120hp Mercarb-rebuild or replace?

[Rhadley]

I have the famous bog and want to get rid of it. I can have it rebuilt for about $150 or get a new one for about $325. Cost to rebuild and evaluate/get boat running great is about $280. <br /><br />I heard this carb is a bad design and that even a new one might not fix the problem. Any advice from a carb expert?<br /><br />Carb is the small accelerator pump version, boat SN 6734398. Carb is 1389-8490, I believe (no markings on carb).<br /><br />Thanks.

 

[novicetech]

Re: 120hp Mercarb-rebuild or replace?

Do you know what year your boat is? I hvae An 1987 3.0 with a mercarb and it runs great. It did not run very good with the fuel filter on the fuel pump all gunked up. Have you changed the filters and done complete tune up? Also there is a post somewhere about some kind of fairy tell where the mechanic removed the tamper proof screw on the idle mixture screw so he could adjust it and it made the boat run much better. You can thank Dons for that story. That tpmper proof screw is only there is your boat is a newer model.

 

[Rhadley]

Re: 120hp Mercarb-rebuild or replace?

Mine is a 1984. I rebuilt the carb last year and replaced all the filters. Ran great for awhile. Also ran great the first 30 minutes of this season, then stated back to its bog. Seems to run better when it is cool.

 

[ron7000]

Re: 120hp Mercarb-rebuild or replace?

have a 2002 3L with mercarb.<br />I fixed mine by rejetting from 1.55mm to 1.65mm and raising float level.<br />If your bog is always there and under slow and easy throttle then it's not due to the small accelerator pump, if it is that model carb.<br />If you are mechanically inclined, the labor would be free, and shipping costs are more than the jets from mercurypartsexpress.com<br /><br />if you go new carb route, look into getting a weber carb.<br /><br />tamper proof screw? didn't notice one on my carb. Although an idle mixture screw would have nothing to do with the bog at 1500-3000 rpms. It's all in the transition circuit

 

[Rhadley]

Re: 120hp Mercarb-rebuild or replace?

Thanks, Ron 7000. I wonder why Mercruiser doesn't change the jets to fix the problem! Yours is very new and it sounds like it is still an issue. Do you know what carb # you have?

 

[retodd]

Re: 120hp Mercarb-rebuild or replace?

I also have the 120 with a mercarb and I too have the "famous" bog condition. I have posted on here too about my problem. I was just googling around and came across a post from 2002 that mentions a Service bulletin# of 88-5. From there I have run into a brick wall, so I am going to call a Mercruiser mechanic tomorrow and see what exactly this service bulletin entails and post the results...Hope this helps all of us with the BOG

 

[novicetech]

Re: 120hp Mercarb-rebuild or replace?

Rhadley, I see you live in texas, do you have any winter down time down there? You can get alot of crud that forms in your gas tank over a months time especially true if the tank is not kept full. I am not trying to say the carb is not at fault, but my boat ran good the first three or four times out this season, and then would bog during during acelleration. I added fuel water seperator and was ready to order carb kit, when I decided I would check the fuel filters. The one in the fuel pump was almost full of crap, I honestly dont know how it ran at all. But anyway, all I am trying to do is save you some money, the manual calls for the filters to be replaced yearly.

 

[novicetech]

Re: 120hp Mercarb-rebuild or replace?

Ron, although for a different problem check this out. http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=24;t=012716#000008

 

[ron7000]

Re: 120hp Mercarb-rebuild or replace?

Originally posted by Rhadley:<br /> Thanks, Ron 7000. I wonder why Mercruiser doesn't change the jets to fix the problem! Do you know what carb # you have?

carb: 807504N<br /><br />engine serial 0M085483<br />engine model 4111021N2<br /><br />I've heard from a mechanic, now retired to FL, that they are "emissions" type carbs. That would be the main reason for the jetting I imagine. Stock jet size at sea level are 1.55mm for mine, and this is listed in service manual #26. I imagine the tamper proof screw is only the Kalifornia carb. Mine will idle down to 500-600 fine, but since it's a 4-cyl its not pretty.

 

[Rhadley]

Re: 120hp Mercarb-rebuild or replace?

NOvicetech, I leave my boat as empty as possible after every use and put in stabil to preserve the fuel. That way I put in fresh gas each use. I replaced the fuel pump filter last year and also installed a fuel/water separator. I fog the carb at the end of the season. <br /><br />Retodd, I will anxiously await the results of your call to Merc. Maybe there is a solution!<br /><br />I don't have a tamper-proof screw. My bog occurs right at acceleration above idle, during transition to high flow of fuel. Once it "catches", I can slam it all the way down.

 

[rbezdon]

Re: 120hp Mercarb-rebuild or replace?

Rhadley, you got the adding stabil right but you got the levels wrong. It is NOT a good idea to leave your tank as empty as possibly. Leave it as full as possible and you'll have less issues in the long run. Larger qualtities of fuel last typically longer. Think about it, it a quart of fuel were to evaporate between you outings, if you had a gollon of fuel this is 25%, if you have 35 gallons this is nothing. Evaporation and time cause the fuel to degrade. The chemical mix of the fuel changes due to evaporation. Leave it long enough and all of the volatile protion evaporates leaving a horrible smelling varnish substance. Also, the air in the tank is full of humidity. If you run on a hot humid day (always somewhat humid around water), the air sucked into the tank is full of water. Now let it stand around and some cool night the water mixes into the fuel. Refill the tank after your run on that hot humid day and you drive all the air sucked in back out of the tank significantly decreasing the water in fuel issue. Use the stabil if you dont run more than once a month but store the boat with a full tank!!

 

[canufixit]

Re: 120hp Mercarb-rebuild or replace?

I have an 87 - bought new. It had the bog when new and I took to a dealer after the first year and they had a known fix - put a change in the carb - runs fine since ....

 

[Rhadley]

Re: 120hp Mercarb-rebuild or replace?

SteveC123--what was the fix? Do you have a phone no. of the dealer? I called a local dealer who did not know of a standard fix.

 

[canufixit]

Re: 120hp Mercarb-rebuild or replace?

God, that was 18 years ago ... I gave you what I had - sorry if it's not enough - but I think any good Dealr should be able to look it -- 1987 120 I/O - carb fixed in 1988 I think ..<br /><br />Sorry ...

 

[retodd]

Re: 120hp Mercarb-rebuild or replace?

Most likely you need one of three things: an accelerator pump adjustable lever if over-fueling on acceleration; if running rich all the time throughout the band you may need a modified venturi cluster; or if flooding in hard corners or rough water, you need the three-piece inlet needle and seat part that allows fuel into the carb. The carb on your engine is a Mercarb, an evolution of the Rochester two-barrel. That unit when installed on the 3L, had a 4cc accelerator pump. Mercury installed the same pump on its 4.3L and 5L engines which worked fine but it's far too rich for the 3L. Mercury has an adjustable accelerator pump linkage that you can purchase. The linkage has three position holes and each hole changes the amount of fuel that's discharged through the accelerator pump by a 1/2cc, so you can reduce the fuel flow by 1cc from one extreme to another. For 3L engines that bog on acceleration there was a Mercury bulletin on a modified venturi cluster. This part installs with three screws. You need to establish which venturi cluster you currently have before ordering.<br />Steve Auger, Mercury Marine

 

[Rhadley]

Re: 120hp Mercarb-rebuild or replace?

Thanks for the real facts, retodd! I have a 2.5L. I assume the problem is even more severe than with a 3L. I have no problems once I get barely past the initial acceleration; I can feel when it is OK to slam down the throttle, even though the rpms haven't picked up yet. <br /><br />I have the smaller sized acceleration pump (carb made with 2 sizes). Is there a way to determine for sure if it is a "too rich" problem? How do I determine which venturi cluster I have?

 

[retodd]

Re: 120hp Mercarb-rebuild or replace?

I wish Iknew the answer to that also as I too have the 2.5. I'm not a tech either but I can google with the best of them...lol I just happened to fint that post somewhere else on the net. Mine also has no go through 1500rpm, although once through that zone i can run wide open all day(studder then surge) I have just adapted so far and idle at 1500 to save going through the multiple restarts and the associated headache> I know this is not correct but it works for now. I also talked to my merc dealer and he said that the passage between the bowl and accelerator pump is bad to get clogged, and to get some real good carb parts cleaner from NAPA. Not the junk in the spray can. I will be doing this tonight and heading to the lake tomorrow and will see what happens. Hope the info was helpful. If this doesn't work I will just buy a new carb for $169.

 

[Rhadley]

Re: 120hp Mercarb-rebuild or replace?

Retodd, Are you going to try the fixes you found? i.e., the adjustable pump linkage and modified venturi?<br /><br />I rebuilt my carb last year and soaked it in a can of the good stuff, but the improvement was short lived and as erratic as ever.<br /><br />Where can you get a new one for $169?

 

[pike hunter]

Re: 120hp Mercarb-rebuild or replace?

I follow the exact procedure in this Service Bulletin from Mercury and cure all my problems:<br /> Mercarb bulletin

 

[Rhadley]

Re: 120hp Mercarb-rebuild or replace?

Thanks, Pike Hunter. That is the real stuff!