Mercruiser 5.7 carbureted wont start when warm

[NotAClue92]

Hey everyone, I have a 98 Mercruiser 5.7(carbureted) that will start fine when cold but when it's warm it wont start. I read about vapor lock and wrapped the fuel line with insulation tape(I know some people say vapor lock doesnt happen on carbureted engines and some say its very common, who the heck knows) It will start fine when its cold but when it gets up to temp(170 degrees) it wont start. When it's running I can see a few drips of fuel coming off the venturis. I ran it at a high idle and shut it off and there is quite a bit of fuel coming from the venturis and it seems like the carb is flooding. This carb was just replaced about a year ago. I think the mechanic put some chinese POS carb on it, but I havent had any issues with it until a few weeks ago. Any suggestions on what to do next other than buy a real carb and just bite the bullet(and not hire another half ass mechanic because I was lazy)? Thanks in advance.

 

[Scott06]

How about an engine serial number or picture of carb so we know what you are working on ?

Fuel dripping of Venturi when you shut off is stuck needle and seat /float or too high fuel pressure. Can measure fuel pressure then put a carb kit in the carb. What carb did he use ?

When this happens if you open the throttle a good bit (pull back quick when it fires) does that help. May be flooding and open throttle to clear it out will help

 

[NotAClue92]

The engine serial number is 0L031915
I didnt see any markings or numbers on the carb. I'll have to check again to make sure.
If I open the throttle all the way it will start within about 5 seconds. I noticed when I shut it off the venturi was leaking enough fuel to form a puddle in the carb and when I opened the throttle the fuel dumped in and started smoking(more of a mist or fog).
How would I measure fuel pressure coming out of the electronic fuel pump if it has a metal fuel line going to the carb?

 

[Scott Danforth]

You need the Mercruiser fuel pressure checking fitting. It's about $45

Merc part number 91-18078

Fuel Fitting Connector 91-18078 - CP Performance

Fuel Fitting Connector 91-18078

www.cpperformance.com

 

[Scott06]

Scott has you steered straight. Buy or rent a fuel pressure tester plug it into the test fitting scott mentions. Alternatively you can disconnect the fuel line from carb and cobble together flare fittings with hose and a T to put it in line between pump and carb. Need 4-6 psi.

you pretty much confirm it is i overflowing the float bowl.
you boat had a mercarb on it originally, which is a common 2 bbl carb. Sierra and others make aftermarket knock off carbs . Either too much fuel pressure or needle and seat is dirty or hung up

if you need carbs parts I get all mine through Mike’s carburetor parts

 

[kenny nunez]

Is the carburetor a 2 or a 4 barrel ?

 

[Scott06]

Is the carburetor a 2 or a 4 barrel ?

By the serial number its a mercarb 2 bbl. Was hoping hed post a pic since it had been replaced

 

[kenny nunez]

I never did like that Mercarb. Too bad his engine does not have a Quadrajet.

 

[NotAClue92]

Okay guys, sorry for the late reply. I spent most of the day yesterday trying to figure out what the heck is going on. I took a picture of the carb so i can post it. I also got a video of what is going on and saw the carb is flooding when the engine is warm and not running. It keeps dumping a ton of fuel into the carb. I took it apart and everything looked clean, but I cleaned the needle and seat anyway. I am not sure what the float level measurements are so I will have to find them to see if it is set correctly. Hopefully I can post a video to show you whats going on. I am going to try to find some fittings to connect a fuel pressure gauge today. Sorry if the images/video arent that clear(I have an android lol)

 

[NotAClue92]

Okay guys, sorry for the late reply. I spent most of the day yesterday trying to figure out what the heck is going on. I took a picture of the carb so i can post it. I also got a video of what is going on and saw the carb is flooding when the engine is warm and not running. It keeps dumping a ton of fuel into the carb. I took it apart and everything looked clean, but I cleaned the needle and seat anyway. I am not sure what the float level measurements are so I will have to find them to see if it is set correctly. Hopefully I can post a video to show you whats going on. I am going to try to find some fittings to connect a fuel pressure gauge today. Sorry if the images/video arent that clear(I have an android lol)

BTW, the video showing the fuel dumping into the carb is with the engine OFF. The engine temp was 170 and the motor was off and it was still dumping fuel

 

[Rick Stephens]

Even if the carb isn't chinesium, looks like the needle valve and seat probably are. And if you still have your original carb I'd be rebuilding it with a high end kit - no way you wore out an OEM carburetor in a couple hundred hours of use. That is where your quality is, the original.

 

[NotAClue92]

I tried rebuilding the original carb with an OEM kit and the check valve would not seat. I even brought it to a reputable boat shop that I have been to before and trust pretty well and they said it can get worn out and just won't seat anymore. Unfortunately the original carb got thrown out with the garage clean up. If I can find a number on the carb I can try a rebuild kit or try setting the float properly because I don't think it's set properly right now. Have you heard of an electric fuel pump going out and putting out too much pressure? I've done some research and some people say they can fail and put too much pressure to the carb.

 

[Rick Stephens]

Problem is in the carb, not the pump. Your pump isn't running when you're shutdown and see all the fuel dribbling.

 

[flashback]

Are you sure the fuel pump isn't running when the motor is not. It sure looks like a stuck float or trash in the needle jet.

 

[NotAClue92]

Okay, I checked the carb, there are no markings or numbers on it, but after doing some research I believe it's a Sierra carb(I thought Sierra was a decent brand, you think they would have some markings or something to identify it) Here is a picture of the carb I BELIEVE is on it. Not 100% sure though

 

[NotAClue92]

Sorry, it would be 18-7376N. The 18-7375N is for a 5.0 not a 5.7.

 

[NotAClue92]

Okay, I took the carb off to check the float height and it seems to be too high(causing the carb to flood). I was looking for the little ruler to measure the float height and found the old carb in the box that the new carb came in. Turns out the new carb is a Sierra 18-7375N. It says on the box it is for a Mercruiser 5.0, the engine in my boat is a 5.7. Could this be the issue? The mechanic insisted he had to order the carb to make sure it was the right one and this one says its for a 5.0, not a 5.7. So I did find out exactly what carb is on the boat now, but I cant find anything on what to set the float height at. I know what the Mercarb should be set to but I am not sure if its the same height as a Sierra. Does anyone know the float height for the Sierra 18-7375N? Any advice on whether or not the 5.0 carb will work with a 5.7. Its been running pretty good until the last few months so I know is was working good at first, but not sure if it will work in the long run. Thank you in advance for your help, I'm ready to be back on the water and I'm sure my daughter is too.

 

[Rick Stephens]

You need to kit that carb. Set the height same as the one for the 5.7. Replace the needle valve and seat cause it leaks. I would expect the 5.0 carb to be calibrated too lean for your 5.7. So get a peak at your plugs and see if they show signs of that. Post pics if you want.

 

[NotAClue92]

I checked the plugs and they look somewhat clean. A little bit brown around the porcelain. I'm going to call the mechanic that installed it and see what the deal is. He specifically said he had to order the parts so they weren't the wrong ones, so for him to F that up really irritates me. I'm going to try my best to get him to replace the carb on his dollar, but we will see how that goes.

 

[Scott06]

I checked the plugs and they look somewhat clean. A little bit brown around the porcelain. I'm going to call the mechanic that installed it and see what the deal is. He specifically said he had to order the parts so they weren't the wrong ones, so for him to F that up really irritates me. I'm going to try my best to get him to replace the carb on his dollar, but we will see how that goes.

unless the original core was bad either rebuild it with a good kit or send it to a place like Mikes Carb Parts For rebuild. I would question a mechanic who couldn’t or wouldn’t rebuild a OEM carb,unless the carb core/body was screwed up, only to put a cheaper aftermarket carb on.
the mechanic would have more billable hours doing the rebuild and less chance of a callback…again makes no sense from my vantage point…mercarb and its older brother Rochester 2 jet are dirt simple carbs that work well if properly maintained .