Sea Ray 5.7L Mercruiser= HEADACHE!!

[Nighthawkk]

I'm about completely fed up with this engine- its a great older boat, but I have nothing but trouble with the 1987 5.7L mercruiser stern drive. Right now the problem is that I experience constant backfire and sputter from the carburetor at anything above 2500rpm. The engine seems to lose all power when under heavier acceleration, and I experienced this issue just today when taking the boat out of winterization, when it was not a problem at all last year. The marina replaced the starter, solenoid, wirings, battery terminal connections, and replaced the prop, and everything was gravy in '08. I can't seem to figure out what the problem is, but I did add 30 gallons of premium with some octane boost and carb/fuel cleaner to no avail, hoping maybe it was bad fuel left in the tank over winter. I've seen other forum posts with people experiencing this same problem, but everyone seems to have different solutions- bad timing, fuel filters or rich fuel, ignition system, "hung up" valves, etc. I really don't want to drop another $4,000 on a boat that has already had copious maintenance and would prefer to have a handy friend do it, so any ideas about where to start would be MUCH appreciated. Thanks!

 

[fc3]

Check base timing, timing advance curve and engine compression. Then get back with the results.

 

[SCORPIO]

Water in the gas maybe?

 

[berth control]

Nighthawk - you are talking about one of the most reliable engines out there - maybe you should take it to a mechanic if you can't get it to run right.

 

[Nighthawkk]

Nighthawk - you are talking about one of the most reliable engines out there - maybe you should take it to a mechanic if you can't get it to run right.

Well it was serviced multiple times, but this is a new problem that just came out of nowhere...just wanted to figure out what it was by asking to see if anyone else experienced similar issues.

 

[Nehalennia]

I would guess timing as well. Do what Frank says and let us know.

 

[Lazy Daze]

This may not help as my 5.7 is EFI (and I assume a different type of ignition system from yours - not sure). However, I had a VERY similar problem last year. For me, I could run all day long under 3,200RPM. But once the engine got good and warm, I couldn't go above it (until it got warm I was fine going WOT). It would start hesitating badly and losing power. If I backed off throttle as soon as it started, and then again kept it below 3,200 I was fine. Turned out to be a bad ignition module - or maybe it's called the spark module??? It is the electronic piece that's inside the distributor cap - mounted directly on the plate that the cap is bolted to.

Maybe there's a way to test this with a DVM????

 

[Chris Dauth]

Nighthawk - you are talking about one of the most reliable engines out there -

Nothing much to go wrong with them. Carby with Thunderbolt IV ignition. No Hi tech stuff to stuff up !

I have one in "Solitude". (1992) First thing, change the spark plugs and use the NGK Iridium ones. They last forever in a boat. The number for the plugs is BR6FIX for the non Vortech cylinder heads which you will have on the 1987. Iridium plugs give better combustion and you'll notice the engine will idle 200rpm faster and much smoother.

Other places to look is the Plug leads, Distributer cap, Pick up inside the dizzy (they rust up and fail) and the Thunderbolt IV module, (should be mounted on the port exhaust elbow)

 

[Tail Chaser 661]

Just coming out of storage makes me think carb issues after sitting.
Maybe a cracked distributor cap. Causing a misfire
Bad coil. May cause a misfire
Coil wire shorting out. Causing a misfire.
Check ignition timing. Distributor may have spun if bolt loosened.

Check engine compression.
Valves may have rusted while stored.
Getting blowby back up through the carb
Spark plugs Black/wet or tan color.?
Carb floats may be sticking. Causing engine to spit and misfire. Had that happen on a 302ci before
If Merc still used the Quadrajunk carb in 87
If so, junk it

600 cfm edelbrock marine carb 300.00 Very nice carb
MSD marine ignition 300.00 Very good ignition
MSD blaster marine coil 35.00?

These parts are a nice upgrade for a carbed 5.7 engine
Change out whats needed.
DIY and save the 100.00 per hr labor rate.

 

[tobnpr]

I agree with the Tail Chaser's carb suggestion above. Has it ever been rebuilt? 22 years is a long time...

It's hard to find an experienced shop to rebuild marine carbs as they're pretty much a dinosaur, but it's not too much more $$ to replace it. I'd slap a new carb on it. Relatively cheap and an easy DIY project.

 

[tdappleman]

I'd start with changing out any fuel filters - fuel/water seps too if you have them. If you have any Ethanol in your gas then it typically will start to break down after a month unless you have added a stabilizer. Adding new gas wont help since the Ethanol goes through phase separation and you most likely have a layer of water at the bottom of the tank. If you can - syphon some gas into a clear container and let it settle and see what you have. You may have other problems (timing/carb/etc) - but start simple and check the quality of your gas first.

If you decide to replace the carb - I went with the Edelbrock 1409 marine version mentioned above. It was $330 from Jegs + another $17 for a jetting kit since I have the 4.3 V6. It has the electric choke and so far has performed flawlessly with only minor adjustments needed out of the box. Not sure the bolt spacing on the Qjet - but the Edelbrock appears to be the same casting as the Weber 9600 which my engines originally came with.

 

[comsnark]

If you have any Ethanol in your gas then it typically will start to break down after a month unless you have added a stabilizer. Adding new gas wont help since the Ethanol goes through phase separation and you most likely have a layer of water at the bottom of the tank. If you can - syphon some gas into a clear container and let it settle and see what you have.

If you have phase seperation in your tank. . .taking a random sample and letting it settle will not tell you anything will it? If you stick it "all the way" to the bottom. . you will get pure water. Somewhere higher and you get . . .something else.

 

[ghuskin]

I had a similar problem last year. I thought it was the carb. Turned out to be the fuel pump. there's a rubber part in there that just doen't hold up to the ethanol. Just a thought!

 

[tdappleman]

If you have phase seperation in your tank. . .taking a random sample and letting it settle will not tell you anything will it? If you stick it "all the way" to the bottom. . you will get pure water. Somewhere higher and you get . . .something else.

Possible - but then again if you get clear water from the bottom of the tank then you pretty much have found the problem. Use a clear hose and you can drain until you get gas. I doubt you'd have several gallons of water so if you draw enough of a sample out it should be a mixture which would settle out - maybe a quart or 2.

 

[Nighthawkk]

I agree with the Tail Chaser's carb suggestion above. Has it ever been rebuilt? 22 years is a long time...

It's hard to find an experienced shop to rebuild marine carbs as they're pretty much a dinosaur, but it's not too much more $$ to replace it. I'd slap a new carb on it. Relatively cheap and an easy DIY project.

Yea it was rebuilt by the previous owner...theres a bunch of new stuff in there along with some chevy parts I think. I guess I could check all the stuff you guys mentioned and then try to have my friend fix it rather than pay the ridiculous $145/hr labor rate at the marina (NY is way overpriced). I suppose either I can have the mechanics try to look at it and point me in the right direction, or I can just troubleshoot by checking the timing, coils, distributer, etc. ourselves

 

[Nighthawkk]

Possible - but then again if you get clear water from the bottom of the tank then you pretty much have found the problem. Use a clear hose and you can drain until you get gas. I doubt you'd have several gallons of water so if you draw enough of a sample out it should be a mixture which would settle out - maybe a quart or 2.

It could be that the 10 gallons in the tank did collect moisture over winter even with the dry gas , but it still doesn't make much sense since I already added 30+ gallons of premium + the octane boosters and carb cleaner. But I suppose it couldn't hurt to rule this out as a possibility. I just feel like it may not be engine related since the V8 was fully serviced last year, and ran wonderfully all year. It had to have been something that affected it during winterization, as we had some very cold days out here for months at a time.

 

[Newbie]

Don't get rid of the Q-jet, they are one of the best carbs out there. When properly tuned, they will perform as well as the best EFI system and far better than an edelbrock.
Did the engine happen to have a tick to it while running? I've had a few lifters that take a while to pump up after sitting for long term. It will keep the valves, whether intake or exhaust from opening all the way and can cuase these symptoms. Need to check your comp, I doubt the timing suddenly changed while sitting for layup.

 

[Nighthawkk]

Don't get rid of the Q-jet, they are one of the best carbs out there. When properly tuned, they will perform as well as the best EFI system and far better than an edelbrock.
Did the engine happen to have a tick to it while running? I've had a few lifters that take a while to pump up after sitting for long term. It will keep the valves, whether intake or exhaust from opening all the way and can cuase these symptoms. Need to check your comp, I doubt the timing suddenly changed while sitting for layup.

Well there wasn't a tick while in idle or running at low speeds, but you could certainly hear the sputter/backfire as the rpms shot down under heavier throttle. She just doesn't want to move beyond 2500rpm, thus I can only cruise at low speeds. A lot of the time if I push it wide open, the rpms shoot up and down (never above the 2500) and will just stall out. I really don't think timing or ignition would be affected by just sitting on a trailer in cold weather. Like I mentioned, the engine was serviced in August '08 and everything was running just fine, so I have to assume it is something that happened during winterization that I may have screwed up myself.

Maybe I should just siphon the tank first to ensure there is no moisture buildup and then run it on fresh gas to see if that eliminates the problem first, then move on to troubleshooting the other possible issues.

 

[Newbie]

I don't know if this is even possible but, if the fuel is bad from water or separation, maybe some is remaining suspended in water after firing and then igniting again on the intake stroke causing the backfire. It would be easy enough to just connect the fuel line before the fuel pump to a portable 6 gallon outboard tank and run it with fresh gas just to eliminate a fuel issue.

 

[bigg_martin]

Don't know if this is available in the uS but here in Sweden we pour a type of alcohol in to the gas at wintertime to dissolve any water in the gas. The water blocks the jets (due to higher surface tension, I believe) and your engine will run lean. So check your plugs, if they are whitish instead of light brown it runs lean. Black, it runs rich.
And if you have points in your distributor check or have it checked that the springloaded ignition advance weights can move freely. They are located under the plate where the points are mounted. If they can't move you run with a static timing advance of 8-9 degrees and when weights can move it changes to 34-36 degrees (total) at around 3000 RPM. I had the same symptoms (how do you spell that?) and it took a while to find out. It's not the most obvious.
But I think you can isolate your troubleshooting to fuel or ignition.

Good Luck

Martin