). Does compression affect idle? The engine has plenty of power at high speed, so I "assumed" compression was good. Nonetheless, this is the kind of help I need!
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
How does it act at idle Does it stumble when to push the throttle? Does it just quit if you idle too low?
Does it seem to gradually load up eventually stalling?
Typical idle rpm is 700 in gear If your trying to idle slower than that it will be difficult.
Carbed motors usually have a "drain"that lets out excess fuel from crankcase usually vented through the intake system
relatively harmless.If that isn't working right I might make it difficult to idle do to excessive fuel in the crank case or too much fuel passing through
the intake.
 

baylazr

Recruit
Joined
Dec 19, 2014
Messages
4
Thanks steelespike. When the engine is cold, I usually have to keep revving (and sometimes rapidly pulling out the choke and pushing it back in) to keep it running. One it is warm, it will accelerate from idle without bogging or stalling. I don't have an RPM gauge, but have a good idea that if I keep the RPMs over 1000 it will idle, maybe a cough every once in a while, but it will recover on it's own. When I adjust the idle screw down to a more normal range (around 700 RPM - in neutral), it will steadily lose RPM and quickly die.

I've never heard of the drain you mentioned, but will check into that more closely. Anybody else know where this drain might be located on this motor?

Sputter, sputter, cough - aghhh...
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
yes compression make a big difference in how well a motor runs, so you can get a loaner tool at most auto part stores and pull all plugs to test all cylinders.
Make sure you pull the kill switch so spark will be killed while doing it.

But sounds like you need to clean the carbs and then reset everything back to where it should be now that you have been jacking with it all.

Follow the service manual steps to sync and link procedure
 

perkdp

Petty Officer 1st Class
Joined
Sep 25, 2002
Messages
237
09 25 ESH similar problem. flat gasket under the plate on top of bottom carb had deteriorated and half coverd the low speed flow. installed a new gasket. reset all the timing, spark advance and idle stops. problems seem to be solved.
 

baylazr

Recruit
Joined
Dec 19, 2014
Messages
4
Thanks perkdp. It sounds like we have different setups - mine has only 1 carb (and no flat plate).

My motor usually cold starts on the third pull, but then "bogs" as I'm trying to warm it up. This leaves me revving and intermittently choking it to keep it running while warming it up. When I can finally get into open water and give full throttle it will cough and sputter, finally "blow it out", then runs decent after that. Generally, it just runs terrible until running at high rpm for a while, then it will run well until it cools down again.

I put new spark plugs in, but it acts as if they're fouled (?). It misses at low rpm until warmed up (and some even then). I'm wondering about timing, spark and all of that. Anybody know if the timing is adjustable / how to check and set it?

Thanks all.
 

baylazr

Recruit
Joined
Dec 19, 2014
Messages
4
Checked compression - 125 psi in one cylinder and 122 psi in the other. I'm fighting this monster harder than ever. It wouldn't start at the launch this weekend, so the problem has gotten worse.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
So you finally did the compression test and that looks good.
now you know the cylinders are sealing properly.
Now spark test with a spark tester and check timing with a timing light to make sure it is in spec
If all that is good then you are left with a fuel problem
 
Top