2000 maximum with 4.3 mercruiser back firing issues

[JohnDumas10]

Hey guys,

I have a 2000 Maximum with a 4.3 Mercruiser with back firing issues. I was cruising around for 40 miles and the tank was pretty low then she started back firing. I filled it back up and tried it again but no dice. She keeps backfiring constantly. I parked it for the last two years due to the fact I have no time to fix it. I check all the fuel lines and the filters and everything was okay. I think she sucked up some dirt in the bottom of the tank and clogged up the jets on the 2-barrel carb. Either that or she skipped on her timing.

Any thoughts on what it could be? Also since she's been parked for the last two years is there anything I have to look out for?

Thanks,

John

 

[Texasmark]

What is backfiring? It's part of the combustion explosion getting back into the intake "circuit". Ok what are the mechanisms? Bang at the wrong time when intake valve open. What are the elements? Spark and cam position. What controls that? Cam chain and distributor position......I guess that engine has a distributor. If not, I'm chasing my tail below.

How would something like that happen on a low fuel condition? Makes no sense. low fuel has nothing to do with it. Crud clogging up the fuel line would wind up in the fuel filter and starve the engine for gas causing it to shut down.

Parked for 2 years.....hmmm. Sticking centrifugal advance in the distributor. I think timing gears for camshaft drive are now solid metal and have been since back back in the 80's maybe. You didn't mention hours on your 4.3 but I'm going to bet, unlike an auto, not going to be equivalent to 200k miles......200k miles at 50 mph = 4000 hrs....no way, 400 maybe on a well used boat! So even if cam gear were nylon tipped as back then, or aluminum (soft) you wouldn't have any appreciable wear or heat caused degradation of the plastic gear tips.

Stuck centrifugal spark advance would put he spark farther ahead on the compression cycle. The intake valve closes on the compression cycle after filling the chamber with fuel/air on the upstroke. If you fire the plug too soon, the intake valve will not have had time to close and, with adequate fuel mix in the cylinder an explosion will occur with part of it coming out the partially open intake valve and wala......back fire. This would cause explosion leakage on all cylinders and would relate to your comment about continuous backfiring which any mechanic that has had his hands on a distributor has inadvertently at one time or another, twisted the distributor far enough advanced to get them all pfarting.

Loose distributor clamp bolt and dist has slipped timing?

Other thing is possibly a cracked/metal coated (internally) track line distributor cap where the crack allows spark meant for one pin tracks over to another pin which does the "firing with the intake valve open" on another cylinder, or a pair of spark plug leads running adjacent, having a high voltage breakdown where the high voltage to the correct plug is diverted to a different plug on the up stroke, with the intake valve still open.

That's what it looks like to me, knowing almost nothing about your boat. HTH.