stalls when hot

hibbert6

Petty Officer 1st Class
Joined
Jul 15, 2006
Messages
323
I have a Merc 125 (1997) that starts well and runs well in the morning. But after the sun starts warming the day past about 85-90 degrees, the engine stalls when putting it into gear. I find myself having to rev the engine in neutral and then quick get it into gear before the revs drop too much. Does this mean my idle is set too low? Any other suggestions?

Dave
 

Barnacle_Bill

Admiral
Joined
Feb 8, 2004
Messages
6,469
Re: stalls when hot

With the boat in the water and in gear it should be idling at 625-725 RPM. When is the last time it had a tune up?
 

hibbert6

Petty Officer 1st Class
Joined
Jul 15, 2006
Messages
323
Re: stalls when hot

With the boat in the water and in gear it should be idling at 625-725 RPM. When is the last time it had a tune up?

I had it tuned up, fully serviced with a sync & link early last summer. Didn't seem to fix the problem, but I didn't realize it because I was boating in the morning on cooler days at first. (I hadn't realized that the problem occurs only after the day and the motor get warmer.) By late June, I was having the "afternoon problem".

My idle seems to be about 700 rpm, but I'll have to remember to check the next time I'm having the problem.

Dave
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: stalls when hot

After it gets hot and you get it in gear and moving along, does it perform normally at WOT?

Mark
 

oldman570

Lieutenant Commander
Joined
Mar 25, 2011
Messages
1,615
Re: stalls when hot

It could be that the idle jets are running lean when the temp is higher. Try opening them a screwdriver blade width at a time to see if that is the trouble. JMO
Oldman570
 

oldman570

Lieutenant Commander
Joined
Mar 25, 2011
Messages
1,615
Re: stalls when hot

It could be that the idle jets are running lean when the temp is higher. Try opening them a screwdriver blade width at a time to see if that is the trouble. JMO
Oldman570
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Re: stalls when hot

Sounds like a vapor lock, switch to a different brand of fuel or if fuel pump has not been serviced replace diaphragm and check discs with new style check disks...
 

hibbert6

Petty Officer 1st Class
Joined
Jul 15, 2006
Messages
323
Re: stalls when hot

Sounds like a vapor lock, switch to a different brand of fuel or if fuel pump has not been serviced replace diaphragm and check discs with new style check disks...

I'll try the idle jets first. I'm not sure I understand how vapor lock would apply here - can you explain? Thanks!

Dave
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Re: stalls when hot

I'm not sure I understand how vapor lock would apply here - can you explain? Thanks!
Black engine cover,internal engine heat and increased outside temp can cause low grade/oxygenated fuel to boil creating vapor which fuel pump cannot over come. The new fuel pump kits has a different check valve and does away with rubber washer in pump. Dont understand the jets going lean as it heats up as they flow same amount cold or hot.... also bottom carbs have no adjustment.
 

hibbert6

Petty Officer 1st Class
Joined
Jul 15, 2006
Messages
323
Re: stalls when hot

Mark,

The engine runs fine at WOT even when hot. Faztbullet, thanks for the explanation. I use what I think is the best fuel available here, Union 76, premium (91 octane). I used Valero gas 2 weeks ago and had the same problem. I've heard that a bottle of Stabil might help; I'm a little apprehensive about all this as I'm taking the boat on a youth group water-sports trip next weekend. If I'm lucky, I can get the boat in the water once or twice between now and then. (3 hours minimum to do this.) I'll have to see if West Marine has the fuel pump parts in stock, put 'em in Tuesday, and try the boat Wednesday. Is this at all tricky?

Thanks,

Dave
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: stalls when hot

Don't know what to tell you about your schedule. But on vapor lock, squeezing the bulb till firm might allow you to get by.

Liquids boil at lower temperatures when the atmospheric pressure is reduced. The fuel is in the tank. The fuel pump is a diaphragm pump that sucks on the fuel line bringing up the fuel. When the volatile fuel is sucked on by the fuel pump there is a reduction in atmospheric pressure caused by the height the pump has to suck the fuel and any restrictions in the line...fittings, the squeeze bulb. Being volatile, it is sensitive to atmospheric pressure and at higher temps it boils making bubbles. Bubbles don't transfer fuel to the carbs and the engine starves and quits.

On which is which, in terms of fuel, I was under the impression the higher octane fuels had more volatile components than lower octane which in my opinion would cause them to boil more easily. I am not a petroleum engineer so I don't know. Maybe googleing the question would get you a professional reply.
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I do know this. I had a Quadrajet carb on a GM car and I wanted a better fuel filter for the inlet fuel since all it has was this little dinky thing where the fuel line attaches. The carb filter is after the fuel pump so the pump is pushing fuel through it, not sucking fuel through it. Big difference. I got the bright idea to install an after market inline fuel filter in the rubber hose preceding the pump.

On vacation the first summer after that, with the car full of kids, camping gear, the boat, AC going full blast rolling down the interstate and all that, in the middle of the afternoon, the engine died.....guess why? Vapor lock. Course I didn't figure that out till later after we got home. We'd sit and let it cool (30 minutes in the heat on the side of the road; no fan, no nothing) drive for 15 minutes or so till it got hot again and repeated the process till we made it home. The pump was sucking hot fuel through the filter I added and it boiled. Didn't take long for the modification to disappear. Boy was I in trouble on that caper. Wife took awhile to get over it.

HTH,
Mark
 
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