Mercruiser 260 won't idle when hot

[thsorens]

For the last 2 years, I have occasionally had trouble starting the boat at idle once out on the water. Starts fine cold, but when warm it's gotten to the point I have to hold the throttle wide open to get it to fire. also won't stay idling when warm pulling into marina and such. Runs great at any other speed and has plenty of power. I rebuilt the original Quadrajet this week and ran it this weekend exactly the same symptom. No difference at all. Either I missed a plugged passageway in the carb or it's not a fuel problem. Really seems like fuel though. I've rebuilt 10 carbs or so in my life so I'm no rookie but certainly no expert either. I really didn't find anything wrong with it when rebuilding it so I wasn't completely surprised it still ran the same. Choke is operating fine. In my experience loading up at idle almost always is a fuel issue. Ignition is more of a back fire, or loss of power under load thing typically. I'll throw some plugs in it this week and change the fuel separator but I'm a little baffled. Just for some background, this is a 1984 260 Mercruiser (350 Chevy) that cracked about 10 years ago so I built a 383 stroker for it with a bit bigger cam and some World Products Sportsman II heads. At that time I rebuilt this same stock carb and put in a bit bigger primary jets as well as different secondary rods and rod hanger, don't remember all the specs now but it ran like a champ for 8 or 9 years and just now is giving me the finger. Hell maybe it needs a valve job I'm not sure, but it doesn't use any oil and still has plenty of power once off idle.
Any suggestions would be appreciated!
Thanks
Tom

 

[alldodge]

It ran great for 8 or 9 years then started acting up. Rebuilt carb and it still does the same thing. I thing you need to try the rebuild again or maybe find a replacement.

 

[NHGuy]

See how it hot starts with a shot of fuel or carb cleaner. Also, if the fuel is getting hot and vaporizing maybe you could run the blower or open the hatch.
What about checking for leaks at the throttle shafts and choke shafts with a squirt of carb cleaner. Does it spin over at the normal speed? If it cranks slowly you might be getting some water in the combustion chambers.
How is the ignition system? Good wires, plugs, sensor, cap, rotor? Has the timing moved on you?

 

[thsorens]

yea, that's what I'm thinking. The other possibility is the thermostat is running hot and heat is boiling the fuel in the carb. 1984 gauges really can't be trusted plus the temp gauge is just the graduating C-H gauge with no numbers on it. Still it seems to work like it always did and shows the temp down in the lower 1/4-1/3 of the gauge. I saw on one of the hot rodders BB's lots of guys stick 1/2" to 1" spacers between the carb and intake for that issue, but hell it ran for 8-9 years fine with this setup.

 

[thsorens]

See how it hot starts with a shot of fuel or carb cleaner. Also, if the fuel is getting hot and vaporizing maybe you could run the blower or open the hatch.
What about checking for leaks at the throttle shafts and choke shafts with a squirt of carb cleaner. Does it spin over at the normal speed? If it cranks slowly you might be getting some water in the combustion chambers.
How is the ignition system? Good wires, plugs, sensor, cap, rotor? Has the timing moved on you?

I'll try shooting some carb cleaner around the shafts, water in the cylinders? Gawd I hope not. cap was replaced 2 years ago ironically, along with the electronic pickup in the distributer. Maybe I do need to check the timing. This weekend I noticed a little extra water in the bilge too, so I probably need shift shaft bellows. Can't find any water dripping from the motor while running. I'm wondering how quick it would sink if I shot a couple rounds of 9MM through the floor out in the middle of the river?

 

[Scott Danforth]

I would clean the carb.

however you mentioned you replaced the pickup in the dizzy? did you use new heat sink grease? could have another bad pickup or ICM

bellows inspection is an annual maintenance item.

 

[airdvr1227]

Asked and answered.

 

[thsorens]

I would clean the carb.

however you mentioned you replaced the pickup in the dizzy? did you use new heat sink grease? could have another bad pickup or ICM

bellows inspection is an annual maintenance item.

Carb is clean, refer to original post. I'll pull the distributer cap and see, no probably didn't add more grease when I replaced the pickup.

 

[thsorens]

Asked and answered.

huh?

 

[Bondo]

I rebuilt the original Quadrajet this week and ran it this weekend exactly the same symptom. No difference at all.

Ayuh,...... Seein's it's a Q-jet, my guess is the welch plugs in the bottom of the bowls are leakin',.....
It's a known issue, that's often fixed by epoxyin' over the welch plugs,....

Especially when ya say ya gotta go Wot to restart it,...
That's SoP for a Flooded motor,.....

 

[thsorens]

I actually did the JB weld trick on all 4 of those plugs. They had been JB'd before as well. I saw there was a bubble test to check they are not leaking which I did NOT do, probably should have. funny thing is it fires right up when cold which to me would indicate there is still fuel in the bowl but maybe its just fuel laying in the intake or on top of the valves? It's totally flooded when this issue happens yes. Has to be a carb problem right? unless I'm boiling the gas but I really don't think the engine is getting hot. Been there a few times when the impeller gets worn. Don't think I'm there now. I'm thinking of just ordering a professionally rebuilt carb for ~$250 and being done with it.

 

[thsorens]

oh hey, just remembered something else. There is a fairly small clear plastic hose that comes from the fuel pump and plugs into the front of the carb, on the airhorn IIRC. Anyway the fitting on the carb was completely plugged with a green goo when I rebuilt it. I got it opened back up, but I wouldn't be surprised if that line or the pump itself is also plugged. is that line an overflow for when the pump goes bad/? What is it's purpose? Could it being plugged be causing me too much fuel pressure?

 

[Bondo]

is that line an overflow for when the pump goes bad/? What is it's purpose? Could it being plugged be causing me too much fuel pressure?

Ayuh,.... It's the vent for the diaphragm,.... Anything is possible,....

 

[thsorens]

I know these Q jets cant take much pressure. There is a hard line all the way from the pump to the carb kinda hard to get my pressure tester in there but I'll figure out a way. Too much pressure would spill gas out the side or top of the carb I would think though and that's not happening. Argghhh!

 

[NHGuy]

You can put a gauge on a temporary line too. Save that OE line, they aren't that easy to replicate.

 

[airdvr1227]

huh?

I thought it might be the choke but then I re-read your earlier posts and got my answer.

 

[thsorens]

You can put a gauge on a temporary line too. Save that OE line, they aren't that easy to replicate.

Roger that

 

[thsorens]

I was at the river this weekend and pulled the top of the carb off. The thing was all the way full of fuel. Like right up to the bottom of the airhorn over the top of the float. I think some even spilled out when the top came off. The float was adusted to 1/4 so I bent it down a bit further, maybe 5/16 or 3/8 just to see if it made any difference. Might have helped but definately didn't fix it. I'm going to fabricate a fuel line so I can hook up my fuel guage this week. must be too much pressure from this old pump.

 

[thsorens]

No Title

Um, I think I found my problem.......

 

[alldodge]

That's a lot of pressure, most won't put out that much if they tried.