Poor Idle

[tomkatesam]

I have a 75 mercury 850 with great compression, 150 psi on each, spark, and the fuel pump works, but I cannot get the engine to idle in gear for more than 5 minutes without having the RPMS at 1000 or not stall when going into gear. I have cleaned and rebuilt the carbs twice. I was trying to tune the engine again last weekend and noticed that when I turn the idle mixture screw out counter clockwise to richen the mixture on the top carb the engine never stumbles. If I turn the screw clockwise it will stuble after the rpms increase. The bottom carbs stumbles when the screw is turned both ways. Can anyone give some advice; this engine is driving me nuts.

 

[Wee Hooker]

Re: Poor Idle

No clue on this particular motor but I have read that some multi carb outboards use only one carb ( low speed jet side) to regulate idle. Can't say myself as i only own single carb motors. Could this be your case though?

 

[tomkatesam]

Re: Poor Idle

From what I have read they should both begin to stall out the engine when they are too rich or too lean.

 

[Chris1956]

Re: Poor Idle

Tom, The carbs on that motor need to be set at the correct spot between rich and lean. In order to idle well and slow and accelerate well. The best way to do this is to start both carbs at 1-1/2 turns out from seated. Now warm the motor in the water, in gear if possible. OK - now adjust the carbs, a little at a time, for smoothest and fastest running.

OK now try to accelerate. If the motor stumbles, open the top carb idle mixture screw 1/8 turn and try again. If it still stumbles open the bottom carb 1/8 turnand try to accelerate. If it still stumbles repeat the top carb, bottomcarb sequence, in that order.

The object is to have the carbs as rich as possible, and still have a 900 Max RPM idle speed.

 

[hkeiner]

Re: Poor Idle

I have cleaned and rebuilt the carbs twice.

noticed that when I turn the idle mixture screw out counter clockwise to richen the mixture on the top carb the engine never stumbles. If I turn the screw clockwise it will stuble after the rpms increase. The bottom carbs stumbles when the screw is turned both ways.

Perhaps the low speed carb passageways and jets are not as clean as you presume. You did not describe your actual "cleaning" procedures so if you did it poorly (even twice), it may not get them clean enough. It is really important to soak the carbs in carb cleaner and then blow out all the passageways complelely. Using a spray carb cleaner from a can and just spraying it through the passageways may not be sufficient. Just my thoughts...

 

[tomkatesam]

Re: Poor Idle

I used carb cleaner and a pipe cleaner to clean the carbs, but I was unable to remove any of the jets because they were stuck and I did not want to strip them. Should the carb stumble and stall if I do turn the screw out too far?

 

[hkeiner]

Re: Poor Idle

I was unable to remove any of the jets because they were stuck

Are you saying that you could not remove the adjustable low speed jet (idle needle)? If so, perhaps you have a damaged jet. They should screw out by hand fairly easily.

If you only meant your can not remove the fixed high speed jet, then that is OK as I also leave this jet alone when I clean my carbs. I don't want to damage these small fixed jets during removal either. I do remove the low speed adjustable jets (idle needle) completely. I also soak the carb body in carb cleaner and clear out all the passageways with compressed air. Using only carb cleaner spray and a pipe cleaner may not do it sufficently. I do not know your particular carb design, but some of the passageways on my carbs are way too small to push a pipe cleaner through.

I am not saying that your carbs are not clean, but your symptoms do indicate that the carbs may not be working correctly at low idle speed. Another troubleshooting test is to spray some fuel mix (1:50 oil:gas) into the carb throat when the motor is stumbling. If the motor responds favorably, it would confirm that the carb(s) is not delivering sufficent fuel at idle. Just a thought.

 

[tomkatesam]

Re: Poor Idle

I picked up a couple bottles of carb cleaner but I did not fully submerge them. Maybe I will just fully submerge them and then clean out. The idle needle can be removed. Thanks for all of the advice. I want to get this engine running well.

 

[Chris1956]

Re: Poor Idle

Tom, You should be able to remove enough of the brass plugs to expose enough of the high and low speed passages to allow the spray carb cleaner to clean up the carb. The tricky thing is to get the carbs idle mixture set properly. Be glad you only have two carbs. Two is a lot easier than three carbs.

The idle passage has small drilled holes in the carb throat, on both sides of the butterfly. Make sure these are clean. Clean the idle restrictor (Idle jet) tube, and you should be good.

 

[hkeiner]

Re: Poor Idle

I picked up a couple bottles of carb cleaner but I did not fully submerge them.

Below is a pic of the carb cleaner I use to soak the carb parts. I use the carb cleaner spray only for rinsing the carb parts after soaking is done.

Another troubleshooting test is to spray some fuel mix (1:50 oil:gas) into the carb throat when the motor is stumbling. If the motor responds favorably, it would confirm that the carb(s) is not delivering sufficent fuel at idle.

Did you get a chance to do the above troubleshooting test? Depending on the results of this test, you may determine that the carbs are not the problem and you do not need to clean them any further.