Retune, rebuild, or replace Mercarb?

[SFT2]

I have a 2 bbl Mercarb on my distributorless 3 liter/Alpha I in an 89 Maxum 1700SR. It's running rich to the point that it loads up and dies at idle. I can rebuild and tune a Keihin CVK in my sleep, but car/boat carbs are not my thing. I haven't touched a 2 barrel since about 1992, and that was a Holley 4 bbl on a blown 403 small block (which would be really cool in a boat, but that's another story). It seems to be doing ok above about 2500 RPM, but no wake zones are a nightmare. I've put in a new plugs (MR43Ts), new wires (8mm Taylors), reset the idle (800 in neutral), and reset the idle mixture (1.25 turns out), and I'm close enough to sea level not to mention the difference. Anyone have any suggestions where I should start? I'm questioning the idle mix setting, but it's where the service manual said to start and I figured it would be close enough to run reliably, if not efficiently.

 

[wncrjb]

Re: Retune, rebuild, or replace Mercarb?

If you have experience with other carbs...... AND you have the RIGHT manual ..... AND you have the right re-build kit, give it a shot. Mercarbs are really not that hard to re-build. Just take it slow and make sure EVERY port is clean. It usually takes me about 3 cans of carb cleaner to do a mercarb.... and triple check you float height, and drop, to make sure that it is correct.

wncrjb

 

[John_S]

Re: Retune, rebuild, or replace Mercarb?

Check to make sure it is not your fuel pump vent hose that is making it run rich. Fuel pump would need replacing. Otherwise, its a good chance the float needle is sticking. That is fairly common withthat carb. A rrebuild is fairly easy.

 

[SFT2]

Re: Retune, rebuild, or replace Mercarb?

Checked the fuel pump sight tube and it's clear (well, it's nasty, but there's no fuel in it). I'm going to read through the carb section of the service manual and see what I can figure out. I'm sure I can handle rebuilding it, but tuning it might be over my head, especially without being able to run up to the lake for a while.

 

[wallydc2]

Re: Retune, rebuild, or replace Mercarb?

You can do it!! Follow the instructions in the manual and you will not go wrong. Don S sent the manual to me for my mercarb and if you can read you can do it yourself, just take your time and you will be fine. If you run into something that you do not understand, ASK these guys, they will be more than willing to help.

 

[Don S]

Re: Retune, rebuild, or replace Mercarb?

I noticed several things in your original post that caught my attention.

1.

It's running rich to the point that it loads up and dies at idle.

An inoperative choke will cause that. Doesn't require a new carb or a rebuild. Fix the wiring or the choke coil.

2.

reset the idle (800 in neutral),

That will be too fast, and will cause dieseling when you shut it off If everything is working right. (Including the choke)

3.

reset the idle mixture (1.25 turns out), and I'm close enough to sea level not to mention the difference.

That 1.25 turns is nothing more than a rough setting to get it running so you can set it properly. It's NOT a final setting by any means.

4.

I figured it would be close enough to run reliably, if not efficiently.

Neither, but it will run untill you adjust it.

Go HERE and download an OEM manual for your engine.
#10 is for your engine, #6 will cover your drive.

 

[SFT2]

Re: Retune, rebuild, or replace Mercarb?

I noticed several things in your original post that caught my attention.

An inoperative choke will cause that. Doesn't require a new carb or a rebuild. Fix the wiring or the choke coil.

Hmm, now that you mention the choke, it does seem to run fine when cold, then rich when hot. Sounds a lot like a stuck choke to me. How do you go about diagnosing the choke, and what controls it? I had assumed that pushing in the key turned on the choke, but it doesn't work that way on this boat.

That will be too fast, and will cause dieseling when you shut it off If everything is working right. (Including the choke)

What's normal? The manual says 600 in some places, 800 in others.

That 1.25 turns is nothing more than a rough setting to get it running so you can set it properly. It's NOT a final setting by any means.

Gotcha. I'll see if I can do any tuning on the muffs, and if that doesn't work I'll try to get to the lake next weekend and do it in the water. Gee, what a shame, I'll have to go to the lake. Oh drat

Go HERE and download an OEM manual for your engine.
#10 is for your engine, #6 will cover your drive.

Got 'em before I picked up the boat, actually, but thanks for the link. I was having trouble finding it again to bookmark.

On another note, I think I want to switch from throttle/shifter on one lever to individual levers for each, so I came up with an idea for a new quadrant. I need a rotary table for the mill before I can really make it happen, but maybe Santa will bring me one and a pile of aluminum plate

 

[Bondo]

Re: Retune, rebuild, or replace Mercarb?

my distributorless 3 liter/Alpha I in an 89 Maxum 1700SR.

Ayuh,........

Since When is Merc building 3.0ls Without a Distributor,..??.......

 

[Don S]

Re: Retune, rebuild, or replace Mercarb?

Bondo, that is the Merc DDIS (Digital Distributorless Ignition System) used in the late 80's and early to mid 90's. I may be wrong about the years.

On another note, I think I want to switch from throttle/shifter on one lever to individual levers for each, so I came up with an idea for a new quadrant.

SFT2, you do realize you have a neutral only button on that single lever control don't you???????
For the idle rpm, look at tuneup specs in the manual.

 

[SFT2]

Re: Retune, rebuild, or replace Mercarb?

Bondo, that is the Merc DDIS (Digital Distributorless Ignition System) used in the late 80's and early to mid 90's. I may be wrong about the years.

SFT2, you do realize you have a neutral only button on that single lever control don't you???????
For the idle rpm, look at tuneup specs in the manual.

The distributor is replaced with a "thing" that apparently works like a crank position sensor. Sits in the distributor hole, runs off the cam, and I believe turns the oil pump, but doesn't have any plug wires going to it. Instead there's a plate with 4 coils (or 2 pairs of dual coils, can't remember the exact configuration).

And the neutral button is kind of annoying. You pull the lever out and it disengages the shifter cable from the throttle. I'm thinking that the throttle lever moving nothing but the throttle cable would tend to have less friction to overcome and would operate more smoothly, which is a problem with the current setup. It's hard to make very small adjustments to the throttle.

 

[Don S]

Re: Retune, rebuild, or replace Mercarb?

I'm thinking that the throttle lever moving nothing but the throttle cable would tend to have less friction to overcome and would operate more smoothly, which is a problem with the current setup.

You could very well have a faulty cable, or improperly adjusted.
They work very smooth when adjusted properly and have good cables that aren't tied down too close to the control or have to tight a bend.

 

[JMRuth72]

Re: Retune, rebuild, or replace Mercarb?

I have not rebuilt the marine version of this carb, but it can not be that much different than the street version. It is a easy carb to rebuild and tune. I would however start with inspecting the choke. I am not familiar with electric chokes. I have only had one engine with it and I never had any problems with it. I am sure that someone on here can talk you through troubleshooting them. The constant problem that I had with the street version of this carb was getting the float set right (of course I was 16 learning through trial and error out of a book and not a good one at that). I finally figured out that the needle valve seat was dirty and the rubber tip of the needle was worn out. I cleaned the seat and replaced the needle valve with a solid metal one. Reset float level and that helped. I then retimed the engine to specs and gas mileage and power went up drastically. The other proplem I had was the mechanical choke. It was a pain to get the linkage set up right when I rebuilt it. The directions were not a lot of help and I couldn't remember where the pieces all went although I did figure it out. What I ended up doing to check choke operation was on a couple of cold days I took off the air cleaner and started it up. While it warmed up I kept checking it to see when it opened until I figured out how it worked. I then kept adjusting it until it opened when I wanted it to. Like I said though I have no idea how a electric choke works. Might try taking off the air cleaner and letting it idle until it warms up and see if the choke opens. If not start troubleshooting there. If it is a choke issue I hope that you have a multimeter. From what I see in the service manual link that someone else provided if you carefully follow the directions step by step it should be easy to find and fix the problem. The directions are much better than what I was working with when I was 16. Good luck and let us know.

 

[flabum]

Re: Retune, rebuild, or replace Mercarb?

The electric choke is easy. First you check to make sure the choke butterfly isn't stuck. Next, there is a round heater coil (enclosed in a round housing) on the side of the carb. There are three little screws holding the cover on. You loosen the three screws and break the cover free from the paint(don't remove it) and with the engine cold, rotate the cover until the choke butterfly is barely closing. Check it's operation by running the engine and ensuring the choke opens after a few minutes.

 

[SFT2]

Re: Retune, rebuild, or replace Mercarb?

You could very well have a faulty cable, or improperly adjusted.
They work very smooth when adjusted properly and have good cables that aren't tied down too close to the control or have to tight a bend.

Suppose I didn't describe it very well. Right now the single lever setup works fine, it's smooth, but heavy. I'm used to the feel of aircraft throttles and would like to get something more like that. Besides, it looks like a fun project. I have a design mostly worked out that has an interlock so the shifter can't move unless the throttle is at idle, and the throttle can't move unless the shifter is in forward, neutral, or reverse, and not at a point in between.

I'm going to spend a couple of hours fiddling tomorrow. After thinking over exactly what the engine was doing I'm suspecting it's just an extremely rich idle mixture. I had a similar problem with a rack of CVKs on an obnoxiously fast bike a couple of years ago. In that case it was slobbering rich at idle to fix the part and full throttle mixture being lean due to small main jets. Going up 2 steps on the mains let me take out a bunch of fuel at idle and make it behave better on the street. I doubt the main jets are too small now, but the idle is definately rich so I'll start with that.

 

[John_S]

Re: Retune, rebuild, or replace Mercarb?

If you find it is not a choke issue, and you are having problems getting a consistant idle, check to see if you have dribbling and pooling of gas. Being careful, remove flame arrester after engine is fully warmed up. Let engine idle. The choke plate should be wide open. Using a flash light, look down the barrels. If you see dribbling and pooling of gas, you probably have a sticking float needle valve.

 

[JMRuth72]

Re: Retune, rebuild, or replace Mercarb?

I do not know if they used a rubber tip on the float's needle valve or a solid metal one. I would pull the top off the carb and check it. If it is the rubber tipped one then I would lean towards a deformed tip where it meets the valve seat. Like I did on my old car I would chuck it for an all metal one. If it is a metal one then I would lean towards a dirty valve seat. Either way I would seriously suggest cleaning the valve seat thoroughly. Carb cleaner and a pipe cleaner work really good for this as you won't run the risk of damaging the seat like you do with a metal brush. If you are not the first owner of this boat I would also strongly recommend setting the float as per the manual as the prior owner may have adjusted it to compensate for fuel drip. Good luck and let us know what happens.

 

[SFT2]

Re: Retune, rebuild, or replace Mercarb?

Well today was no fun. It rained all day until about an hour before dark, and I didn't want to try to work on the boat with no light, so I put up Christmas lights instead. Hopefully tomorrow I'll have a chance.

First I'll check the choke by running the motor on the muffs until warm. I'm hoping that with the engine cover off and the bilge blower on the whole time I'll get away with leaving the flame arrestor off. I keep an extinguisher in the boat at all times, and have 2 more at either side of the garage door, so I'll be prepared if bad things happen.

Once the motor's warm I'll shut down, disconnect the throttle cable, set the idle to 800, and fiddle with the idle mix until it's in the middle of the range before the tach drops from either lean or rich mixture. After that I'll reconnect the cable and reset the idle to 700.

At some point in all of that I'm planning on pulling the filter out of the fuel pump to see if anything odd is going on there. I'll do that after everything else so I don't fill the bilge with fumes before having to work on everything. I fixed the fuel leak that came with the boat already so I'm confident I won't become America's next astronaut.

I ran across an MCM service bulletin that mentioned 2.5 and 3 liter engines going lean in hard right turns with the spring loaded needle, and mine does exactly that, so I'm going to guess that's the one I have.

 

[Don S]

Re: Retune, rebuild, or replace Mercarb?

I ran across an MCM service bulletin that mentioned 2.5 and 3 liter engines going lean in hard right turns with the spring loaded needle, and mine does exactly that, so I'm going to guess that's the one I have.

Just because a service bulletin is for a 3.0L engine doesn't mean it's for ALL 3.0L engines. The bulletin is from 84, you have an 89. The new style needle was installed in all the newer carbs from the time they found the problem.

First I'll check the choke by running the motor on the muffs until warm.

You should have an electric choke on that engine, you don't have to run the engine, just turn the ignition switch to the run position and the choke should slowly open.
Also make sure you have power and ground TO the choke.

Running your engine with the flame arrestor off will not cause a fire in itself. It prevents a flame from shooting out of the carb and igniting anything that will burn should you have a backfire. Having the engine cover open prevents a build up of fumes IF you have a fuel leak. If you don't have a fuel leak or fumes in the bilge, you won't have an explosion. Use that fume detector over your lip before you start messing with it.

 

[SFT2]

Re: Retune, rebuild, or replace Mercarb?

Just because a service bulletin is for a 3.0L engine doesn't mean it's for ALL 3.0L engines. The bulletin is from 84, you have an 89. The new style needle was installed in all the newer carbs from the time they found the problem.

Yes, but since the new style needle is what they say causes the lean stumble in hard starboard turns, I'm guessing that's what I have before tearing into the float bowl.

You should have an electric choke on that engine, you don't have to run the engine, just turn the ignition switch to the run position and the choke should slowly open.
Also make sure you have power and ground TO the choke.

I read somewhere that the choke's hot wire went straight to the alternator, and until the engine was at 800 RPM it wouldn't produce enough juice to open the choke.

Running your engine with the flame arrestor off will not cause a fire in itself. It prevents a flame from shooting out of the carb and igniting anything that will burn should you have a backfire. Having the engine cover open prevents a build up of fumes IF you have a fuel leak. If you don't have a fuel leak or fumes in the bilge, you won't have an explosion. Use that fume detector over your lip before you start messing with it.

I'm just a bit paranoid, especially after discovering a fuel leak in the middle of the lake a couple weeks ago (now fixed). I'm considering spending $100 for a fume detector just for a little peace of mind.

 

[SFT2]

Re: Retune, rebuild, or replace Mercarb?

A bit of an update. Pulled off the flame arrestor and played with the choke for a while. Discovered that it was set too high, so I turned it down to where it should be. Then I turned on the key, stuck the meter on there, and got full battery voltage. Plugged it back in and waited, but nothing. Choke didn't move. So I fiddled some more, pulled the cover off, and watched the coil move. Then I realized there isn't a spring on the choke plate, it's opened by airflow, put it back together, and it's working right.

Unfortunately I couldn't start the motor thanks to a broken v-belt. Apparently the previous owner put too narrow of a belt on, and the spare it ziptied around the motor mount is too wide and short. No chance of it going on. So now I'm digging around in the service manual for the right size belt, and not having much luck.