Engine Surging Around 3600-3900 RPM

[msd58]

I was operating my 1995 5.0 4 bbl Rochester carbed Mercruiser today for about an hour and 1/2 at about 3600-3900 rpms. Normal rpms for planing speed of 26-28 mph for my boat. I noticed that it began to surge slightly varying about a couple of hundred rpm down then back up again. When I gave it heavy throttle it seems to make matters worse really bogging down. When I backed off throttle the bogging would go away and I could get it back up to normal rpms again and the surging was intermittent and again only did it around 3600-3900 rpms Everything else seemed fine; starts quickly, no miss fires or rough running.

Any suggestions??

 

[Bondo]

Re: Engine Surging Around 3600-3900 RPM

Any suggestions??

Ayuh,.... Change the Fuel Filters....

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

The spin off filter is easy the one, on the carb is tough to undo. What the technique there? I don't want to damage something.

 

[6meter]

Re: Engine Surging Around 3600-3900 RPM

The spin off filter is easy the one, on the carb is tough to undo. What the technique there? I don't want to damage something.

You need a 1" open end wrench on the big nut and a good 5/8" flare nut wrench for the fuel line. A good quick rap on the 5/8" should pop it loose. If the filters don't cure it, it might need the carb rebuilt to clean out the insides.

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

You need a 1" open end wrench on the big nut and a good 5/8" flare nut wrench for the fuel line. A good quick rap on the 5/8" should pop it loose. If the filters don't cure it, it might need the carb rebuilt to clean out the insides.

Thanks picked up the wrench needed at NAPA

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

Changed both filter today. No real change. The first time I experience the surging boat was running for at least one hour plus before anything was noticeable. This time less than 1/2 hour. No worse, just happened sooner. When you floor throttle it bogs out out till you back it off. Then it behaves for a while and symptoms return. There has been no stalling, back firing, black smoke or gas smell.

 

[21cuddy]

Re: Engine Surging Around 3600-3900 RPM

I am having the same problem. I have an 87 wellcraft with a 260 mercruiser/alpha one gen one. Hei distributor w vacuum advance, edelbrock 650 carb, holley electric fuel pump. I can see i'm getting plenty of fuel in my clean glass fuel filter, the carb is new and the engine timing is set at 8 deg before tdc at idle. The cap,rotor,plugs and wires are new. It takes forever to get my boat to plane but once it does i can run 4200 rpms all day and it idles fine. only under a load at midgrange power it shutters. Please help anyone got any ideas? i was going to check the new plugs to see if they have water on them, thought maybe my exhaust manifolds had a water leak somewhere and water was getting to the plugs. I have to floor the boat to get it to start after it runs for a while. Please help

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

Please don't hijack thread. Your problems seem much different issue than mine.

 

[Bondo]

Re: Engine Surging Around 3600-3900 RPM

Changed both filter today. No real change.

Ayuh,... Did you find foreign matter in 'em,..?? Water,..??

Could need to dig deeper into the Carb..

I am having the same problem.

Ayuh,.... Welcome Aboard,....
You don't have the same problem,... Nor the same motor/ drive...
The vacuum distributer is a Red Flag, that your motor is Sub-standard,...
Start your own thread, 'n We'll talk about it...

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

Ayuh,... Did you find foreign matter in 'em,..?? Water,..??

Could need to dig deeper into the Carb..



Ayuh,.... Welcome Aboard,....
You don't have the same problem,... Nor the same motor/ drive...
The vacuum distributer is a Red Flag, that your motor is Sub-standard,...
Start your own thread, 'n We'll talk about it...

THANKS!


Some dirt particles in the 1397-8767Q filter. Not much. No water in spin off.

Seems like After its warmed up when hit full throttle it bogs down drops off plane. When you pull throttle back he seems to run better.

 

[NHGuy]

Re: Engine Surging Around 3600-3900 RPM

I had some 4000 to 4500 rpm surging last season, it was gone this year after a set of plugs, cap, rotor, water separator, timing correction & fuel filter. I also replaced my original 22 year old fuel lines and cleaned out the anti siphon valve. I think the tank to pump line was a likely problem, it was cracked and was probably allowing air in where fuel belongs. Just an idea.

 

[user64]

Re: Engine Surging Around 3600-3900 RPM

Does your engine have distributor with mechanical advance springs? If so, check to see if they are rusted. I had these symptoms and thought it was fuel. I replaced the springs and was good to go. Good luck

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

Does your engine have distributor with mechanical advance springs? If so, check to see if they are rusted. I had these symptoms and thought it was fuel. I replaced the springs and was good to go. Good luck

Must be mechanical no vacuum advance hose.

 

[Adirondack]

Re: Engine Surging Around 3600-3900 RPM

Just a couple quick thoughts. Tired fuel pump. Kink in a supply hose from the tank. definitly sounds like its starving for fuel. Throw a fuel pressure gage on it, for pressure, and just be sure there's decent volume coming up to the carb when you crank it.

 

[mcimichell]

Re: Engine Surging Around 3600-3900 RPM

I'd bet that it's related to the fuel flow. My 7.4L did the same when some dumb mechanic put a small car fuel filter in the line... now you have to find out why it happens only after running 1.5 hours, or now it happens that also immediately?
Do you seldom reach that high RPM or you often go full speed every time you take your boat?
I would check the fuel filters too...

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

It seems to run fine most of the time. I mean you start out bringing up on plane and up to cruising speed which for my boat is around 3200-3800 rpm depending on conditions. It begins to drop and few hundred rpms then back up again if you go full throttle it bogs down drops off plane. If you pull back on throttle it ceases to bog down and you can repeat the process. I have not run the boat at a consistent 3600 rpm since problem began but I bet it would run at that speed ok except for the surging down. I remember very early in the season after coming thru an extended no wake area and slamming down the throttle to open up secondaries and it bogged down for a 1/2 a second and then jumped right up. Boat never did that before. Spark plugs were fresh this spring. Both filters were changed last week; the small filter inside carb and spin off type to no avail.

 

[mcimichell]

Re: Engine Surging Around 3600-3900 RPM

My 7.4L does the same... it seems to choke for too much gas and then changes sound and burns the wheels!
What about a dirty sprayer (the little things that spray the gasoline inside the 3rd and 4th body)?? That should start working only when you open up the full throttle, while it could get dirt inside if you never go full trhottle, like I have to go almost always! :'(

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

I will found out tomorrow and post resolve.

 

[msd58]

Re: Engine Surging Around 3600-3900 RPM

Just a couple quick thoughts. Tired fuel pump. Kink in a supply hose from the tank. definitely sounds like its starving for fuel. Throw a fuel pressure gage on it, for pressure, and just be sure there's decent volume coming up to the carb when you crank it.

Well the problem was a clogged fuel pick up inside the fuel tank. There is a screen on the end of the tube inside there. He said it was junk in there probably from when the tank was made.