Mercarb 2 bl mixture not responsive - help!

New to board and i've spent weeks working on this problem and researching the net for ideas and tips. I'm hoping to get some fresh ideas on what to do next.<br><br>New to me 87 4 winns with a 305 and merc outdrive.  A month or so ago, on a hot day, it heat soaked and I decided to take a look at the carb and replace the fuel separator while i was there. Overall ran well before i took it off. Carb wasn't too dirty but given that i didn't know when it was last rebuilt i decided to do maintenance. Bought a rebuild kit, replaced the float, acc pump, all gaskets. Took everything apart except the choke, soaked in ultrasonic carb cleaner, out it all back together.<br><br>Idles fine, and shoots fuel from acc pump but idle mixture is not responsive. All of this on muffs.<br><br>Have the idle down low (500-600 rpm) so it shouldn't be pulling from main circuit. I sprayed around the carb to look for leaks but couldn't find any (and wouldn't this have caused idle to be high?)<br><br>I did notice that my choke isn't fully opened - would that cause any issues? I will fix that.<br><br>The kit didn't include the thick (1/4") base gasket. I originally installed the carb with the thin gasket and then read on net that people have had trouble tuning with it so i reinstalled the thick one. No difference.  Also reinstalled the original mixture screw since the new one looked a little different - no difference.<br><br>I double checked the float level with the ruler and it's spot on. No visible drips of fuel into the bowl when idling.<br><br>What i don't know if whether it matters which needle assembly to use. Kit shows 2 methods: one with a spring and one without. when i took mine apart it didn't include the spring so i adjusted the float level without the spring based on the rebuild sheet. Would a bad float level cause my problem and wouldn't i see fuel dripping from venturi if so?<br><br>I haven't checked the timing but seems like i should be able to adjust the mixture even if the timing was a bit off. I'll check that too.<br><br>The only thing i can think of is that the idle mixture seat got damaged and there is fuel passing through the circuit even though the screw is in. Seems a bit far fetched to me - can it be that enlarged?<br><br>Out of ideas and very frustrated with this carb. Welcome any feedback and ideas.<br>Thanks in advance<br>Patrick<br><br><br>

Are you using a vacuum gauge to adjust the mixture ???? FWIW, the last time I set the mixture on my engine, I just set the needle to the factory default number of turns specified and called it good....and the engine now runs great @ low RPMs ( idle to 1300). My chronic issue with my MERCARB is a hard deposit that accumulates in the emulsion block jets ( the long ones that are fixed to the block).... the method that finally gave me the results I wanted ( I don't have an ultrasonic cleaner) was to gently insert and twist with my fingers a #70 drill bit in those jets.... A surprising amount of crap came out.

Thanks for the quick response!
There is no vacuum port on this carb. if the emulsion jets were clogged wouldn't that inhibit idling at all?
Engine seems to idle well regardless of the where the mixture screw is at (i don't sense a change in idle quality or speed when turning it in or out).

On my 5.7, the vacuum port is a pipe plug on top of the intake manifold as is likely on yours.
Re: clogged emulsion jets..... depends on how clogged.. one jet 99% clogged, engine once warmed up (I have no choke, i.e. idle enrichment circuit, TKS, version) engine won't run in gear much under 1300 RPM at all. Only minor clogging, just fluky throttle response in the 800 to 1200 RPM range.
The idle mixture screw has a very fine thread and manual adjustments of it make little RPM difference.
If your carb is 100% squeaky clean, set it to the factory default # of turns out and call it good, otherwise you will need a vacuum gauge to see the difference @ idle RPM. I've done it both ways, and with a totally cleaned out emulsion jets and clean everywhere else, the default setting was bang on over the 650 to 1500 RPM range. I had set it with a vacuum gauge before and 1/4 turn was all it was off from default and it did not run the way I wanted it to in that low RPM range.... and 1/4 turn did not make much of a diff on the gauge... But... that's just my engine... a roller tappet engine with tight compression numbers whose valves that were set to spec clearance about 50 hours previously.

Thanks again for the reply!
I get the idea of set it and forget it but i'm still not quite understanding why it's behaving the way it is... It doesn't stumble or lean out when i accelerate, but i'm worried that the issue is that it's too rich, especially at idle.
Fundamental question: shouldn't my warmed up engine (all stock 305) at idle stumble or die when the mixture screw is turned in all the way?

pferat said:

Thanks again for the reply!
I get the idea of set it and forget it but i'm still not quite understanding why it's behaving the way it is... It doesn't stumble or lean out when i accelerate, but i'm worried that the issue is that it's too rich, especially at idle.
Fundamental question: shouldn't my warmed up engine (all stock 305) at idle stumble or die when the mixture screw is turned in all the way?

Click to expand...

yes it should lean out and eventually die when turned all the way in unless the throttle plates aren't closing and exposuing the idle transition circut to vacum

I had a similar issue with the mercarb on my 3.0, came down to which of the two base gaskets in the carb kit I used. Cant explain why but my manifold had a single oval hole in it yet carb base gasket had two holes... So i used the gasket in the kit that matched the hole in the intake. Mixture screw was unresponsive.... Put the two hole gasket on and it worked normally.

Your description suggests clogged idle emulsion tubes. When you screw the mixture screw in all the way it should kill the engine; if not it’s getting fuel from something other than the idle system. What can happen is that the idle gets raised to compensate for rough running and then the carb is getting fuel from the idle transfer slots because the throttle plate is now open too far. If the idle mix screws are unresponsive the carb still isn’t clean enough.

Dieter said:

yes it should lean out and eventually die when turned all the way in unless the throttle plates aren't closing and exposuing the idle transition circut to vacum

I had a similar issue with the mercarb on my 3.0, came down to which of the two base gaskets in the carb kit I used. Cant explain why but my manifold had a single oval hole in it yet carb base gasket had two holes... So i used the gasket in the kit that matched the hole in the intake. Mixture screw was unresponsive.... Put the two hole gasket on and it worked normally.

Click to expand...

IIRC my kit only came with one thin base gasket. As i mentioned in the OP, i put that one on and it made no difference.

louc said:

Your description suggests clogged idle emulsion tubes. When you screw the mixture screw in all the way it should kill the engine; if not it’s getting fuel from something other than the idle system. What can happen is that the idle gets raised to compensate for rough running and then the carb is getting fuel from the idle transfer slots because the throttle plate is now open too far. If the idle mix screws are unresponsive the carb still isn’t clean enough.

Click to expand...

OK, makes sense, but i can get the idle down pretty low (at least by ear) by turning the idle speed screw out. I did peak in the butterflies and my recollection was that the blades were mostly closed but i'll look at it again. Maybe it's not low enough and it's getting fuel from the idle xfer as you say.
I will take it apart again and play close attention to the tubes. How can i tell whether they are clogged and how do i clean them? I had them in a carb cleaner with ultrasonics and blew them out with compressed air already. Do i need a pin to shove in there to see if anything is caked in there?
Thanks for the help everyone!
Patrick

Re: " Do i need a pin to shove in there to see if anything is caked in there?"..... reread my post #2....

sandkicker said:

Re: " Do i need a pin to shove in there to see if anything is caked in there?"..... reread my post #2....

Click to expand...

Yup - you said it earlier - thanks!

When I did my Quadrajet I soaked it in carb cleaner and blasted it out with a real air compressor not just canned air. You can use a thin wire to ream it out, just be careful. This pic shows where they are on the 4bbl Quadrajet, you have to figure it out for your carb.

[h=2]Mercarb 2 bl mixture not responsive[/h]This carb will only work right with the correct mercruiser base (thick) gasket. The mixture screw adjustment is non-responsive w/out it.

pferat said:

Yup - you said it earlier - thanks!

Click to expand...

UPDATE: Finally got back to the boat to work on the carb. There was no debris/blockage in the diffusers (is that what they're called, can't remember). While i was there i also made sure the tiny holes were also clear - everything looked good there too.

Now here's the interesting thing. When i took the lid off the carb, there was no fuel in the bowl. It's been a few months since i ran the motor, but is that expected? Floats are properly adjusted... Could there be a leak? no leaks on outside... this leak into the motor could explain why adjusting the mixture had no effect on idle.

Anyone have ideas on where this problem might be? When i get the engine running, i can get it to idle low by turning the idle screw down so that suggests it's not pulling from the mains, right? But again, at that low idle, idle mixture screws don't do anything... It's getting gas somewhere else but I don't know where or how.

My mercarb's bowl will dry out after a sitting idle for a week in warm weather, or 2 weeks when cool..... evaporation.
As posted before.... you need a vacuum gauge to adjust the mixture.... and make sure the carb throttle linkage is disconnected from the throttle cable.

sandkicker said:

My mercarb's bowl will dry out after a sitting idle for a week in warm weather, or 2 weeks when cool..... evaporation.
As posted before.... you need a vacuum gauge to adjust the mixture.... and make sure the carb throttle linkage is disconnected from the throttle cable.

Click to expand...

OK re bowls going dry. I'll disconnect the throttle when adjusting (can't remember if i was doing that before or not).
I'll try this again today and report back.

Well, today was a bust. Disconnected the throttle cable and got the boat running but it was pig rich. turned it off then noticed that the idle mixture was all the way IN. Screwed it out a few turns but then couldn't get the motor to fire up - i think it was flooded.
I'm at wits end. Can I send the carb out to get rebuilt or do just buy a reman unit?

pferat said:

Well, today was a bust. Disconnected the throttle cable and got the boat running but it was pig rich. turned it off then noticed that the idle mixture was all the way IN. Screwed it out a few turns but then couldn't get the motor to fire up - i think it was flooded.
I'm at wits end. Can I send the carb out to get rebuilt or do just buy a reman unit?

Click to expand...

id send it out for rebuild to a place like mikes carbs or other reputable shop. Then you know you have the correct parts in the carb for your engine.

did you verify fuel pressure is ok ? If fuel pressure is too high fuel level in bowl will over flow and dribble into carb.

Dieter
Thanks for the suggestion to look into the fuel pressure. I haven't checked that but i can tell you that when i was diagnosing this issue a few months ago with the engine running, I did look down the venturis and there was no gas dribbling into the bores. That tells me that the float level was OK, right?
I will call Mike's though - prob my best bet to have this thing rebuilt by a pro. I am obviously missing something...

pferat said:

Dieter
Thanks for the suggestion to look into the fuel pressure. I haven't checked that but i can tell you that when i was diagnosing this issue a few months ago with the engine running, I did look down the venturis and there was no gas dribbling into the bores. That tells me that the float level was OK, right?
I will call Mike's though - prob my best bet to have this thing rebuilt by a pro. I am obviously missing something...

Click to expand...

It wouldn't verify the float level/fuel level in bowl is correct, just that it isn't so high its overflowing the fuel bowl and dribbling out the vents or venturi's into the engine. Only mentioned the fuel pressure check because if you have over 5-6 lbs you can replace or rebuild the carb but it will still over flow.

Based on what you are seeing- that the idle mix screw is completely closed off and it still runs, it has to be getting fuel from else where otherwise it would die. So if your fuel pressure is ok then yes I would go through the carb again or have someone legit rebuild yours professionally

When I finally got my mercarb cleaned 100% etc... I set the idle mixture to the default setting ( 3 1/2 turns out..... for my engine) and the engine ran fine.

Reviving this thread... Appreciate all the advice you all have given me to date!

Took my time taking it apart again and now working on adjusting the linkages. Have another issue that might be related to the problems i've been having... I noticed that when i had my acc pump rod attached, I couldn't open the throttle all the way. So i disconnected the acc rod, and was able to properly adjust the choke pull down. So far so good. When I put the acc lever back on, i adjusted the rod so that it has the recommended distance between the top of the carb and the top of the acc lever (29 mm i think). (Note, i have the early style and there is only 1 hole in the acc lever). So far so good. But again, when i try to open the throttle all the way, it seems like the acc pump is bottoming out and the throttle doesn't open all the way.

Do I have the wrong acc rod? From videos I've seen, it *looks* like the right one but maybe i'm not doing something right...
Wow, this carb is kicking by butt.

Bend acc link rod for max wide open throttle first.

Do not worry about the measurement.

once you have wide open throttle the objective is to have acc pump/link rod movement as soon as throttle is moved.

If the acc pump bottoms out the bend the tang on the acc lever with one hole.
My guess is you need about ~1/8" or less to get it right.

Also if you think you are good enough, You can drill a second hole in acc lever and my guess would be about 1/16 away from original hole closer to the pivot of the acc lever

kghost said:

Bend acc link rod for max wide open throttle first.

Do not worry about the measurement.

once you have wide open throttle the objective is to have acc pump/link rod movement as soon as throttle is moved.

If the acc pump bottoms out the bend the tang on the acc lever with one hole.
My guess is you need about ~1/8" or less to get it right.

Also if you think you are good enough, You can drill a second hole in acc lever and my guess would be about 1/16 away from original hole closer to the pivot of the acc lever

Click to expand...

Thanks - i'll try this out

Re" "move as soon as"... This is true in the most literal interpretation of "as soon as".... When carb is built at the factory, the paint acts as a "zero clearance bearing" on the linkage joint. Once you take the carb apart you destroy this "zero clearance bearing". Judiciously bending the "Z" shaped rod to take ALL the slop out of the linkage is vital.

It's been a while since i worked on the boat but I did recently and wanted to give you guys an update.
Turns out, as some of you pointed out, that my idle was too high and had the xfer slots too exposed. My carb rebuild(s) seem to have been OK all along. Now I can turn the mixture screw and get the engine to stumble when closing it. Thank you all for your help!

I still need to get a proper tach installed - the one on the dash isn't very accurate. I'm also going to install a fuel pressure gauge in the line to monitor it.