Engine Performance Problem

[AHAB KS1]

My friend has a 1994 5.0 MerCruiser with 2400 hours. I noticed earilier in the season while at cruising speed the rpms were dropping off, picking back up and consistantly repeating the process. I wasn't getting much respect from the others, especially on the way out if you know what I mean. By the end of the season, the symptom is definitely noticable to everyone and viewable on the tach. It would start out at 3300 on quickly dropoff to 2500 and back up. I can think of three problems of which two are valid.

Problem #1 - bad gas (not likely, 20+ gallons added every week)

#2 - carborator rebuild (probable)

#3 - gas filter (highly likely. If you know my friend he's a "if it

ain't broke, don't need fix'n")

Any thoughts or ideas you guys might have would be greatly appreciated.

[Phishy]

buddy has/had a boat w/ similar sized engines, new plugs, new wires, cap and reset the timing fixed the issue...she probaly just needs a tune up is all,

after he tuned both they purred like new

[luremaker]

Could be a bad filter or even a bad pickup screen in the tank. Dose it come and go or is it there all the time? Is it more on take off or just cruising? could be a fuel pump starting to go if the fillers are clean, with a carb it would spit a sputter on take off and normally you wouldn't have that kind of RMP drop if you are just cruising , check for a bad (craked ) wire or cap if it is shorting out you will drop that plug and then you are firing on 7. I also would do a compression check and see where you are at, you may have a weak one or two. And it may be time to rebuild. Engine are fun to figure out but you only need to things to make them run gas and electric one can't run well with out the other.

I hope that this helps and you find the issue.

Luremaker

[Nailer]

It's also possible the distributor is getting wore out.

You/he should be changing the fuel/water separator filter every year.

[AHAB KS1]

No sputtering. The problem is during cruising speed, trolling is smooth.

[luremaker]

my best guess is not in the carb but its always good to rebuild every 5 or so years in case you have varnish or junk in the bowl, there is a test for the fuel pump, if you look up testing a pump in auto I think It will tell you . Sounds like a blockage or in the electric firing .

Luremaker

[Priority1]

No sputtering. The problem is during cruising speed, trolling is smooth.

It leads me to think it's a fuel filter. It doesn't need as much fuel at trolling speed but requires a lot more at cruising speed or under load when taking off.

A good tune up should be done regardless. You want the rig to be all that it can be especially venturing out on the BIG water. Convince your friend for safety sake.

[sherman51]

it sounds like its in the fuel system i believe. sounds like either a filter or the fuel pump. if you,ve changed the fuel filter and the fuel water seperator then you need to check the fuel pump out.

sherman

[luremaker]

I would still check both tune up wires and cap and also the fuel system I to and leaning to the fuel system but it doesn't hurt to check everything .funny about boats you fix on thing and another thing gos.

[prop nut]

if a properly done tune up does not fix it it should tell you wahts wrong

all good tune ups should always include a fuel sample and a leak down test

[AHAB KS1]

What is a leak down test?

[danthebuilder]

You try any carb cleaner?

[Priority1]

What is a leak down test?

A Leak Down test is a step above a compression test. It will show a percentage of leak down and pinpoints problems such as rings, valves etc. A compressed air source is needed to perform the test. Like a compression test each cylinder is tested separately.

[spoonfed]

Had the same problem. We changed the distributor plugs and wires. That fixed it. The distributor was wore out.

[2UNREEL]

Could be all of the above mentioned. Check the Fuel Filter screen where the fuel line connects to the carburetor. Some carbs have a screen that is often missed and can get debris clogging it up. Also check for a vacuum leak at the carburetor, loose carb or incorrect gaskets or bad gaskets can cause this symptom as well. Or the flip side the flame arrestor might need a good cleaning too. Air, spark, fuel, compression and exhaust....have fun a lot of us have been there.

[Ace HI]

Timing chain or worn cam.

[RONDAYVOO]

I have a twin engine and went through the same problems with each engine at different times. What I found was I had rubber hose for pick up tubes wich the ethenaol ate through and had a hole in the pick up tube but just enough suction to run at low RPMS, but at higher Rpms I was not getting enough fuel to run. So the engine would go up and down in rpms. Then I had a bunch of rubber in my tank that was picking up in the fuel filters. Shortly after changeing out the pick up lines to copper the problem started again. After rebuilding and cleaning the carb and a tune up wich did not help. The fuel pump I check the PSI out put and it was close about 2lbs from what it should be, I thought it was ok it was still in range but on the low end. Could not find anything wrong so I spent the money and put a new fuel pump in problem solved. Also make sure you check all your filters also the little in the carb, I have seen that be the problem on many boats its the one many people do not know is there. Hope this helps

[Grant Sadler]

Jeff; I have a 5.0 in my boat too and it was doing what you have stated, water seperator filter always took care mine, I keep a spare on the boat all the time.

Also my boat has a 70 gal. tank so at half tank I fill up and I put 1 bottle of dry gas in haven't had a problem in so time.

As PropNut said the hole check up thing would be money well spent being 15yrs old, better then row-row your boat acrossed the lake, LOL