Carbed Mercruiser 260 5.7L Missing Above 4000 RPMs

[Kola16]

Hello all. My Mercruiser 260 GM 5.7L I/O with the MC-1 Drive sounds like it is missing, or starving for fuel at higher RPMs (4000+). It does it when already on plane, and/or out of the hole. It is happy as a clam at idling and cruising speeds (750-3500 RPMs).

Here is all the back info:
- All parts and engine have less than 75 hours on them over 5 years from the rebuild.
- I just put a well rebuilt Edelbrock 1409 on it (5/1) with the adapter plate. I had a Rochester Quadrajet on it, but it was always giving me fits. The person who rebuilt the Quadrajet did a very poor job. The Quadrajet also had the problem it is having now.
- It has a Pertronix electronic ignition built from the old original distributor from the 1979 block. The Pertronix was added 5 years ago with the new rebuild.
- The ignition coil is new and I tried another one just to make sure. Still the same results.
- I will be changing the spark plugs today. Hopefully changing the plug wires soon once I find the right part numbers.
- Rotor is new 2 years ago now. I'll be replacing that any day.
- Cap is probably 5 years old with the new rebuild so I should replace that too any day.
- I checked my fuel line from the tank pickup to the carb and all is clean. Even ran it on a 6 gallon plastic tank and still the same missing sound. Checked the vent line too. I run non-ethanol gas too, and there was no water in the fuel-water separator.
-The boat sits on a trailer, no bottom growth to worry about or anything.
-The problem has been hit and miss for 2 years now and I haven't been able to pin it.
-It does not always do it. Usually it happens after being on the water for more than 20 minutes or so...which is very frustrating because it wont happen for many hours sometimes, and I will think it is gone from whatever I did, but then all of the sudden........IT'S BACK!!!

• Could it be the mixture screws on the carb? I never adjusted those on the new carb, but I had the same missing with the old carb. I did not play with the mixture screws with the Edelbrock since I have never done it before, but I could learn how to do that...
• Could it be the old distributor body conversion to electronic ignition?
• Could it be the mechanical fuel pump going out? I took it apart and all of the rubber looked good, no frays...
• Could it be me not changing the wires, cap, plugs etc.? I will be doing that ASAP and will update once complete...

I have the video of it missing below, but it is hard to hear in the video. Much easier in person. Usually I am able to get to 4500-4600, but it was only allowing me to get to 4200-4300. I never run at that speed, but I throttle down for getting out of the hole so that is where the problem really hits is in the hole shot.

Thanks for any help or ideas!

 

[wshekar]

I had a random miss for a couple months only to figure out the ICM was going bad. Had a friend with the same engine so I used his and the problem went away. Replaced the ICM and haven't had the problem since. Costly part though.

 

[Rick Stephens]

• Could it be the mixture screws on the carb? I never adjusted those on the new carb, but I had the same missing with the old carb. I did not play with the mixture screws with the Edelbrock since I have never done it before, but I could learn how to do that...
• Could it be the old distributor body conversion to electronic ignition?
• Could it be the mechanical fuel pump going out? I took it apart and all of the rubber looked good, no frays...
• Could it be me not changing the wires, cap, plugs etc.? I will be doing that ASAP and will update once complete...

Mixture screws are only for idle.

Certainly could be an ignition issue. Cross that bridge after figuring out whether it is fuel.

Probably not wires plugs or cap. Doesn't hurt to swap em out.

But my bet is something fuel related. Might be worthwhile to put a 'T' in your fuel line and hook up a fuel pressure gauge. See if it is dropping off.

 

[nola mike]

Hello all. My Mercruiser 260 GM 5.7L I/O with the MC-1 Drive sounds like it is missing, or starving for fuel at higher RPMs (4000+). It does it when already on plane, and/or out of the hole. It is happy as a clam at idling and cruising speeds (750-3500 RPMs).

Here is all the back info:
- All parts and engine have less than 75 hours on them over 5 years from the rebuild.
- I just put a well rebuilt Edelbrock 1409 on it (5/1) with the adapter plate. I had a Rochester Quadrajet on it, but it was always giving me fits. The person who rebuilt the Quadrajet did a very poor job. The Quadrajet also had the problem it is having now.
- It has a Pertronix electronic ignition built from the old original distributor from the 1979 block. The Pertronix was added 5 years ago with the new rebuild.
- The ignition coil is new and I tried another one just to make sure. Still the same results.
- I will be changing the spark plugs today. Hopefully changing the plug wires soon once I find the right part numbers.
- Rotor is new 2 years ago now. I'll be replacing that any day.
- Cap is probably 5 years old with the new rebuild so I should replace that too any day.
- I checked my fuel line from the tank pickup to the carb and all is clean. Even ran it on a 6 gallon plastic tank and still the same missing sound. Checked the vent line too. I run non-ethanol gas too, and there was no water in the fuel-water separator.
-The boat sits on a trailer, no bottom growth to worry about or anything.
-The problem has been hit and miss for 2 years now and I haven't been able to pin it.
-It does not always do it. Usually it happens after being on the water for more than 20 minutes or so...which is very frustrating because it wont happen for many hours sometimes, and I will think it is gone from whatever I did, but then all of the sudden........IT'S BACK!!!

• Could it be the mixture screws on the carb? I never adjusted those on the new carb, but I had the same missing with the old carb. I did not play with the mixture screws with the Edelbrock since I have never done it before, but I could learn how to do that...
• Could it be the old distributor body conversion to electronic ignition?
• Could it be the mechanical fuel pump going out? I took it apart and all of the rubber looked good, no frays...
• Could it be me not changing the wires, cap, plugs etc.? I will be doing that ASAP and will update once complete...

I have the video of it missing below, but it is hard to hear in the video. Much easier in person. Usually I am able to get to 4500-4600, but it was only allowing me to get to 4200-4300. I never run at that speed, but I throttle down for getting out of the hole so that is where the problem really hits is in the hole shot.

Thanks for any help or ideas!

How is the 1409 jetted and set up?

 

[Kola16]

I had a random miss for a couple months only to figure out the ICM was going bad. Had a friend with the same engine so I used his and the problem went away. Replaced the ICM and haven't had the problem since. Costly part though.

Hmm that could be an option. The wiring harness looks like the original. I thought the ICM was only for starting, but shows you how much I know.

 

[Kola16]

Mixture screws are only for idle.

Certainly could be an ignition issue. Cross that bridge after figuring out whether it is fuel.

Probably not wires plugs or cap. Doesn't hurt to swap em out.

But my bet is something fuel related. Might be worthwhile to put a 'T' in your fuel line and hook up a fuel pressure gauge. See if it is dropping off.

That is a good idea, I'll have to get a fuel pressure checker and test it.

 

[Kola16]

How is the 1409 jetted and set up?

Yikes! I will send the company that rebuilt the carb a message.

 

[kenny nunez]

This is something else to check with a flush out. Start the engine then put it in forward, turn off the engine and push the throttle as far as it will go. Go and look at the plate where the shift linkage is, look at the shift interrupter switch with the lever and plastic roller, the roller must in the center of the V. Sometimes the shift linkage is going too far into forward gear and causes the plate with the V to move just enough to cause a high speed stumble.

 

[Kola16]

How is the 1409 jetted and set up?

"This one comes with .098/.101 jets (primary/secondary) and .068x.047 rods and the orange step up spring as intended from Edelbrock." From the rebuilder...

 

[Kola16]

This is something else to check with a flush out. Start the engine then put it in forward, turn off the engine and push the throttle as far as it will go. Go and look at the plate where the shift linkage is, look at the shift interrupter switch with the lever and plastic roller, the roller must in the center of the V. Sometimes the shift linkage is going too far into forward gear and causes the plate with the V to move just enough to cause a high speed stumble.

I know what you are talking about i will for sure check it out, thanks!

 

[Scott Danforth]

use a fuel pressure gauge and check fuel pressure. that will tell you the health of your fuel pump

is the carb inlet filter in good condition? is it clean?

which pertronix kit? igniter - the one with the magnetic sleeve that sits on the cam lobe, igniter II - the one that reads the cam lobes? if you have the igniter and there is debris stuck to the magnet, that can cause a miss

did you remove the ballast wire when you installed the pertronix kit 5 years ago? is it getting the full 12 volts, or only the 9 volts after the ballast wire?

the igniter module could be failing, pull the pertronix and drop in a set of points (if you followed the instructions, you still have the points as backup)

 

[Kola16]

So I changed the spark plugs and they were gross. I ran the boat for 6 or so hours and no misses. Of course it runs fine when we're watching. We were trying to see if the interrupt switch was being tripped but we weren't having any misses.
The boat was; however, not cruising at its normal cruising speed and could not hit the top speed. It's normal cruising is 29 mph at 3500 RPMs and 41 mph at 4400 RPMs. It was only hitting 26/27 mph at 3500 RPMs and 37 at 4400 RPMs. The boat was pretty weighted, but I did not think it was that weighted! I think it is unrelated though. Maybe prop related or something...

use a fuel pressure gauge and check fuel pressure. that will tell you the health of your fuel pump

is the carb inlet filter in good condition? is it clean?

which pertronix kit? igniter - the one with the magnetic sleeve that sits on the cam lobe, igniter II - the one that reads the cam lobes? if you have the igniter and there is debris stuck to the magnet, that can cause a miss

did you remove the ballast wire when you installed the pertronix kit 5 years ago? is it getting the full 12 volts, or only the 9 volts after the ballast wire?

the igniter module could be failing, pull the pertronix and drop in a set of points (if you followed the instructions, you still have the points as backup)

I did buy a fuel pressure gauge from NAPA, but then I realized it was for fuel injected so the gauge read to way too high of pressures! DOH! I ordered another for lower pressures that will be here in a few days.

The Pertronix kit is the Pertronix II. It is getting 9 volts I believe the last time I tested it, but I will test again to verify. I do not still have the points. Someone else did the swap for me. The points were old, old, oooooold, and gross. Before the rebuild the boat sat for 30 years under tarps. So the distributor sat for a looooong time and I was debating getting a new distributor...

 

[Rick Stephens]

Running slow at a given RPM has nothing to do with your motor. Bent prop, nasty hull, or water logged, but not motor.

 

[Kola16]

Running slow at a given RPM has nothing to do with your motor. Bent prop, nasty hull, or water logged, but not motor.

That's what I figured. Thanks! My guess is the prop and weight...

 

[Scott Danforth]

So I changed the spark plugs and they were gross. I ran the boat for 6 or so hours and no misses. Of course it runs fine when we're watching. We were trying to see if the interrupt switch was being tripped but we weren't having any misses.
The boat was; however, not cruising at its normal cruising speed and could not hit the top speed. It's normal cruising is 29 mph at 3500 RPMs and 41 mph at 4400 RPMs. It was only hitting 26/27 mph at 3500 RPMs and 37 at 4400 RPMs. The boat was pretty weighted, but I did not think it was that weighted! I think it is unrelated though. Maybe prop related or something...

I did buy a fuel pressure gauge from NAPA, but then I realized it was for fuel injected so the gauge read to way too high of pressures! DOH! I ordered another for lower pressures that will be here in a few days.

The Pertronix kit is the Pertronix II. It is getting 9 volts I believe the last time I tested it, but I will test again to verify. I do not still have the points. Someone else did the swap for me. The points were old, old, oooooold, and gross. Before the rebuild the boat sat for 30 years under tarps. So the distributor sat for a looooong time and I was debating getting a new distributor...

you may also need the mercruiser fuel test fitting

if you are only getting 9 volts, you still have the ballast wire in the system. follow the igniter II instructions.

however, as I stated prior, I would rebuild the distributors vs spending money on new ones.

 

[Kola16]

you may also need the mercruiser fuel test fitting

if you are only getting 9 volts, you still have the ballast wire in the system. follow the igniter II instructions.

however, as I stated prior, I would rebuild the distributors vs spending money on new ones.

My mistake. It is getting 12v. I will test the fuel pressure as soon as that kit gets to me (shipping says Sunday).

 

[Kola16]

Okay long story. So I took it to a mechanic just to run it on the hose real quick to see if he notices anything. He noticed there was rubber dust clogging up my flame arrestor. So while it is running, I take the flame arrestor off, and the RPMs pick up a little at idle. I thought it was an "aw ha" moment. The power steering belt had come loose and rubber dust from it had clogged the flame arrestor causing the carb to not be able to suck in air. So we replace and tension the power steering belt, and clean out the flame arrestor and leave the mechanic's to hit the water with high hopes. I mentioned to the mechanic that I thought it may be the fuel pump causing the missing, and his response was, "maybe, but I doubt it since mechanical fuel pumps are pretty bullet proof."

So we hit the water and same thing. Could barely get on plane and missing, and couldn't make it past 3500 RPMs under load, same problem. In neutral we could hit 5000+ RPMs just fine like always, but not in gear.

So I switched out the plug wires, cap, rotor, and plugs. The plugs I had just put in a few weeks prior. The plugs I had just put in had a lot of black on them. Weird. They looked like crap and they were in there for 6ish hours of running (the boat ran fine for those 6 hours too, but then it ran like crap again). Ran the boat after swapping out everything, and still the same thing.

So I hooked up the fuel pressure gauge to the system, and it was reading 4 PSI. It should read 5-7 PSI right? Is 4 too low? I am going to get better fittings for the fuel pressure gauge tomorrow because I may not have had a good enough fit, but I am not sure. I will update tomorrow.

So how often do those mechanical fuel pumps go bad? From what everyone has told me, they are pretty reliable, but then again why would electric fuel pumps exist? Trying to fix hard starts? UGHH BOATS!

 

[dubs283]

I hooked up the fuel pressure gauge to the system, and it was reading 4 PSI. It should read 5-7 PSI right? Is 4 too low?

4 psi might be okay for lower rpm speeds but once the boat planes or trying to get on plane pressure should be a steady 6-7 psi

it is in your interest to check fuel system vacuum as well, should be no more than 2 in/hg at all times

 

[tank1949]

That is a good idea, I'll have to get a fuel pressure checker and test it.

Yours may have clogged inline fuel filters. Mine did. Pain in the butt to get them out....

 

[Kola16]

Oh also I know it is not the shift interrupt switch because I held the little lever spring thing open so it would not trigger it and still the same thing.

4 psi might be okay for lower rpm speeds but once the boat planes or trying to get on plane pressure should be a steady 6-7 psi

it is in your interest to check fuel system vacuum as well, should be no more than 2 in/hg at all times

Okay I will try to check this. I have never heard of a fuel system vacuum before so I will have to Youtube that...I checked my tanks air vent and it was clear. Can gas block the air vent line if the tank is too full of gas?

Yours may have clogged inline fuel filters. Mine did. Pain in the butt to get them out....

I only have one inline fuel filter that is directly before the carburetor, and it is brand new as of 3 weeks ago. My pickup line from the tank does not have a fuel filter.


My spark plugs were black and sooty though. Doesn't that mean I am running rich, and I am not getting enough air? Or one cylinder isn't igniting?

BTW thank you for all of the suggestions!