Need help 5.0 mercuries I/o problems

[66mstgfsbk]

Ok so I got a 2002 mercruiser 5.0 with the alpha drive. The problem is the engine backfires out of the carb like a lean misfire. Had the carb rebuilt and flow benched all works good. I have 6-7 pounds field pressure at carb inlet. The free flow from the pump was a cup of fuel in 4 seconds. Plugs good, cap good, rotor good, firing order verified, compression at 170 all around. Even after the carb work still pops out intake. Has the thunderbolt ignition module that bolts to the riser. The timing before was at idle 4 degrees with 12 total, I set timing according to manual....I'd did not say anything about grounding a purple wire I checked twice. Set to 8 degrees and now have total of 16-18. I have chased my self round and round. Runs great in neutral with no load on muffs, in gear on muffs works great. As soon as it hits the water, Pops out the intake. After I reset the timing it has popped way more out intake. Fuel in tanks is fresh. This has happened once or twice over the years, give it throttle and it acted like it would clear out and run great. Now 3 years later it is unusable, great while idling in water but once throttle is applied pops and won't clear out or gain power. Any help is appreciated.

 

[66mstgfsbk]

Forgot to mention there is no knock sensors or module in ignition.

 

[keith2k455]

Try retarding timing and see what happens. Are you. sure you have good spark? It's either got to be that or the carb

 

[66mstgfsbk]

Retarding the timing it will run slightly better but still backfire out of the carb. I have ruled out the carb at this point since it has been rebuilt and flow benched. I am just not familiar with the thunderbolt ignition system.

 

[Fastatv]

Retarding the timing it will run slightly better but still backfire out of the carb. I have ruled out the carb at this point since it has been rebuilt and flow benched. I am just not familiar with the thunderbolt ignition system.

Couple of suggestions for you. Are you 100% sure that it is wired correctly.....plug wires that is? And, if it has a water/fuel separator, have you sampled it for water....changed it this season?

 

[Scott Danforth]

is it a new cap and rotor, or did you visually just look at it? carbon tracing can cause what you are describing

 

[Bt Doctur]

back fire out the carb means a intake valve is open at the wrong time. It could be:
broken/weak valve springs
coil going bad
cam lobe going bad
intake valve not seating properly
cross-firing
Have you done a cylinder balance test or a leakdown test

 

[66mstgfsbk]

I've looked more into it I know for sure I did a compression test it all checked good. It was 175- 185. I've not heard any ticking or knocking out heads. I checked the cap for crackes and cleaned contacts on rotor and cap. Ohmed wires they checked good. I don't remember if I did a leak down before. The part that gets me is it did this once in a while before then would clear out. Now it's every time. Wires are in correct spots. I have 16 years in auto field but this is new but same basics. I'm kinda leaning toward the thunderbolt v ICM but have no way to test it. I do have a compleate est ignition that I may hook up to eliminate any ignition parts, but it is out of a vehicle. Yes I know some parts are protected and some are not, but this would be for testing only purposes. I know a coil that is damaged will also do the same thing. I've just have no way to test tb v system. Thanks for all the help.

 

[keith2k455]

You're like me, experience with auto trying to apply to boats. I say trust your instincts, it's still always fuel and spark, just a bunch of other stuff gets in the way to control fuel and spark.