engine luls and backfires over 4500 rpm only

[WhereBob]

I'm running a mercruiser 3.0L that luls and an ocassional backfire when doing over 4500 rpm. I can drive all day and it runs smooth with no problems till I kick it up over 4500.

I had and possibly still have water in the tank, I added a fuel/water seperator (mercury/quicksilver fuel/water filter) and seafoam to the tank. It was backfiring and dieing on me at any speed. Could it be that the fuel/water filter cannot filter everything when the RPM's reach a certain level?

EDIT:
Also I just did a search on max RPM at WOT for my 3.0L alpha one drive and its 4400 - 4800 RPM max. I was hitting 5100 - 5200 and holding for 5 minutes at a time (I was adjusting/experimenting the trim for max speed). It held smooth for at least 3 passes but then started acting up after 15 - 20 minutes of that. After that every time a put it over 4500 it lulled and backfired then kept going, then lul again, etc.

Could I have put the motor under stress which tweeked the timing at high RPM? Temp gauge stayed steady at 150 the whole time.

Any other ideas on what might be the cause?

Update: 5/8/09
Finally took it in to my LBS and they put it on the dyno. My tach is acurate and the motor is capable of cranking up to 5300 rpm at WOT. It started cutting and lulling and he said that it has a rev limiter and that is what is cutting the engine and is the cause which is good. The motor should not go over 4800 rpm.


Thanks for the help

 

[chiefalen]

Re: engine luls and backfires over 4500 rpm only

Run it on a external tank of good fresh fuel if problem still presents itself than post and we will go from there.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Run it on a external tank of good fresh fuel if problem still presents itself than post and we will go from there.

Great idea, only problem is I don't have an external tank and or the fittings to hook up.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Is it even possible that the fuel/water seperator filter can't keep up or do they work at any rate? Seems that I read somewhere that some filters are rated at so much fuel per hour or something

 

[TilliamWe]

Re: engine luls and backfires over 4500 rpm only

Is the bottom of the boat fouled with growth of any kind?

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Is the bottom of the boat fouled with growth of any kind?

Externally no. Internally there was slit in the engine bay at one time but I cleaned and then flushed it out the rest through the drain hole.

Why do you ask, what could this mean?

 

[chiefalen]

Re: engine luls and backfires over 4500 rpm only

Ok you don't want it the easy way.

First on the trailer raise the bow of the boat as far up as you can.

Go to a auto store and buy the cheapest electric fuel pump you can get your hands on.

Buy 10-15 feet of hose that will fit the barbs on the fuel pump.

Buy 2 clamps to hold the hose on the fuel pump.

Cut the hose in half and install hose and clamps on each end of the pump.

Take off your fuel pickup tube and anti-siphon valve its the one when you follow the hose from your fuel pump enters the tank.

Look for the arrow on the pump and stick the end that sucks all the way in the back of the gas tank.

Put the other end into a clear container, and hook up the pump, pump till you get to clear fuel.

Water is heavier than fuel will sink to the bottom of the tank with all the gunk.

When you get to clear fuel youll see it shut off the pump.

Now look at the fuel pickup tube has a screen.

Is it clean?

The fitting is a anti-siphon valve blow thru it, it working ok?

Install the valve and hose back on the tank.

You have a fuel filter in the fuel pump? Clean it.

You have a fuel filter in the fitting bye the carb.Clean it.

Vent is clear?

Ok now run it and see if the problem still presents itself.

Post your results.

Oh quit adding stuff to the tank never liked that snake oil sea foam.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

I thought about doing that orginally when I first discovered water in the tank Chiefalen but my boat mechanic suggested what I did so I went that route. Thanks for all your input!

Also I just did a search on max RPM at WOT for my 3.0L alpha one drive and its 4400 - 4800 RPM max. I was hitting 5100 - 5200 and holding for 5 minutes at a time (I was adjusting/experimenting the trim for max speed). It held smooth for at least 3 passes but then started acting up after 15 - 20 minutes of that. After that every time a put it over 4500 it lulled and backfired then keep going, then lul again, etc.

Could I have put the motor under stress which tweeked the timing at high RPM? Temp gauge stayed steady at 150 the whole time.

 

[krisnowicki]

Re: engine luls and backfires over 4500 rpm only

check your tach first and make sure you were hitting 5200 with that little of an engine and the boat you have it would be a little wierd if you werre using a decent size prop. Second, what kind of prop are you using. You will want to raise in pitch 2-4 inches in prop size to get your wot in range if your tach is right. Each inch will result in 150-200 in rpms so up in pitch down in rpms...

 

[Bondo]

Re: engine luls and backfires over 4500 rpm only

Could I have put the motor under stress which tweeked the timing at high RPM? Temp gauge stayed steady at 150 the whole time.

Nope,....

It's suffering from Fuel Starvation.....

Chiefalen told you how to fix it,... But you don't even want to try.....

Great idea, only problem is I don't have an external tank and or the fittings to hook up.

 

[chiefalen]

Re: engine luls and backfires over 4500 rpm only

I thought wot is 4200-4600 for that motor.

You do a compression check?

Check the plugs?

You didn't think the piece of information about running 600 rpm over wot is something you should have included in your original post?

Post back the result will take all of a half hour, to perform the test and pull the plugs.

 

[bomar76]

Re: engine luls and backfires over 4500 rpm only

Ok you don't want it the easy way.

First on the trailer raise the bow of the boat as far up as you can.

Go to a auto store and buy the cheapest electric fuel pump you can get your hands on.

Buy 10-15 feet of hose that will fit the barbs on the fuel pump.

Buy 2 clamps to hold the hose on the fuel pump.............
yadda yadda yadda..............

.

Time for a reality check.
The OP doesn't want to buy a tank or fittings to diagnose....what makes you think he would buy all this stuff (or anything else)?

 

[chiefalen]

Re: engine luls and backfires over 4500 rpm only

Thanks guys i needed that. Bomar thanks.

I really thought this guy wants to fix his boat.

I didn't rest till i had mine running good. Month and a half but this weekend i take my son fishing.

Still dismantling the donor boat. All kinds of stuff i don't even want.

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

I really thought this guy wants to fix his boat.

I do want to fix my boat guys please give me a break. I work 2 jobs and don't have a lot of time and the time I have I like to spend with my 7 year old son. I planned on tackling this on my only day off, this Sunday.

Time for a reality check.
The OP doesn't want to buy a tank or fittings to diagnose....what makes you think he would buy all this stuff (or anything else)?

Since I only have basically 8 hours on Sunday's I try not to waste time and money on going to the store buying stuff if I will not need to. This is why I wait to get several replies/opinions before I jump into something.

BTW I did buy 12ft of hose, a pump and fittings when I first discovered water in the tank but ended up not using them as I solved the problem another way.Some money wasted there.......
But that was another thread
http://forums.iboats.com/showthread.php?t=274370

Nope,....

It's suffering from Fuel Starvation.....

Chiefalen told you how to fix it,... But you don't even want to try.....

I talked to 2 marine mechanics today about it and they both agreed that I was over reving the engine so it might not be fuel starvation. If it was water or bad gas the problem would be at any RPM (which is kind of what I figured and why I kept asking if the filter wasn't keeping up at high RPM's). But the filter can filter at those speeds my mechanic said.
How do you know what I am and am not trying anyway? Because I ask more questions you assume I haven't done anything?

You didn't think the piece of information about running 600 rpm over wot is something you should have included in your original post?

Sorry my inexperience showing there and I appologize. I didn't think about it till later. I put it in as soon as I realized.

Once again please understand that I do want my boat running good but I have very limited amount of time to do things. I like to seek all avenues and opinions before I tackle a job. I am learning over the years that this saves me time and money, though I do get impatient sometimes jump the gun.

Thanks for all your help and guys please don't flame me if you don't know what I have or haven't done or going to do or my situation in general.

 

[Bondo]

Re: engine luls and backfires over 4500 rpm only

I'm running a mercruiser 3.0L that luls and an ocassional backfire when doing over 4500 rpm.

Ayuh,.... OK Bob,....

Your motor is suffering from Fuel Starvation,...... Period.....

Either the Tank, or it's plumbing, the fuel pump, or I suspect the Carb has crap in it that's restricting the fuel flow.....

The fuel filter, unless it's plugged, isn't the issue,...
Nor is the fact that you're running a mere 600rpms over the recommended rpms....

The motor just Isn't getting the required Flow of fuel at WOT.....
Find the restriction,+ you've found the Problem....

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Ayuh,.... OK Bob,....

Your motor is suffering from Fuel Starvation,...... Period.....

Nor is the fact that you're running a mere 600rpms over the recommended rpms....

The motor just Isn't getting the required Flow of fuel at WOT.....
Find the restriction,+ you've found the Problem....

Thanks for the reply!

My mechanic said something about when you over rev the engine the valves can start to float. He mentioned a couple other things but I didn't understand what he was saying.

I'm going to work on it tomorrow. I have not had any luck so far reproducing the problem in my driveway, always runs perfect which makes it hard to trace the issue.

 

[drrm123]

Re: engine luls and backfires over 4500 rpm only

Reving boat out of water is not good i'm sure a few other people on this board will chime in on that one, no load one engine will not produce problem your having on the water. you'll need to take boat out to get it to check to see if trouble is fixed. I'm sure also that you have a fuel starvation problem, engine pops when starved for fuel and if it was ignition it would just die with no popping. could be a little piece of junk in carb passage or filter plugged up restricting flow of fuel when under heavy load when it needs the most. you may need to get a tach to verify rpm is correct also. i dont think 600rpm would hurt anything either. if you want you can check all the basic stuff first with should not cost you anything. pull plugs make sure they are all burning the same check timing and points gap is you have points, use a ohm meter to check plug wires, i thing it should be about 3 to 11 thousand ohms per foot of wire depending on the type of wires you have also. all fuel filters resistance on coil. since you already bought 12ft of fuel line get a 5 gallon fuel can and put about 2 or 3 gallons in it and run hose into it and to fuel pump and run wide open to see if it pops. when was last time boat was tuned up?

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Reving boat out of water is not good i'm sure a few other people on this board will chime in on that one, no load one engine will not produce problem your having on the water.

Oh man no I wouldn't rev it above 2000 RPM without load and I am using rabbit ears hooked to a hose. Thats why its hard to reproduce the problem. Nearest lake I can run on is 25 miles away and unfortunately we have wind advisories today and tomorrow. I'll have to just take things apart and look over everything.

Thanks for your reply also

 

[TilliamWe]

Re: engine luls and backfires over 4500 rpm only

Bob, I agree with the others you are having a fuel starvation issue. The reason I asked about the bottom of the boat being dirty is that a fouled bottom creates a bunch of extra drag and can cause detonation and a poor running engine. Some people could mistake that for "engine luls and backfires over 4500 rpm only "

 

[WhereBob]

Re: engine luls and backfires over 4500 rpm only

Finally had some time to look it over. Carb looks nice and clean and no fouling. Pulled the plugs and they almost look new. No signs of fouling, not black (too much fuel) or white (lack of fuel). All rubber gas lines look clear, couldn't see through the hard gas line but I blew through it. Gas return line had crud in it so I replaced that but I don't think that would have anything to do with gas intake.

The only filter is a water/fuel seperator and thats new, I just put it in before I ran it last week so thats good and I don't think water gets by that so it wouldn't be a water problem right? I may borrow a gas can, drop a few gallons in it, run a hose and see what happens.

thought wot is 4200-4600 for that motor.

Your right, says that on the air filter cover.

Probably will check the tac and see how on it is. If its acurate I'm just going to keep it under 4600 and if it does this again I'll take it in and have a pro look it over for me.



Thanks everyone!