rough running engine at startup

n6xb

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Aug 23, 2011
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I know this topic has been beat to death, but I still need help! At startup, engine runs real rough. After warm up, then it seems fine. Now I assumed it could be the Idle Air Control Valve, but I cannot find it! I have attached a couple of carb pictures. Does anyone know the correct part number?

However now, even after the engine has warmed up, it will run for a minute or so and then just stall out. Very strange!

All plugs were replaced as were the wires and distributor cap. I also ran Seafoam thru the gas tank.

Engine is a mercruiser 7.4L Bravo III, 1995, serial number 0f413209

Now it has become a hair puller!...:) Thank you!
 

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bruceb58

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Mar 5, 2006
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You have a carb. Why would you think you have a IAC valve?

What is your choke doing during your cold starts?
 
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thumpar

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Jun 21, 2007
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IAC valves are on fuel injected motors. Maybe time for a carb rebuild and check all your filters.
 
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n6xb

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Aug 23, 2011
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Oh... that explains why I could not find the IAC! As for filters, I have replaced the large water/fuel separator. Is there another one that should be replaced?

As for the choke, it is closed during a cold start. As you can see in the picture, then it slowly opens. Should it open fully at normal temperature? The picture shows it about 20 minutes after engine shutdown.
 

bruceb58

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Is it possible the choke is not opening fast enough? Maybe try adjusting the choke so it is not quite closed at startup so it gets a head start for opening.
 

n6xb

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Aug 23, 2011
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Yes, it could be possible the choke is not adjusted correctly. How is it adjusted? Since it appears to be working, maybe I can assume it is not bad. I don't know how often those go bad. All of the linkages around the choke seem to operate freely.
 

Bondo

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Apr 17, 2002
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At startup, engine runs real rough. After warm up, then it seems fine.

However now, even after the engine has warmed up, it will run for a minute or so and then just stall out.

Ayuh,.... Possibly 2 different issues,..??

Yer choke adjustment is by loosenin' the screws holdin' the plastic round thingy on the forward starboard-side corner of yer carb,....
The plastic parts turns slightly in the housin',....

The stallin' out after a minute or so, sounds like the fuel pump power, oil pressure safety switch is bad,...
It's down low under the exhaust manifold mid-way port-side of the block, just aft of center, ahead of the oil filter boss,.....
 

n6xb

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Aug 23, 2011
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18
I will play around with those two items and see what I can figure out. Thank you!
 

n6xb

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Aug 23, 2011
Messages
18
My rough running engine problem is evolving and I am collecting more data. Now it seems that the engine runs good for about 15-20 minutes. Then without warning, the engine stalls out. I can restart, but it is hard. It will then run ok for a few minutes and die again. After re-starting a couple more times, then it is impossible to start. It just grinds. However, if I use starting fluid, it will run for about 5 seconds and then stall out again.

It seems like the fuel delivery is stopping. I believe the fuel pump is mechanically driven. What could stop the fuel from flowing? Oil pressure and voltage gauges are normal when the engine stops. The beeper does turn on and runs at a higher frequency when the engine stops. It is a higher frequency than when just turning the key to ON.

Any ideas Gents?
 

indybleck

Petty Officer 2nd Class
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Sep 29, 2011
Messages
136
Check the filter in the fuel line going into the carb. There should be a screen for a filter. Where the fuel line screws into a housing there should be filter in there. Take the housing off the carb filter and spring.
 

Silvertip

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Sep 22, 2003
Messages
28,758
I hope you adjusted the choke on a stone cold engine and not when the engine is hot. Playing around with stuff is not repairing. You need a service manual for your specific engine.
 
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