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1992 3.0 stumbling under loan

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daunderwood

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1992 3.0 stumbling under load

I have a 1992 3.0 four cylinder Mercruiser that has always been quite reliable. Until last summer, that is. One day it began really struggling when pulling a tube. The engine will run up to nearly 5000 rpm with several people in the boat with no trouble, but once I put something in the water to pull I can't get much over 3000 rpm before the engine begins stumbling, losing power, and backfiring.

I've checked the inline filter and have replaced the fuel line from the mechanical pump to the carbuerator (on the advice of a mechanic friend who thought I might be pulling in air through an old cracked fuel line) but I still have the same symptoms. The only change I've made was to replace the spark plugs last year. I gapped them a couple of hundredths too wide - something I didn't find out until I'd replaced them all. Could a too-wide gap cause the trouble under load that I'm experiencing, or does anyone have an idea of some other way I should go?

Thanks in adavance,
David
 
Last edited:
4200-4600 rpm no more on the water under way, never in neutral above 1500 rpm. Proves nothing.

Check compression post back. Warm motor all the plugs out and with the throttle fully down.
 
Thanks for the reply, but I have to tell you that I don't understand what you're getting at.

My problem is that the engine performs normally when driving just the boat but stumbles and loses power at around 3200 rpm when I'm pulling a tube or skier.

Does anyone have any ideas?

Thanks,
David
 
You ain't supposed to be running that motor past recommended wot. Look on your valve cover will tell you what recommended wot is.

You don't understand do a compression test ?
 
Could a too-wide gap cause the trouble under load that I'm experiencing...
Click to expand...

How wide? Do as 427435 suggested; a 0.005" over sized gap will work but it makes the ignition system work a little harder and/or can cause the spark to seek another ground other than the spark plug electrode, i.e., through the side of the spark wire. Why do you think engineers spec. a certain gap?

As chiefalen stated
Check compression
Click to expand...
; it will tell you alot about the power of the engine.
 
chiefalen said:
You ain't supposed to be running that motor past recommended wot. Look on your valve cover will tell you what recommended wot is.

You don't understand do a compression test ?
Click to expand...

Chiefalen,

I know not to run an engine to redline. My point was supposed to be that the engine would rev smoothly to whatever I asked it to unless I was pulling a tube or skier. I do not routinely run it anywhere near that. I self limit the engine to less than 4200 rpm. I just don't have a desire to run any faster than that.

I understand a compression test, but I didn't understand the sentence "Check compression post back". I understand simple mechanics but that sentence didn't make clear what you were saying.

I truly appreciate you taking time to try to help me.

David
 
guyjg said:
How wide? Do as 427435 suggested; a 0.005" over sized gap will work but it makes the ignition system work a little harder and/or can cause the spark to seek another ground other than the spark plug electrode, i.e., through the side of the spark wire. Why do you think engineers spec. a certain gap?

As chiefalen stated ; it will tell you alot about the power of the engine.
Click to expand...

Guyjg,

I recon that the engineers specify a certain gap because that's where the engine will perform the best under a wide variety engine speeds and loads. I double checked the gap and they were at 0.037 instead of the specified 0.035. It seems that a widening of 0.002 wouldn't cause my problems. I suspect that I was sold the wrong plugs and that the neck may be too long or short. Before I find a compression tester I'm going to verify the plugs are the correct items.

Thanks for your help. I benefited from it.

David
 
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