Carburetor issue??? Cut out at high RPMs

[sirchad]

I have fought an issue for a few years and I simply gave up on it. But now the symptom has changed.
Symptom: engine cuts out above 4K RPMs. This would only happen once or twice every trip to the lake.
I just rebuilt my carburetor and now it happens almost every time I get above 4K RPMS.
I also noticed that if during this cut out time I leave the throttle where it is the engine will die. If I pull back during this cut out the engine will not die.
This leads me to think it is a fuel starvation issue, just my thought.
When I rebuilt my carburetor I went by the book. However the mixture setting seems subjective to me. I am not a carburetor expert.
2002 4.3L Mercruiser. Thunderbolt IV

 

[sirchad]

Fuel filter/separator is changed every year. and the inline screen filter at the carb is clean and clear. I looked down into the gas tank and didn't see anything.

 

[Bondo]

This leads me to think it is a fuel starvation issue, just my thought.

Ayuh,..... I completely agree,.....

Fuel filter/separator is changed every year. and

Do ya dump it into a clear container, let it settle, 'n look for Anything but clean fresh gasoline,..??
If not, try it, I use a quart freezer baggie,....

How 'bout crud in the anti-siphon valve,..??
It's the hose barb at the tank's outlet,....

How 'bout the tank's diptube,..??
A crack in it up higher will allow air into the fuel flow,....

 

[kenny nunez]

One way to narrow down the problem is to use a 6 gallon out board tank and connect it where the original fuel line is connected.

 

[sirchad]

Ok so since I don't have an outboard fuel tank anymore I decided to open up the check valve/anti-siphon valve. Big mistake. The anti-siphon valve seized into the pickup fitting. Since they are both made of aluminum they broke as I tried to open it up. I have new parts on order to replace them. however the inside of the check valve had no signs of any buildup/sludge or marks on the ball. The fuel pickup tube screen was also spotless clean as if it were new. I guess the stabil has been doing its job.
However inside the fuel line I noticed the inside layer of clear looking plastic that looked bad. I looks to be peeling away from the rest of the inside. So maybe the lines themselves are causing the issue by restricting flow. I purchased new a new fuel line but I am still waiting on the anti-siphon valve and fuel pickup fitting.
Does anyone know the average life span of fuel lines. I have owned this boat 5 yrs. Not sure if they are original or not. There are no external signs of wear on the old fuel line. I have heard that ethanol is what is eating up the fuel lines. The good news going forward is that I now have an ethanol free gas station in town.

 

[sirchad]

I also don't see any cracks in the dip tube or fuel pickup tube but I new one comes with the fitting so it will be replaced.

 

[bman440440]

if your fuel lines look anything like these (Ethanol issue) the lines are your problem

 

[wellcraft-classic210]

The issue you describe does sound like fuel starvation. Have you considered or tried monitoring the fuel pressure at the carb inlet while the problem is occurring ?

Life span of rubber parts can be variable -- But I would expect 10 years in many cases -- If your hoses are deteriorating from the inside (vs dry rot & cracking form the outside _ it would seem the material the hoses were made of and fuel are not 100% compatible. Hard to say if its a defect or design issue. Ethanol could be part of the problem also.

 

[sirchad]

yes bman440440 that is how the lines look. This makes me optimistic that my problem will be resolved. Without buying a fuel pump because I assume that would be next.

 

[sirchad]

actually wellcraft-classic210 that is what I was thinking about this morning. Maybe I could find a guage and fittings at an autoparts store. The difficult part may be the fact that I have a steel fuel line going to the carb.

 

[wellcraft-classic210]

Yes -- I know what you mean --Its not a good idea to cut into the existing line --

Not sure if its easier to bypass the entire existing steel line with a temporary line and tee -- or add in something using a Female-Union Inverted Flare Nut, 3/8", etc. I guess it may depend on what you have on hand.

It seems you may not need to do the pressure test given what you found on your fuel lines / assuming that fixes the problem.

 

[sirchad]

I installed the new fuel line, anti-siphon valve and with fuel pickup fitting. I did a test run and it ran really well. 4300RPM at 49MPH. 850lbs in the boat. Problem appears to be solved. The issue was apparently the fuel lines. If this problem reoccurs I will update this thread otherwise I considered it closed. Thank you everyone for your help. Have a great boating season.