Mercruiser 5.7 carbureted wont start when warm

[NotAClue92]

I'm not sure how to tell if the original carb was worn out or not but I rebuilt it specifically as the rebuild instructions said and it still had issues which is why I called a mechanic. I got in touch with the mechanic and he said even IF the was the carb for the 5.0 it wouldn't have any issues unless I was at WOT a lot, and I rarely have this old boat at WOT. I ended up getting the float specs so I'll set the float height and see if it makes a difference. Luckily a good rebuild kit for this Sierra carb is pretty cheap, so if I need to I can rebuild it and see if that fixes the issue since a new carb runs about $400. Or have the original carb professionally inspected and rebuilt.

 

[Scott06]

I'm not sure how to tell if the original carb was worn out or not but I rebuilt it specifically as the rebuild instructions said and it still had issues which is why I called a mechanic. I got in touch with the mechanic and he said even IF the was the carb for the 5.0 it wouldn't have any issues unless I was at WOT a lot, and I rarely have this old boat at WOT. I ended up getting the float specs so I'll set the float height and see if it makes a difference. Luckily a good rebuild kit for this Sierra carb is pretty cheap, so if I need to I can rebuild it and see if that fixes the issue since a new carb runs about $400. Or have the original carb professionally inspected and rebuilt.

What was it doing that caused you to rebuild it. normally the biggest issue is getting all the passages clean

heres a couple of screen shits of manual pages showing setup for mercarb would assume sierra uses the same carb set ups. Check float isnt water logged

 

[tank1949]

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.

sounds like timing too advanced.

 

[nola mike]

sounds like timing too advanced.

Sounds more like he's running rich. Did you read the thread?

 

[NotAClue92]

I believe it was bogging down unless you very slowly gave it throttle. I rebuilt it and had issues with it still so I called a mechanic and he said it was the carb, so he replaced the carb. I pulled the carb and cleaned the needle and seat but I couldn't get the seat to come out, it was going to break trying to get it out so I just sprayed a bunch of carb clean in there and hopefully it cleaned it. I set the float height the same as I would a Mercarb so I will have to see if that solves the flooding issue, then I can check to see why it's not wanting to start when it's warm.

 

[Baylinerchuck]

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.

Going back to this post….the ball check, (if that’s what you’re talking about), is for the accelerator pump. During a rebuild one of the little known tricks is to form a new seat for the stainless steel ball.
1. Ensure the seat is clean. Use a-tips, carb cleaner, compressed air.
2. Inspect the old ball making sure it’s clean and not pitted.
3. Drop the ball in the chamber.
4. Use a small pin punch and hammer and sharply smack the ball into the seat. Just a light smack will do, don’t smash it.
5. Remove the old ball, install the new ball, spring and cap.
If the ball does not seat, you’ll have a stumble when accelerating. It’s a simple problem to resolve. Also rebuilding the original carb is a MUST. That carb was specifically tuned for your engine. After market carbs are tuned for a happy medium. Get a quality kit and rebuild the oem mercarb. Just my advice.

 

[Rick Stephens]

While you play around with the chinesium carburetor, send the OEM one off to a real carburetor shop.

 

[NotAClue92]

I have a good shop that I trust so I'll try bringing the old carb to them and have it rebuilt. I've been told mixed things about whether or not this new carb will work on my engine. Some say it's fine, some say it will need to be jetted or replaced. I still am trying to figure out the original issue with it not starting when it's warm, but that could have been an issue with the carb this entire time(it flooding the engine). I'll bring the old carb to my trusted shop and see what they can do. Thanks for the advice. I'll post an update once I get a chance to test the boat and see if the float adjustment helped or if I end up just swapping back to the old carb once it's rebuilt.

 

[tank1949]

Sounds more like he's running rich. Did you read the thread?

I didn't have too. I have experience timing issues too many times. Cheap or stale gas gets too much advance, and a hot motor can't crank well.

 

[Scott06]

I have a good shop that I trust so I'll try bringing the old carb to them and have it rebuilt. I've been told mixed things about whether or not this new carb will work on my engine. Some say it's fine, some say it will need to be jetted or replaced. I still am trying to figure out the original issue with it not starting when it's warm, but that could have been an issue with the carb this entire time(it flooding the engine). I'll bring the old carb to my trusted shop and see what they can do. Thanks for the advice. I'll post an update once I get a chance to test the boat and see if the float adjustment helped or if I end up just swapping back to the old carb once it's rebuilt.

What you originally described hot restart issue but would restart if u opened throttle is a flooded engine from fuel spilling in after shutoff. So likely if you fixed needle and seat and fuel pump pressure is ok it will work. Plugs may be fouled and in need of cleaning.

in the attachments in post 22 factory service manual shows same jet size and power valve for the 5.0 and 5.7, only accelerator pump nozzles are bigger diameter. So if the carb is maintaining a proper bowl level should work ok, maybe adjust accelerator pump lever hole up one to compensate.

 

[nola mike]

I didn't have too. I have experience timing issues too many times. Cheap or stale gas gets too much advance, and a hot motor can't crank well.

 

[Scott06]

I didn't have too. I have experience timing issues too many times. Cheap or stale gas gets too much advance, and a hot motor can't crank well.

His carb is dumping a huge amount of fuel down out of the float bowl after shutoff.

 

[Lou C]

There was so much fuel being discharged from the venturis you’d think the electric fuel pump was still running! To check to see if the needle valve holds flip the carb over and try blowing thru the intake port. If it’s holding you should not be able to blow thru it. If you can the float is sticking/rubbing against the bowl or the needle & seat are defective.

 

[tank1949]

His carb is dumping a huge amount of fuel down out of the float bowl after shutoff.

That will do it too!

 

[NotAClue92]

Alright, I ordered the fuel pressure tester(I can use it for other things as well) and it was supposed to be here yesterday and didn't arrive. Anyway, I decided to swap carbs and see what happened. I cleaned up the old carb and put it on just to see if I still had the same issue. The boat fired up right away and I let it get up to temp while checking for fuel leaking from the venturis(I didn't see any fuel dripping). I shut it off and let it sit for 30 minutes just like I did before. Went to start it and it fired right up. Gave it a few light taps from the throttle and it sounded good. Seems like the issue was in the other carb. Idk where I messed up at when cleaning it but I'm going to leave the original carb on there and lake test it this weekend and see how things go. I'll probably order a rebuild kit for both just to have it. But for today, it seems like its running good. I have to adjust the mixture screw a little but it sounds much better than before. Thank you for all of your help and I will post an update once I lake test it and see how it runs. Luckily all this project cost me was a $7 roll of insulation tape, about $15 for the carb clean, and about $50 in beer. Hopefully she runs well. Again, thank you for your help, you saved me a lot of troubleshooting and money.