Mercarb fuel mixture problem/concern

[mia9516]

I'm running a 4.3L Merc Alpha One with a Mercarb. Last week, while on the lake, I noticed a lot of black smoke coming out the exhaust. This concerned me because the previous day my boat had been running fine with no black smoke whatsoever. I started by making sure I was in the appropriate RPM range (650-750RPM's). I then started on the fuel mixture adjustment. I put her in forward gear and noticed there was still black smoke coming out of the exhaust. I started by turning the adjustment needle clockwise. There was no drop in RPM's and the black smoke stopped once I threaded the needle all the way in. I don't think that it's normal for the needle to be all the way in like it is. After adjusting, we took her out on the lake and she ran fine. Am I possibly looking at a dirty carb? Or maybe a damaged needle? There is a spring providing tension on the needle so I am hesitant to remove it for fear of loosing the spring. Is there a more "sailor proof" way of checking the needle? Thanks.

 

[HT32BSX115]

Re: Mercarb fuel mixture problem/concern

Howdy,


Your description sounds a lot like a stuck/maladjusted choke.


Remove the flame arrester and adjust the choke out of the picture and see how it does.

Regards,


Rick

 

[mia9516]

Re: Mercarb fuel mixture problem/concern

Howdy,


Your description sounds a lot like a stuck/maladjusted choke.


Remove the flame arrester and adjust the choke out of the picture and see how it does.

Regards,


Rick

Hi Rick,

By "out of the picture" I presume you mean adjust the choke plate so that it's vertical and perpendicular to the two plates inside the carb? I'm still reacquainting myself with carburators. It's been a long time ;-)

 

[John_S]

Re: Mercarb fuel mixture problem/concern

Check to make sure fuel pump site/safety line doesn't have gas in it. (if you don't have an elec pump) If there is gas, diaphram is ruptured and need to replace fuel pump.

Mercarbs have sensitive float needles. If you see gas dribbling down the barrels during idle, then carb will need to be cleaned, kit installed, and adjusted. Note, that a fuel pump putting out too much pressure will cause the same symptom.

 

[mia9516]

Re: Mercarb fuel mixture problem/concern

Check to make sure fuel pump site/safety line doesn't have gas in it. (if you don't have an elec pump) If there is gas, diaphram is ruptured and need to replace fuel pump.

Mercarbs have sensitive float needles. If you see gas dribbling down the barrels during idle, then carb will need to be cleaned, kit installed, and adjusted. Note, that a fuel pump putting out too much pressure will cause the same symptom.

John,

Say there is gas dribbling down the barrels, how would you know if it's because of the needle of too much fuel pressure?

 

[John_S]

Re: Mercarb fuel mixture problem/concern

You have to rig, via a T, a fuel pressure gauge to the fuel line. Not the easiest to do with a hard metal fuel line. Measure the pressure at idle. Max Merc spec I believe is 9psi, but anything close to 8psi, should be suspect. Most carbs don't like 8.

What I did on mine, was remove fuel line from carb and push the nut up the metal line. I pushed some rubber fuel line over the flared end and hose clamped. The carb end I installed the typical car type fitting that accepts rubber hose. Then there is just the T and more hose clamps.

 

[mia9516]

Re: Mercarb fuel mixture problem/concern

You have to rig, via a T, a fuel pressure gauge to the fuel line. Not the easiest to do with a hard metal fuel line. Measure the pressure at idle. Max Merc spec I believe is 9psi, but anything close to 8psi, should be suspect. Most carbs don't like 8.

What I did on mine, was remove fuel line from carb and push the nut up the metal line. I pushed some rubber fuel line over the flared end and hose clamped. The carb end I installed the typical car type fitting that accepts rubber hose. Then there is just the T and more hose clamps.

I noticed last week, while performining a pre-op prior to taking my boat out on the lake, that there is a small amount of fuel dribbling into the barrels while idling. I have an electric fuel pump so this leads to me to believe that there is a possible issue with the mixture needle and/or the carb needs to be rebuilt. Is this a correct diagnosis?

 

[John_S]

Re: Mercarb fuel mixture problem/concern

Just because you have an elec pump, doesn't mean you can eliminate it. If you don't want to measure the pressure and feel more comfortable doing the carb 1st, OK. If it hasn't been done in a few years, probably due.