Mercruiser 5.7 thunderbolt ignition running issues

[Rufdod]

I've searched tons of threads and can't seem to find exactly what I'm looking for .

I have a mercruiser 5.7l 350 with thunderbolt ignition and alpha drive , with a 2 barrel carb in a 1996 21' Marada open bow.

Starting off last season I decided to do a carb rebuild due to some rough running early season , and wanted some better performance / fuel economy and figured on a 20 year old boat it wouldn't be a bad idea .

After the rebuild I was have intermittent running issues that seemed to result in sputtering , popping then shutting down, it would eventually restart after letting it sit . When the boat was running it ran fine and strong until this occurred . I basically ended up chasing my tail and replacing about everything within reason that I could with no improvement . Towards the end of the season the boat ran consistently just slow, about 300 rpm lower that it did all season . This was after I repropped the boat to get the rps where they should be, prior to this I ran 3500rpm across the board

The list of parts are as follows :
Carb rebuild
New alternator
New battery
New cap and rotor
New plug wires including to coil
New coil
New fuel/water separator
New tank pickup line
Checked and cleaned tank vent
Installed online fuel gauge to check fuel pressure (checked out fine )
Adjusted the shift cables which would rule out the shift interrupter because I was having that issue prior to adjustment
Cleaned spark arrestor
New starter (don't think that's relevant )

Attempted to replace the ignition sensor under the cap, but when it was replaced the boat ran terrible and wouldn't rev past maybe 2000rpm

Checked the timing and seemed fine

So far the only thing I have done this season is ran a compression test on all cylinders getting 150psi on all but #6 which was 142

So I guess my question is can anyone point me in a direction of something I haven't done? My thoughts are either a carb related issue or something related to the new ignition sensor not working / making the boat run worse

 

[alldodge]

I think you need to redo the carb again.
Pull some plugs and tell us what they look like

Also are you setting the timing by putting the motor in base timing mode?

 

[Rufdod]

All the plugs looked the same , a wetish black, but the plug on the cylinder that was 142psi was more brown and dry .

I've had the carb apart about 5 times , the last time having a shop set the float. But I'm not ruling out the carb..

 

[alldodge]

wetish black

This means your running rich, and is probably the reason you have to let it sit a while if it won't start. The sitting allows things to dry out so it will start. The more brown or tan color is what you want.

If you have a mechanical fuel pump make sure its not dumping fuel in the carb from a ruptured diaphragm

 

[Fun Times]

I don't see new spark plugs on the parts replaced list. What plug number are you running?

Since the spark plugs are rich looking, Carb float level might be off too. Did you change or readjust the float at all? Are you comparing the Carb measurements to the service manual for your engine?
What did the carb rebuild consist of?

 

[Rufdod]

I did readjust the float level and had the shop adjust it to the specs for the engine.
I did replace the plugs the beginning of last season and gapped them to spec , I forget the exact numbers on the plugs , when I get them I will post . As for the fuel pump, how do I tell if it's dumping fuel from a ruptured diaphragm ?

A different shop today tod me to check several things , including kill switch and ignition switch trying to diagnose if it's a fuel issue or ignition issue

 

[alldodge]

Boat was running sort of ok before the carb rebuild. Now after the rebuild it does not run correctly, plugs are black, its the carb.

The fuel pump will have a clear tube running from it to the carb. If the diaphragm ruptures, it start pushing fuel up this tube and in the carb. Being a 96 model it might be electric. What's your motor serial number?

 

[Rufdod]

Ok yeah, it's def mechanical and there's no fuel coming through the clear hose. I forgot about that hose. I agree it would make sense being the carb..

 

[Rufdod]

The boat ran the first season relatively fine all season, last season being the second season owing it , the first time I put it in the water I had an alarm going off which is what started the process of replacing parts, turned out to be the depth alarm not a temp or oil alarm, which led into doing a carb rebuild because it was running very rough the first day I had it in the water and with the alarm I didn't want to run it much so I never got a accurate idea on how it was going to run, I just jumped in and pulled the carb

 

[Rufdod]

As for the engine serial number I don't have it handy at the moment , but it's something very close to if not exactly 0F76999

 

[alldodge]

not exactly 0F76999

Close enough, your motor is here
http://www.mercruiserparts.com/5-7lx...-1996-0f601000

Has a mechanical block mounted fuel pump and Merc 2 bbl carb
http://www.mercruiserparts.com/bam/subassembly/31890/1939/100

 

[Rufdod]

Yep that's where I was getting my info

 

[Fun Times]

The fuel pump will have a clear tube running from it to the carb. If the diaphragm ruptures, it start pushing fuel up this tube and in the carb.

Rufdod, bear in mind that often the inner hole of the fuel pump hose bib "fitting" where the tube connects to becomes heavily gummed up to a solid mass blocking any potential excessive fuel flow back to the carb. You'll want to inspect the tube and fitting for blockage.

The list above mentions you checked fuel pressure,

Installed online fuel gauge to check fuel pressure (checked out fine )

What did the fuel pressure read? You may want to keep the gauge connected until you solve the running rich issue.

Since the spark plugs are fuel fouled and sort of old now due to running them after the season you readjusted the carb's float level, They should be either cleaned until you figure out the issue or new ones installed < especially once you get the engine running better.

Towards the end of the season the boat ran consistently just slow, about 300 rpm lower that it did all season . This was after I repropped the boat to get the RPM's where they should be. Prior to this I ran 3500rpm across the board

So at one point you where only able to achieve 3500 RPM at full throttle? What RPM are you reaching now at full throttle?

 

[Rufdod]

I have checked to make sure everything wasn't gummed up or clogged , it was somewhat clogged early last season.

Installing new plugs this season

As for the 3500rpm , that was with a 21p prop, after trial and error I finished the season with a 19p prop and around 45-5000 rpm

After last season I assume if I had read more and learned a lot more , I could have probably avoided a lot of wasted time and money by not messing with the carb. If I had figured out the depth alarm was the only thing going off , I could have actually ran the boat to see if there were any issues , and maybe learned about props and figured out the low rps were a prop issue not an engine run/tune issue.

 

[Fun Times]

Do you recall what the fuel pressure was? You should still consider keeping an eye on the psi till you figure something out..

So at this time would your game plan be put new spark plugs in, check everything you can visually, start the engine on the garden hose, see how it runs., Recheck your new spark plugs and depending on how they look go run the boat to see how the boat performs? Or?

Keep us updated and don't forget to mention what spark plugs you're currently running.

 

[Rufdod]

Thanks everyone I really appreciate the responses

The psi from what I remember last year was 10-14 , somewhere in there which I thought seemed high for a mechanical carb motor .

I will post back when I get the spark plugs, having some weather moving in the next few days so beginning of next week I should be able to get some more info

 

[alldodge]

If your getting 10-14psi that is way to high, and would be the reason the carb is flooding. It should be 9 psi max, and would be better around 6 or 7

 

[Rufdod]

That's sorta what I thought, but I haven't read anything that said a bad pump could cause higher psi, everything I read said to just make sure you are getting enough pressure . What would cause high psi? Or is the simple answer just a bad pump? Or is there a way to fix that?

 

[alldodge]

Might have the wrong pump for your motor. Most output 4 to 7 psi, but some are designed to output more in the case of an fuel injected system. They will output 15 psi which is pushing fuel to a high pressure pump which in turns builds to 40 or more. So you can get a regulator and place in line to adjust the pressure down, or find the correct pump.
https://www.summitracing.com/parts/...D51YaedmGVDNkDQSrBVG6v5iiucJOgFxhUaAlf48P8HAQ
It should have been 97401A8 and replaced by 8M0058164, which I think was made by carter but don't quote me

 

[Fun Times]

Has the fuel pump ever been replaced with new or remounted with new gaskets or?

Using the wrong size/type pump mounting gaskets has been known to change the OEM fuel pressure readings. Bad pump could cause to high of pressure too. Take note of the mounting gaskets and plate, (Items 2, 5, 6) http://www.mercruiserparts.com/bam/s.../31890/1939/60

Again, you need to ensure what the fuel pressure is or you'll just be wasting a new set of spark plugs.