1988 Mercruiser 4.3L 4BBL Bogging and Sputtering over 3K

daman4469

Petty Officer 2nd Class
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Feb 22, 2009
Messages
107
Hi all!
I have searched the forums and got some good tips on this; my situation is slightly different so I wanted to post. I have a 1988 Dynasty Apollo with a 4.3L Mercruiser Alpha I with a 4 BBL Carb. I bought the boat last summer and had it tuned up and it ran great all summer long with a top speed of 52MPH at 4500 RPM. I winterized it according to manual (treated fuel, etc), and de-winterized this spring. I took it out to the lake last weekend. Started right up, and off I went. As I hit open water, I pegged the throttle and got to 49 MPH at 4400 RPM. A minute later, the engine bogged down. As the boat rocked back and forth, the engine would bog, then catch up, then bog again, slowly losing RPM every time. I found that if I backed off to around 3K, it ran great with 0 issues. If I stop and sit for a minute and try again, it gets right up on plane, runs good for a minute, then does the same thing again. The engine will rev right up when not under load. I suspected a fuel issue, so I did some quick troubleshooting on the lake: Up to this point, I have: checked the linkages to be sure they are functioning (4 barrels opening, etc), made sure fuel is getting to carb.
I have seen some tips on cleaning/changing fuel filters and water separators, fuel pumps, points, and even replacing the carb. I'm not above taking it to a shop, but I like to know whats going on and know its something I can't fix before I go taking it somewhere and paying big bucks. I personally suspect a fuel filter (specifically, the one right by the carb) but its just a hunch) but wanted to check what others think before I go ripping in to it this weekend. I live an hour from the lake so its hard to 'try one thing then test', especially since its only happening under load, so Im trying to get the most likely suspicions out of the way.
I appreciate you all, as always!

Rob
 

Bondo

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Apr 17, 2002
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70,468
Ayuh,..... My guess is, with the boatin', Rockin',... the water bubble in the carb's bowls were also Rockin', causin' some of yer issue,....

Water in the fuel sounds the most likely suspect,....

Have ya got a canister filter in line before the fuel pump,..??
If so, inspect it's Contents for water, 'n crud,...
If not, Get one, 'n install it,....

Ya might have to rebuild the carb it get the the issue, checkin' the filters comes 1st though,....

That's the ole Quadra-jet ain't it,..??
 

daman4469

Petty Officer 2nd Class
Joined
Feb 22, 2009
Messages
107
Well, it's a 4.3 LX. One other piece of info...initially, when the problem started I had a little less than half a tank. I pulled the boat from the water and filled er up, so I'm not so sure it would be water in my gas...or would it? 👍🏻
 

Bondo

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so I'm not so sure it would be water in my gas...or would it?

Ayuh,.... That's why I say, Check the Contents of the fuel filter,.....

The fuel filter is not only a filter, but also a Diagnostic tool,....
 

daman4469

Petty Officer 2nd Class
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Feb 22, 2009
Messages
107
Well I will do as you said; ripping in to it today...I'll keep you posted.
 

daman4469

Petty Officer 2nd Class
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Feb 22, 2009
Messages
107
Small filter by carb was really gunked up. Bad. Cleaned it real good. However, can't get fuel pump assembly back on!
 

daman4469

Petty Officer 2nd Class
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Feb 22, 2009
Messages
107
Got all back together, took for test ride. Ran good for a half hour then it started acting up again. Wondering if it clogged up again...
 

NHGuy

Captain
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May 21, 2009
Messages
3,631
If you pull the filter and it's full of crap again you'll need to clean out the fuel system. Ugh!
If it's not full of crud see if the ignition wires are bad. My early 5.7 did that when it had old wires.
 
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daman4469

Petty Officer 2nd Class
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Feb 22, 2009
Messages
107
Well, the filter is dirty but not nearly as bad as it was. Not sure if something else is going on; I can't imagine the carb being out of spec as it was perfect last year and nothing has changed, although maybe there are some clogged jets. All 4 barrels are opening as they should. Fuel delivery is definitely sub par; my wife even mentioned 'why is it hesitating again?'. Taking to a shop tomorrow hoping it's a quick fix, although I'm worried as it only does it under load and not while sitting in the driveway. I'll post back.
 

daman4469

Petty Officer 2nd Class
Joined
Feb 22, 2009
Messages
107
Boat is done at the shop. Bad (weak) ICM. Looking back, I had a similar issue on my old OMC 5.0 and the resolution was the same.
 

nateo

Petty Officer 1st Class
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Apr 13, 2014
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352
I love when people say it was working nothing has changed then immediately say its broke now. Wouldnt that itself be a change?
 

daman4469

Petty Officer 2nd Class
Joined
Feb 22, 2009
Messages
107
Not playing semantics here, just looking for help, which I got. Thanks all for your help!
I love when people say it was working nothing has changed then immediately say its broke now. Wouldnt that itself be a change?
 
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