Carbed Mercruiser 260 5.7L Missing Above 4000 RPMs

Kola16

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Big WOW looking at my engine tonight! The person who installed my engine in my boat or the engine builder (whatever one) did not install the bolt that is on the front of the block used to hold the pushrod to the fuel pump up during install of the fuel pump! That part of the block must not be that high of pressure if there was little to no oil coming out of it? Definitely plugging that and doing an oil change....
 

Kola16

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Well I think I found the problem. At least I found "a" problem. I was about to assemble the fuel pump and gave it a one-over one last time. Then I found this: 20210604_182840.jpg

That crack in the gasket is just barely on the side going to the carb; therefore, the side seeing the 6 PSI. Going to pick up a new fuel pump tomorrow and see how it goes! Fingers crossed!
 

Kola16

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Well that did not fix "the" problem, but it did fix a problem down the road. Also my fuel pressure does not go down now when I hit the throttle and the pressure stays up when I turn the engine off, so that is good. I still am having the same problem not even being able to get to 4000 RPMs/missing etc. This was yesterday:

I am going to take the carb apart today to check for cloggage, and hopefully be able to buy a timing light and spark plug tester at NAPA.

If the timing was off on my engine though, why would it run good for a while, then not good at other times? If the timing was off, wouldn't it always run poorly?

I also found a leak in my Y-exhaust pipes while out yesterday...the problems are mounting 😭
 

Kola16

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I went to get a timing light and a spark gap jumping tester, and the local auto stores did not have a good timing light or a spark gap jumping tester at all.

So I took the carb apart and I need some help on understanding how the Edelbrock 1409 carb works. Does the fuel at higher RPMs get siphoned through the bottom of the bowl, and then up and out the jets? I found a decent amount of some weird sediment, which I am assuming are corrosion products from the carb. How does fuel get to the jets from the bowl of the carb? If the fuel has to go through some port at the bottom of the bowl, then that would definitely cause problems from all of the sediment being at the bottom.

Starboard side bowl (mucky gas from sediment):
Snapchat-1440411313.jpg
Port side bowl (much cleaner):
Snapchat-1930521542.jpg
Sediment at bottom of bowl after fuel evaporated:
Snapchat-522627197.jpg
 

Scott06

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Heres a tuning guide that shows the theory of operations. Yes siphoned through the jets and out the venturis primary during low speed and as rpm increases as airflow requires the secondaries start to get airflow the sediment you see is probably clogging and causing some of your issues

 

Kola16

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Heres a tuning guide that shows the theory of operations. Yes siphoned through the jets and out the venturis primary during low speed and as rpm increases as airflow requires the secondaries start to get airflow the sediment you see is probably clogging and causing some of your issues

Thank you that is exactly what i was looking for! I was calling the jets the wrong thing. The jets are at the bottom of the bowl and that is what the fuel goes through which transfers the fuel out the nozzles. I was calling the nozzles the jets 🥴

I will try to blow out and clear the carb, and if the boat runs good i will have to get a brand new carb. No rebuilts this time, expensive lesson learned i guess. I will also find a better timing light nearby to check what my advance is at just to at least know....
 

Scott06

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Thank you that is exactly what i was looking for! I was calling the jets the wrong thing. The jets are at the bottom of the bowl and that is what the fuel goes through which transfers the fuel out the nozzles. I was calling the nozzles the jets 🥴

I will try to blow out and clear the carb, and if the boat runs good i will have to get a brand new carb. No rebuilts this time, expensive lesson learned i guess. I will also find a better timing light nearby to check what my advance is at just to at least know....
No need for a new carb just clean and rebuild yours. places like mikes carb parts have rebuild kits (gaskets needle seat and accelerator pump). If this carb only worked 3-4 times then had issues you have fuel system contamination , buy a new carb and its passages will be plugged soon too. Need to clean out tank and lines of sediment and if you have a fuel filter it isnt working
 

Kola16

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No need for a new carb just clean and rebuild yours. places like mikes carb parts have rebuild kits (gaskets needle seat and accelerator pump). If this carb only worked 3-4 times then had issues you have fuel system contamination , buy a new carb and its passages will be plugged soon too. Need to clean out tank and lines of sediment and if you have a fuel filter it isnt working
The corrosion and particulates did not come from the fuel tank they came from the carb. I know this because the same corrosion products were on the non-fuel parts of the carb as well. They stripped the nickel plating and corrosion resistant stuff off the carb after they rebuilt it. Thats why it quit working after the first time using it in the saltwater. The mechanic that cleaned my flame arrestor looked at my carb and told me the thing was bad and that he sees it all the time. People buying rebuilt carbs that only work a few times. He said to only buy new carbs or carbs that have their corrosion resistance. I cut open the inline fuel filter I installed when I installed the rebuilt carb and it was clean as a whistle.
 

Kola16

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Update, so I took apart the carb and blew out on the channels and "stuff" with an air compressor. Got it all clean and particulate free. Put the carb back together to go test it on the lake. Still a no go for the higher RPMs. I could barely plane even with putting the trim tabs down.

So I finally found a decent timing light and checked the timing. (I still need to make sure that every cylinder is sparking. Just gotta find where the hell I put that tool I just bought 😂 ) The timing was 12° at idle (750-800 RPMs) and 32° at ~3000-3150 RPMs. That is weird. I have never checked the timing on this engine before so I am not sure what it started out at, but it was supposed to be set correctly. I have about 100 hours on the engine since the rebuild and most of the time it has ran flawlessly. So now I am curious if the timing was correct, then all of the sudden got messed up on its own somehow, or if it has always been that way? When it had the Quadrajet carb, it was always hard to start. When I put the Edelbrock on it was easy to start, until it quit working properly then it was hard to start. Definitely a sign of too advanced timing, but really not sure what happened.

Anyway, I set the timing to 8° at idle, which gave me 27° at ~3000-3150 RPMs. I have not run the boat on the lake yet to test it, although I am doubtful the timing being off by 4° too advanced is the difference between 21 mph and 41 mph at WOT. Stupid work not letting me play with my POS toys...
 

Kola16

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I tested the boat at the lake yesterday. Retarding the timing did seem to help the performance slightly, but definitely did not fix it, as expected. All cylinders had spark too. I am at a loss at this point. I will pull the carb apart again to see if any corrosion is back in there. I will also do a compression test next.

So far I've checked fuel delivery, air delivery, spark, time of the spark...going to check compression...does a clogged exhaust ever happen? I am running out of stones to turn and need ideas... If the distributor was not working properly, then I would have an improper advance at high RPMs granted that my idle timing is set properly, correct? If the distributor springs were worn out or loose, the advance at 3000 RPMs would be off? My advance at 3000 RPMs (27°) seem to be where it should be...
 

Scott06

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I tested the boat at the lake yesterday. Retarding the timing did seem to help the performance slightly, but definitely did not fix it, as expected. All cylinders had spark too. I am at a loss at this point. I will pull the carb apart again to see if any corrosion is back in there. I will also do a compression test next.

So far I've checked fuel delivery, air delivery, spark, time of the spark...going to check compression...does a clogged exhaust ever happen? I am running out of stones to turn and need ideas... If the distributor was not working properly, then I would have an improper advance at high RPMs granted that my idle timing is set properly, correct? If the distributor springs were worn out or loose, the advance at 3000 RPMs would be off? My advance at 3000 RPMs (27°) seem to be where it should be...
clogged exhaust is possible if an exhust shutter falls down into the wye pipe. They can make their way into the bellows and restrict exhaust. If you had the drive off recently and looked in the exhust bellows and that was clear look into the tops of the wye pipe.

I cant recall if you measured the fuel pressure while running at speed or just at idle. Yes would do compression test and recheck the carb. Carbs can be ticky to get clean for sure

Agree the ignition advance is correct, really just looking for 25-30 degrees at 28000 or so as a rule thumb
 

Kola16

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clogged exhaust is possible if an exhust shutter falls down into the wye pipe. They can make their way into the bellows and restrict exhaust. If you had the drive off recently and looked in the exhust bellows and that was clear look into the tops of the wye pipe.

I cant recall if you measured the fuel pressure while running at speed or just at idle. Yes would do compression test and recheck the carb. Carbs can be ticky to get clean for sure

Agree the ignition advance is correct, really just looking for 25-30 degrees at 28000 or so as a rule thumb
I did check the fuel pressure at WOT on the lake. I think it was 6.5 PSI or so, so that checked out. Held pressure well too with the new fuel pump. The old fuel pump did not hold the pressure for very long so good thing I replaced it.

I will do the compression test and definitely check the Y pipes. There is a leak in the port Y pipe at the lower joint, so that needs to be fixed anyway. I have gaskets laying around for taking the drive off so no problems with checking the exhaust completely.
 
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Kola16

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Something weird with testing today. When I was measuring the timing today, sometimes the timing light would not light up on the number 1 plug. So I put the timing light on different plug wires for testing and the timing light flashed just fine. Put it back on plug 1 and it would be sporadic when it flashed. When I put the timing light clip right next to the distributor cap for plug number 1, it flashed fine. Kinda weird I think. I put the inline spark tester on the number 1 plug again and it seemed to flash fine and consistent. Plugs 1 and 2 had carbon fouling. All other plugs where a little white/cream colored on the tips. I did look at the plug colors before I started pulling plugs. Keep in mind these are brand new plugs.

When the timing light would consistently flash, it read at 12° at idle again, even though I set it back to 8° and tightened the bolt down, and rechecked to make sure that it was at 8° after tightening. Is it normal for an engine's timing to do that? Could the teeth on the distributor be bad or something? I checked the advance at 3000 RPMs when it was at 12° at idle and it was at 30.5°.

The compression test for cylinders 1 through 8 respectively were: 145, 120, 135, 143, 120, 140, 130, and 160 PSI. I tested the 120 and 160 values twice to make sure that is what they were. Having 2 cylinders at 120 and 1 at 160 PSI is kind of concerning, but I think I am okay. Any thoughts?

Also I was hearing a popping noise on the port side of the engine only. I am not sure what the noise was...

I will be checking the exhaust system once the engine cools down...

Popping noise:

Number 2 carbon-fouled plug:
20210617_203705.jpg
 

Kola16

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I took the port side Y exhaust pipe off and inspected the shutters. They were right where they should be in good working condition. The leak was easy to find the cause of, the 3 allen screws had come loose so I tightened those back up when I reinstalled the pipe.

I am taking the carb apart again even though I am pretty sure it is clean. Probably time to take it to a real mechanic....
 

Kola16

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Wow, it is a lot harder to get a mechanic to work on my boat than I thought! So I have to try to fix it on my own. I was doing some research and I found out that I needed to do the compression test a different way. So this time I took the compression test with all of the spark plugs out, the throttle wide open, and the engine warm. None of which I did last time, but it still lead to the same general readings in relation to each other. This time I got:
1 - 165 PSI
2 - 142.5 PSI
3 - 160 PSI
4 - 162.5 PSI
5 - 140 PSI
6 - 165 PSI
7 - 160 PSI
8 - 190 PSI

I then did a wet test on 2 and 5 and got:
2 - 157.5 PSI
5 - 160 PSI

So it looks like I have low compression in 2 cylinders, and high compression in 1 cylinder. OUCH with only 100 hours on the engine! So I guess I am pulling the engine and watching Youtube on how to rebuild an engine.

I think the cause was running lean from the old carburetor, but that is just a wild guess...
 

kenny nunez

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How about doing a wet compression test on all the cylinders before you start pulling the engine apart. Also the low reading may be caused by a cylinder not firing from a ignition or possibly the carburetor leaking through the float chamber plugs.
There are places that specialize in Quadrajet carburetors who can rebuild it better than new with the latest modifications.
 

Scott06

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How about doing a wet compression test on all the cylinders before you start pulling the engine apart. Also the low reading may be caused by a cylinder not firing from a ignition or possibly the carburetor leaking through the float chamber plugs.
There are places that specialize in Quadrajet carburetors who can rebuild it better than new with the latest modifications.
Agreed none of those compression numbers would make me think the engine needs to come apart.
 

Kola16

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How about doing a wet compression test on all the cylinders before you start pulling the engine apart. Also the low reading may be caused by a cylinder not firing from a ignition or possibly the carburetor leaking through the float chamber plugs.
There are places that specialize in Quadrajet carburetors who can rebuild it better than new with the latest modifications.
Agreed none of those compression numbers would make me think the engine needs to come apart.
Really? Even with 190 PSI as my high and 140 PSI as my low? That's a 30% difference and 50 PSI difference. I have an Edelbrock 1409 on there now. I am just going to buy a new carb just to be safe after I finally get the darned thing running. I will do a wet test on all of the cylinders tonight.
 

Scott06

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Really? Even with 190 PSI as my high and 140 PSI as my low? That's a 30% difference and 50 PSI difference. I have an Edelbrock 1409 on there now. I am just going to buy a new carb just to be safe after I finally get the darned thing running. I will do a wet test on all of the cylinders tonight.
yes I suspect you have some guage accuracy issues around 150 psi is new, unless you had water or gas in the high one I suspect thats an outlier. Typically I think you start getting dead cylinders below 120 psi.

The carbon fouled plug is way rich I suspect that may be yoyur issue, although as weve seen hard to diagnos over the keyboard. the clicking noise is either a leaking exhaust manifold gasket or the flappers clicking in the wye pipe.

You verifyied the firing order is correct?I thought on these engines swapping 5 and 7 the engine would run ok just not have peak performance and not backfire....
 

Kola16

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yes I suspect you have some guage accuracy issues around 150 psi is new, unless you had water or gas in the high one I suspect thats an outlier. Typically I think you start getting dead cylinders below 120 psi.

The carbon fouled plug is way rich I suspect that may be yoyur issue, although as weve seen hard to diagnos over the keyboard. the clicking noise is either a leaking exhaust manifold gasket or the flappers clicking in the wye pipe.

You verifyied the firing order is correct?I thought on these engines swapping 5 and 7 the engine would run ok just not have peak performance and not backfire....
I did not verify the ignition timing, but I have no reason to believe it is off since it was running great, and then all of the sudden not without touching a thing. I will verify tonight anyway. I would not disagree that I have gauge accuracy issues, but I believe the readings in relation to each other are accurate, therefore the differences between them are accurate and precise. The highs and lows were the same between the 2 tests where I left the plugs in and engine cold with the throttle open versus the plugs out when the engine was hot with the throttle open on the 2 different days.
 
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