170hp mercruiser (help)

lewski

Seaman Apprentice
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Mar 23, 2011
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35
hi, my 1986 170 merc backfires when I tried pulling a skier up the other day and feels like it is lacking power.Ive done a complete tuneup....new plugs,points,wires,timing,dwel,rotor,condesor,dist cap,even ran carb cleaner.Rus very well when warm ,idles nice, but backfires when you give it throtle fast.Any ideas will be apprec iated.
 

86Rinker19ft

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Apr 16, 2011
Messages
38
Re: 170hp mercruiser (help)

backfiring through the carb is almost always a sign of timing, at least in a car engine. Have you ever changed the timing chain? May not even be that involved if the distributor has loosened possibly when you did all you tune up stuff. If you are familiar with a timing gun and how to use it, maybe start there. I suppose it's possible that you have a carb issue, but I would think less likely.
 

86Rinker19ft

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Apr 16, 2011
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38
Re: 170hp mercruiser (help)

Another possible though is your point gap, are you 100% that you changed that correctly?
 

scv76

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Jun 17, 2011
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Re: 170hp mercruiser (help)

My 86 has always bogged and sometimes backfires up through the carb. under hard throttle. Advancing the timing on mine helped, but did not solve the problem 100% of the time. I read somewhere that the accelerator pump on these carbs is suspect. Not enough gas at initial nailing of the throttle, then normal gas and a backfire sometimes occurs. If I remember correctly, there is a replacement accelerator pump that delivers a larger volumn of fuel. This is all hearsay at this point. I will be watching this thread to see what the experts have to say as I have the same thing happening. BTW, If I don't nail her, she has no problems with bog or backfire
 

stonyloam

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Mar 13, 2009
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Re: 170hp mercruiser (help)

Mine was a head gasket, loss of power and backfiring through carb. Might want to do a compression check if the timing does not help. A quick check would be to pull the plug wires one at a time to see if you have a weak cylinder. Good luck.
 

stonyloam

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Mar 13, 2009
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5,827
Re: 170hp mercruiser (help)

I read somewhere that the accelerator pump on these carbs is suspect. Not enough gas at initial nailing of the throttle, then normal gas and a backfire sometimes occurs. If I remember correctly, there is a replacement accelerator pump that delivers a larger volumn of fuel.

Right about 1986 they redesigned the carb and went from a 2cc to a 4cc accelerator pump. They are not interchangeable, but if it is the pump, a rebuild might help.
 
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