Total Advance Ignition Timing

[Don Brock]

I have a 1985 Wellcraft 23 Nova XL with M260. It's a Chevy 350 cid (5.7L) 260 HP (w/Rochester quadrajet). Owned it less than a year - my first IO. <br /><br />Three questions:<br />1. Books says initial timing should be set at 8 degrees BTDC. I didn't see any mention of Total Advance in the Seloc manual. My local marine dealer tells me that I should set Total Advance to 33 degrees at 3000 RPM. I've done that and my initial advance ends up at about 17 degrees instead of 8 degrees. I'm real sensitive to this because I just finished doing a head job to repair some badly damaged exhaust valves. I'm told that pre-ignition, too much timing advance, and lean carb are contributors to this. I don't want a repeat of the problem, so want to get this right the first time.<br /><br />2. I was also advised to rejet my carburetor to the specs of the 330 HP engine. The mechanic said the richer mixture would sacrifice a little fuel economy but would correct the overly lean condition from the factory and would help cool the exhaust valves thus ensuring their longevity. Is this a good idea?<br /><br />3. One more related question. I believe too much heat and possibly preignition were the culprits that caused my problem. What temp should I see at WOT on this engine? When the problem occurred the initial symptoms came after I had been running at WOT for about 10 minutes showing a steady 183 degrees on the temp guage, at 4500 RPM. I noticed the speed beginning to decrease very gradually so throttled back and cruised at 2500 RPM back to pier with some difficulty. Changed plugs and went back out, but the engine ran terrible - hard starting and barely able to get on plane. A subsequent compression check showed two cyls were down to 15 psi. Pulled heads and #2 & 8 cyls had badly cupped and cracked exhaust valves (note that these cyls are farthest from carb) Interestingly all other cyls were OK - including #1 & 7. I was really lucky - it could have been much worse. So got the valve job. However, if 180 degrees at WOT is too hot, then I need to fix a cooling problem as well or I'll be doing this again real soon.

 

[rodbolt]

Re: Total Advance Ignition Timing

hello<br /> without going to the manual to see which ignition system yopu have I will go out on a limb. boats do not have any vaucum advance. all the advance is either mechanial on older systems or electronic on EST and some thunderbolts. set the initial timing and lock the distributor. at 3000 rpm in the wat4er under a load allow the timing to stabilize and read it then. dont try to free rev it on a flusher. if it wont come up to spec then check advance weights or Modules.normally if its a lean mix then at least 4 valves will show it. the Q jet cannot be rejetted for hign speed. you can change the primary metering rods and jets some but the secodary is controled strictly by the tapered rods.be carefull when playing with the rods and jets. retarded ignition likes to eat valves faster than advanced. if the timing is retarded then the comustion temp has a tendancy to climb outta site. a friend of mine made his impala manifolds glow cherry red by forgetting to tighten the dist once and it rotated right around> . smoked a nice 327. I dont try to rengineer the things I just try to make them run as designed. remeber your boat runs at its rated power most the time. your car rarely does. most cars would not last if you hooked a 2 ton trailer and ran them at the redline for hours on end.<br />so when checking your total advance remeber you cannot do it by reving on a trailer nor rotating the distributer at 3000 rpm. there are specific reasons why.<br /> good luck and keep posting

 

[Buttanic]

Re: Total Advance Ignition Timing

I race 350 Chevys and have experience setting distributor advance curves. 34 to 36 degree at or above 3000 RPM at full load is safe. The reason you are seeing 17 degrees at idle is because there is not a wide enough advance curve in the distributor. If you want to set the advance at 8 degrees at idle than you need 26 degrees built into the distributor. Your distributor only has 16 degrees built in. Find a distributor shop or tune up shop with a Sun distributor machine and they can make ajustments to it to get 26 degrees. Check with any local speed shops or racers they can probably point you in the right direction. In the late 70's and early 80's the manufactures started lowering advance built into the distributor to meet government emission mandates. Increasing it puts you back to the way things were before the government got involved in building engines<br />Buttanic

 

[Don Brock]

Re: Total Advance Ignition Timing

My ignition is a Thunderbolt IV. I dont recall seeing any weights in the distributor, but I could have missed them since I wasnt looking for them. Sounds like the gouge is to set the Total Advance under load in the water, NOT in the driveway. 34-36 deg at 3000 RPM or higher. <br /><br />Dont mess with the carb jets.<br /><br />I'll check with the local speed shops for a Sun machine. Set the built in dist advance to 26 degrees. Thanks guys. <br /><br />Rodbolt, I can relate to the story about forgetting to lock down the distributor and it twisting at RPM and the cherry redhot exhausts. I did the exact same thing with a Dodge Superbee. I finished timing it and during the test drive it was running great, but it suddenly changed. I limped home and in the dark the exhaust manifolds were glowing cherry red - they looked transparent. Lucked out and the motor survived. Lesson learned: Retarded ignition generates terrific heat that can destroy the valves.<br /><br />What about the temp at WOT? What should it be?

 

[merc 140 pontoon]

Re: Total Advance Ignition Timing

Have you looked at Mercruiser Service Bulletin 95-11?<br /> http://www.boatfix.com/merc/Bullet/95/95_11.pdf

 

[Bondo]

Re: Total Advance Ignition Timing

Good Vibes,<br />You've been given No Bad Advice here, But with your added Info, I'll give it a Shot......<br /><br />1) Set your Base Timing at Idle to 8*BTC ..... <br />The Merc. ThunderBolt IV module has the advance built into it... It's stamped on the module somewhere... Forget the Dist. Machine.... You have NO Springs or Weights..... You can CHECK the Total Advance at WOT, with a Full Load... But Don't try to Set it that way.... Total Advance can be changed by changing to a different module, But I Don't think this is a Issue you need to deal with.... Using the link provided by merc 140 pontoon, You can see the differences.... Any of the modules from the V6s Or the V8s will run on your motor, But, Like I said, The Stock 1 you have is the 1 You Need.......At this point it seems you lack the knowledge to start playing with these Super Tuning Issues.... Go with the Stock Settings,+ Get It Right..!!!.....<br /><br />2) I was also advised to rejet my carburetor to the specs of the 330 HP engine. The mechanic said the richer mixture would sacrifice a little fuel economy but would correct the overly lean condition from the factory and would help cool the exhaust valves thus ensuring their longevity. Is this a good idea? <br />NO......<br />Factory Settings are to the Rich side if anything... <br />I'm Not a Rochester Quadrajet guy, But Any Carb is Tuneable.... <br />I Prefer the Merc/Weber/Carter carbs myself, their Very Easy to Tune....<br />Tuning the carb is done by Checking the Plugs after a run... What colors are they ?? Black? Lean it out a little..... White? Richen it some... You're looking for a nice Cream + Coffee Color....(there Is more to it than that, But this is the Basics)<br />So, Either Learn to Tune Your carb, or Get 1 You Can, or Get Someone who Knows, to do it for you.....<br /><br />3)Yes, I Agree... However, Your Heat Problem was Cylinder/Exhaust Gas Temptures were Way Too High..... NOT Overall Engines Temps... You can Burn-Up a motor that's Stone Cold...... 180* is Fine...<br />To Lower These Temps, See #s 1 + 2.....<br /><br />I Believe your Problems Started with a Poorly Tuned Motor being Run Too Hard.....(10 MINUTES at WOT ????)<br /><br />You Don't need to over richen the carb, or set the timing at WOT, or set Anything to Any Other Motor's Spec.s.... <br />Just set Your Motor up RIGHT......

 

[Don Brock]

Re: Total Advance Ignition Timing

Is it unwise to run this engine at WOT for 10-20 minutes (assuming its correctly tuned)? I expected that it was designed to take it, but I'm getting a different opinion after this experience, talking to folks, and reading the forum.<br /><br />In any case this is exactly what I needed to hear. Assume the Mercruiser engineers know their "stuff" - trust 'em and just stick to the basics. Good advice - I won't get fancy until I have more experience. Pontoon, the Mercruiser Tech link is what I was looking for. After the timing is correct, I'll experiment with tuning the carb as bondo suggests. I really want to keep the engine stock - I've read enough other forum postings to understand that basic tenant. Too bad there isn't a cyl temp gauge to go along with coolant gauge. Sounds like the spark plugs will tell the story well enough. Thanks gents.

 

[airman]

Re: Total Advance Ignition Timing

I'm curious - Why does the engine have to be under load to set total advance? On an older (spring and flyweight) distributor at least it seems to me that simply getting the revs up should be all you need. What am I missing? Obviously you can't do it on a flusher for fear of collapsing the garden hose. It seems a lot safer to me to poke around a high revving engine when the boat is securely tied to a dock.

 

[rodbolt]

Re: Total Advance Ignition Timing

Hello<br /> airman.<br /> I dont like to rev engines with no load. bad things can and do happen that way.some modules will not react instantly. some require a period of time to stabilize. each ignition system has its own method of testing and setting. I dont like the old tie it down and wind it up method because it sometimes grabs things off the bottom or pulls docklines in from 3 slips down. If I cannot use my dyno or a test wheel then I have a trusted friend that can operate the boat for me while I play with the engine. I have only been tossed out once in 20 some odd years . that helmsman got fired that day . the I/O engines will run years and years at the 4000-4200 rpm range as long as everthing else is up to par, but let the fuel,cooling or ignition system degrade and it will self destruct. the worst yet was a customer who tried to power time his volvo EFI. by the time I got it it had 3 misshapen intake valves and the intial timing was about 36 degrees. I tried to tell them you could not do it . <br /> good luck and keep posting.

 

[Bondo]

Re: Total Advance Ignition Timing

The Internal Dynamics of a Free Wheeling engine,+ a Fully Loaded engine are Just Not the Same.....

 

[airman]

Re: Total Advance Ignition Timing

Bondo,<br />I'm not thinking of the whole engine here, just the distributor. Obviously an electronic module will "know" that the engine is unloaded and mess things up. However, my old merc 470 has flyweight advance so I don't see how 3500rpm loaded vs. unloaded would make any difference. In the spring I want to check the advance curve and make sure it is sitting right for my normal cruise (3300-3500rpm). I realize rodbolt disagrees, and he's welcome to, but I'm more comfortable sticking my fingers near high speed machinery when I'm tied to the dock. I'd rather risk the engine than myself, especially when I've seen them on ebay for 500 bucks.

 

[Buttanic]

Re: Total Advance Ignition Timing

Airman <br />What you plan to do will work with your mechanical advance distributor. I do it all the time on my race car. A Sun distributor machine does the same thing only it turns the distributor at half engine speed because a distributor runs at one half crankshaft speed in the engine. This is old tech. Only electronicly<br />controlled advance needs to be under load because manifold absolute pressure sensors, trottle position position sensors, engine speed sensors, temperature sensors,knock sensors and others input to the electronic module that controls what the advance will be at any instant.<br /><br />Buttanic

 

[airman]

Re: Total Advance Ignition Timing

Buttanic, <br />Thanks. All the engines I've worked on (2-stroke, 4-stroke& diesel) are mechanical and there's lots to like about them. I'll go ahead this spring as planned. Unless my advance curve is really bad I'd rather have the timing out a little bit at idle than at cruise.

 

[Buttanic]

Re: Total Advance Ignition Timing

Airman, only thing you have to watch for is that it isn't advanced too much a idle. It won't damage anything but it can make the engine hard to crank especially when hot. You will think you have a low battery. I wouldn't let it get over 12 degrees at idle. <br /><br />Buttanic

 

[airman]

Re: Total Advance Ignition Timing

Thanks. I think if I was near 12 degrees at idle I would fix the advance mechanism. My concern would be too much advance as I climb out of the hole. I would be worried about pre-ignition, especially with a heavy load.