Backfire at idle

blefever · 03-27-2009, 03:23 PM

This was kinda weird, at least I've never had this happen to me.

My boat has a pair of 454's carburated. MSD billet distributor, 6al box, Taylor 8mm wires.

Made a run to Catalina Island and back. Boat ran fine, 3500 to 4000 rpm's most of the way. When we got back to Newport and came down to idle, about 7-800 rpm, the starboard motor back fired, or just kinda poped out/in the exhust. It would just go pop every few seconds. If I brought up the rpm to 1100 or so, no pop, it went away. The motor didn't stumble or seem to run rough, no black smoke, just a little pop.

Wa da ya tink? Bad plug, wire, cap, box?

tx911 · 03-27-2009, 10:52 PM

do you have a billet roller cam?

blefever · 03-27-2009, 11:19 PM

I did not do the motor work so not sure what the cam is other then it is a mild roller. The guy I got the boat from who had the work done told me that it was a 502 grind.

Boat Tech · 03-27-2009, 11:21 PM

it kind of sounds like it could be something like a broken,bent,stuck or tuluped valve some where.how old it the engine? might have a leaking exhaust riser that could hurt the engine.also how old was some of the old gas or did you buy gas at the island.could be water in there fuel over there right now do to all the rain we have had.

blefever · 03-28-2009, 11:06 PM

Boat teck, The boat was full of gas when I left home, Shell 91 octane, from a good station. I would be very suprised if there was water in the fuel. Fuel filter/water seperator is brand new NAPA. I normally run 87, but had a half tank that sat for 6 months, so I put the higher octane in to make up for any loss.

The engine has 330 hrs total, not sure how many since the motor work was done. He did not touch the lower end, added Edelbrock alum heads, 502 roller, intake and carbs, MSG ign, EMI Thunder exhaust. I have had the boat for over a year now and have ran it anywhere from the ocean to 6200 ft on Crowley Lake near my home. It has always run great, (a little fat and down on power at Crowley but that is to be expected at 6200ft) so I feel good that the motor's were done right. Not saying that it's not a valve issue, but again would be very suprised. I know that no matter how good the motor man, stuff happens.

You mention exhaust riser leak. Are you thinking sucking air and causing the pop?

The pop was not consistant and did go away when I brought the idle up.

Thanks for the help.

jeffswav · 03-29-2009, 01:51 PM

Was the old 87 fuel E10 ethanol blend. Adding new 91 will not correct bad fuel if the fuel separated. I wasted the rod bearings on a fresh engine with bad fuel (most likley cause). This may not be your problem. I would do a search on E10 ehanol and educate yourself on the subject. Many threads have been posted.

blefever · 03-30-2009, 02:53 AM

Jeffswav, I should have mentioned in my post that when the boat was put up for winter, I did add the proper amount of fuel stablizer. Also, the boat ran perfect up untill the return to the harbor. From the launch to the ocean is a 45 minute idle, to the island is 40 minute's and back is the same. The poping happened as we entered the harbor after the return trip, so I'm thinking bad gas would have been a problem from the start.

Boat Tech · 03-30-2009, 03:56 AM

well it could have been the old gas from 6 months ago and stablizer just finally geting to the engine lowering the octane and that is what you heard.I would change you filters.did the noise go a way be for you got to the trailer and shut it off ? if yes or no I would start the engine on the trailer soon and see if it comes back.becous now you have remixed the fuel up good on the trailer driving home,and on the water you where on plane for some time leting all the bad gas flow to the rear of the gas tank where the pick up is and suked up the old fuel witch is heavyer.if the noise did not go away then I'm willing to bet it is a valve problem. p.s check to make shur you'r distributor is tight.is your engine oil new ? on the exhaust, no I mean a water leak between the manifold and the pipe that lets the water out of the engine if that gasket gets bad then water will enter you [cyl] and hurt a valve. try pulling the oil fill cap and look in side for water,rust. good luck.

blefever · 03-31-2009, 06:07 PM

I'm going to check it out this weekend in the drive on the hose. Thanks for all the ideas.

Blueabyss · 04-02-2009, 05:37 PM

Sounds Like a Dist Cap and Rotor Button problem. Most 454's have the cap located toward the rear of the engine and the drips from the back of the hatch can drip right on them depending on your installation. Plus if you have not changed them in a couple years then its time anyway. When I had that motor I changed mine every year when I changed my oil.. The one that worked the best for me was called Blue Streak .18-25 per cap. The worst ones were The Stock Merc ones..

Chris