merc 5.0 backfiring help

[bolo78910]

2000 merc 5.0 4 barrel carb runs great all season long, on the ten mile journey back it start to backfire from 3000 rpm all the way to 4000 ect, when coming of plane it pop pop again at 1500 rpm . i change water separator filter, filter in the carb is clean as new. now what i notice the riser to manifols gasket has saltwater trase around both sides, both riser/manifold is new this season.

engine still has strong power i also notice the tach wander up down a little .

of to work now guys . let me get it. thanks

 

[Aloysius]

Re: merc 5.0 backfiring help

Ignition. If you've got a flat top cap distributor, they're known problem.

 

[bolo78910]

Re: merc 5.0 backfiring help

Ignition. If you've got a flat top cap distributor, they're known problem.

thanks so ur saying it might be the distributor or the cap.

 

[dingdongs]

Re: merc 5.0 backfiring help

a plug may be on its way out.if not new it may be time to treat the ol' girl.

 

[Bondo]

Re: merc 5.0 backfiring help

Back firing (not just an engine miss) at higher RMPs screams a lean fuel mixture to me and is the first thing I would check.

Ayuh,.. Big ole Ditto on That....

 

[Aloysius]

Re: merc 5.0 backfiring help

Those flat top distributor caps are notorious for developing internal flaws, allowing crossfire between cylinders.Additionally, quality control on the rotors is poor..I've seen 3/16" gap between rotor and terminals in the cap.

Too wide a plug gap, or high wire resistance, causes the secondary voltage to rise higher than necessary, creating crossfire within the caps.

 

[Don S]

Re: merc 5.0 backfiring help

The "Flat" caps were only used on the later MPI engines, not carbed. You have the Thunderbolt V ignition system.
While lean does cause backfires, I don't see a mixture setting suddenly being the problem at 3-4,000 rpm.
I would still at the cap and rotor, and also pull the spark plugs and see what they tell you.
Have you removed and checked the fuel filter for water?

 

[Aloysius]

Re: merc 5.0 backfiring help

Water is a good possibility, but it would probably die at idle if the float bowl had alot of water in it. Otherwise, I doubt there's a carb problem that just materialized..especially one the creates problems from crossover to close to full throttle.

Tach is a pulse counter..rpms could indeed be wavering, or there may be an ignition pickup issue.

Do you have a low voltage issue/loose wire to the distributor?

From my experience, sudden backfiring is almost always ignition..unless you've gotta mechanical issue..intake valve!

 

[bolo78910]

Re: merc 5.0 backfiring help

wow lots of info here guys thanks, went back to the boat tonight didnt do much th yard mechanic says he bet his life its the module under the cap the problem.
anyway tonight i start it up and as usual it start fine idle like a kitten the tach was not wandering as before. i was in my slip tied up with strong ropes so i bump it in gear and bring the rpm up to 3000 as it reach there it pops pops from the carb every time. i will pull the plugs and look before i perchase that module thanks any more hint tips guys

 

[bolo78910]

Re: merc 5.0 backfiring help

helo again, i get this from another site where a bad altinator can cause this, what i notice tonight and sunday, is that the my charging use to be 13. something now it is reading over 15 never reads that high before. your thought.

 

[bolo78910]

Re: merc 5.0 backfiring help

The "Flat" caps were only used on the later MPI engines, not carbed. You have the Thunderbolt V ignition system.
While lean does cause backfires, I don't see a mixture setting suddenly being the problem at 3-4,000 rpm.
I would still at the cap and rotor, and also pull the spark plugs and see what they tell you.
Have you removed and checked the fuel filter for water?

don i did not pour the gas from the filter in a glass to see if water was in it, i will do that next time at the boat. have to pull those plug also what a pain in the zzzz to get at those plugs .

 

[Don S]

Re: merc 5.0 backfiring help

Find and fix your charging problem first. If you have an ignition problem caused by excessive voltage, no reason to chance damage of your new ignition parts.
Use your DVM to check the output of the alternator, do not depend on your helm volt meter.

 

[Aloysius]

Re: merc 5.0 backfiring help

I'll bet the charging problem is the issue. solid state stuff is very sensitive to voltage fluctuations or a/c ripple.

 

[bolo78910]

Re: merc 5.0 backfiring help

Find and fix your charging problem first. If you have an ignition problem caused by excessive voltage, no reason to chance damage of your new ignition parts.
Use your DVM to check the output of the alternator, do not depend on your helm volt meter.

make a lot of sense to me don s thanks

 

[bolo78910]

Re: merc 5.0 backfiring help

update, tonight i went back i barro a distributor module from a friend, still backfire. take the spark plugs out they all look normal, wirel has no cuts. another friend show up and told me he has a spare mercarb in his basement, of we went to get it and install , with this carb bolt on still tied in the dock i has no more backfire, so i think the culprit was the carb. will test run it tomorro.

 

[bolo78910]

Re: merc 5.0 backfiring help

update here guys, took the boat for a test run this evening, water calm , at 4000 rmp trim out real nice i was doing 27 knots with no more backfire, but if i tri to give it a little more it would kinda boggs slightly , i back of a little and keeps it at 4000 rpm and it was running great. so my problem was most def the carb, my original carb has a spacer plate the one my friend lent me has no adapter plate so i use the one from mines and the gasket was a little mess up so i will need two gaskets . maybe that why it idle a little up and down losing air from the bad spacer gasket and manifold gasket. so problem solved guys thanks to everyone who helps on this .. have a bless holiday.

 

[Bondo]

Re: merc 5.0 backfiring help

Ayuh,... A Bad gasket will No doubt cause a Lean condition,...

Good Luck with it Bolo....