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Discussion Starter · #1 ·
Hi,

I'm having trouble tracking down an engine issue. Boat is a 1986 Sea Ray with a 5.0L 230HP mercruiser engine.

The problem is that the boat will only run above 3000 RPM for about 10 seconds before it starts to hesitate and buck. I don't hear an electrical miss. Watching the tach - I will see it drop several hundred RPM and then bounce back up and all over until I drop the throttle back and get it under about 3000 RPM. The water seperater/fuel filter only has about 20 hours on it. The in-lin fuel filter is brand new and the cap/rotor/wires/plugs all look like new.

Can a fuel pump just get weak or something? Any suggestions?

Thanks for looking!
 

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QUOTE(Blu @ Aug 14 2009, 04:26 PM) <{POST_SNAPBACK}>Hi,

I'm having trouble tracking down an engine issue. Boat is a 1986 Sea Ray with a 5.0L 230HP mercruiser engine.

The problem is that the boat will only run above 3000 RPM for about 10 seconds before it starts to hesitate and buck. I don't hear an electrical miss. Watching the tach - I will see it drop several hundred RPM and then bounce back up and all over until I drop the throttle back and get it under about 3000 RPM. The water seperater/fuel filter only has about 20 hours on it. The in-lin fuel filter is brand new and the cap/rotor/wires/plugs all look like new.

Can a fuel pump just get weak or something? Any suggestions?

Thanks for looking!

How old is the gas in the tank. Filters and seperators won't always get water and/or dirty gas. The first place I look with those symptoms is the gas itself. If it isn't fresh ie. a fair amount of gas run through the tank It is worth checking. Contaminated gas can clog filters quite quickly.

I did have a coil wire crack that also produced similar symptoms on the same engine you have so after gas look to ignition. Fuel pump wouldn't be my first guess as it seems you are pumping fuel fine up to 3000RPM and should maintain enough fuel pressure in the lines.

I start with the simple fixes and then look elsewhere if the problem isn't found. Hope you find the issue soon enough.
 

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So many things it could be. So under load (3000RPM+) is when you are having the issue but do you notice it more after the engine gets warm? If yes, then it could be ignition/electrical related as electrical components have increased resistance when hot.

It could be a fuel issue. Check seperator, drain into a glass and inspect. You might try to run the boat off an aux tank to eliminate the option of bad fuel. If fuel is ok then it might be a case where it's simply not getting enough fuel which would point you to the carb (assuming not fuel injected), a possible stuck anti-siphone valve comming from the tank to the fuel pump. Pressure check the pump will rule this item out. How long since the carb has been rebuilt? Possible sticking fuel float or gum and tarnish in the ports. Note: A lean running fuel starved engine will backfire, do you have this condition?

Electrical: Never hurts to remove the cap and rotor and inspect for arcing. A visual check of the wires is a long shot, you really need to test with an Ohm meter to determine resistance. Another possibility but I doubt it would be a slow dying coil. Is this a "can" type coil?..more likely to fail then the High Energy Ignition coils. Can't hurt to check resistance over the primary winding. Need more info on ignition system, is it Thunderbolt or Delco?

Last but not least....I had the exact same problem not long ago, lack of power over 3000RPM and a jumping tach. The fix>> a bad shift intureptor switch. The swich is designed to "kill" the engine and remove torque from the drive unit during shifting. The problem was that the switch kept changing state while driving and it resulted in the exact issue you have. (lack of power, backfire,etc)

Next time out, have a buddy drive while you disconnect the switch and jump with a paper clip. I did and it was an immediate improvment.. all issues disappeared. Now you won't be able to take her out of gear until you reinstall the switch to eliminate the bypass so don't head to the dock yet. While the switch is out check with an Ohm meter..you should have a steady open or closed circuit as you cycle the switch plunger. A bad switch will get worse when warm.

Good luck...keep us posted.
 

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Mercruiser had a problem with the brain back then after it would get hot it would brake down and change the motor timming all over. Check your timming cold then after it starts to hesitate and buck check the timming again to see if it is all over the place.
 

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Sounds similar to this: 4.3L Stalls/Kills at Higher RPMs (mariahownersclub.com). In summary: Turned out to be the guy's carburetor.

E10 gasoline is not a good thing for marine engines and fuel systems. I treat the gasoline in both our boats with StaBil marine formulation.

Jim
 

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Thanks for all the replies! I have already checked the Cap/rotor/ plugs and wires. All looked good. Also, when the tach jumps, it's not a false reading. The engine is bogging down for a sec then ramping back up. Like it's going to stall then comes back to life - repeatedly. It has never backfired either.

I'll start with removing the water seperator and checking it for water. I'll probably add some Sea Foam to the fuel too.

Is it a big job to remove and clean the carb?

Doesn't seem to be heat related. Will happen as soon as I'm coming out of the river from port.
 

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Discussion Starter · #7 ·
QUOTE(SEMIJim @ Aug 15 2009, 08:44 AM) <{POST_SNAPBACK}>Sounds similar to this: 4.3L Stalls/Kills at Higher RPMs (mariahownersclub.com). In summary: Turned out to be the guy's carburetor.

E10 gasoline is not a good thing for marine engines and fuel systems. I treat the gasoline in both our boats with StaBil marine formulation.

Jim

That does sound very similar. Only difference is that the engine doesn't die. Just keeps jumping up and down in RPM.
 

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QUOTE(Blu @ Aug 15 2009, 09:27 AM) <{POST_SNAPBACK}>That does sound very similar. Only difference is that the engine doesn't die. Just keeps jumping up and down in RPM.
Yup. Could be a difference in degree of carb dirtiness or corrosion.

Note: Some people have reported good results from adding SeaFoam gasoline treatment.

As to carburetor rebuilding... well... I used to do it on my cars, back in the day. Since you have the 5L MerCruiser, I'm guessing you have either a Rochester QuadraJet, Weber, or possibly Holly, spread-bore 4bbl carburetor. Probably the Rochester. Not for the faint-of-heart. Easy to screw them up badly unless you know what you're about. The QuadraJets, in particular, are twitchy. (Ask me how I know
) I would do it myself, but, as I said, I used to do it all the time, back in my Woodward street-racing days
.

I would start simple, with fuel filter, water separator (if you have one), fuel tank, fuel tank pickup, etc., before considering addressing possible carburetor problems. Could also be a failing fuel pump.

Jim
 

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QUOTE(Blu @ Aug 15 2009, 09:24 AM) <{POST_SNAPBACK}>Thanks for all the replies! I have already checked the Cap/rotor/ plugs and wires. All looked good. Also, when the tach jumps, it's not a false reading. The engine is bogging down for a sec then ramping back up. Like it's going to stall then comes back to life - repeatedly. It has never backfired either.

I'll start with removing the water seperator and checking it for water. I'll probably add some Sea Foam to the fuel too.

Is it a big job to remove and clean the carb?

Doesn't seem to be heat related. Will happen as soon as I'm coming out of the river from port.

The sea Foam is a great idea and really good stuff. I'd give that a chance to work. I would not rush in to the carb. until you have checked fuel and ignition issues as mentioned above. Given the year and make of the engine it is probably a Quadra Jet carb. They are a good carburetor but can get real finicky when messed with. There are only a few people who are really good rebuilding them and I have seen times where messing with them leads to buying a replacement for big bucks. You are on the right track with the low cost easy fixes. You are not spending a lot of money and everything you are doing is good maintenance for your boat/engine. Check all that stuff out before any removal or disassembly of components. Chances are you will clear it up before getting in too deep. Good luck.
 

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Blu
if you pull off the fuel water seperator and find water again? check the o-ring on the gas cap!
I had the same problem on my 21 searay, couldn't get it on plane. change seperator, put in some drygas and good for a few weeks. then it returned. did same steps as before but drained the 50 gal fuel tank, and all ok again for a few weeks. well come to find out with the cover on the boat rain water runs right over the gas cap, and guess what i found? the $2 o-ring was gone. just one more thing to check.
 

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Discussion Starter · #12 ·
QUOTE(F350 @ Aug 17 2009, 09:46 AM) <{POST_SNAPBACK}>Blu
if you pull off the fuel water seperator and find water again? check the o-ring on the gas cap!
I had the same problem on my 21 searay, couldn't get it on plane. change seperator, put in some drygas and good for a few weeks. then it returned. did same steps as before but drained the 50 gal fuel tank, and all ok again for a few weeks. well come to find out with the cover on the boat rain water runs right over the gas cap, and guess what i found? the $2 o-ring was gone. just one more thing to check.

That was a good catch! Thanks for the idea - I'll check the condition of mine.
 

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Discussion Starter · #14 ·
I finally had a chance to work on the boat today. I believe I found the problem. There is a wire mesh filter inside of a fitting that is on the fuel tank pick-up tube (the fitting the anti-syphon valve screws into). It was about 90% clogged. I could barely see light through it. Got it all cleaned out (marina didn't have a replacement) and put back together just before I needed to go pick up the kids
. So, I will water test tomorrow morning and report the results. Keeping my fingers crossed.
 

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Given the symptoms you described and your description of the clogged fuel pick-up screen that you cleaned it is my guess you found the problem. Fingers crossed as well but high probability that was your culprit.

Just a note, it is not uncommon for this type of problem to reoccur until you can run out any contamination in the tank. Not always but it can happen again. Bad thing about boats in this region, they have to sit too long in the winter. They would behave so much better if we ran them all year. At least you will know where to start if it acts up again.

Looking forward to you report.
 

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This is not a fuel issue.
What you are discribing makes me think that you are "losing" the hub on the flywheel or the propeller.
Call me @ 586.954.3330 We'll talk about it!
Have a safe and blessed weekend!
DON
 

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QUOTE(SCMSDON @ Sep 6 2009, 09:56 PM) <{POST_SNAPBACK}>This is not a fuel issue.
What you are discribing makes me think that you are "losing" the hub on the flywheel or the propeller.
Call me @ 586.954.3330 We'll talk about it!
Have a safe and blessed weekend!
DON
Good news. I made it out this evening and she ran like a champ!
Thanks for everyones input.
 

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Discussion Starter · #20 ·
QUOTE(SCMSDON @ Sep 6 2009, 09:56 PM) <{POST_SNAPBACK}>This is not a fuel issue.
What you are discribing makes me think that you are "losing" the hub on the flywheel or the propeller.
Call me @ 586.954.3330 We'll talk about it!
Have a safe and blessed weekend!
DON

Thanks for the offer, but the issue has been resolved. It was fuel starvation due to a clogged filter on the pick-up tube from the fuel tank.
 
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