Backfiring 5.0 Merc

[1994 5.0LMercuryAlphaOne]

Marina tells me they don't know why I continue to have the problem. When I first put the boat in water this spring it ran rough and backfired. Marina rebuilt carb. Continued to have symptoms: backfires around 3800 rpm, looses power when it pops, does not seem to want to "run out" smooth as in past (power), diesels at times after running and turning off without idling.
Took back to marina and they checked timing. Said they adjusted 2 degrees. Checked compression. Said this is not the problem or concern.
Marina said it could be the ethynol in the gas. I have ran tank empty and filled with non-ethynol gas. Same symptoms.
I believe this is a backfire from the carb and not an after fire from the exhaust as described in another post.
Man this is frustrating.

 

[Bondo]

Re: Backfiring 5.0 Merc

Ayuh,... It sounds like a fuel delivery issue, or lack of it...
It could be the tank, the plumbing, the fuelpump, or the carb...

 

[Aloysius]

Re: Backfiring 5.0 Merc

Backfiring through the carb has only 3 possible causes..lean mixture, high voltage spark distribution problem, or intake valve sticking or floating. don't rule out weak valve springs..the stock springs are only single springs, and do degrade with time and use.

The ignition system..especially cap, rotor, wires, is in good shape and squeeky clean, with no build-up inside the cap, isn't it??

 

[Adirondack]

Re: Backfiring 5.0 Merc

Flat lobe on a cam shaft can also cause what you describe withut necessarily showing up on a compression test. Make sure you have good rocker action on all your exhaust valves. Just food for thought.

 

[Alpheus]

Re: Backfiring 5.0 Merc

Im betcha its a clogged filter,delaminated hoses or bad fuel pump. Its runnin lean. If it was spark or flat cam it would show up at lower RPM's as well and wouldn't be dieseling...

 

[6meter]

Re: Backfiring 5.0 Merc

Didn't see this replaced....took my boat out for the first time of the season and was popping through the carb at higher rpms. Did everything you had done. Turned out to be the coil.

 

[LAC_STS]

Re: Backfiring 5.0 Merc

Mine was doing the same thing and popping through the carb. It was also backfiring through the exhaust sometimes. It would bogg very bad after 34 throttle and about 50% of the time it would not go over 3K RPM.

I replaced everything fuel related, tuned up the dist, and everything else I could think of. Replaced the coil and it ended up being the resistor wire tha goes to the coils + terminal.

The wire was heating up like its supposed to when at idle and low RPMs but when you went to higher RPMs it was not cooling down to let more power in.

Everyone who looked at the boat thought it was fuel related including me. As soon as I replaced that resistor wire with a ballast resistor it ran great.

Oh yea you have a 94 so you prob dont have a points dist so you dont have a resistor wire. But check your coil.

 

[Aloysius]

Re: Backfiring 5.0 Merc

LAC is correct..non-points distributors must have 12 volts to the pickup. You can't run the distributor hot lead off a resisted circuit. A non-ballasted coil must have a resistor, though.

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

I can't say all this is clean? I shouldn't assume it though. I will check tomorrow afternoon when I see the boat. What should I be looking for under the dist. cap? I have owned the boat 2 years and never changed or seen fuel filter on any work order as being changed. How often should these be changed (besides now)? What hoses could be delaminating? Should I replace the fuel pump if after replacing filter I still have symptoms?

You guys are great for responding. I will post what I find.
Is it OK to say what lake my boat floats?

 

[LAC_STS]

Re: Backfiring 5.0 Merc

Well you should ask them about the fuel filter. Its actually a fuel water separator filter. It should be changed at least once a year.

You can change it. You should dump it in a clear bucket and see if you see any water in the fuel. If you have water in there you need to pump the tank out and put fresh fuel in.

But if you have been having all of your work done by a marina I would call them tom and ask them if they have changed it.

 

[Alpheus]

Re: Backfiring 5.0 Merc

What kind of carb are you running? If You have a Rochester or a Mercarb there is a fuel filter in the inlet side of the carb. If you have a weber, then you have 2 very small cylindrical screen filters under your float needles.

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

I will try to get brand of carb and ask about filter.

 

[Alpheus]

Re: Backfiring 5.0 Merc

Pick one.

Rochester Quadrajet

Weber/Edelbrock

Mercruiser Mercarb

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

Looks like the Rochester.

 

[Aloysius]

Re: Backfiring 5.0 Merc

Quadrajets have very small float bowls, and will drain the bowl quickly if fuel flow is impaired.

Remember, a carburetor is a lot like a toilet, where the float lets fuel or water in. Reduced fuel flow (plugged filter for instance) will take several seconds to show up because of this reserve fuel in the bowl. Any problem that happens instantly is not fuel flow related.

 

[chaparall villain 2]

Re: Backfiring 5.0 Merc

well on my 350 i had problems with fuel flow at wot and it didnt backfire it just died back like it was out of gas ... but i am with youall i would look into fuel system and filters first then fuel pump ... good luck

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

Took my boat to second mechanic. He actually drove the boat and feels sure it is the "resistor wire". Waiting to have it delivered and installed. Man I hope this is it! Other repairs, as mentioned in posts, are some big deer.

 

[Alpheus]

Re: Backfiring 5.0 Merc

Its pretty easy to rule out a bad resistor wire without throwing a new one at it.

That mechanic should have known how to do that...

 

[Don S]

Re: Backfiring 5.0 Merc

Took my boat to second mechanic. He actually drove the boat and feels sure it is the "resistor wire". Waiting to have it delivered and installed. Man I hope this is it! Other repairs, as mentioned in posts, are some big deer.

Find a new mechanic, a 94 Mercruiser does not have a resistor wire.

 

[GLENN M]

Re: Backfiring 5.0 Merc

it would appear good mechanics are as hard to find as good women,if your lucky enough to find one hang on for dear life.