3.0 merc i/o with bad spark knock

[Dunaruna]

Re: 3.0 merc i/o with bad spark knock

Have a close look at your multi-meter. The 'multi' means it does many things (sorry, couldn't resist ).<br /><br />Does it have a switch on it that allows you to choose 4 - 6 - or 8 cylinders?<br /><br />Aldo

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Nope, it's more for repairing electronics. Should I get one from the marina, or goto the local autozone?

 

[Dunaruna]

Re: 3.0 merc i/o with bad spark knock

If you lived in Melbourne, Australia I could point you in the right direction. <br /><br />If your marinas are anything like ours, they charge like a wounded bull. Autozone sounds good. A multi-meter is the same whether it's for a boat or a car.<br /><br />Aldo

 

[ziggy]

Re: 3.0 merc i/o with bad spark knock

ya need a tool called a tach-dwell. it has a tachometer in it, and does dwell for usually 4,6, or 8 cyl. engines. got mine at sears. i don't think a multi meter will check dwell. a multi meter will let you see electricity(so to speak). VAC, VDC, and resisitance (ohms)usually.

 

[trog100]

Re: 3.0 merc i/o with bad spark knock

dwell aint the magical mystery all these pro guys would have to believe mike.. its simply the percentage of time the points are closed..<br /><br />it they aint closed for long enough the coil dosnt have enough time to charge at high revs..<br /><br />in simple understandable terms.. the narrower the points gap the longer the points remain closed the wider the points gap the shorter time they remain closed..<br /><br />points are a total compromise.. the correct gap for low revs aint the same as the correct gap for high revs.. the manufacturers recommended gap should give u the right dwell angle if the system isnt worn.. however useing a dwell meter tells u for sure..<br /><br />in an electronically controlled system the dwell angle would vary depending on engine revs.. low revs the dwell angle would be less.. high revs it would be greater.. points cant manage any of those fancy adjustments so they get set at a middle point..<br /><br />it works like this.. u set your gap to wide.. less dwell.. points closed for less time.. current passing thru coil primary for less time.. at high revs your engine will missfire and not be happy cos the coil wont have time to charge before it fires the spark..<br /><br />at 5000 rpm the points are closed charging the coil ready to fire the plug only 20% of the time they are at 1000 rpm..<br /><br />u set your gap too narrow.. more dwell.. points closed for longer.. too much current passing thru coil priamry when it aint needed at low revs.. points burn out quicker.. engine quite happy at high revs but things not so happy at low revs..<br /><br />the basic idea with points is to have em as close as u can.. maximum dwell angle needed for high revs.. commensurate with a nice clean sharp opening..the plugs fire at the opening..if it aint a qucik clean opening the plugs dont fore as well.. points have basic mechanical limitations.. spring bounce.. arcing and all the other kind of mechanical things engine valves (for example) have at high revs.. and they have to last a certain time.. they do limit the revs an engine can manage.. okay at the 5000 or so a boat engine does.. but would cause problems with higher revs.. cos of the dwell angle (points closed) not being long enough factor.. <br /><br />there aint no magical mystery to any of this stuff.. neither is there with the timing.. u run too much advance and your engine will knock.. u run slightly retarded and u have a happy engine but lose out slightly on economy and power output..<br /><br />back in the dark ages when these old cast iron headed boat engines were designed we had leaded gas.. leaded gas had a higher octane rating than todays unleaded stuff.. 100 octane stuff was the best.. today 100 octane gas dosnt exist.. regular unleaded stuff is 95 at best.. premium unleaded (for engines that aint happy on regular) is 96 i think..<br /><br />95 octane leaded gas would have been the gas put in tractors back when these engine were dreamed up. not cars.. he he<br /><br />these are UK octane ratings i think the US uses a different octane measuring system.. but the principle still applies.. todays gas aint as good as it was due to the no-lead factor.. <br /><br />u could probably have cured your knocking problem simply by useing better gas if such a thing existed.. sadly it dont.. <br /><br />in your case mike.. u did cure the promlem.. u didnt "mask" it.. the promlem was engine damaging pre-ignition or knocking.. <br /><br />as yet the cause which is of seconday importance we dont know.. it could be any one of the things i mentioned in my first post..<br /><br />i think i am beginning to gain insight into the follow the manual mantra the pros keep screaming on this forum.. they have seen and had to put right too many amateur foul ups.. merely knowing a none pro has been near an engine means checking everything cos it could well be set any old how.. thinking he might not have followed a "manual" must be a nightmare.. he he <br /><br />basically if a would be boat mechanic really has no understanding of "how things work".. he can follow a manual step by step and in theory get things to work.. this is how the pros would have u do it.. dont ask why or how.. u dont need to know.. just follow the manual..<br /><br />personally i think a basic understanding of "how" things work".. should come before any would be mechanic is let near a spanner/wrench or a manual..<br /><br />the first thing to do aint buy a manual.. which is designed to tell pros setting and such and make their life a little easier..<br /><br />the first thing to do is to learn a few basics about how the thing u are about to take apart works..what it does.. how it does it and what overall part in the sytem an individaul part plays.. if u dont know the basics no manual designed to be read by guys who do know (proffessionals) is gonna be any use at all to u..<br /><br />manuals are usefull.. but u do need a little knowledge before u can make best use of one..<br /><br />and i really am gonna stop feeling guilty about not following the book and trying to explain things in simple terms..<br /><br />trog100

 

[waterone1@aol.com]

Re: 3.0 merc i/o with bad spark knock

Ya know Trogg, I have to agree with you to some point. I refer to this as the McDonald's syndrome. Years ago the management at McDonalds decided that if their employees were allowed to think for themselves, they would not have the same, consistant result all over the planet. So they devised a repeatable formula for all of their resturants to follow, step by step. Completely eliminating any chance for variation because someone thought it should be done a different or perhaps, better way. And what did they get ? The most successful, repeatable, fool-proof way of serving a hamburger in several countries, with 100's thousands of employees doing it all the same way. I have had a burger in Helsinki, Finland....it was exactly what I expected....just the same as down the street from my house. Can you blame Mercruiser or Volvo for wanting their engines to be serviced the same way ? If we were building a racing boat, throw the manual out the window and tune that particular engine for the most performance it is capable of, but for day to day repeatable results that will work on any production engine....follow the manual, it will work. If you are a "super tech" and can read beyond the lines.....go your own way, but the manual is a safety net that will get the job done right.

 

[Dunaruna]

Re: 3.0 merc i/o with bad spark knock

Your analogy is flawed waterone1, it ain't the same worldwide. In Japan you need to upsize just to get what you and I consider regular, in Italy when you order coffee you get COFFEE!! . <br /><br />Aldo

 

[waterone1@aol.com]

Re: 3.0 merc i/o with bad spark knock

Oh, I forgot, if you are in Moscow, or somewhere like that, only take one napkin....otherwise the police will be after you.

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

And don't forget that things/specs DO need to be customized for a specific application or need. Example: Order a fish sandwich down here in the southern states and it's catfish, order the EXACT same sandwich in the northern states and you get whitefish. A perfect example. I'm not afraid of "working outside of the lines." All I want is the best performance and the best economy my little boat has to offer. Tha's why we have more than one prop to choose from when ordering a replacement, not just the one the manufacturer said was the best. I have learned alot from this chat room, but please remember I have had prior automotive experience, however limited it was, and I do have one of those diplomas around here for electrical engineering. Give me a circuit board and stand back, give me points, and watch me say "hunh?" Thanks guys, mike.

 

[rodbolt]

Re: 3.0 merc i/o with bad spark knock

mikebc<br /> best advice I can offer is forget about almopst allyou learned about automotive work. marine engines work in a totally diffent environment. its closer to a a roundy round race car pulling a fully loaded travel trailer.a subsystem does not have to fail it just has to degrade. kinda like a circuit card, it may work but a component has degraded enough that the noise renders the output usless.<br /> retard the ign timing a few degrees and run it at its rated horsepower,remember the load never goes away on a marine engine, for very long and the 2.5 and 3.o motors like to get hotspots around the head gasket area and the gasket fails. run it with a bit much advance and they tend to eat the piston at the ring land. the 3.o likes to crack heads if run hot. it also idles a bit rougher than the 2.5 with a rochester. while a lot of the principles of operation remain the same the operating environment is much harsher and less tolerant. some times I am biased cause all I see is dead motors on a daily baisis. just remember that "spark knock" results in uncontroled and rapid rise in combustion pressure and heat. kinda like a voltage spike on your circuit card. the card may put up with some but not a stady series of them. the "knock" or "ping" you hear is actually the metal in the cyl head viberating.retarded ign timing leads to excessive heat at the head gasket,exhaust valves and ex manifold. over advanced can lead to "ping" and loss of top end power. the base ign timing ,on that motor, also controls the amount of max timing advance. if its off at idle it will be off at WOT.I bet you dont repair circuit cards on the wing? I would imagine you at least look up the tolerance specs and the performance specs of the replacement components. all the points are is a mechanical swith that controls and inductive transformer. when the primary field collapses the secondary has a voltage induced in it. its gotta go someplace so the pressure causes the electrons to head for the spark gap. hopefully at the correct time in relationship to the piston and the valve train. on some modern EFI motors max ign timing is controled byseveral inputs from various sensors, some use a piezioelectric sensor in the block to detect spark knock and will constatly advance the spark until knock is detected then back the timing off. if knock is detected it can start decreasing spark advance and increasing the injecter time on pulse width. what I am trying to tell you is your power plant has many subsystems. the 3 that fail the most and cause the mechanical assy failure are ign,fuel and cooling. they dont have to fail just degrade and bad things happen. I am or have been certified on merc,volvo and am aurtherized for warrenty repairs on marine power,indmar and pleasure craft.plus a bunch of outboards. I may start the day diagnoesing a 8.1 MPI merc followed by a TBI volvo 5.7 followed by a 4 hp yam followed by a HPDI yam or an EFI yam or just a set of carbs on a 25 hp. I cant remember it all but I know where the service manual is. I do usually remember how it works just not all the service specs and procedures.

 

[trog100]

Re: 3.0 merc i/o with bad spark knock

this "howstuffworks website is my favourite.. it should be required reading for every would be meddler.. <br /><br /> http://www.howstuffworks.com/big.htm <br /><br />it dont say much about getting the right macdonalds burger thow.. he he<br /><br />macdonalds are a special case thow.. they have even put new words into the english language.. "macjobs" for example to typify low wage dead end jobs..<br /><br />praps the good old adage is true after all.. "pay peanuts get monkeys"..<br /><br />

<br /><br /><br />trog100

 

[trog100]

Re: 3.0 merc i/o with bad spark knock

good stuff rodbolt.. i would have to dissagree with just one point.. <br /><br />"retarded ign timing leads to excessive heat at the head gasket"<br /><br />we are talking sensible margins here.. not retarded by a mile stuff..<br /><br />basically the only downside is slight loss of power and fuel economy.. the engine is under less stress..dosnt get as hot and will in fact llst longer.. <br /><br />dont get retarded ignition timing mixed up with weak mixture symptoms.. the too are different things.. in short no engine will be damaged in the slightest by running the timimg slightly retarded from the optimum power setting.. <br /><br />trog100

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Well,<br /> <br /> I found a conversion kit that gets rid of the old point system, for about the cost of a 1/2 way decent dwell meter. Any thoughts on these upgrade kits?? Best part- I would never have to set dwell!! Feed me whatever information you guys have on this! Thanks again for your knowledge, Mike.

 

[Don S]

Re: 3.0 merc i/o with bad spark knock

I found a conversion kit that gets rid of the old point system, for about the cost of a 1/2 way decent dwell meter. Any thoughts on these upgrade kits??

Some are really good, some are pure JUNK. <br />What kind of conversion kit did you get?<br />A good usable dwell meter is 30 bucks.

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Pertronics "BREAKER-POINT-TO-ELECTRONIC IGNITION CONVERSION KIT" For just over $60 after a few discounts and coupons. Seeing that I put in new plugs and wires late last fall, and now with this "upgrade" should I replace anything else, just to cover all of my bases?

 

[trog100]

Re: 3.0 merc i/o with bad spark knock

not really.. the kit u are talking about is a common one.. in truth there are a zillion things u can check and worry about.. but fit the thing and see if it works okay.. i think u know the basics by now..<br /><br />apart from the possibility of your timing being a litle over advanced your other problem wasnt an ignition one anyways..<br /><br />do a visual check on your plugs make sure they aint been running too hot for whatever reason.. in one of these engines a nice brownish colour should be about right.. they aint lean burn technology.. make sure they aint white-ish.. which would be sign of too hot a plug or lean mixture..<br /><br />u should do this immediately after a run by the way.. doing it after a period of idle or no-wake speeds will give false results..<br /><br />u could also increase your plug gap by about five thou.. say 35 to 40.. this will take better advantage of your new ignition system.. <br /><br />trog100<br /><br />ps.. and if u really want to step outta the box.. try and find some extended nose plugs.. they work better than the stanard nose ones that the manual recommends..<br /><br />my 3.0 pulls an extra 150 rpm at WOT (about 4900-ish) with electronic ignition.. 40 thou plugs gaps and extended nose plugs.. all part of a mod package i did to my engine.. which bit did the most i dunno but as package its improved things all round..

 

[Don S]

Re: 3.0 merc i/o with bad spark knock

The Pertronics are good units, and a lot of people have them. You still have to make sure the timing is set correct and that the advance timing is working.<br />Before you install the kit, check the advance weights and springs and make sure they are in good shape and not froze up and all rusty.<br />Also check the bushings in the distributor that they are not worn out. (They will only wear out in one direction, so wiggle the shaft all different directions) If you have a lot of movement, you need either new bushings or a new distributor.