# 99 25hp Merc Primer knob



## richwalker71 (Feb 11, 2010)

This is the first motor I have owned with this knob. Read the Merc manual as far as using it (starting procedure). Is it basically, pull to choke and a fast idle when you turn it open (clockwise)? I assume this is to help the engine warm up, right? Do i need to turn it back down before opening up the throttle?
Thanks,
TRW


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## mark_gardner (Mar 18, 2009)

not really, all that knob does is bump the idle up a hair. it's possible that motor could use an idle adjustment anyways


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## love2flyfish (Feb 25, 2007)

> This is the first motor I have owned with this knob. Read the Merc manual as far as using it (starting procedure). Is it basically, pull to choke and a fast idle when you turn it open (clockwise)? I assume this is to help the engine warm up, right? Do i need to turn it back down before opening up the throttle?
> Thanks,
> TRW


Like SB said, its just adjusts the throttle a hair. I would say if it sounds like its idling high, turn it back down. Also be aware that the choke on that motor is a pump to choke, if you look closer, all the choke knob does is push down a built in primer pump.

Good luck with your motor!


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## richwalker71 (Feb 11, 2010)

Pretty much what I thought. Thanks guys.
TRW


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## Un-shore (Sep 27, 2007)

Wait. comeback!

I'm not near my motor to check but I think it also advances the timing. I don't know if it would damage the motor running it like that but you never know


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## mark_gardner (Mar 18, 2009)

> Wait. comeback!
> 
> I'm not near my motor to check but I think it also advances the timing. I don't know if it would damage the motor running it like that but you never know


 that advanced timing rod is a joke , i've run the piss out of these motors without one ( timing advance linkage) if your motor sounds like its wanting to die out while idleing on the water hose just bump the adjustment screw under the cowling and you'll be good to go


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## richwalker71 (Feb 11, 2010)

The only problem I've had is the motor will "bog down" and die every once in a while when I open the throttle quickly. Seem like it floods out. It seems like this happens less frequently if the knob is "open" part way. Other than that it runs like a top.
Someone else suggested this could be a timing issue. I think it's fuel/carb related. Any suggestions?

TRW


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## noeettica (Sep 23, 2007)

WHILE WERE AT it how in the %^&* do you set the timing and why do these motors buck just above idle ... Dave


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## mark_gardner (Mar 18, 2009)

never messed with the timing on mine as i never needed to but that timing advance linkage has a lot of slop to it so i cant really say whether they're really all that effective or not :-/ mine will bog a little sometimes also just feed it slow and steady till she perks up then lite your candles


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## Weedy (Dec 11, 2006)

I lost that timming advance linkage on mine (05 motor), she still will run without it!!!


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## richwalker71 (Feb 11, 2010)

Think I figured out the bogging problem: My exhaust dips below the water coming out of the hole. The extra back pressure is causing the problem. If I trim the motor up a bit, she spins right up. 

PS- Found a new school of fish during testing this afternoon


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## WannabeDeepSea (Jul 12, 2008)

> Think I figured out the bogging problem: My exhaust dips below the water coming out of the hole. The extra back pressure is causing the problem. If I trim the motor up a bit, she spins right up.
> 
> PS- Found a new school of fish during testing this afternoon


Not that it is...but it could also be running a little rich. If you have fixed your issues ignore, if it continues to be a problem then:

There is a fuel/ air mixture screw located on the carb in the front of the motor. It is the one located above the primer knob by maybe 2" and has it's own access hole through the air shield. Anyway, if you are idling rough or bogging, it might need to have an adjustment(please only adjust if necessary- a motor that runs rich is better than one that runs lean when it comes to the life of the motor).

Adjustment:
After proper idle speed adjustment. Start the engine, put it in gear at idle. Adjust the mixture screw counterclockwise until it misfires(due to being too rich). Then count the turns while adjusting it clockwise till it once again misfires(this time too lean). Divide the turns you counted by 2 and turn the screw counterclockwise to find the happy medium. Too rich is better than too lean.

I am not an outboard mechanic and do not claim to be, but having the same issue with a 25hp carb I installed on my 20hp, this cleared it up. The information was a summary of what was written in a manual I used to tune the new carb- worked great.


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## richwalker71 (Feb 11, 2010)

Thanks. Plan on fishing pretty hard this weekend so if the problem persists I may give that a shot. 
TRW


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## mark_gardner (Mar 18, 2009)

agreed, thats good info.... thanks


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## WannabeDeepSea (Jul 12, 2008)

One quick note I forgot, be sure to run the motor a little to warm it up to full operating temp before adjusting, otherwise it will not be an accurate adjustment.


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## Un-shore (Sep 27, 2007)

A little late to the dance, but it should normally start with the exhauast lowered so it is indicating a small adjustment is needed.


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## richwalker71 (Feb 11, 2010)

*Re: 99 25hp Merc Primer knob UPDATE*

Went ahead and leaned her out a tiny bit. Results were no more bogging in the hole shot, smoother idle, and I swear it starts even easier than before. Thanks for the suggestion.


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## mark_gardner (Mar 18, 2009)

so how abouts did you get to running lean? did you just mix the oil lean or an adjustment on the carb ? the leaner you run it the hotter she's gonna run just dont want to scorch a piston  but if you can find her sweet spot you should be good to go


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## cutrunner (Jun 8, 2010)

lean is mean. give it all the timing you can  if everythings under control, your going to slow. Mario Andretti


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## richwalker71 (Feb 11, 2010)

I did the adjustment suggested by Wannabe'. Wound up with the adjustment screw about an 1/8 of a turn closer to the lean side. Still running standard mix on the oil/gas. 

I'm no mechanic so now you guys got me worried. :-/
Am I in danger of damaging the motor?


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## cutrunner (Jun 8, 2010)

na your fine. if it starts detonating then you have a problem


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## mark_gardner (Mar 18, 2009)

i'm sure your fine however running a motor too lean can do damge, i doubt anything you've done will be a problem


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## WannabeDeepSea (Jul 12, 2008)

It is a matter of what you call lean.  There are two different references I have heard from folks.  

-You can be "lean" or "rich" on oil to gas mixture that you are responsible for.  Don't mess with this, the oil should still be mixed to specifications for your particular motor(50:1 for merc).   

-You can also have "lean" or "rich" fuel to air mixture in the carb.  The adjustment that was described using the screw is a fuel/air mixture adjustment.  It is better for motor life, according to the manual, to have a more "rich" fuel/ air mixture than "lean", but "proper" adjustment should help the bog(if that was the issue, which appears to be).  That does not mean you are running too lean.  Motor life would only shorten if it were too lean as cut runner was referring to.


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## richwalker71 (Feb 11, 2010)

I think I'm OK. I'm running 50:1 and it was only a small adjustment to the fuel/air mixture. Thanks!


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