Mercruiser bogging

Renken89

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Aug 9, 2013
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I'm new posting here but have been a reader on the site for a few years now.. I must say it awesome what you guys do here.. I find myself posting after having had bogging issues with my mercruiser 120 4.3 on a 89 renken..after replacing the coil, condenser and points along with exhaust bellows due to the boat not starting after winter... Since correcting that issue the motor was hard to start a few minor adjustments to the carb problem solved(was bogging on idle). Took the boat for a test run Thursday. Boat started off dock great, got on plane pulled about 5k rpm. Ran the motor 10 to 15 minutes and After slowing idle speed momentarily and attempting to accelerate away the motor bogs. Start the boat cranks fine, give it gas bogs down shuts off. Repeated same thing a couple times and decided to adjust the carb a bit more. After a little tinkering was able to get started back up on plane. Shot around the lake for another couple hours. The stall still happened but was fewer and farther between. Assuming bad gas was the culprit left the lake filled up with 87 no ethanol gas and added Seafoam. Went back to the lake to day to test it out and it's way worse. Hard to start without giving a lot of throttle. Bogs under load low and high rpms. Boat will no longer plane and motor will not reach 3k rpms. Kinda has me puzzles. Tried to be thorough with the post. Help!!!
 

bonzoscott

Senior Chief Petty Officer
Joined
Jul 26, 2008
Messages
745
Re: Mercruiser bogging

Welcome to iboats! First thing/s to check are tank vent and all fuel filters. Motor is starving for fuel. Usually there is one filter in the fuel pump and one in the carb behind the fitting where the fuel line attaches to the carb. Also, not sure what motor you have. 120, if that's HP, would be a four cylinder but a 4.3 is a V6. The experts here are gonna need your engine model and serial number to help.
 

RichRab

Petty Officer 1st Class
Joined
Jun 10, 2010
Messages
249
Re: Mercruiser bogging

I guess you have a carb. And there is a diaphragm called an accelerator pump. After you first shut motor off so that there is fuel pressure look down carb and open throttle and gas should immediately spray. If it does not you could have low fuel pressure due to a filter or fuel pump or the diaphragm could be bad or sometimes just needs to be adjusted by changing the travel of the linkage by bending or changing the hole on the linkage so that fuel is sprayed sooner. I do not remember but I think it should spray until throttle is opened about half way. You did gap points and plugs to correct settings? Good luck.
 

Renken89

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Aug 9, 2013
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Re: Mercruiser bogging

It's the 120 3.0 sorry about that, was reading a lot of info and had a brain fart apparently. I have read a lot about the fuel filters and possible causes of these issues. Wanted to start a post for a little walk through type deal. Plan on checking filters and hoses today. And possibly the antisyphon filter on the tank.. Thanks for the responses. Ill post serial numbers when I get outside today. Thanks again.
 

Renken89

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Aug 9, 2013
Messages
44
Re: Mercruiser bogging

I guess you have a carb. And there is a diaphragm called an accelerator pump. After you first shut motor off so that there is fuel pressure look down carb and open throttle and gas should immediately spray. If it does not you could have low fuel pressure due to a filter or fuel pump or the diaphragm could be bad or sometimes just needs to be adjusted by changing the travel of the linkage by bending or changing the hole on the linkage so that fuel is sprayed sooner. I do not remember but I think it should spray until throttle is opened about half way. You did gap points and plugs to correct settings? Good luck.

I did set the gap on the points. Didn't not gap or replace the plugs (yet). I can say the boat ran like a charm last year with the same plugs.
 

NHGuy

Captain
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May 21, 2009
Messages
3,631
Re: Mercruiser bogging

Not quite but close on the accelerator pump. You can always pump the throttle and expect a shot of gas in the carb throat. If it does not squirt there could be low fuel in the fuel bowl, or the actual accelerator pump could have failed too.
Definitely check contents of all fuel filters. Pour them into (separate if multiple) jars, and watch for sediment or water contamination in your fuel. If it's there it could have got in the carburetor too.
If all that is good try running with the gas cap loose, that will free up fuel flow if there is a clogged tank vent. My guess is the carb has a clog in the fuel inlet at the needle and seat, or a stuck float that won't let it fill with enough gas. Under heavy use the carb isn't getting enough gas, motor goes lean and poops out.
 

Renken89

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Aug 9, 2013
Messages
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Re: Mercruiser bogging

Thanks for the response. I was going over the fuel system earlier today. Found the vacuum was not completely attached to the carb from what I think is the fuel pump and had a small tear towards the carb side. This could be a simple fix however I do have some confusion about the fuel system it's self. I obtained the boat from a neglectful previous owner who apparently had zero mechanical knowledge and even less motivation to take care of his property. That said I've run into some weird issues through the rebuilding of this boat. The boat ran awesome last year for a good 10 or 12 trips until beeing winterized. I don't not see a vent on the gas tank anywhere. Neither do I see any external fuel filters in the fuel system nor any kind of fuel shut off to use for inspection or maintenance. Beeing a first time boat owner many aspects of this machine are new to me. I removed the hard fuel line to the carb thinking I would find an internal filter system and instead found a spring and small golden colored thimble shape and size metallic piece which only increased my confusion. I replaced the piece as it was removed and covered the boat from the approaching storm. That about all I was able to accomplish today before the rain hit. Thanks for all the help.
 

NHGuy

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May 21, 2009
Messages
3,631
Re: Mercruiser bogging

That clear line is a tell tale, if it gets gas inside it, it's time to replace the fuel pump. Just put on a fresh piece of clear fuel rated line. BTW that might be your running problem. It's a vacuum leak til you fix it.
The thimble shaped thing in the fuel inlet is porous, it's your fuel filter. Clean it out or replace it. Since there is no other filter I can say there's no fuel/water separator. On motors that have them they go in a fixture before the fuel pump and are spin on cartridges. They look like a car oil filter.
Since your motor does not have one there's the possibility water got to the carb. And if the fuel filter isn't very fine other stuff could have gotten in there too.
I'd just get all the gas out of that tank, right to the bottom, then pull the sending unit and look in there. Get it really pristine, check the carb too if you can.

If the gas tank is too full and you can't get rid of it or use it elsewhere, you can polish the fuel by repeatedly filtering it between containers. You'll spend the cost of a tank of gas getting the polishing stuff together though. Watch for water at the bottom each time, gas stays on top.

Marine fuel line is really hard to remove from the brass barbs it's attached to, I use extra line when I replace it so I can cut it each time I need to take it off.
 
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Renken89

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Aug 9, 2013
Messages
44
Re: Mercruiser bogging

That clear line is a tell tale, if it gets gas inside it, it's time to replace the fuel pump. Just put on a fresh piece of clear fuel rated line. BTW that might be your running problem. It's a vacuum leak til you fix it.
The thimble shaped thing in the fuel inlet is porous, it's your fuel filter. Clean it out or replace it. Since there is no other filter I can say there's no fuel/water separator. On motors that have them they go in a fixture before the fuel pump and are spin on cartridges. They look like a car oil filter.
Since your motor does not have one there's the possibility water got to the carb. And if the fuel filter isn't very fine other stuff could have gotten in there too.
I'd just get all the gas out of that tank, right to the bottom, then pull the sending unit and look in there. Get it really pristine, check the carb too if you can.

If the gas tank is too full and you can't get rid of it or use it elsewhere, you can polish the fuel by repeatedly filtering it between containers. You'll spend the cost of a tank of gas getting the polishing stuff together though. Watch for water at the bottom each time, gas stays on top.

Marine fuel line is really hard to remove from the brass barbs it's attached to, I use extra line when I replace it so I can cut it each time I need to take it off.

I took the tank off and cleaned it thoroughly last year and is new as compared to the rest of the boat,also had the carb rebuilt. So it was a filter? It seemed solid and not very pourus. Had small amount of gunk in it will get something to soak it in tomorrow or replace it completely. Someone posted earlier that the fuel pump possibly had an interior filter. Mine has the what I believe to be the bowl on top with a flat head screw on top. Looks different than others I've seen so it made me a bit apprehensive to take it apart.
 

NHGuy

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Re: Mercruiser bogging

Well since you got the tank really clean recently that's a good thing. And since the carb is a recent rebuild that has run well up to now, that's another good thing. If the gunk you see is directly over the center of the inlet it could be your solution. But if that occurred since you cleaned up the fuel tank and rebuilt the carb, maybe you should redo the tank cleanup. All you can do is go over the stuff you know, like the timing, the dwell, the point gap the fuel system.
While you have the filter off the carb try to get anything in the fuel inlet to back out, maybe a shot of compressed air in the fuel bowl vent if it has one. Might free up something, and it won't hurt anything if you just do a few puffs of air.
Your carb needs to have idle down to where it belongs and mixture screws turned in til it stumbles, then out til it runs bad, then split the difference.
If that stuff doesn't work put a separate tank on there and see how it does.
 

Renken89

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Re: Mercruiser bogging

Awesome. Will double check everything for sure. The tank and such was cleaned prior to the rebuilding. the carb was packed full of junk when rebuilt wouldnt be surprised if some residue was left. Any suggestions on taking the fuel pump apart to check the filter? Or is it just as simple as loosening the top screw catching the fuel and inspecting the contents of the filter?
 
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NHGuy

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Re: Mercruiser bogging

I bet you there is junk in the pump. There must be serviceability to it. I went in the parts diagram for the old style pump and found a replacement diaphragm listed (49089A 1) I Don't know when they changed styles so go on there and confirm it by appearance or part numbers or something.
Would you like the parts lookup link? It shows everything nicely. Here it is
https://webapps.brunswick.com/literature/literatureSearch.do You go in by either serial number which is best, or by year and model. I normally look up my parts there and then google, e-bay and amazon them to see which is the way to go.

I woke up and was hanging around today and I got to thinking about your bog. I am pretty convinced that you should chase down fuel system cleanliness. No need to reinvent the wheel on the other stuff till the most likely culprit has been addressed.
 
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Renken89

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Aug 9, 2013
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Re: Mercruiser bogging

Awesome buddy. Thanks a lot. Hope to get some work down today after yesterday's down pour. It's all under load issues so I intend to do a much work as possible in the yard before making the hour trip to the boat dock and hopefully save myself some headaches on the water. Thanks again. Will post results soon
 

Renken89

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Aug 9, 2013
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Re: Mercruiser bogging

What's good to soak these filters in btw? Serial number 6060070
 
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Renken89

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Aug 9, 2013
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Re: Mercruiser bogging

image.jpg

New and old filter. Managed to drop the old one when I took it out. Also the new one came with a gasket. Which end does the gasket go on and which side does the spring push against? Sorry for all the newb questions lol
 

Renken89

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Aug 9, 2013
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Re: Mercruiser bogging

image.jpg

What I found in the fuel pump filter.. Suggestions from here welcome lol
 

Renken89

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Aug 9, 2013
Messages
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Re: Mercruiser bogging

image.jpg

This is under the rubber washer it is caked in there.. I'm gunna try and clean what I can get too. If this is frugal and the pump needs to just be replaced someone let me know.
 

Renken89

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Aug 9, 2013
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Re: Mercruiser bogging

It's obvious I'm going to need to upgrade this fuel filtration system. As far as applying a fuel water separator what part and filter type should I look at installing? Along with the water fuel separator what else if anything can I do to prevent this in the future. It's very likely this is the first time these filters have been changed since the 90's.
 

NHGuy

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Re: Mercruiser bogging

Well if you can get it clean just do the kit. Use a brush and some solvent on the old pump. Mercruiser kits usually come with an instruction sheet. I would guess they will suggest that you use Mercruiser Perfect Seal compound on the gasket where the pump bolts up to the block. That stuff is magic, if they say to use it, get some.
If a new pump doesn't cost too much maybe just replace it unless it looks inferior to OE.

There are fuel/water separator kits with brackets, search for one that mimics the factory one. You want one with a changeable filter. Whatever you find, get one that bolts up firmly to the motor, and can be serviced easily.

The carburetor fuel filter goes in the housing with the little cone facing back toward the pump, the spring goes around the cone, the round gasket goes just before the fitting that you tighten into the fuel inlet. By the way, use a real flare nut wrench on that nut or you'll round it off and need a new fuel line. Also, hold the fuel inlet housing with a wrench as you tighten the fuel fitting to avoid twisting the carburetor. Just go buy a real flare nut wrench at Sears or from a tool truck, avoid the hardware store or parts store for that wrench. You need a real wrench to tighten that nut, they are really easy to damage. Ask me how I know!
 
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Renken89

Seaman Apprentice
Joined
Aug 9, 2013
Messages
44
Re: Mercruiser bogging

Well if you can get it clean just do the kit. Use a brush and some solvent on the old pump. Mercruiser kits usually come with an instruction sheet. I would guess they will suggest that you use Mercruiser Perfect Seal compound on the gasket where the pump bolts up to the block. That stuff is magic, if they say to use it, get some.
If a new pump doesn't cost too much maybe just replace it unless it looks inferior to OE.

There are fuel/water separator kits with brackets, search for one that mimics the factory one. You want one with a changeable filter. Whatever you find, get one that bolts up firmly to the motor, and can be serviced easily.

The carburetor fuel filter goes in the housing with the little cone facing back toward the pump, the spring goes around the cone, the round gasket goes just before the fitting that you tighten into the fuel inlet. By the way, use a real flare nut wrench on that nut or you'll round it off and need a new fuel line. Also, hold the fuel inlet housing with a wrench as you tighten the fuel fitting to avoid twisting the carburetor. Just go buy a real flare nut wrench at Sears or from a tool truck, avoid the hardware store or parts store for that wrench. You need a real wrench to tighten that nut, they are really easy to damage. Ask me how I know!

Yea apparently the previous owner already ruined that for me its almost round. The hard fuel line is going to be on the purchase list along with the pump. While I'm at it how do I go about testing the fuel pump. I know it needs 5psi how do I go about the testing itself. Appreciate all the help thus far!
 
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