# 1999 9.9 Mercury 4 stroke idle issue



## tubejig (Apr 5, 2010)

Rebuilt the carb completely. Starts very quickly but will not stay running. I have to rev up the motor in neutral and then almost slam the motor in gear to get moving. Very tough to keep running and not have the engine idle at high rate. Problem occurs on no wake areas, I run a little faster than most to keep motor running not stalling. When at full throttle I experience no problems. Any suggestions would help, I'm thinking throttle adjustment but it seem more than just screw adjustment. Thanks in advance


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## firemanmike2127 (Mar 17, 2013)

There have been a couple of older threads on 9.9 Mercs of this age & a bit newer having idle issues. Might do a search on the Boats & Motors section. Several guys have posted on what worked well for their engine idling issues. Mike


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## Shortdrift (Apr 5, 2004)

Low speed jets need cleaning. The passages are very small and foul easily. Try running Sea Foam through the motor. Do you use a fuel additive regularly? If not, that could eliminated future problems.


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## fastwater (Apr 1, 2014)

Good call Shortdrift.

Low idle passages are extremely small in these carbs.and when rebuilding them, they should be soaked in carb cleaning solution for several hrs. then all passages meticulously blown out with compressed air. These carbs are notorious for the low idle passages to become clogged and guys rebuild the carb without soaking them and paying particular attention to the small passages, reassembling them and having low idle issues.
Would try the Seafoam in your fuel to try and help clean the internal passages up but if that doesn't work, I'd tear the carb back down and soak the snot out of it.


Also, check to make sure there are no air leaks in fuel lines and connections from the fuel tank to the carb.


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## firemanmike2127 (Mar 17, 2013)

Tubejig, look closely at the bore of the low speed jet & pay particular attention to the low speed circuit in the carb body. If you look closely in the bore of the carb at the 12 o'clock position right behind the throttle plate you will see several small holes that get 'uncovered' as the throttle plate is opened. Those are part of the low speed circuit that are pretty important. Usually you can spray an aerosol cleaner into the low speed jet passage to check these small transition orifices to ensure that they are clear. On some carbs (most OMC's have this) there is a soft welch plug above those openings to allow for a more thorough cleaning. 
Fuel additive is definitely part of my regimen & I encourage all my customers to use Mercury's Quickcare treatment or an ethanol rated stabilizer. The comments others have made about low speed jets & passages needing further attention is good advice IMO. 
I routinely do carb rebuilds & here are a few of my techniques that generally work well. Jets & needles ALL get soaked in Berryman's Chemtool. I keep a small glass jar of it on the workbench specifically for that purpose. The bore of a jet can be 'narrowed' by accumulated varnish or old/cruddy fuel residue. I usually spray the carb body & internal passages with a cheap aerosol carb cleaner. I generally use the generic stuff from Walmart that I buy by the case (Super Tech @ $ 2.29 per can) because it is inexpensive & won't break down paint (like when I'm doing a Mercruiser carb). I almost never soak a complete carb body unless the thing is in just awful shape. Reason for that is because I had an instructor at an OMC service school (a long time ago) tell me that in their OB aluminum carb bodies it COULD cause porosity, which might potentially ruin the casting. I'm not sure he was totally correct, but I rarely find it necessary to soak a carb body. With a lot of the newer plastic components I won't expose them to anything harsh or caustic for fear of ruining them. If I choose to dunk a metal casting, I'll use Berryman's ChemDip, which can be bought by the gallon. Berryman's aerosol carb cleaner is awesome too, but I reserve it for my high dollar Holley & Edelbrock carbs since it is considerably more expensive & WILL REMOVE PAINT. That causes major headaches when doing a painted Mercruiser carb. I know this sounds a bit like a Berryman's commercial but I'very been using it for a long time & their products are excellent. Mike


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## fastwater (Apr 1, 2014)

firemanmike2127 said:


> Tubejig, look closely at the bore of the low speed jet & pay particular attention to the low speed circuit in the carb body. If you look closely in the bore of the carb at the 12 o'clock position right behind the throttle plate you will see several small holes that get 'uncovered' as the throttle plate is opened. Those are part of the low speed circuit that are pretty important. Usually you can spray an aerosol cleaner into the low speed jet passage to check these small transition orifices to ensure that they are clear. On some carbs (most OMC's have this) there is a soft welch plug above those openings to allow for a more thorough cleaning.
> Fuel additive is definitely part of my regimen & I encourage all my customers to use Mercury's Quickcare treatment or an ethanol rated stabilizer. The comments others have made about low speed jets & passages needing further attention is good advice IMO.
> I routinely do carb rebuilds & here are a few of my techniques that generally work well. Jets & needles ALL get soaked in Berryman's Chemtool. I keep a small glass jar of it on the workbench specifically for that purpose. The bore of a jet can be 'narrowed' by accumulated varnish or old/cruddy fuel residue. I usually spray the carb body & internal passages with a cheap aerosol carb cleaner. I generally use the generic stuff from Walmart that I buy by the case (Super Tech @ $ 2.29 per can) because it is inexpensive & won't break down paint (like when I'm doing a Mercruiser carb). I almost never soak a complete carb body unless the thing is in just awful shape. Reason for that is because I had an instructor at an OMC service school (a long time ago) tell me that in their OB aluminum carb bodies it COULD cause porosity, which might potentially ruin the casting. I'm not sure he was totally correct, but I rarely find it necessary to soak a carb body. With a lot of the newer plastic components I won't expose them to anything harsh or caustic for fear of ruining them. If I choose to dunk a metal casting, I'll use Berryman's ChemDip, which can be bought by the gallon. Berryman's aerosol carb cleaner is awesome too, but I reserve it for my high dollar Holley & Edelbrock carbs since it is considerably more expensive & WILL REMOVE PAINT. That causes major headaches when doing a painted Mercruiser carb. I know this sounds a bit like a Berryman's commercial but I'very been using it for a long time & their products are excellent. Mike


Agree Berryman's is a good carb. cleaning agent. 
Also, along the lines of soaking the carb body, not doubting what your instructor told you about causing porosity but have been soaking stripped carb bodies for better than 40 yrs and have never had an issue with any products designed and made to soak carbs in.

I do believe that if a carb body had a regular diet of some of these more harsh cleaning agents, there could possibly be an issue.


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## firemanmike2127 (Mar 17, 2013)

Fastwater, I've never ruined a casting by soaking it either. ChemDip has gotten pretty expensive too, especially if you buy a 5 gallon pail. I just keep a gallon size at the shop these days. We used to soak EVERYTHING years ago. With most outboard carbs I'll just soak the interior of a bowl because of accumulated crud. In Tubejig's case, he might not need to do anything more extensive than a thorough recleaning. Aside from the pick-up tube, the rest of the low speed passages are all in the upper part of the carb body. Mike


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## steelneyes2 (Jul 19, 2011)

Mike or Fastwater, are the 1999's part of the powerheads that include the "non adjustable" idle air mixture screws? If so pulling the welch plug allows access to the mixture screw which was "non-adjustable" in order to meet regulations at the time. Many of the Yamahas and Mercs of that era were tuned so lean it was hard to keep them idling. Depending on the number of hours on the motor a small tweak of the mixture screw to the richer side can make a fussy motor purr like a kitten. 
If that's the case with the OP's motor, it won't idle correctly even after a perfect cleaning and few of the manuals in my experience cover the adjustment because it was "illegal" to tamper with emissions to make your motor run correctly.


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## firemanmike2127 (Mar 17, 2013)

I'm not sure but I recently purchased a 1998 Mercury 15 HP 4 stroke. I'll take a look at it & see how it's set up. Interesting that you mention an air screw as opposed to a fuel mixture screw. You don't see that too often in the older engines but idle air screws or orifices are definitely out there. Not sure how Mercury Marine set up those smaller 4 stroke motors from the late 90's. Mike


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## tubejig (Apr 5, 2010)

I appreciate all of the advice. I did not soak the carb thoroughly for certain during my first cleaning of the carb. Keep your comments coming if you have them, I will not do another cleaning until Saturday. Appreciate the gas line comments as well.


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## fastwater (Apr 1, 2014)

steelneyes2 said:


> Mike or Fastwater, are the 1999's part of the powerheads that include the "non adjustable" idle air mixture screws? If so pulling the welch plug allows access to the mixture screw which was "non-adjustable" in order to meet regulations at the time. Many of the Yamahas and Mercs of that era were tuned so lean it was hard to keep them idling. Depending on the number of hours on the motor a small tweak of the mixture screw to the richer side can make a fussy motor purr like a kitten.
> If that's the case with the OP's motor, it won't idle correctly even after a perfect cleaning and few of the manuals in my experience cover the adjustment because it was "illegal" to tamper with emissions to make your motor run correctly.


Like Mike, am not sure about the years but I do know you are correct and they started plugging the air mixture screws. You are also correct that it is possible to carefully remove the plug and expose the screw for a better adjustment. But again, not sure when they started plugging them. 




tubejig said:


> I appreciate all of the advice. I did not soak the carb thoroughly for certain during my first cleaning of the carb. Keep your comments coming if you have them, I will not do another cleaning until Saturday. Appreciate the gas line comments as well.


Mike is correct and his advice is spot on for what you ought to try at this point since you've already rebuilt the carb. Opening the throat of the carb, locating the small low speed orifice's/holes and using the small nozzle on your can of carb cleaner, blast cleaner down those holes.


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## lil goose (Oct 16, 2009)

I had the exact same thing on a Mercury 15 hp. They rebuild the carb and everything ended up a small pin hole in the side of the carb. Had to replace it with a new one and it's been great. Good luck.


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## tubejig (Apr 5, 2010)

Cleaned all components again along with trimming the fuel feed line to the carb. back 1/2 inch and everything runs good again. not sure if I didn't clean good enough the first time or had a small vacuum leak in line but all is well. Thanks for the help.


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