Mercruiser 5.7L warm start challenges

[opticalhighway]

Hi there, my 5.7L Carb 2BBL starts perfectly cold. Issue is when warm after taking out on the plane and then left sitting for 10-30mins it will take 5-8secs of cranking at full WOT (in neutral) to start. So seems like a flooded engine scenarios. I have tried:-

1) two different ICMs
2) two different coils
3) carb rebuild with carefully set float to prevent boiling gas flowing in after shutdown
4) lagged the fuel lines to reduce vapour lock

One question I do have is around the choke which works as expected. Set correctly for cold start and then opens over 2-3 mins as engine warms. Does anyone have experience of the choke closing over say 30mins even though the engine is warm and then when trying to restart the air is then starved hence creating a flooded engine scenario?

Also I have seen a thread on here that sometime an older starter battery may be a problem as the hotter engine can pull more for the battery and if not great will then exhibit a poorer spark? Not sure on this one.

Any insight or comments appreciated......

 

[Scott Danforth]

First, welcome aboard

next, if its flooded. ICM's and coils wont help. as ignition wont cause a flooded situation. Its also almost never the coil for ignition problems

lets start with a year of your 5.7, what kind of carb, or an engine serial number

if the carb is an old rochester, the welch plugs may be leaking fuel. or it could be a bad needle and seat.

vapor lock almost never happens in a boat unless the engine is overheating

so we are back to the carb.

 

[kenny nunez]

Before re-starting the hot engine remove the flame arrestor and look if the choke is closed and as SD suggested look down the air horn to look for fuel leaking.
Also connect a volt meter to the + or bat side of the coil to measure the voltage when the starter is engaged. There should be full battery voltage at that point. If not then the by-pass circuit in the solenoid is the problem.

 

[opticalhighway]

First, welcome aboard

next, if its flooded. ICM's and coils wont help. as ignition wont cause a flooded situation. Its also almost never the coil for ignition problems

lets start with a year of your 5.7, what kind of carb, or an engine serial number

if the carb is an old rochester, the welch plugs may be leaking fuel. or it could be a bad needle and seat.

vapor lock almost never happens in a boat unless the engine is overheating

so we are back to the carb.

Thanks for the reply. Much appreciated. It looks like a Rochester to me. I've attached some pics of it when I did a service on it. New needle and seat and float last August. Didn't have a clue about the welch plugs then though...

 

[opticalhighway]

Before re-starting the hot engine remove the flame arrestor and look if the choke is closed and as SD suggested look down the air horn to look for fuel leaking.
Also connect a volt meter to the + or bat side of the coil to measure the voltage when the starter is engaged. There should be full battery voltage at that point. If not then the by-pass circuit in the solenoid is the problem.

Thanks I will check that out.

Thanks for the reply. Much appreciated. It looks like a Rochester to me. I've attached some pics of it when I did a service on it. New needle and seat and float last August. Didn't have a clue about the welch plugs then though...

Sorry I should have said it look original and if so is 1996.

 

[kenny nunez]

How many idle mixture screws does it have? The Rochesters have 2 and the Mercarbs have 1. Personally I never cared for the Mercarb.

 

[opticalhighway]

Hi there. It has one at the bottom.

 

[Scott Danforth]

Mercarb, and they are not that great of a carb.

On a 1996 5.7, I would have a 650-700 CFM 4-barrel

 

[QBhoy]

Odds on to be the choke sticking or half way between. Very common issue.

 

[tank1949]

Hi there, my 5.7L Carb 2BBL starts perfectly cold. Issue is when warm after taking out on the plane and then left sitting for 10-30mins it will take 5-8secs of cranking at full WOT (in neutral) to start. So seems like a flooded engine scenarios. I have tried:-

1) two different ICMs
2) two different coils
3) carb rebuild with carefully set float to prevent boiling gas flowing in after shutdown
4) lagged the fuel lines to reduce vapour lock

One question I do have is around the choke which works as expected. Set correctly for cold start and then opens over 2-3 mins as engine warms. Does anyone have experience of the choke closing over say 30mins even though the engine is warm and then when trying to restart the air is then starved hence creating a flooded engine scenario?

Also I have seen a thread on here that sometime an older starter battery may be a problem as the hotter engine can pull more for the battery and if not great will then exhibit a poorer spark? Not sure on this one.

Any insight or comments appreciated......

Hook up a timing light and see if distributor is advancing as you rev motor. But, first check all battery connections!!!!!!!!!!!!!!!!!!!!!!!!!

 

[stresspoint]

what happens if you don't advance the throttle ? ,

have you recently or before this issue started changed anything IE: timing carb settings , thermostat , fuel source /octane ?.

was the carb kit OEM ?.

have you checked ( as above) cranking voltage ?.

have you looked down the carb when stopped to see if fuel is dripping ?.