Mercruiser 5.0 backfire up intake

[ibrw1]

L-345059 Thought it was fixed. Backfire intake manifold. had boat for three years, then backfire issue starts and I did the following; Set Base timing by jumping purple with white stripe to ground, then starting engine. Idle is truly 650 at set timing to 10 degrees before. NOTE timing chain is good, strobe NOT jumping all over, indicates chain is ok. Bought rebuilt carburetor and tuned it. Idles great, lots of power. Still backfires. I put in all new AC MR43LTS as requested on air cleaner. Still backfires. Bought new rotor and distributer cap. Kit #805759Q3 QuickSilver OEM parts. Still backfires. I quadruple checked spark plug wires too. They are going to correct locations. I bought a new gas filter/water separator. Issue is getting worse. Before, engine had to warm up, and had to be running near WOT Then, next trip, it fails between 2900-3500 rpm Only after engine warm up. If I idle or go slow for 10 minutes, no problem. Runs great, then back it comes. If at slow speed and I put the hammer down, it never backfires. You know, going through the danger zone. (2900-3500). So all new plugs, carb, filter, rotor, distributer cap, setting special base timing and adjusting fuel mixture to exact specs. New gas, no clue here. thoughts? All OEM parts. I am very meticulous. Outdrive is 2 years old. All new parts.

 

[Watermann]

Sounds like a tough nut to crack,. Just trying to think why it would be firing through the intake after you did all that work. Have you verified that the the timing hasn't jumped on you, yeah the chain may be good but it's easy to recheck... Another thing to think about is the mark being a little off on the pulley, that happens with age. Have you tried dropping back a couple degrees to see if that might fix the issue?

 

[alldodge]

Your not getting any backfiring at idle. Back firing is caused at high rpm by a lean condition or valves not sealing. If it's not fuel then I would start checking valve guides and springs. The simple test is for compression and leak down

 

[TCATTC]

You could have a bent pushrod or broken rocker.

 

[ibrw1]

You could have a bent pushrod or broken rocker.

Bent push rod or broken rocker is loud clucking noise, Engine runs quiet and lots of power. I had a bent push rod before on my 440 Mag in my car. Not sure if broken rocker has sound or not? THANKS for reply!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[ibrw1]

Your not getting any backfiring at idle. Back firing is caused at high rpm by a lean condition or valves not sealing. If it's not fuel then I would start checking valve guides and springs. The simple test is for compression and leak down

I will do a compression test ASAP. Funny though, engine idles perfect, Boat has never run better or faster in the 3 years that I've had it.Backfire only between 2900-3500 RPM and ONLY after the engine warms up. that has to be a clue.......

 

[alldodge]

ONLY after the engine warms up

That is a big clue, I now think it has a better possibility that its running lean. Check you float level setting and if it is correct then check what size jets are in the carb

 

[ibrw1]

Sounds like a tough nut to crack,. Just trying to think why it would be firing through the intake after you did all that work. Have you verified that the the timing hasn't jumped on you, yeah the chain may be good but it's easy to recheck... Another thing to think about is the mark being a little off on the pulley, that happens with age. Have you tried dropping back a couple degrees to see if that might fix the issue?

So, my questions...............If it jumped time, why would it run so well, Why would it have to warm up then fail, then why would from idle run well right through the bad speed window without so much as a hickup, why does slowing down for 5 minutes buy me 5 minutes of trouble free operation? THANKS FOR ALL YOUR HELP. Im not saying you are wrong, Im asking because I wish to learn from the masses. Every reply either chips away at the issue or points a new direction, I have a list of what its not, eventually what ever is left is the issue THANKS EVERYONE

 

[Watermann]

Here's another SWAG... Carbon build up on the piston gets hot and when fuel is added it fires off while intake valve is still open. Power tune or Seafoam to clean up carbon. Can't hurt.

 

[ibrw1]

So to recap, AFTER ISSUE STARTED new plugs AC-MR43LTS, New rebuilt Carburetor at some factory in Florida, New Gas/Water separator, New Rotor, New Cap ALL OEM no aftermarket, Set idle to 650, IN BASE TIMING MODE I set timing and adjusted fuel mixture, rechecked idle and adjusted back down to 650, then adjust Carb again. It never ever ran this good, idles great, runs fast, pops up the intake AFTER WARM UP, Sometimes up to 15 minutes before issue starts. If I go slow for 5 minutes, then no problems at all for about 5 minutes. DURING period when issue is active, If I run from idle through bad range no problem at all, not a single pop, ISSUE STARED ALL AT ONCE, so cant be valve guides. Because it only happens in a small range, its not a valve. Pretty sure. Boat engine does not burn/use oil THANKS Everyone

 

[alldodge]

And another piece of the problem, thought you were going flat out and you were just crusing

why does slowing down for 5 minutes buy me 5 minutes of trouble free operation?

You are running lean IMO, because you get a back fire at higher rpm and slowing down allows the carb to fill back up with fuel. You take back off and the fuel pump can not keep up so the carb starts running low.

Start by checking your fuel pump pressure (4 to 7 psi). If you can leave the gauge on the line so as you drive you can see if the pump is keeping the pressure within the range. If the pump test low, you probably need a fuel pump or you have a restriction in the line

 

[ibrw1]

That is a big clue, I now think it has a better possibility that its running lean. Check you float level setting and if it is correct then check what size jets are in the carb

I replaced the Carb after the issue started. I replaced it with the exact same model, and it is the model called out by Mercruiser using the engine serial number. BUT I think you are on to something, My procedure for the carb adjustment was this, MIND YOU only one idle mixture screw, Put in base timing, START ENGINE, engine is at operating temperature, boat is in the MISSISSIPPI river, I adjust IDLE to 650, I adjust TIMING to 10 degrees before, I adjust IDLE, again, back down to 650, I turn the ONLY IDLE mixture screw in until the engine stumbles, I adjust out 1/2 turn. I adjust IDLE back up to 650. I turn idle mixture screw in about 1/2 turn and it stumbles, I adjust back out 1/2 turn, I shut engine off, I take out of base timing, Take off, engine, backfires after warm up.AND THANKS, Really got me thinking again. I will also get exact engine water temperature next run in the river.

FUNNY THOUGH, The issue came like someone turned on a light switch. POW,,,,That takes away any wear issues, This is the second Carb, No Change, so I think that the jets are okay, Its not fuel supply issue because If I Go Real Fast, its okay. By the Way, This boat goes faster then ever, Im going to have to go to a 21" I can easily clear 4800 RPM, that's bad..... I bought the boat from a dealer. Mechanic said, timing is perfect, He says everything is perfect, do not touch, NOW a 19"x14.3 is not enough, going to get a 21x14,3, after everything is fixed..

 

[Scott Danforth]

have you checked the anti-siphon valve? could be debris in the tank and a partially plugged anti-siphon valve.

when you changed the fuel/water filter, did you empty the contents into a clear container to inspect? could be water in the fuel making it to the carb.



I thought of making a comment on the boat being in the Mississippi (Too thin to plow, too thick to drink) however you probably heard them before.

 

[TCATTC]

Bent push rod or broken rocker is loud clucking noise, Engine runs quiet and lots of power. I had a bent push rod before on my 440 Mag in my car. Not sure if broken rocker has sound or not? THANKS for reply!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Sometimes no noise at all. I would pull the valve covers to be sure.

 

[ibrw1]

have you checked the anti-siphon valve? could be debris in the tank and a partially plugged anti-siphon valve.

when you changed the fuel/water filter, did you empty the contents into a clear container to inspect? could be water in the fuel making it to the carb.



I thought of making a comment on the boat being in the Mississippi (Too thin to plow, too thick to drink) however you probably heard them before.

Anti who? Thought you were messing with me, Sea Bats, Bucket of Steam, 50 feet of chow line. Heh heh. Looked it up. Anyway, Goes fast though? I looked at fuel, was cloudy, I'm gonna take all that Jazz apart tonight.Thanks, thanks a lot.

Mississippi, no, live a couple hours away, funny stuff.

 

[ibrw1]

Sometimes no noise at all. I would pull the valve covers to be sure.

I'm going to compression test it, Normally if bent or broken, idle is rough and plugs look different, Popping the plugs out tonight, I'm going to check out what you say. Idle is good, but still, something is going on in Funky Town. Gotta look, right?. THANKS

 

[ibrw1]

I'm going to compression test it, Normally if bent or broken, idle is rough and plugs look different, Popping the plugs out tonight, I'm going to check out what you say. Idle is good, but still, something is going on in Funky Town. Gotta look, right?. THANKS

So, Lets just say I got money to burn, I mean I got a boat, right? Any who, I'm thinking I get new Spark Plug Wires? if so, L345059 5.0L 2 bbl Mercruiser Thunderbolt V, What wires are the absolute best hands down holly crap I wish I had these, wires? What about Coil, Heat related issue???? I never use Sierra, was told to deep six em. Thoughts?

 

[ibrw1]

Sometimes no noise at all. I would pull the valve covers to be sure.

Ok, Will check, Idle is good, I'm taking the plugs out tonight. Looking at them, possible story there. Looking at anti siphon Valve and so on too. Will compression test on Friday. You would think that bent or broken would show up on compression test?

 

[ibrw1]

And another piece of the problem, thought you were going flat out and you were just crusing


You are running lean IMO, because you get a back fire at higher rpm and slowing down allows the carb to fill back up with fuel. You take back off and the fuel pump can not keep up so the carb starts running low.

Start by checking your fuel pump pressure (4 to 7 psi). If you can leave the gauge on the line so as you drive you can see if the pump is keeping the pressure within the range. If the pump test low, you probably need a fuel pump or you have a restriction in the line

WOW, I've learned Lots today, I'm going to buy that asap to test fuel pressure. I'm going to have more test equipment in my boat then in the space shuttle. THANKS

 

[alldodge]

At least its cheap
http://www.walmart.com/ip/14644669?...=pla&wl6=55136637649&veh=sem?reviews_limit=9&