3.0l Mercruiser Intermittent Carb Issue

[Roadblock007]

Hey all. I've been having this issue for the past few years and it's really bugging me. I'm pretty sure the float in the carb is sticking every once in a while. I got it all ready last weekend and de winterized it and it was running great. I fired it up today and it was running like garbage, was pouring fuel down the carb like mad. Whenever it does this I have noticed that I can turn the mixture screw at the side of the carb all the way closed and it has zero effect. I have a new fuel filter and a water separator. Sometimes it's ok and sometimes its crap. I rebuilt the carb 2 seasons ago and it made no difference

 

[Scott06]

Measure your fuel pressure if above 6 psi it can over come the float and needle and seat. Also check the fuel pump diaphragm rupture line which is a 1/4” yellowish clear Tyson tube up from fuel pump to carb air horn. If fuel pump diaphragm ruptures this will dump fuel in the carb. Did you replace the float when you rebuilt the carb. These carbs use a delicate Plastic covered foam Float easy to break when you handle it.

 

[Roadblock007]

Fuel pump diaphragm is ok, no fuel in the return line. I used a new float. This issue is intermittent. I'll tow it to the lake and itll be great, then go out again next week and its flooding. I don't have a gage to measure fuel pressure. Why would turning the mixture screw all the way closed have no effect?

 

[Rick Stephens]

Vacuum/Pressure gauges are used to test fuel line pressure. They are really inexpensive. Any auto parts store will have them. Cheaper than the wasted drive to the lake.

If the carburetor is pouring fuel down the intake like crazy, that has no relationship to the tiny little idle ports sitting right above the throttle plates that are controlled by the mixture screw. Sounds like you need to first test pressure, if that isn't it, rebuild the carburetor and find the gummed up check valve or crud in the needle valve. Carburetor rebuilds are the most often repeated job. All it takes is missing one thing and the rebuild isn't good enough. And there are a lot of little things can be missed or skipped.

 

[Roadblock007]

Vacuum/Pressure gauges are used to test fuel line pressure. They are really inexpensive. Any auto parts store will have them. Cheaper than the wasted drive to the lake.

If the carburetor is pouring fuel down the intake like crazy, that has no relationship to the tiny little idle ports sitting right above the throttle plates that are controlled by the mixture screw. Sounds like you need to first test pressure, if that isn't it, rebuild the carburetor and find the gummed up check valve or crud in the needle valve. Carburetor rebuilds are the most often repeated job. All it takes is missing one thing and the rebuild isn't good enough. And there are a lot of little things can be missed or skipped.

I have a vacuum gage but wasn't aware that was the tool to use. I didn't notice a test nipple. Any idea where to tap in?

 

[havoc_squad]

I have a vacuum gage but wasn't aware that was the tool to use. I didn't notice a test nipple. Any idea where to tap in?

Most are a dual vacuum gauge and low pressure fuel gauge, with measurement markings for both.

The tool you need I believe is 91-18078 made by quicksliver or equivalent part, called out at least in the OEM service manual for EST carbed based motors, which I believe is around 1989 or 1990 and up.

You install this item on the fuel line between the fuel pump and the carb. Then install the vacuum/low pressure fuel gauge with some clamp on the fitting to prevent leaks.

I would definitely recommend you get a temporary flexiable fuel rated fuel hose with the appropriate fuel connectors to allow you to bypass the metal fuel line.

It's a real pain in the rear to attempt to install pressure test fitting without some flexible fuel line. I've done this and I hated every minute trying to convince it to fit.