'92 M25C2 WOT is not as high as before

[LukeFishWalker]

When I first got this used outboard it would run in the 5100 range with 12p wheel.
Then, after using it with very little problems I started noticing that at WOT it would
sometimes drop to 4800 for 15 - 20 seconds and go back to 5100.
Now it stays at 4800 even after switching to an 11P wheel.
Compression is still in the 115/120psi range, timing is correct 25BTDC at WOT,
no air in the fuel lines, new fuel pump and it delivers a good stream when jerked
off the barb while running, cleaned the carb and it was spotless.
However, upon disassembly I failed to screw in the air/fuel jet to determine the
exact revolutions of the air/fuel jet and I now wonder if I have it set properly.
When at idle, there is a pretty fair amount of degrees of turning both ways
(Guessing 270degrees) before engine idle is affected.
Should the air/fuel jet be set more towards closed or open?

Thank You For Your Time & Patience,
Lukus

 

[Sea Rider]

Air/fuel mixture screw doesn't affect wot rpm, it's just for idle/ralenti adjutment at neutral. Have to set screw till OB revs evenly doesn't matter if slightly at higher revs, from there adjust with vertical screw till revs lowers to around 900-1000 rpm. That OB likes running bit rich at idle. Carb adjustments must be performed with OB well warmed up and on water, not barrel or muffs which gives different readings.

Anyway at 4800-5100 wot rpm, OB is at the lugging side which is bad. Wot range is 5200-5850. Does carb opens fully when timing marks stops at 25 deg ? Check props, if old or beaten could be on their way to become kaputt (spun) Perform a hub slip test on both props to rule that out.

Are spark plugs in good working order ? Run some spray decarbonizer as Engine Tuner or equivalent product through carb's throat, follow can instructions. Most times regains lost agility.

Happy Boating

 

[LukeFishWalker]

Hey Sea Rider, long time no sea.
Yeah, the plugs are new and the old ones (1 year old) were sorta black but not damp.
Yes, the butterfly is at 90 degrees with the throttle wide open at 25BTDC.
The 2 props (11P & 12P) are perfect and neither prop makes a difference in RPM.
In fact, the 11p is brand new right out the box.
When I first got the OB it came with the 12P and it scooted nicely.
Then, after 6 months it started to vary at WOT from 5100 - 4800 and back to 5100.
It would run fine for 8 minutes then dip to 4800 for about 15 - 20 seconds and back to 5100.
Now it just stays at 4800 and there's no real reason, meaning no additional weight.

The first time it happened I took the carb off to rule out the possibility of stuck float or
clogged jet. Everything appeared to be just fine, in fact it was almost perfect.

When you say: "That OB likes to run rich at idle" I guess that means the air/fuel
screw needs to be more to the open side?

I am still using the Tohatsu TC3 oil, I know it's a long shot but could that be an issue?

I never use the ethanol fuel, I always use the non-ethanol fuel from the same place.
That way I know where to go if there's ever any water, so far so good.

Thanks For Your Time & Patience,
Lukus

 

[pvanv]

Lukus, It takes patience, a good ear, and a very good shop tach to set pilot screw mixture. Once you have the idle stop screw at the correct idle (motor on boat in water), turn the pilot screw in (lean) until speed is lost. Then back out until speed is lost. The middle area is the sweet spot. Recheck idle stop. Go through the idle stop and mixture resetting a couple of times until you are confident that you have the best mixture at the correct idle speed. Then tun counterclockwise (rich) about a quarter turn for stability.

WOT is 4800-5500 for that motor. I would prefer it top out over 5000.

In addition to checking timing: Run the TC-W3 oil at 50:1. Too much oil will slow the WOT RPM. Check that the crankcase recirculation tube is in good condition. If all that checks out, you may want to whip out the meters and start testing the ignition components.

 

[Sea Rider]

Lukus,

Have you modified that 25 to a 30 HP ? if already at 25? with carb fully opened at 180?, change carb gasket for the 30 HP model to unrestrict air flow to carb and pass it to a 30 HP. A friend of mine with a 30 HP low hours used, started doing same, changed prop from a 13 pitch to a 11 one, tach doesn't want to rev higher than 5200 wot revs with a light load, which is strange, have ordered a new induction tach to check current one. But his OB wasn't hesitating as yours.

Troubleshooting electrical parts when having hesitation issues is the worst of all, as the electrical component is still alive in comparison to testing dead ones. Check if metal contacts inside both plug caps are in shinny state, if corroded, darkened could produce electrical hesitating issues.

Is fuel hose internal O'ring connectors specially the one located at fuel line entrance is in good condition ? could be dried, worn letting air in and OB to miss/hesitate. Try a used fuel hose in good working order to rule that out.

Happy Boating