Mercarb Idle speed vs. idle mixture adjustment

[Yachtsman1304]

Does anybody know if the idle SPEED can be adjusted independently of the idle MIXTURE? To make a long story short I did a "hmm, wonder what this screw does" number to my idle speed adjustment. Ya, I know the "if you don't know what it does....." Anyway, the idle mixture was not touched and I am wondering if I need to redo the whole adjustment process or if I can can just get it warm in the water and set my idle speed to my 550-600 RPM.

1997 Mercruiser 4.3LX, 2 bbl Mercarb.

Thanks.

 

[JustJason]

Re: Mercarb Idle speed vs. idle mixture adjustment

If you never turned the mixture screw before you cranked the idle down, and your carb is fine, (ie-clean) then you should be able to just set the idle speed screw to the proper rpms.

 

[Yachtsman1304]

Re: Mercarb Idle speed vs. idle mixture adjustment

Very good...now, just to make sure I am on the right path. The idle SPEED adjustment is the one on the rear of the carb, and it rests against the 1/2 circle (for lack of a better description) thing. From my reading of manuals I am to back that off so it is not touching, then tighten just until it touches, then two turns more. Again, I am well aware I have no business playing with the carb. with my mechanical skills!

Thanks for the reply.

 

[JustJason]

Re: Mercarb Idle speed vs. idle mixture adjustment

Technically you verify your compression, ignition system as a whole, then you back the idle speed screw down as low as she'll go without dying, then you turn your idle mixture screw in slowly until the motor wants to cough crap and die, then you turn it slowly out until the same, then you set it 1/2 way between those 2 points. After that you turn the idle speed screw back in until your idle rpms are correct when the boat is in the water, under a load, and in foward gear....

That's the technical way to do it correctly

 

[John_S]

Re: Mercarb Idle speed vs. idle mixture adjustment

Very good...now, just to make sure I am on the right path. The idle SPEED adjustment is the one on the rear of the carb, and it rests against the 1/2 circle (for lack of a better description) thing. From my reading of manuals I am to back that off so it is not touching, then tighten just until it touches, then two turns more. Again, I am well aware I have no business playing with the carb. with my mechanical skills!

Thanks for the reply.

I'd recheck the maual. A '97 would probably have T5 ignition, and need to be in base mode. Should be set to a specific rpm, not mechanical turns (outside of a starting point). Some can be done in neutral, others called out in gear.

 

[Yachtsman1304]

Re: Mercarb Idle speed vs. idle mixture adjustment

From what I see in the manual it is a T IV. I will double check that though. It is the V6.

 

[Yachtsman1304]

Re: Mercarb Idle speed vs. idle mixture adjustment

John,
I did some checking and you are correct, it is a T5. So, I would need to be in base mode even if I was just adjusting the idle SPEED only? The manual does not say anything about the base mode (5B, the Mercarb 2 bbl manual).

 

[John_S]

Re: Mercarb Idle speed vs. idle mixture adjustment

John,
I did some checking and you are correct, it is a T5. So, I would need to be in base mode even if I was just adjusting the idle SPEED only? The manual does not say anything about the base mode (5B, the Mercarb 2 bbl manual).

I can't access manual from here. I will check this evening and post back. I am familiar with the SBC V8 but doubt the V6 is significantly different. Just knowing that the T5 makes small timing changes at idle, tells me it would have to be in base mode to make any adjustment.

 

[180shabah]

Re: Mercarb Idle speed vs. idle mixture adjustment

Just backing up a little - what was the problem before all of this?

 

[Yachtsman1304]

Re: Mercarb Idle speed vs. idle mixture adjustment

Well, the problem is I thought the choke was sticking and before I could control myself I started fiddling with the idle speed screw. I had no business doing that and I have business playing in a carb.

The issue that even made me start even thinking about stuff was when I come off of plane and back to idle speed the boat sputters and acts like it wants to stall (it did once). Give it gas and it stays running. So, I did the fuel/water separator change, spark plug change, fixed a crappy corroded connection on the alternator. I was checking the flame arrestor and saw that screw that looked like it was binding the choke and I could not stop myself from dinking with it.

 

[smarks]

Re: Mercarb Idle speed vs. idle mixture adjustment

It dosen't sound like your likly to have an external tachometer so if you think your boats tach is close you can use that one and set your idle around 600 to 625 rpm after the motor is hot and in gear.

And yes it's the adjustment by the "1/2 circle thing ". Turning the screw inward will increase the idle rpm's and turning the screw out will decrease the idle RPM's.

Steve

 

[John_S]

Re: Mercarb Idle speed vs. idle mixture adjustment

While I can't access the SM, I did find this in a '94 service bulletin: http://www.boatfix.com/merc/Bullet/94/94_02.pdf


Timing Engine or Adjusting Carburetor:
Because the Thunderbolt V Ignition Module has Idle Speed Spark Control, it has to be locked in ?Base Timing Mode? whenever you:
-Set or Check The Timing
-Adjust Engine Idle RPM
-Adjust Idle Mixture

BEFORE turning ignition key on or trying to start the engine, connect a jumper wire from the Timing Lead (PUR/WHT wire, see wiring diagram) to a good ground. This locks the Module into ?Base Timing Mode?. Now any of those three adjustments can be made.

IMPORTANT: The jumper wire must be connected to the Timing Lead and ground BEFORE turning ignition key on, otherwise the Module will not go into the ?Base Timing Mode?.

Adjustments are made the same way they were done on Thunderbolt IV engines.
The Idle Speed Spark Control in the Module is only active within a certain RPM range. Because of this, engine idle RPM has to be set exactly to the specified RPM shown and in NEUTRAL GEAR. The boat can be either in the water or on a trailer.

IMPORTANT: Use an accurate shop tachometer when setting engine RPM because it has to be at an exact RPM in forward gear.

Timing at Idle RPM: 10? BTDC
Idle RPM in Neutral Gear: 650

IMPORTANT: Be sure to disconnect the jumper wire between ground and timing lead or ignition will remain in ?Base Timing Mode?.