Carbed Mercruiser 260 5.7L Missing Above 4000 RPMs

[nola mike]

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.

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?

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!

If you have a mechanical fuel pump there's another filter in the fuel pump assembly. Sooty plugs can be an ignition problem as well, if you're not igniting the mixture. One cylinder problem would be plug problem in that cylinder only.

 

[Kola16]

If you have a mechanical fuel pump there's another filter in the fuel pump assembly. Sooty plugs can be an ignition problem as well, if you're not igniting the mixture. One cylinder problem would be plug problem in that cylinder only.

So take the fuel pump apart and look for a filter in it?

I ran it today to get a video of the fuel pressure. At idle it was between 4 and 5 PSI. But when i hit the throttle it would drop to 2.5-3 PSI as you can see in the video https://youtube.com/shorts/wZyFibjlJoU?feature=share

 

[Kola16]

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

I tried to check the vacuum, but the fuel pump had metal lines running to it on the intake from the fuel water separator. So I took the reading between the tank and the fuel water separator. Is that an okay spot to test it at? It said 4 in Hg. Although I question this cause the gauge needle does not sit at 0 when disconnected ...

Also I tried running the engine on a separate auxiliary today with no avail. Same can't get up to power missing thing...

 

[dubs283]

At idle it was between 4 and 5 PSI. But when i hit the throttle it would drop to 2.5-3 PSI as you can see in the video

that's not good, pressure should not be dropping when rpm increases

also, get rid of that in line fuel filter, there should be no in line filter on the output side of the fuel pump

as far as the vacuum, it's fine that you checked it between the tank and the separator, 4 in/hg is too high but if the gauge is static at two then its okay. good check to run on auxiliary tank, best guess at this point is to replace the fuel pump and abate the filter and test

 

[Mcfltfyter]

I wish mechanical pumps were actually as reliable as your mech thinks they are.

 

[tank1949]

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.

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?

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!

U should also have a canister type water/fuel separator filter, at least all of my carbureted Mercruiser had them. Go buy a cheap automotive electric fuel pump and pump some gas into bucket. If a 6-7 fuel E pump starves for fuel, I'd suspect something is plugged up. Just my 2 cents. Good luck!

 

[Kola16]

that's not good, pressure should not be dropping when rpm increases

also, get rid of that in line fuel filter, there should be no in line filter on the output side of the fuel pump

as far as the vacuum, it's fine that you checked it between the tank and the separator, 4 in/hg is too high but if the gauge is static at two then its okay. good check to run on auxiliary tank, best guess at this point is to replace the fuel pump and abate the filter and test

Really get rid of the fuel filter? The Edelbrock carb install manual calls for it since the Edelbrock does not have a filter in the carb like the Rochester had. If I get rid of that one then the only filter would be the one that is supposedly in the pump. How fine of a filter is the one in the pump?

 

[nola mike]

Really get rid of the fuel filter? The Edelbrock carb install manual calls for it since the Edelbrock does not have a filter in the carb like the Rochester had. If I get rid of that one then the only filter would be the one that is supposedly in the pump. How fine of a filter is the one in the pump?

Some applications came with an inline filter, though I don't think any v8s did. Also I don't think any were on the pressure side of the pump. None of your setup looks stock to me, but I may be wrong. Usually hard lines to the carb, any flex lines need to be uscg approved. A spin on type filter on the suction side of the pump is the "official" upgrade. You need to check your pump filt #2, inside #1

 

[dubs283]

Really get rid of the fuel filter? The Edelbrock carb install manual calls for it since the Edelbrock does not have a filter in the carb like the Rochester had. If I get rid of that one then the only filter would be the one that is supposedly in the pump. How fine of a filter is the one in the pump?

yes, it's not a good idea to have a filter on the pressure side of the pump. also, as mike states hard line should be used on the pressure side, or braided approved fuel line with flared connections.

if you are concerned about filtering the fuel you can add one prior to the pump, but a water separator and the filter in the pump should be sufficient

 

[Kola16]

yes, it's not a good idea to have a filter on the pressure side of the pump. also, as mike states hard line should be used on the pressure side, or braided approved fuel line with flared connections.

if you are concerned about filtering the fuel you can add one prior to the pump, but a water separator and the filter in the pump should be sufficient

Got it, thanks for the clarification on the fuel water separator and fuel pump fuel filter being enough filtering. The braided flex hose is CG approved. I'll ditch the fuel filter and rubber hose before the carb and remove the fuel pump to check the filter this evening. The majority of my engine is stock. The Edelbrock with the inline fuel filter and the braided flex hose are about the only aftermarket parts I think.

I'll have to get the part number from my fuel pump when I take it off because mine does not look like the one pictured above. Plus there is a glass bowl on mine, which is empty like it should be. I don't think it has 3 gaskets either. Should it though? It seems like most mechanical SBC fuel pumps should bolt up pretty much the same minus how the fuel lines are routed. The boat is from 1979, so I bet it is older than the one pictured above I am unsure where the fuel pump came from as the engine was rebuilt 5 +/- years ago, but the mechanic had a fuel pump in stock that looked just like mine, which he said was the "old" style.

 

[Kola16]

This is my setup. The Mercruiser fuel pump part number is 86246. I have the diagram attached and a picture of what my fuel pump actually looks like. I Will take it apart tonight to look at the filter, but knowing the history of my boat, I bet it is clogged as can be since when the Rochester carb was in there, the fuel filter in the Rochester carb would get a little bit of stuff in it, so I can imagine that a fuel filter before that one will definitely trap some crap!

 

[Kola16]

So I realized that the pressure gauge was definitely not calibrated and was reading lower than it should have for delivery pressure and higher for vacuum. The needle sat at an atmospheric pressure at 3 in. of Hg. I first took the inline fuel filter out. I then calibrated the gauge (calibrated to 0 PSI at atmospheric) and ran the engine on the hose, and the gauge was reading about 6 PSI at idle. When I thottled down, it would drop a little, but not as much, and then come back up to 6 PSI. That was on the hose though, so I bet it would drop a lot more in the water under load, but maybe not. I also took the fuel line from the outlet of the fuel pump and pumped it into a bucket and it seemed like a solid stream.

I then took the fuel pump off and like I thought, there was no filter in the fuel pump so the fuel pump was definitely not clogged. That means that the only filter I have in my system now is the fuel/water separator. Is that enough on its own? I bought a replacement fuel/water separator since that one had been in there for about 4 years.

I think my next step will be to put in a new fuel pump. Man those suckers are not cheap compared to your standard SBC car mechanical fuel pump

Calibrated fuel pressure gauge running on the hose in the driveway:

Fuel pumping into bucket video:

Fuel pump disassembled video:

 

[Scott06]

If you are getting 4-6 psi of fuel pressure the pump , filter , and delivery system are doing their job and a new fuel pump wont help you.

 

[Kola16]

If you are getting 4-6 psi of fuel pressure the pump , filter , and delivery system are doing their job and a new fuel pump wont help you.

I get that, but I am concerned that the pump is working intermittently. I have had this same problem for a looooong time. My boat will run fine for a long time then all of the sudden I can't get it to run at 3500 RPMs, just idle. I thought it was the old carburetor I had on there. So I took the old Rochester carb out and put a rebuilt Edelbrock 1409 in and it ran great for 3 trips, but on the fourth trip I got the same problem I got with the Rochester. The Rochester was always hard starting it took lots of cranking (coil spring choke). The Edelbrock (electric choke) always fired up right away until the fourth trip it took tons of cranking like with the Rochester and it was also that fourth trip that it didnt want to run properly...

Additionally, I am not sure if the fuel pump is the original fuel pump that came with the boat from 1979, or if it was replaced in the rebuild 4ish years ago. I'm thinking it is the original since it definitely had the original glass indicator and it had a tiny label playe of it with a serial number that led to nothing when I put it into Google

 

[Kola16]

41421293F
Looks ancient

 

[Kola16]

I am not sold that it is the fuel pump though. It might be the newly rebuilt carb and here is why I think it is possible. One day I was at the lake when I had the Rochester carb and the boat was doing the same thing. It could idle, but it could not plane and sounded like it was missing at high RPMs. I put the boat on the trailer, disassembled the entire carb, put it back together, and the boat ran great when I put the Rochester back together and in the boat. In fact it started right up even with the bowl being empty from disassembling it.

Fast forward to now with the newly rebuilt Edelbrock 1409. It had this problem the first trip AFTER I ran the boat in salt water for an entire day of fishing (2 weeks of sitting in between). Yes saltwater can sprinkle onto the motor, now that I type that I realize I need to fix that... The mechanic said that he does not trust rebuilt carbs because he has seen many rebuilt that cannot keep corrosion at bay since they loose their anti-corrosion properties after rebuilds. Well this is what my newly rebuilt 1409 carb looks like now:


To me it looks like I threw $300 into the hole in my hull because when I put this carb in in April, it was sparkly, shiny clean and looked brand spanking new.

What do y'all think or have opinions on? Carb or fuel pump? I am replacing the fuel pump regardless just for my sanity...

 

[Rick Stephens]

Looks like that carb has all the protective coating washed off it. Edelbrock says only use Simple Green to clean their carburetor and never carb cleaner or a tanking. Whoever did the rebuild doesn't know Edelbrock. Too bad, they are really nice carburetors.

 

[Scott06]

I get that, but I am concerned that the pump is working intermittently. I have had this same problem for a looooong time. My boat will run fine for a long time then all of the sudden I can't get it to run at 3500 RPMs, just idle. I thought it was the old carburetor I had on there. So I took the old Rochester carb out and put a rebuilt Edelbrock 1409 in and it ran great for 3 trips, but on the fourth trip I got the same problem I got with the Rochester. The Rochester was always hard starting it took lots of cranking (coil spring choke). The Edelbrock (electric choke) always fired up right away until the fourth trip it took tons of cranking like with the Rochester and it was also that fourth trip that it didnt want to run properly...

Additionally, I am not sure if the fuel pump is the original fuel pump that came with the boat from 1979, or if it was replaced in the rebuild 4ish years ago. I'm thinking it is the original since it definitely had the original glass indicator and it had a tiny label playe of it with a serial number that led to nothing when I put it into Google

A couple of troubleshooting thoughts come to mind.

- measure fuel pressure when this happens
- you mention the new carb worked for three times then started acting the same- possible that debris in fuel system pugged this carb same as the Q-jet? Might be worth lifting the fuel bowl cover to see what it looks like in there.
-have you measured the strength of the spark both when it runs well and when it acts up. Simple spark gap tester should be nice blue spark jumping at least 3/8" gap, and also check timing and advance with a timing light.

Hard to diagnos from the keyboard but if new carb is clean I would lean more towards ignition issue. Maybe replace the pertronix module and or verify spark looks right on timing light (aka not missing) and the total advance isnt getting hung up. I assume you have a mechanical advance possibel corrosion etc is hangin it up. Id think you would want total of like 28-30 degrees at 2800-3200 rpm. Without proper advance you wont make power at high rpms

 

[Kola16]

A couple of troubleshooting thoughts come to mind.

- measure fuel pressure when this happens
- you mention the new carb worked for three times then started acting the same- possible that debris in fuel system pugged this carb same as the Q-jet? Might be worth lifting the fuel bowl cover to see what it looks like in there.
-have you measured the strength of the spark both when it runs well and when it acts up. Simple spark gap tester should be nice blue spark jumping at least 3/8" gap, and also check timing and advance with a timing light.

Hard to diagnos from the keyboard but if new carb is clean I would lean more towards ignition issue. Maybe replace the pertronix module and or verify spark looks right on timing light (aka not missing) and the total advance isnt getting hung up. I assume you have a mechanical advance possibel corrosion etc is hangin it up. Id think you would want total of like 28-30 degrees at 2800-3200 rpm. Without proper advance you wont make power at high rpms

-I will keep the fuel pressure gauge on there from now on. I'm going to order a more permanent one so I can just leave it in there.

-I do not think it is debris in the fuel line because when i pulled the Q-jet apart I did not find a speck of anything and it was very clean. I also found little to no debris in the Q-jet's internal filter. And i have pulled the straw off the top of the fuel tank to look in there and it is very clean in there.

-I will have to replace my timing light. I can't find the connections for it and it is very old anyway so I will make sure the timing is on.

Hopefully I can check the above this weekend...

 

[Kola16]

Looks like that carb has all the protective coating washed off it. Edelbrock says only use Simple Green to clean their carburetor and never carb cleaner or a tanking. Whoever did the rebuild doesn't know Edelbrock. Too bad, they are really nice carburetors.

I agree. Not impressed at all. Last time I buy a rebuilt carb.