Mercruiser 5.7LX backfire through carb.

[sjajf16]

I know this has been discussed before, but?
Been researching a lot here, but now I need to put something out for some help/advice. Quick backstory and history. Mid-season last year picked up a 23 foot maxum with a Mercruiser 5.7LX, 4 bbl with a Thunderbolt V ignition. It had a random backfire through the carb at higher RMP?s (3400+) that was attributed at the time to be old fuel?as in maybe a year or more without treatment and stored off season (and boat was barely used during this time as well). Engine would recover with a slight pull back on the throttle, but could never get to WOT without the backfire. Prior to buying had a mechanic do a tune up (plugs, cap & rotor), and give a once over (compression, leaks, etc). Got a good report in that the motor was mechanically sound, and issue seemed to become more infrequent, but again could not quite get to WOT. During this time was adding Startron additive to fuel and replaced fuel filter/water separator. Took a chance as I could not pass up for the price!!

Flash forward to this season, coming out of storage (indoor, climate controlled..I live in Michigan) it ran horrible! Constant back fire that started at 2800 RPM and got worse as engine warmed up. Since this was more consistent, I did some research and looked more closely at ignition. Long and short, I found a HEAVILY corroded ignition sensor (looks to be original) mounted in the distributor, as well as nasty looking wires and coil. Replace all with OEM, interesting the sensor PN has been updated and replaced six times since the one I was replacing. It is now completely encapsulated in epoxy and has an added ground wire. While I was at it, I removed and cleaned all ignition grounds as well as remove and cleaned the connections to the Ignition Control Module and Knock Control Module mounted to side of distributer. Took her out and ran MUCH better, good cruise at around 3400-3600, smooth acceleration?..but?I?m back to where I was last year, getting a random backfire through carb at higher RPM?s and can?t get to WOT, engine recovers with slight throttle pull back. Also seems that anything above 3000 I need to gradually increase throttle or I get a random backfire?.seems like it?s a lean condition now maybe, carb rebuild? I know really old fuel left to sit in a carb can gum up and varnish passages, jets, float, needle & seat.

Sorry for lengthy explanation, but want to provide as much info as possible. While I am no marine expert, I consider myself a pretty good backyard mechanic, but am getting pretty frustrated.

 

[Bondo]

.seems like it?s a lean condition now maybe, carb rebuild? I know really old fuel left to sit in a carb can gum up and varnish passages, jets, float, needle & seat.

Ayuh,..... Welcome Aboard,.... This quote sounds to be True to Me,....

 

[sjajf16]

Thanks for the welcome, I've always seen some of the better advice here!

Heading on vaca for a week in July. Thinking of just running as much treated fuel through her as possible til then if nothing else I'll pull carb for rebuild while I'm gone.

 

[Watermann]

Have you checked and verified the timing? How about checking the "mechanics" work to be sure the plug wires aren't maybe off by one. Normally the backfire through the carb is a retarded timing issue.

 

[NHGuy]

Yes, for example if you used a non marine tech he may not have known that a wire needs to be shunted to ground when you set the timing on a TB V. You can also test it with the tach (gray) wire disconnected at the coil. A dodgy tach can mess up a Mercruiser ignition.

 

[sjajf16]

Thanks all, timing is next. Found in documentation that this system needs to be placed in a Base Timing Mode by grounding the PUR/WHT wire prior to turning ignition key on. Looks to be 10 degrees BTDC @ idle RPM of 650.

I know the wires are correct in that I replaced those with the coil and ignition sensor. Just to be sure I had a friend that was helping check my work...and that chevy firing order has stuck with me since high school...18436572

NHGuy, when you say "test it" do you mean test run it with the gray wires disconnected from (-) side of coil? Without this reference at coil will that cause other issues?

 

[Fun Times]

There should be two gray wires to the coil, one to the ignition control module and the other runs into the main wire harness that goes to the helm gauges.

Be sure to disconnect the one that goes to the helm as the test or you can go directly to the back side of the tachometer and disconnect the gray wire there as well....Whatever is easier to do but at the coil is better incase the gray wire is grounded out along the way someplace.

Did you install a new rotor when you changed the sensor under the cap. You want to ensure the rotor wheel isn't touching the sensor.

As a starting point, Incorrect timing sounds to be the main issues here though.

Good luck.

 

[sjajf16]

OK, big step backward...verified timing and went out for another run. Hit around 2800 and she starts backfiring through carb with power fall off, as soon as I retard throttle slightly, it recovers. If I throttle hard, smooth run up to 2600 and back fire, if I ease throttle up, I can milk it up to maybe 3000, and backfire. And I don't mean a quick pop, it's a rapid pop pop pop. Everything runs fine up to around 2600 no matter what I do with throttle....so, any other ideas? Anyone have any knowledge of the TB V ignition module or knock sensor module mounted on distributed? I know there is a redesigned replacement that runs $400+. Should I pull carb for rebuild? Seems like hit and miss troubleshooting. Thanks for any help or suggestions

 

[sjajf16]

Real quick run down on what's been done...plugs, wires, cap, rotor, coil, sensor module, timing, verify plug wire routing, cleaned all ignition grounds, unplug and clean ignition and knock modules and using startron fuel additive in the last few tank fulls.

 

[NHGuy]

By any chance did you use an aftermarket cap or rotor or sensor? I'd go straight Mercruiser for those pieces. Is the rotor touching the inside of the cap? The rotor is supposed to be locktighted down on the distributor shaft, did you do that? Is the distributor bushing tight, no distributor shaft wiggle? Is the prop short enough, like maybe a 17 or 15 pitch? That's a lot of boat for a 350.
What about your ignition wires? We had a boater on here last year that got the wires backwards on the cap and plugs, his broke down at higher rpm too. Make sure you have the ends with the external grip contacts on the spark plugs, and the internal grip ends on the cap.

 

[NHGuy]

You also should know that after you set the timing you shut off the engine before removing the shunt wire. Next time you start it will have full knock sensed advance.

 

[sjajf16]

NHGuy, thanks...yes all parts are OEM. Also when I did timing I grounded shunt wire prior to starting, set Idle RPM and timing per documentation, shut down and removed wire.

I'm going to try compression test tonight. At least check it off the list, especially since problem has never completely gone away....For some reason I am still leaning toward ignition/timing. Driving me nuts! :frusty:

 

[sjajf16]

I've also got some pretty good documentation on the Thunderbolt ignition system as well. I may start pulling harnesses apart and verify all wiring.

 

[FreeBeeTony]

I had a similar problem a few years back. Went through the same things you did and even replaced the carb.
Mine started to "miss" @ 3200RPM like clockwork.....wouldn't really call it a backfire.
I even replaced the heads thinking it was a head gasket, and when I did the #3 piston was clean compared to the rest, this was due to unburned fuel (fuel wash) caused by a lifter hanging up and keeping an intake valve open. I eventually found this when the engine self destructed, valve hit the piston.....

I was able to pin point the cylinder by shorting out each plug wire while the condition was present. A friend of a friend suggested checking the valve adjustment. When I did I found them to be too tight. I readjusted and the problem went away, unfortunately only temporarily as the engine blew later on as stated above.
I think there was a problem with the lifter bore. I bought the boat knowing it had a problem.

Anyway, if you search on my name you can find the thread, was probably almost 10 years ago.
I would go through the carb and if that doesn't work I would do the test I did to see if it's one cylinder causing a problem.

Hope this helps....good luck!!

 

[sjajf16]

Mercruiser Thunderbolt V ignition module conversion

I have another thread going with an issue on my 5.7LX 4bbl..serial number is 0k016752.

Just a general question here...Has anyone done the conversion from the original Ignition Control Module, PN 806957T4/806957-4 to the new module PN 807264A01?

If so, what was the problem/symptoms that you were trying to address? Did this fix problem? Was the conversion pretty straight forward?

If you want to look at my other topic you'll see what I am getting at.

Thanks!

 

[Bondo]

I have another thread going with an issue on my 5.7LX 4bbl..serial number is 0k016752.

Just a general question here...Has anyone done the conversion from the original Ignition Control Module, PN 806957T4/806957-4 to the new module PN 807264A01?

If so, what was the problem/symptoms that you were trying to address? Did this fix problem? Was the conversion pretty straight forward?

If you want to look at my other topic you'll see what I am getting at.

Thanks!

Ayuh,.... No need for a new thread, let's keep all yer info in one place,.. Ok,..??

What conversion are ya talkin' 'bout,..??

The T-Bolt IVs, 'n T-Bolt Vs are very interchangeable,... The distributor don't care which module ya use,....

In my experience, the modules either work,..... or they don't,......

 

[old bird]

I also believe it's fuel related. I'd rebuild the carb. and recheck/replace all filters.

 

[Watermann]

Now that you know the engine has the timing close you should look into the carb.

Backfiring through the carb (spitting or coughing) beside being a lean condition is commonly caused by the accelerator pump in the carb not squirting enough fuel before the main jets start working.

 

[81 Checkmate]

Just throwing it out there......Ive seen and experience a number of 80 ish small block chevy's where the cam lobes have worn down and it's not opening the exhaust valve on a cylinder or cyclinders. They all had the same Symptom as you.
Might want to pop off your valve covers and see if all the rockers are moving.

 

[sjajf16]

Well....discard this thread. Compression check showed 30 psi in #2 cylinder. At least it's something definite to start with, anyone have any ideas where to start. I'm thinking valve cover and take a look at springs, rocker arms, pushrods....maybe roll it around a bit see if cam lobe is toast.