Backfired .....
 

Twin 454 mags ..one with 10 hours one with 30 or so .... at WOT or close one motor will backfire ... so far only twice ..and just once per occurnace .... I am guessing carb but dont really know which motor .. Everthing seems ok oil pressure temps ect .Suggestions on where to start ? Thanks ..

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What type of carb do you have?

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I would say that you have a valve hanging up or going bad.
At WOT I dought the carb could be the cause.
If it was a lean condition it could cause the valves to get TOO hot and that could cause a backfire.

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Possibly you had valve float.

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I'd be looking at the cap and rotor. It's quick and easy.

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Joe .. I think they are Carters ..what ever was stock .... Please dont say valve ...

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Is it popping back through the carb or out the exhaust? A lean mixture will cause the carb to pop at sustained high RPM as the engine starves for fuel.
Usually if a cap or rotor is bad the idle will not be good, but not always. Are both engines pulling the same RPM?
How much time is on the valve springs?

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Ed ..this is a 311 with silent Thunder ....and its only happened twice ..first time I wasnt even sure what it was ...maybe hit something or ? ..But today Def. a backfire ..even my wife knew what it was ...and I think its thru the carb but not sure ....The Starboard motor is Brand new.. (reman jasper) .. which was replaced by Jasper due to old motor (16 months) sticking Valves ...The Port motor is also Jasper (about 30 hours) ..I dont know which motor it is ...But seems port side ( but just a guess) .. going 60-65 at 4500 plus ...kinda hard to pinpoint .. and yes both motors seem ok as far as both at same rpm at speed ...thanks for the input ... I know I'll get blasted for this but I have put 18K in these motors since I got the boat in May ..could have done alot better I think for my $$ than this ..... Valve springs ...the 30 hour motor had heads redone this summer due to reversion ...the new motor is ...well new ...<10 hours ...

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After a good run I would pull a few spark plugs and see if they read lean. Are these Gen V engines with the mechanical fuel pump mounted on the raw water pump?
You may want to hook-up temporary fuel pressure gauges to each engine and see if one side has a bad or clogged fuel pick-up in the tank. Did you have the carbs gone through?
I wish you were here in SE Florida where I could help you HANDS ON!

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If you can not verify that the motor is leaning out try the ignition system. Had the same problem. It was the coil. Watch the tachs while you are coming up to WOT rpm. You will see the tach bobble on the motor that pops. Change the coil on that motor (hell or both motors, coils are cheap). Also change/check plug wires, plugs, distributor cap and rotor.

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What is nice about having two engines is that you can switch one component at a time from one engine to the other and see if the problem jumps over to the other engine. From his posts I guessed he doesn't know what engine is doing it yet.

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Mike, you've probably got Rochester Quadrajets for carbs. Check a couple of plugs that are reasonably easy to get to and check to see if they are running lean. I'm guessing it's somewhere in the ignition system.

I tend to agree with the coil going bad idea. It might be just starting to break down a bit as it gets hot.

Except for wet sanding on the transom, got Outlaw finished up this afternoon. Gonna crank the motors over tomorrow on the hose and double check everything then try to drop it in at the local ramp for a little test run.

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I would agree with Russ on this as well, check for the simple things first, plug wires, ignition wires, hell even a bad ground can cause a backfire. If your losing spark, and then reconnecting its lighting too much fuel. Especially if you just repalced the motor, go over it and check all your connections. I think we all tend to think worse case scenario right off the bat.

I had a situation on my dually where both my low beam lights went out at the same time, I was convinced it was something serious, as the bulbs looked fine, drove for a month blinding people, then while at one of my GM dealers, had them check it, sure enough, both bulbs were bad. I quoted Homer Simpson...DOH!!!

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Check the timing on the offending engine when you find out which one it is. Check the distributer cap for carbon tracks. Also check your coil for carbon spots from the negative terminal to the coil wire. 4500 RPM is kind of vanilla to float a valve so I don't think this is the problem. As mentioned in a post above if it is too lean it could cause a backfire thru the carb. if all checks okay and the problem still persist hook up a vacuum gauge and make sure things are smooth. If you have a burnt valve it will show up here.

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I had the exact same thing happen years ago to my 1979 Coupe de Ville. Maybe we are slamming our hoods closed too hard?

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[QUOTE=offshore-drillin] I think we all tend to think worse case scenario right off the bat.

It's a boat. It's because of all our experiences that we COMPLETELY AND TOTALLY PANIC WHEN SOMETHING GOES WRONG. All we hear is "ka-ching, ka-ching, ka-ching" and then the mechanic telling us, "it's worse than we originally thought".

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The first thing I think of with a backfire is lean fuel mixture. It can be caused by several problems. Check the easiest first, filters, collapsed lines, etc.

As far as a coil being bad, I couldn't tell you how many times I've convinced myself I had a bad coil, bought a new one, and it has never yet fixed the problem. I'm sure I have a couple spare coils laying around.

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Sounds like a personal problem, and without question more information than I needed to know. :D :D

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Same here; it either works or it doesn't - No in between for me. I have also never had a fuel filter restrict my fuel flow enough to cause a low pressure problem.

Roby

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Had a similar problem, one of my engines "backfired" thru the exhaust at 4000 rpm. It was the the electrical connector (socket) on the engine that was bad and caused the engine to shut down just enough to get fuel/air mixture into the exhust and in the next rev it fired with a pop in the exhuast. I cleaned the connector and separeted the pins going into the socket and then it worked :D :D

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Thanks guys .. got a ot of checking to do ...will keep ya posted ...

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Yoo Hoo ! cant wait to hear a report on how it sounds and does in the water ....I dodnt know for sure but I would think you need to kinda take it ez first time out because motors have been idle so long ? like we can take it EZ .... :D

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fuel starvation can cause a lean backfire at WOT.
bad coil will usually not result in a backfire, but more of a simple miss that gets worse with additional heat.
bad plug wires will result in a miss, and sometimes a midrange backfire, but not a wot backfire.
rotor and cap can cause just about any sort of pop, miss, or backfire.
weak or broken valve spring can result in WOT backfire.

that's a short list.

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Don't forget to check the connections on the distributer module if you have the Merc thundervolt ignition. I had a backfire problem with one of my engines after a rebuild and the cause was a loose connection on one of the module post.

Good Luck!!!!!!

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Make sure the distributor is tight and didnt move. Check the ignition timing. Also check the timing advance at wot. You will need timing tape on the harmonic balancer or a timing light capable of reading advance. I once had a dist. a little loose and it turned just enough to lower the advance enough that it started poping at 4000 too.

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Took the boat out today for some test runs ...Put Teressa in with the motors and made a few WOT runs .... Port motor ....it is thru the carb ...you have to be at 4900-5000 for a cpl of mins and pop ..... So i guess start swapping stuff out ... gona do Filters ,Cap and rotor ....see what happens ...This is sorta good news as this is the older of the 2 motors ..< 50hrs ... Starboard has about 10 or so ... There is another isssue with this motor ....burns or sumthin about 1/2 quart an hour ....it was full when we started and now shows about 2 quarts low ...we put between 4-5 hours ...This same motor got water in it this past summer and we pulled it ..had heads checked and valves done ...has new gil manifolds with Stainless marine pipes .... there are no leaks that I know of ..

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Mike:
What color are the spark plugs after the run?
If the center porcelain is beige or brown that's good. If they're white the popping is from a leaning-out. Compare the plugs to the other engine too. Should look similar.
Ed

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will do ...one thing ...can I pull em now after a 10 min idle back to the house? ..or should I run a WOT ....shut em down and pull em ??? sure would be nice to be able to let it cool down in there ......

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Call me stubborn but I still think it is leaning out. See how the plugs look now after the idling. If it started popping immediately at 4900 then that's one thing, but it took some time, as you described, before it started to happen. This tells me that it's either heat related to an electrical component or the demand is exceeding the supply of fuel. Keep me posted; I love this stuff.
Ed

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I certainly hope you didn't close the hatch cover all the way....:D

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was wondering if someone would pick up on that .... :D we just cracked the cover and she peeped in from the back seat ........ :drink:

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If it is a lean condition it would take time for the valves to heat up and cause the POP.
If it was a coil or wires it would just be a MISS not a POP.
It has to be something related to the valves - lean mixture, valve float, sticking valve.

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going to take some plug readings this weekend ...go from there ....

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My money is on leaning out. Experienced the same think after having the wore out 454's that where in the boat when I bought it rebuilt. You know, the ones that are supposed to have low time on them and run great :( , that's another story.

Anyway, after total rebuild of everything, engines carbs, alternators, distributors etc. heard that pop through the carb, mostly on the port engine. Well after the inital 15 hours of messing around swapping parts etc., finally got around to plug and vacuum readings (like a nubmer of OSO guys were suggesting) which quickly revealed the problem; way, way to lean! Probably the only reason they ran before the rebuild was they were so wore out when I bought the boat, the blow by was causing teh plugs to be black, but even then they pooped once in a while above 4000 rpm. After total rebuild, plugs went instantly to stark white :eek: . Long story made short, started re-jetting the 750 dual pump Holley's two steps at a time; went from #70's on the primary to eventully #88's and the secondaries went from #80's to #90's. Plugs started turning beige and they've ran great (no popping), well except that unknown to me, the damage had been done to those nice freshly rebuilt 500 hp 454's.

Since I had messed around with it for those first 15 hours or so before I found the lean issue, by the time I resolved the problem, I had about 25 hours on them; port engine dropped a SS Manley intake valve at 35 hours , went from 3800 rpm to zero in about 3 seconds and locked it up! :mad: You guessed it, total rebuild due to hydro lock; cracked pistons, bent rods, two crack cylinders, it wasn't pretty. The mechanic that had rebuilt it 35 hours earlier called and said, "man, these things have been running lean, found play in a couple of valve guides after only 35 hours...........". That made me pull the heads on the other engine and had them rebuilt again; found play in a few of those valve guides and excessive wear on the seats, replaced guides, valves, seats and everything just to avoid possibility of maybe dropping a valve in that engine and causing the same major damage. That was another $5,500.00 not counting the 1.5 karat ring I had to by my wife since I had now spent about $15,000.00 on boat engines I'd only had for about 10 months; she calls it "her boat motor". It was worth it, didn't have to hear about it anymore after that :D !

What I'm saying is, CHECK THOSE PLUGS AND CHECK THEM NOW! Re jet while taking vacuum readings a various rpm's, write them down; vacuum will drop with each re-jetting until you get it right and then vacuum will begin to increase when you've gone pass the right jets, that means you got to rich. Change jets 2 steps at a time, if you've got 70's go to 72's take vacuum and plug readings again. I know, dyno it and be done with it. That's great except I can't afford a dyno and didn't know anyone that had one, besides nothing like seeing it under real (load) conditions assuming you don't take 25 hours to correct the problem of course :mad:. With #70's and #80's, I had 6 inches of vacuum at 3000 rpm when I started rejetting; with #88's & #90's, the vacuum readings were 12 inches at 3,000 rpms, no more popping, plugs beige.

Where did I get all this wisdom, right here on OSO, (thanks Stephan30, Formula31 and all the other guys that posted for me) the greatest source of information you'll ever get on this boat stuff and it's free! :D

Hey, if this isn't the problem great, at least I've past on my experience and hopefully someone benefits from the post just like I did about 6 months ago. Good luck and let us how you make out.

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pulled cpl of plugs ... if anything Starboard side (right) looks leaner ...going to change the fuel filters ....and on the oil consumption there are oil coolers ....anyone heard of cooler developing pin holes ? I am loosing about 1/2 Quart an hour ...

sorry bout the pics ...not very good

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Can't tell much from the pic about the plug on the left, but the one on the right is running lean and hot. I see enough from that to tell you have a fuel supply issue to that cylinder (fuel pressure at WOT, or some other fuel delivery issue).

The one on the left, can't tell much.
You'll need to put in fresh plugs, and then do a couple of hard passes and then show us some pix.

But don't do any pulls until you determine the cause of your lean-ness.
Fit a fuel pressure gauge near the carb. Have somebody watch it while you increase speed gradually to WOT. You should maintain 5-7 psi from idle to WOT. It would be great if somebody could verify what your FP is when the motor starts popping.

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Can't see the tips in your photos.

If your oil cooler is where you are losing the oil, you should see it in your exhaust water.
The oil pressure is higher than the water pressure, so the oil should get added to the water rather than the other way around.

It's usually a mess in the water and easy to spot.

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I had my wife show me how to save and lighten your photo.
Mcollinstn is correct with his diagnosis.

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http://dragstuff.com/techarticles/ho...ead-plugs.html

The color of the porcelain means very little if anything.

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From what I can tell it looks like the one on the left is burnng a little rich, looks a little carboned up, and the one on the right looks okay. Maybe I'm just seeing it different on my screen.

I always thought that a plug should be tan in color, more light light tan, light grey or white meant running too lean. Black obviously carboned up/too rich.

When I dialed in the smoothbore carbs on my ex-bike, I went through two coffee cans of plugs to get them exactly right. Once set, I ran the bike 25 thousand plus miles and NEVER adjusted the carbs again.