92 OMC Cobra 5.8L low rpm after 20 min cruising

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cheers731

Petty Officer 3rd Class
Joined
Jul 4, 2009
Messages
85
Hi folks,

Need some help troubleshooting on this repeating problem. It's a 92 OMC Cobra 5.8L engine, easy start up on cold engine and runs well at WOT after warm up and cruising over around 15-20 min. Then the rpm will fall gradually to 1500 and if I push more gas, it will stall. Then it gets worse, I can only run it at idle speed. At this point, I can smell some unburnt gas at exhaust. If I let the boat sit overnight, it will run fine for about 15-20 min. I had plugs, wires and fuel pump replaced a year ago and no improvement. Is the ignition coil get overheated or something else. I appreciate any suggestion or advice to get this problem resolved.

Thanks,

Frank
 
Bondo

Bondo

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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

cheers731 said:
Hi folks,

Need some help troubleshooting on this repeating problem. It's a 92 OMC Cobra 5.8L engine, easy start up on cold engine and runs well at WOT after warm up and cruising over around 15-20 min. Then the rpm will fall gradually to 1500 and if I push more gas, it will stall. Then it gets worse, I can only run it at idle speed. At this point, I can smell some unburnt gas at exhaust. If I let the boat sit overnight, it will run fine for about 15-20 min. I had plugs, wires and fuel pump replaced a year ago and no improvement. Is the ignition coil get overheated or something else. I appreciate any suggestion or advice to get this problem resolved.

Thanks,

Frank
Click to expand...

Ayuh,.... Sounds like a Fuel Delivery Issue,...

Start at the Tank vent, 'n work yer way to the Carb,... Everything is Suspect,...
 
C

cheers731

Petty Officer 3rd Class
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Messages
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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

Thanks for the reply.

I checked the entire system last year when it first started. New water separator, fuel pump and checked anti-siphen valve and make sure it deliver without restriction. The funny thing is it happens repeatedly after 20 min or so of running. I read more on the forum, could it be the timing off? I have replaced cap, rotor, wires and plugs but not done other parts of tune-up, such as condenser and contact set. They are probably 6 years old.

Thanks again,

Frank
 
Bondo

Bondo

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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

cheers731 said:
Thanks for the reply.

I checked the entire system last year when it first started. New water separator, fuel pump and checked anti-siphen valve and make sure it deliver without restriction. The funny thing is it happens repeatedly after 20 min or so of running. I read more on the forum, could it be the timing off? I have replaced cap, rotor, wires and plugs but not done other parts of tune-up, such as condenser and contact set. They are probably 6 years old.

Thanks again,

Frank
Click to expand...

Ayuh,... The Points, should be changed Every Spring,....
 
tfitch03

tfitch03

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Messages
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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

I had a similar problem in an airplane. We could fly for a while then the engine would sputter and die. Deadstick landing, fill up with gas (hissing noise when we opened gas cap), engine started back up, ran great, then sputtered and died. After a formal investigation we found that a mud dauber had clogged the vent tube. As fuel drained out it would create more and more suction eventually effecting engine performance and then total fuel starvation.

Like Bond-O mentioned, everything is suspect but I would take a close look at the tank vent.
 
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cheers731

Petty Officer 3rd Class
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Messages
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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

Many thanks for all the advice, really really appreciated it.

Frank
 
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cheers731

Petty Officer 3rd Class
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Jul 4, 2009
Messages
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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

Ok, I have eliminated a couple possibilities and will trace down the vent hose to make sure it performs correctly.
Here is my question: how do I know if I have points system? Does points or condenser gets overheated if timing is off?

Again, your input is highly appreciated.

Frank
 
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bonzoscott

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Jul 26, 2008
Messages
745
Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

I believe all OMC went CDI ignition '90 and some maybe '91. And a little crazy - this is what you could do. Do you know where the vent is? Should be close to fill. Drill a 3/8 hole through a tennis ball. Place it on the fill. Stick your compressed air nozzle in the outer hole and give it 125 psi. It should be whistling outta your vent.
 
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cheers731

Petty Officer 3rd Class
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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

Great idea, will give it a try.

Can't say enough thanks for the helpful ideas.

Frank
 
jerryjerry05

jerryjerry05

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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

Blowing compressed air into a gas tank isin't good idea.Something about static electricity and blowin up???
Most newer boats have a vent with a filter in it?
Post a pic of yours?
Unhook the vent line and blow through it.You'll be able to tell if it's clogged?
After it runs for a while, touch the coil and see how hot it gets.
It should be warm but not so hot you can't touch it.
You should have electronic ignition.
 
Bondo

Bondo

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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

bonzoscott said:
I believe all OMC went CDI ignition '90 and some maybe '91. And a little crazy - this is what you could do. Do you know where the vent is? Should be close to fill. Drill a 3/8 hole through a tennis ball. Place it on the fill. Stick your compressed air nozzle in the outer hole and give it 125 psi. It should be whistling outta your vent.
Click to expand...

Ayuh,... That's abit Excessive, 'n could split the tank, Wide Open,....
 
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bonzoscott

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Messages
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Re: 92 OMC Cobra 5.8L low rpm after 20 min cruising

Thanks for the advise jerryjerry05, and Bond-O. I used that method to drain 20 gal of 6 year old fuel from a project. Won't do it again!
 
M

Mattrick2oo3

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Apr 2, 2014
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This is the exact same problem I am having! Did you ever figure out what was wrong with it?

Thanks,

-Matt
 
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Maclin

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May 27, 2007
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6,761
The simplest thing to try is loosen the main fill cap, let it grab air until it stops hissing, then leave it less than full tight, start 'er back up and drive on.
 
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Maclin

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Oops, left this part out... If that works the FIX THE VENT AS SOON AS YOU CAN :)
 
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Mattrick2oo3

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Thanks,

Can you tell me where the vent is located on the tank? I didn't see one when I removed the anti siphon valve to inspect it.
 
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Mattrick2oo3

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Apr 2, 2014
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Nevermind, found it... The bottom screen looks a little plugged up and does this position (vent holes facing downward) make it easier for water to be pulled into the fuel system?
 
MikDee

MikDee

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Can't see the Pix! Use the little camera icon for pix off your computer. Use the clipboard for photobucket..
 
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