1998 9.9 Mercury 2 stroke idling issue

Ian045

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Jul 12, 2025
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I have 9.9 mercury and I can get it to run but only when it is full throttle and I’ll pull the cord. I can’t get it to idle and it’ll die after about 3 minutes. The carb to my knowledge is clean, I’ve tried spraying it and no help. Any suggestions on what it could be.
 

jimmbo

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Does it die after 3 minutes at all speeds?
Spraying Carb Cleaner into a Carb does Eff all, except clean the Throat and Throttle Valve.
If the Motor can't Idle, it is likely the Idle Circuit is Dirty, and a Teardown and proper Cleaning required
Other Things... Is the Spark strong enough at both Plugs to Jump a 3/8" Gap?
What are the Compression Numbers?
 

Ian045

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Does it die after 3 minutes at all speeds?
Spraying Carb Cleaner into a Carb does Eff all, except clean the Throat and Throttle Valve.
If the Motor can't Idle, it is likely the Idle Circuit is Dirty, and a Teardown and proper Cleaning required
Other Things... Is the Spark strong enough at both Plugs to Jump a 3/8" Gap?
What are the Compression Numbers?

Okay, so the motor will only start when in gear I can’t get it to start in neutral only at full throttle. I get good spark but I don’t know the gap of the plugs. And I don’t know how to check the compression number. I most likely don’t have the correct tools to check it. I feel like it has something to do with the fuel or something like that because my bulb won’t stay hard, I don’t know if my fuel line is too long or something like that.
 

jimmbo

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The Gap on the Plugs is in the Neighbourhood of 0.035" I asked can the Spark jump a 3/8" Gap, much larger, cause a Spark at 0.035" at Atmospheric Pressure really means nothing as little to no Stress is placed on the Ignition System.
So the Motor will only Start if it is at Full Throttle? Are you using the Choke at all attempts to start, fast or slow?
What else have you Tinkered With/Adjusted? Did you fiddle with any part of the Throttle Linkage, as adjusting Screws?

While a Carb may look Clean, it has many Fuel Circuits that can be Dirty or Plugged that require disassembly to Clean .
 

airshot

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And a good factory carb rebuild kit with new seats and gaskets plus the info to set the carb up correctly to start. Well worth the time and money ! If you think the carb is good and clean then it is time for a rebuild....
 

Ian045

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The Gap on the Plugs is in the Neighbourhood of 0.035" I asked can the Spark jump a 3/8" Gap, much larger, cause a Spark at 0.035" at Atmospheric Pressure really means nothing as little to no Stress is placed on the Ignition System.
So the Motor will only Start if it is at Full Throttle? Are you using the Choke at all attempts to start, fast or slow?
What else have you Tinkered With/Adjusted? Did you fiddle with any part of the Throttle Linkage, as adjusting Screws?

While a Carb may look Clean, it has many Fuel Circuits that can be Dirty or Plugged that require disassembly to Clean .
I do choke it just to get the fuel running through it but then I turn it off, I’ve run it on fast idle most of the time. I’ve messed with the idle screw, I went off factory settings which is screw it down til it touches and then 1- 1 1/2 turns past. And the mixture screw on the carb, I know not to turn it too much to not break it. I’ll try this week some time to take the carb off and get some pictures maybe if I can get good quality ones.
 

Ian045

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And a good factory carb rebuild kit with new seats and gaskets plus the info to set the carb up correctly to start. Well worth the time and money ! If you think the carb is good and clean then it is time for a rebuild....
I’ll see what I can find online about a rebuild kit. Because like I said to my knowledge (which is not much) it doesn’t have any dirty spots or clogs that I could see.
 

Ian045

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And a good factory carb rebuild kit with new seats and gaskets plus the info to set the carb up correctly to start. Well worth the time and money ! If you think the carb is good and clean then it is time for a rebuild....

Is this a rebuild kit or is this something else?
 

jimmbo

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I do not know which Screw you are referring to as the Idle Screw. Turning the wrong one can throw the Sync between the Carb and Ignition Advance out the Window.
As for the Idle mixture Screw, 1 1/2 Turns out from "lightly seated" is the usual Initial setting. It will be overly Rich, but will allow the motor to start and run. Final Adjustment on it is done with engine warm, in gear. Set for Highest Rpm, but sometimes, an 1/8 turn richer could reduce any Lean Stumble/Hesitation
 

jimmbo

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I’ll see what I can find online about a rebuild kit. Because like I said to my knowledge (which is not much) it doesn’t have any dirty spots or clogs that I could see.
Unless you have Kal-El's X-Ray Vision, you aren't going to be able to see a clogged or dirty Fuel Circuit, until you take the Carb apart, that includes any Covers(look like small frost Plugs)
 

Ian045

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Unless you have Kal-El's X-Ray Vision, you aren't going to be able to see a clogged or dirty Fuel Circuit, until you take the Carb apart, that includes any Covers(look like small frost Plugs)
I’ve taken it apart but I don’t really know what to look for I was just looking for like dirt or stains in it. I’ll take it apart tomorrow evening sometime and see if I can find something else. Anything I should look for?
 

Ian045

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I do not know which Screw you are referring to as the Idle Screw. Turning the wrong one can throw the Sync between the Carb and Ignition Advance out the Window.
As for the Idle mixture Screw, 1 1/2 Turns out from "lightly seated" is the usual Initial setting. It will be overly Rich, but will allow the motor to start and run. Final Adjustment on it is done with engine warm, in gear. Set for Highest Rpm, but sometimes, an 1/8 turn richer could reduce any Lean Stumble/Hesitation
Okay so I found that it is called the idle speed screw and it sits on the cam follower. It moves when you turn the throttle. I can post the link if you would like to the website with all the stuff I’ve been trying to go off of.


Website link Page 133
 

airshot

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You can talk to the folks at that source about your wants and needs, they have always been helpful to me. Sounds like you don't have much experience with small engines, so ask them for the correct parts, just have you model and serial numbers handy to get the correct parts. Look for a factory service manual for your motor, as it will show you everything you need to know and lots of pics to help you out.
 

airshot

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I’ll see what I can find online about a rebuild kit. Because like I said to my knowledge (which is not much) it doesn’t have any dirty spots or clogs that I could see.
Most of the tiney passages are to small to see if they are clogged, and rebuild kits come with needles and seats along with new gaskets. Using the old stuff rarely works...
 

Ian045

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Most of the tiney passages are to small to see if they are clogged, and rebuild kits come with needles and seats along with new gaskets. Using the old stuff rarely works...
Thank you very much. And yea I don’t have too much knowledge on engine, just someone eager to learn about them.
 
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