Engine losses power at 3200RPM? Mercury 5.7 Magnum

[delasem]

Hey everyone,

Hope you can help with this one?

Let me give you the technical stuff first:

Caravelle 1999 BR, 5.7 Mercury Magnum. The carb is a Rochester quadrajet 4 brl NEW.

Installed the carb yesterday and took the boat for a spin, my idle issue is gone and I now can shift without the engine stalling, the previous carb had the idle circuit damaged.

Here is my new dilemma, boat takes off great and has a very fast level trim take off, like a dream. Engine sounds strong and it pushes this bad boy nicelly until RPM gets to 3200, all of a sudden the engine falls to about 3000RPM than within seconds and without me touching the throttle it rev's back up to 3200 and back to 3000 and back to 3200...you get the point. What the heck is going on?

The boat has had a full tune up two weeks ago that included the plugs, wires, rotor, distributor cap, magnetic timing sensor, PBC, coil and fuel pump, during this time we did the timing and it is at 8 degrees as per the manufacturer specifications.

The touchy thing I did not do was touch the carb, out of the box it went on to the engine and no tuning was done. This was because it started fine and idles great so I figure why mess with it.

Here is the kicker as long as I don't rev the engine over 3200Rpm the boat runs and sounds great, better than ever.

Can anyone point me in the right direction with this problem?

Thank you

 

[Don S]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Caravelle 1999 BR, 5.7 Mercury Magnum.

Mercury makes outboards, Mercruiser does IO's and inboards.
Might want to post the Mercruiser serial number for your engine (on a tag just above and aft of the starter). Mercruiser didn't use the Quadrajet in 99. Nor did they use a 4 bbl. So it's time to do some investigating.

Along with the serial number, you might want to post some pictures of the engine. Especially the carb and intake manifold.

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Hello Don,

Engine number: 14093638

I have tons of pictures and would love to attached, u fortunately I don't know how to do this with my iPad?

Can I provide any other info?

 

[Don S]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Engine number: 14093638

That is a block casting number, I need the Mercruiser serial number. Click on the thumbnail



Pictures are easy, read this thread.

http://forums.iboats.com/showthread.php?t=468676

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Don,

Thanks for the infomation.

Here is the Engine serial number, got it from the Mercruiser filter cover: 0F273490
Here are some pictures of the engine as requested:

Additionally When no load is applied the engine revs perfectly and sustains RPM without any issues, the problem appears only under load?

 

[joewithaboat]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

I doubt that this is your problem but it would be a good idea to separate those ignition wires a little.

Visually check the carb at WOT to see if the linkage is allowing the air horn to open at the top rear. If all of the linkage is not adjusted just right sometimes it binds up. You maybe getting full fuel but not full air flow.

 

[Don S]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Your 0F273490 serial number is for a 1994 350 Mag model 4350100GS, not a 99. That explains a lot.

When you get to 3500 rpm, might want to look and see if the secondaries are opening. They are vacuum operated, so you will have to be under load to check.

You might also want to replace that steel fuel line. Clamping a hose to it like that is a no-no. if the hose slips off the tubing, you will spray gas all over a hot engine until the carb runs out of fuel. Real safety issue.

 

[Bondo]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Your 0F273490 serial number is for a 1994 350 Mag model 4350100GS, not a 99. That explains a lot.

Ayuh,... I thought rochesters were long dead, 'n gone by '94.... intake might be even older than the serial #s...

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Thanks everyone for the great advise, some really good feedback. Particularly from Don regarding safety. I have my familly on this boat and god forbid...

I will check all that was suggested, one thing I did notice is that the rear air horns do not open at all no matter what rpm even over 4500rpm (no load applied)? Should they?

Also what is WOT?

Thanks

 

[Bondo]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

one thing I did notice is that the rear air horns do not open at all no matter what rpm even over 4500rpm (no load applied)? Should they?

Also what is WOT?

Nope,... No openin' without a Load....

Wide
Open
Throttle...

 

[joewithaboat]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

WOT= Wide Open Throttle

If the air horn doors are not opening, there is your problem.

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

So how do I fix it? Any thoughts.

 

[Don S]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

You have to test them under load. Without a load and just revving the engine in neutral the secondaries will not open.

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

If this was the problem and since I will be out in the water what can be done to repair or test? Any ideas would be great

Thanks

 

[Don S]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Pull the carb apart and find out what is plugged up and not working right. Could be any number of things wrong with it, including put together wrong.

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Just got back from the water ( I'm at the cottage until tomorrow), carb secondary is opening just fine at WOT. I also checked the filter in the carb and the fule line all the way to the T fitting, all looks good but problem continues.

I'm stumped, what could it be?

Any thoughts?

Thx

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

After reading a few other discussions in this forum it appears that this is a known issue, one thing that I am considering is perhaps replacing the electronic pump with a mechanical. If I do this do I have to by-pass the fuel filter that sits between the fuel tank and the carburettor? Will the mechanical pump have enough psi to handle the large fuel/water filter?

Any thoughts?

Also, I've noticed that the new carburetor is not adjusted properly as it relates to fuel air mixture, during idle there is signs of black smoke which I did not have with my previous carburettor. How can I adjust the mixture. I did try adjusting the 2 air screws on the front of the carburettor and spent an hour moving the from fully closed and turning quarter increment to the fully open position and nothing changed. At minimum I should have seen an idling change in the motor but nothing? This same condition occurred with my original carburetor.

The main difference between my old and new carburetor is that the new one idles like a charm and shifting no longer stalls the engine, but I still have the same oscillation at 3200rpm and the black smoke.

E

 

[delasem]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

Problem Solved!

I followed everyone's advise, bought a new original low pressure fuel pump and the beast came alive, 4500rpm at a top speed of 67 km/h.

Thanks everyone for all the help and support, could have not done it with your help.

Now I'm working on getting the mixture just right. I set the timing to its proper setting but need to adjust the air/fuel intake. Any thoughts would be great.

E

 

[NHGuy]

Re: Engine losses power at 3200RPM? Mercury 5.7 Magnum

With the motor warm you set the idle for 650 rpm. Then one at a time you turn the idle mix screws down til the motor stumbles and then back out til it runs poorly. Then split the difference. Lastly reset the idle for the 650 to 700. It's supposed to be 650 in gear.