Mercruiser 5.0 Liter Backfires - Help Please!

[Jubillee]

"I own a 1992 Bayliner 2452 Ciera powered with a 5.0 liter (Chevy 305) Mercruiser Alpha outdrive combo. It runs quite well with one exception. It sometimes backfires when powering out just before planing and also when I back off the throttle and start coming off plane. This is at aproximately 3,000 to 3500 RPM. The engine has electronic ignition, but not fuel injection. <br />I have checked the plug wires, replaced the plugs, replaced the distributer cap, rebuilt the carb., checked the timing and none of these things have cured the problem. On rare occasion when cruising at a constant planing speed it will backfire, but most often it happens either accelerating or decelerating. <br /><br />Just about the time I think the problem has disappeared it does it again and makes me uncomfortable. I have spoken with several Marina mechanics and they have no definite answer. I hate to pay hours and hours of labor if they don't have a good idea of what the problem is. Has anybody encountered this problem before? <br />[*]null

 

[DJ]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

Jubilee,<br /><br />Is it runing cold? What temp. do you have?

 

[Jubillee]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

The temp. is normal, 180 f. It is fresh water cooled. I run it in Puget Sound and the San Juan Islands.

 

[Walter]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

180 degrees is "normal"?? Are you sure? That sounds a bit high to me. Of course, I'm basing my opinion on what operating temp my 1980 MerCruiser GM 305 ran at...that was 150 degrees. My manual says the thermostat should be fully open at 143 degrees. Again though....this is a 1980 model.<br /><br />Guys...assuming his gauge is accurate...do you think he's running a bit hot? Or is this normal for his model year? Just curious...<br /><br />Walt

 

[Walter]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

Jubillee...<br /><br />I just remembered we're in the BOATING forum. You'll receive many more replies if you post this topic in the ENGINE/MOTORS forum.<br /><br />FYI<br /><br />Walt

 

[Joecol]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

Is there any engine noise coming from the top of engine?? Is there a ticking noise coming from beneath one or both of the rocker arm covers? What I'm getting at,is Chevy 305's are known to have soft camshafts,meaning one or more of the lobes on the shaft have prematurally warn down.Is it a loud popping sound from intake or low miss out the exhaust.Quickest way to check would be to remove rocker covers and have someone crank over engine.(Disable iqnition system first so engine won't run,remove coil wire} Watch for rocker arm movement or any visible problems like loose rocker arm ECT.... Just my 2 cents but make sure you get back to me at Cshwetz@nwconx.net or post a reply!!! <br /><br /> Joseph in Canada { Nipigon, Ontario}

 

[f_inscreenname]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

I used to have a chevy that if you backed off gas to fast it would back fire out the tail pipe. It ended up being a cracked valve. <br />I have a question. You said it happens when you come off a plane. Is that when the ass end sinks down in the water? If so..... You may want to check the carb. The floats may be set right at the top. When you come off a plane (and tilt your motor back) it may be loading up.<br /> With boats its never simple!<br /> Mark<br />P.S. 160 to 180(180 when running at speed) is normal any hotter though could be bad.

 

[Jubillee]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

Thanks for the responses to date. I will check my manual in the morning, but I have a 180 degree thermostat in it if I remember right. It maintains a temp of 175 to 180 through the full range of speed.<br /><br />The float comment is interesting. I have had someone mention that before the carb was rebuilt. The fellow that rebuilt it says the float level is right where it is supposed to be. Maybe we should try setting it lower. <br /><br />Yes, I do mean when I am coming off plane and the bow rises and the inverse as well. <br /><br />It shouldn't be the cam. Although it is an older boat, it had very little use and the engine only has 250 hours on on the meter. Again, worth checking however. I'll start my troubleshooting as time permits. Hope it isn't a cracked valve.<br /><br />Thanks for the comments. Keep them coming.<br /><br />Bill H.

 

[petryshyn]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

The variable here is that backfiring occurs in two different places for two different reasons....<br /><br />Are you seeing fire come out of the carb? or is the bacfire you refer to from the exhaust......2 totally different issues that must be addressed first....

 

[Trent]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

Schematic is correct.. Really they are two diffent things. It really is called backfiring and afterfiring. Backfiring comes out the carb. Afterfiring out the exhaust.<br /><br />If it were mine I would check the compression both wet and dry. You said this was fresh water cooled..Do you mean a closed cooling? Or raw water? Also the parts books only list a 142 or 160 degree thermostat for the 5.0 depending on year and what kind of cooling system.<br /><br />Also have you checked for excessive carbon deposits in the cylinder? Weak valve springs?

 

[DJ]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

Schematic and Trent are right. Two different animals with different potential causes.<br /><br />Exhaust "afterfiring" is typically caused by:<br /><br />1. Rich mixture that collects in the manifolds, etc., combustion chambers, back sides of valves. and is set off by hot spots (carbon buildup). <br /><br />Backfire through carb. is typically caused by:<br /><br />1. Lean mixture.<br /><br />2. Cold engine. I do not think this is your problem.<br /><br />The least expensive thing to try first is to go to a GM dealer and ask for some engine cleaner. They used to call it "Top Engine Cleaner". Follow the instructions on the container.<br /><br />You may have a carbon buildup in the combustion chambers causing hot spots that fire the fuel/air mixture at the wrong time.

 

[petryshyn]

Re: Mercruiser 5.0 Liter Backfires - Help Please!

If the carb is a 2bbl rochester, the vacuum power piston in the top of the carb has a tendency to bind from uneven wear. This can cause lean on acceleration and rich on decell.(if it shakes loose while at high speed) I've cleaned and polished many, but eventually required replacement, as the piston bore is at fault. This may not be your problem, as marine apps may be different, but this has been my automotive experience. Also have a good look inside the distributor cap. Check for carbon traces(resembling pencil drawing) This can cause cross firing and your exact problem as well.....Cross firing can also occur from plug wires running parallel to each other. Try spraying down each plug wire with a water spray bottle to look for leaks to other wires or ground. Do this at idle, and listen for drops in RPM when doing each wire.<br /><br />These are all shots in the dark. You must let us know whether its backfiring through the carb, or exhaust befor we can REALLY troubleshoot this thing. I hate GUESSING!!<br />