4.3 Mercruiser backfire

ASD

Cadet
Joined
Jun 9, 2010
Messages
11
if there might be anything else I could try to fix the back fire through the carb from cylinder #6.


4.3 175 HP MERCURY CRUISER 6 CYLINDER

In the fall and early spring no problems. My mechanic informed me that the starboard side manifold needed to be replaced(original and it should be replaced 5-7 years) which I did and once it was replaced, I have been having a popping noise (back fire )from cylinder #6only.Performed pressure and leak down test with good results. I can run boat with out the wire on the cylinder and it will not back fire. I have already replaced wires and cap in hopes it might be that.
When running boat in gear at 2800 -3000 rpms popping will happen sporadically. sometimes it stops for about 30 -45 seconds. I have not pushed engine in fear of blowing rod.

we have removed both heads and found no problems (well starboard had an exhaust rod that was mushroomed) when we had them tested. We have check carb, rotor cap wires, gas ect. and performed all tests.

any other things it might be?


Thank you in advance for your help.
 
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6meter

Chief Petty Officer
Joined
May 15, 2010
Messages
525
Re: Need help!!!!!

Re: Need help!!!!!

I'm guessing you mean the top of a exhust valve was mushroomed. Thats a problem! The rocker arm in junk also. The valve won't open enough to let the exhust gases out so when the intake opens, it escapes then, with the pop.
 

zbnutcase

Commander
Joined
Sep 19, 2009
Messages
2,055
Re: Need help!!!!!

Re: Need help!!!!!

It HAS to be a valve sealing issue, NOTHING ELSE WILL CAUSE BACKFIRING FROM ONLY ONE CYLINDER.
 

ASD

Cadet
Joined
Jun 9, 2010
Messages
11
Re: 4.3 Mercruiser backfire

Both the exhaust and intake vavles were replaced and still having issue.
Could it be timeing chain?
 

ASD

Cadet
Joined
Jun 9, 2010
Messages
11
Re:UPDATE 4.3 Mercruiser backfire

Re:UPDATE 4.3 Mercruiser backfire

Update.

Problem fixed, it was the new starboard side exhaust manifold that was installed. It seems that when it was manufactored, the casting was incorrect and there was a blockage inside that opening.

Dont know if this is common, mechanic stated never seens this in all the years he has been working on boats.

If anyone else is having a popping noises from carb, make sure the exhaust manifold is clear of any obstructions.

Thanks for all your help.
 
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