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'95 carb'd 4.3LX idles great, maybe fuel starved in 10 minutes @ 3K rpm - help please

D

den458

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My ‘95 carb’d Mercruiser 4.3L s/n OF751007 idles great all day but acts fuel starved on the water after about 10-15 minutes at 3000 rpm. Engine gradually loses power & may stall. Usually restarts instantly, then struggles to maintain 2000 rpm, stalls again, & I idle back to the launch. Bottle of Seafoam in the tank hasn’t helped yet. New fuel pickup down-tube with anti-siphon valve & all new fuel line hoses 4 years ago. New fuel/water spin-on filter yesterday. Have ordered a new electric fuel pump just-in-case. I’m pretty sure I have a fuel delivery problem. I think the OEM fuel pump Merc# 861155A6 (obsolete) pressure spec is low (6-8 psi).

Not comfortable pulling a carb, can’t test fuel pressure here at home.
Will replace fuel pump tomorrow (yes, I’m throwing some $$ at it, hoping to save $$$ in shop).

Fuel tank not venting? I can try to leave cap unlocked.
Anything else I can do to troubleshoot this, before it’s over my head, & into the shop ?

Newby 1st post here. Thanks in advance for any suggestions.
 
I had a similar problem, turned out to be a faulty oil pressure switch. I connected a 12volt DC LED light in parallel with the fuel pump wiring. Started engine and checked for light on, at idle or slightly higher RPM the light stayed on indicating power to fuel pump. When I started to increase RPM’s the light started to flicker on & off finally stayed off and within 10 seconds the engine stalled. Replaced the oil pressure switch and all is good (oil pressure switch not the oil pressure sending unit)
 
Good place to start looking. Another is the anti-siphon valve at the tank. They tend to get clogged and shut off fuel flow.

Jeff
 
GlastronGary said:
I had a similar problem, turned out to be a faulty oil pressure switch. (oil pressure switch not the oil pressure sending unit)
Click to expand...
Thanks, GlastronGary: I'd previously bypassed the oil pressure switch with a jumper, so the oil pressure switch is not a factor at this time.
 
Thanks guys: I may take a look at the anti-siphon valve next (a pain to get to). The anti-siphon valve was replaced maybe 4 years ago when I replaced the pickup tube & all fuel lines, but agree it could be the problem here. Plumbing-in a temporary fresh fuel tank would be a bigger pain, but I sure see the wisdom in it. New fuel pump should arrive today. I may have an update later today.
 
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Tikitech said:
Is this a TKS (Turn Key Start) system motor?
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Well yes, I turn a key, it starts.
And yes, loosening the gas cap I'll do next outing on the water.
But previously I ran it for an hour on the garden hose & muff in my driveway, fuel cap snugged down & it never struggled at an idle.
It ran at 3000 rpm for maybe 15 minutes before it acted starved for fuel.
Probably won't try 3000 rpm unloaded for long in my driveway.
New fuel pump arrived, got busy with other stuff today, back to it tomorrow.
 
den458 said:
Well yes, I turn a key, it starts.
And yes, loosening the gas cap I'll do next outing on the water.
But previously I ran it for an hour on the garden hose & muff in my driveway, fuel cap snugged down & it never struggled at an idle.
It ran at 3000 rpm for maybe 15 minutes before it acted starved for fuel.
Probably won't try 3000 rpm unloaded for long in my driveway.
New fuel pump arrived, got busy with other stuff today, back to it tomorrow.
Click to expand...


That's not what TKS means... TKS carbs do not have a choke... They have an enrichment jet circuit instead.
 
Don't know if I have a TKS carb or not, I have whatever was OEM on my '95 engine in my '96 boat. Is the carb relevant to my issue that it runs 20 minutes at 3000 rpm before it starts acting fuel starved?

I did get the new fuel pump installed today ($41 fuel pump deal on Amazon), ran the engine on the muff @ 2000 rpm in my driveway for maybe 20 minutes, ran fine, but learned nothing new. May get out in the river Sunday, stay tuned.
 
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den458 said:
Don't know if I have a TKS carb or not, I have whatever was OEM on my '95 engine in my '96 boat. Is the carb relevant to my issue that it runs 20 minutes at 3000 rpm before it starts acting fuel starved?

I did get the new fuel pump installed today ($41 fuel pump deal on Amazon), ran the engine on the muff @ 2000 rpm in my driveway for maybe 20 minutes, ran fine, but learned nothing new. May get out in the river Sunday, stay tuned.
Click to expand...

Hence the reason I asked if it's a TKS system, if the enrichener on a TKS system isn't functioning properly it will act exactly as you describe. The TKS system enrichener takes approximately 10-12min to reach temp and close the valve completely.
 
With new fuel pump, the boat ran fine for an hour before fuel-starvation returned, it really puzzles me.
Think, how can it run fine for an hour then act fuel-starved?

The boat ran fine at 3400 rpm for about 20 minutes to well up river...
Then ran fine 3000 rpm about 40 more minutes kneeboarding & wakeboarding, so 60 minutes all normal, then....

It began to act fuel starved again. Opened the gas cap, listening for a vacuum seal, heard nothing.
Left the gas cap loose rest of day, engine acted fuel starved whenever engine reached 2000 - 2400 rpm, then it'd lose power.
Tried letting it sit 30 minutes while just floating & chilling, then tried tubing but engine lost power again at 2000 rpm.
Idles great all day. We idled back downriver to the ramp.

So problems are not the fuel-water separator / filter, not the fuel pump, not the fuel tank venting.
Could possibly be a gummed-up fuel-line anti-siphon check-valve, but I was hoping fresh fuel + the Seafoam would eliminate that.
Could possibly be a clogging fuel pickup tube, I suppose.
Could be the carb, I guess, though the boat running fine for an hour before fuel-starvation, well it really puzzles me.
Someone tell me if taking & posting a carb pic here, or hunting for a brand on the carb would help us know something important...
Yes, using a temporary fuel tank plumbed-in might eliminate something.
Should I remove & examine the anti-siphon valve? We know it works well enough for engine to idle all day, run 2000 rpm 20 minutes in my driveway.
I understand the valve may be mandated, but should I leave the valve off, bypassed for a couple hour outing to condemn it? It was new 4-years ago.
Next step suggestions appreciated.
 
As I read more about anti-siphon valves, they appear only necessary when the fuel tank is located higher than any fuel lines, so that fuel cannot siphon out of the tank if a fuel line opened below tank level. An anti-siphon valve has been connected inline on my fuel tank since new 24 years ago. But I have no fuel lines that route below tank level. My fuel lines & fuel go only up, relative to the tank level. So I could remove & eliminate that. Certainly worth a try.
 
Due to weather, put cover on yesterday, driving to FL Thursday for a few days, so back at it late next week, will post a pic of the carb then, thanks.
 
Similar issues have been posted over time.
Some of them, the cause was the has tank pick up tube, it typically goes down to almost the floor of the tank. Typically there is a screen of some type at end of tube.
It is not uncommon for that to fall off and find its wat to the gas tube and cause a clog. Symptoms always seem to be runs great for 15-40 minutes then stalls.

So if you can confirm NO fuel at carb immedeatly after it stalls then the gas pick up in the tank may be worth looking at.
Typically you can remove the gas tank sending unit to get a view of inside of tank
 
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