Brand New Engine Backfires !!!

OK----this has been over 2 year project----replaced a 6.2 Merc w/ a 502/502 custom engine----everything brand new-did not use any parts / accessories from my blown engine----Have 9 hours on engine--all running like a top----went to start the other day---huge backfires out the exhaust--would not start----ran fine a few days before-------distributor was tight, had not moved--all wires tight---did some re-search and the minds told me it was my MaxVolt-mv-d6 ignition box-----(same as MSD) Talked to the company I purchased same from--they kinda agreed--installed a brand new one on today---SAME BACKFIRES???? Replaced my brand new MSD coil w/ another MSD coil----same---check inside the brand new distributor----all fine???????????? Any thoughts???? Mike--Tampa Bay !!!!!!

When you say ran fine a few days before, you mean out running the boat around?

Yes Ratickle--- I have 9 hours on the engine----probably 6 hours in the water vs the engine stand-----and she ran GREAT!!!!! I mailed the Max Volt blue box back to KE Performance where I bought it----but they have not tested it yet !!!! It is the newer Digital Box w/ RPM limits and it changes timing when you start your engine???/ Fires 12 sparks each time to the plug--why??-AND if it is the box---I also have a brabd new one they sent me also on the boat that does not work!!!???? anyways MSD has now the same system and they have alot of problems with them---at least that is what I read online---nobody talks about the Max Volt box!!???
SOOoooooo I'm waiting for them to test same and go from there !!!!

Okay, thanks. Make sure to let us know if that's it. It doesn't make much sense that it could be anything else at this point, but if the box is good, than we need to do some more digging.

OK---after several weeks of waiting-KE Performance phoned and told me the ignition box was no good--wheeeew!!!!! I'm glad it was that-----So they are sending me a MSD anolog vs. the new digital boxes so when I install this box-I will let you know if the problem is fixed!!!! Thanks---Mike-Tampa Bay

Great to hear. Did they say why it took so long?

At least you're not up here where you'd have to wait until spring to find out if it was really the issue.....

OK---changed over the ignition system to the MSD system they sent me----SAME F------- PROBLEM....backfires out the exhaust----then just spins like a top---give her a squirt of gas---boooom out the exhaust...........this is the 3rd brand new ignition box I have put on her--it can't be the box?????? Frustrated !!!!!!!!!!!!

Have you found TDC on 1 and double checked to make sure something in the timing system did not slip or skip?

timing chain slip
firing order incorrect
distributor turned and slipped
crossfire occuring under the cap?
cracked cap full of moisture
distibutor gear trouble?? roll pin on distributor shaft get get sheared and "relocated"
spark jumping across wires???

RSchap1---right now I guess it has to be anyone of the above you listed--my engine tech does not want me to touch her anymore and to bring her in for him to look at---he believed it was the ignition boxes also-but we were wrong----AND RATICKLE you are right--thank goodness I live in Florida and the boat is not in storage for the winter!!!
Thanks--everyone---Mike--Tampa Bay

OK---long story short---found a piece of a broken snap ring inside the distributor---stuck/attached to the magnetic pick up-----it would move and lock up the spinning gear----bottom gear was brand new--keyway was good--Distributors don't have snap rings, so we determined it was picked up on the table by mistake when they were building the distributor at the factory----re-installed-re-timed ran good on the hose here for about an hour---will test run her in the water tomorrow------YES it is still warm in in Florida !!!1LOL------Thanks for all the idea's and comments------------Mike--Tampa Bay !!!!!!!!!