centrifugal spark advance / carb setup

[vinnie1234]

My Gm 3.0l 4 cylinder engine has a points ignition system and centrifugal advance mechanism. When setting up the ignition, the spark advance at idle is supposed to be 0* BTDC. To start, I set a test light on the points with #1 cylinder at top dead center and rotated the distributor until the light came on. Then I installed the cap and checked the timing with my light. It now sits around 8 to 10 degrees BTDC when idling. I think that this would indicate the springs in the advance mechanism are fatigued.

My theory is that this has screwed up my carb settings a bit, as I set the throttle opening and idle mixture screws with the timing set this way. I left it at this setting because (in theory) set this way I should have correct advance at full throttle. If I retard by 10 degrees, then full advance would be retarded as well.

I notice that there is a bit more variance in idle speed with engine temperature than I would like. My theory (again)is that this is a result of an increased throttle opening due to setting up with a little more advance than what the engine and carb are designed for.
I believe the right answer is to replace the springs, however my better half is a little touchy about spending money on the boat. In her view I can fix anything so there should be no need to spend money on it. lol. Anyway, I wondered if there were any tricks that you guys have come up with to MacGyver this. Such as springs from something else... a tension spec for the springs or torque spec for the rotor - something like that so I can "modify" the existing spring setup to maintain 0* at idle and still achieve full advance when appropriate.

Fyi, idle mixture screws are set at 1&3/4 turns out. I don't have a spec for throttle opening at idle, however the boat comes out of the hole nicely and goes really fast. Plugs are tan-light gray, and all plugs are burning the same. Dwell at approx. 30-34* (my dwell meter is for an 8 cylinder so I have to double the number on the scale which hovers at 15-17 degrees).

As a boating noob, I have been taking it out and testing in stages - haven't really hit WOT for more than a few seconds, as I don't think that paddling this thing back to shore would be a lot of fun by myself. I want to make sure I wont blow it up before I get cocky. I also want to learn how to properly trim the boat before I test RPM at WOT. I figure if the bow is down too much then that will slow her down some and throw off the numbers I get.
The boat was a basket case that I essentially got for free, (sat for about 10 years) so I am nervous that there is something hiding underneath that will come up and bite me in the tush. My objective is to get it tuned perfectly and running like a new boat, one issue at a time.



Thanks for your help.

 

[bruceb58]

What is your total centrifugal advance? This would be your final advance minus initial advance. Should be around 28°

You understand that the ignition fires once the points open right?

Since you have a stringer drive, you can't do any trim unless you have the optional trim that moves the front of the motor up and down. Do you have that? Not all do.

 

[Maclin]

Regarding the advance mechanism springs, what does it feel like when you move the rotor? Is there a good amount of resistance? And a good snap back? If so the springs are ok. They usually no not fatigue, so do the rotor snap-back test first before assuming they need replaced.

The initial timing should be set by the timing light with engine running, not by a test light (do you work on older motorcycles?), and at low enough RPM that the advance weights are held in by the springs. After setting initial then check the total at the higher rpm.

Do you use a dial-back type timing light?

 

[vinnie1234]

Hi Bruce, Yes, I have selec-trim on the boat and it works now, after I rebuilt the trim motor. (*&%#$).
Yes, I do understand that spark fires when the points break. I don't think that setting the initial timing this way would cause the initial timing discrepancy that I see. I have done this before on cars and motorcycles with success.
With the distributor shaft only turning at 1/2 engine RPM and considering time delay for spark to occur in the plug (I would think) that the difference would only be 10ths of a degree, but I confess I haven't done the math for that one. Perhaps that is it?

The water pump and front engine mount obscure my view of the crank pulley when the spark advances, so I am not sure that I achieve any specific degrees of advance. The engine performs well off idle and I have a good plug reading. Operating temp is also right around 160* - so I am thinking final advance must be close.

Maclin, I don't have a dial back light. the reason I set initial timing this way is because the boat was a no start and I didnt want to fart around. I wanted to eliminate timing as a cause and if it don't fire at 0 it aint the timing! lol.

I had one guy tell me that an "oldtimer" told him that a timing light was no good on those things. Not sure why that would be the case, but I was considering just setting the timing "by ear" and running the boat to see. Having said that, it runs so well off idle right now that I don't want to mess with it. the idle is the only thing that gets me. I have maybe a 75 rpm difference between warm and hot - so it isn't bad, but I am a fussy bastard and I want it to be perfect. When I say warm and hot, I am referring to choke-off condition. In my view, once the choke is off - if all is set up properly I shouldn't get an appreciable difference in idling rpm. (+- 20?)
As for the spring/weight mech it feels perfectly normal to me. Idle rpm range is currently 500-600 rpm off choke.

 

[Scott Danforth]

I think your over thinking it. engine RPM at 600, adjust dwell, readjust idle speed, set base timing to 8 degrees, go boating.

or if you want it perfect pull the points and condensor, install a pertronix, set the idle to 600, set base timing to 8 degrees, go boating.

 

[vinnie1234]

lol! You speak the truth Scott. As for the electronic ignition, this may be something in the future for when I can put some money into it, but for the time being I have to go with what I've got. As far as the points, the good thing about them vs and electronic system is that sandpaper and a matchbook can get you up and running whereas if the ignition module goes for a toot it's down to a paddle and a workout. I was thinking I would get the petronix and a new distributor and keep the old points setup in a cubby on the boat just in case.

I got a hold of a manual for 1986 and up omc. I realize the outdrive is different, but there are specs in there for the gm 3L with Rochester 2bbl carb that say 0* base timing. In my experience there are some vehicles with elec spark advance, (such as a mid 90's gm 350) that require base timing at 0, but this is for the ignition module. You disconnect the EST wire and set base timing, and then the module sets it automatically at 8*. Before I got this book, I figured 8* was where I would start. It is at around 8* right now, but I got to thinking about the carb settings and wondered if this was the problem.

In my logic, i know that the engine will idle faster with retarded timing, so with advanced timing, it idles slower. to get the engine to correct idle speed, idle mixture would have to be enriched as the throttle plate has been opened to compensate for the lowered rpm and vacuum has been reduced below the throttle plate.
As the engine heats up and the air in the intake heats, there is less demand for fuel as there is less available oxygen by volume. This brings the idle down in the same way that backing out the idle screw does. I figure that with the timing retarded, (to spec) mixture screws in a touch and throttle plate opening smaller (with higher manifold vacuum), there would be less cfm at idle, less fuel metered and as a result less of a difference in idle speeds between warm and hot engine settings due to oxygen levels in the charge.

Does my logic make sense? To me it seems worth the effort if perfection is just a tiny tweak away. You can call me crazy, but you can't call me lazy.

 

[bruceb58]

8° is too much. Its going to be either 4 or 0.

 

[vinnie1234]

It just dawned on me, that I could lock the springs closed so there is NO centrifugal advance, start it up and see if the timing changes. That would at least tell me if I am wasting my (and everybody elses) time with this idea. I think the sun is out now.

 

[Maclin]

"In my logic, i know that the engine will idle faster with retarded timing, so with advanced timing, it idles slower."

This is backwards, advancing timing will increase idle rpm's.

 

[Maclin]

If the rotor snaps back with authority then I do not see any need to do anything but keep the idle below 800 and set the timing. If the timing mark is still moving around then maybe the dist shaft bushings are worn, or the cam timing belt/gears/tensioner have some play,

 

[vinnie1234]

The timing mark is very consistent, but the degrees indicator on the block is rusted all to hell. 0 is about the only thing I can figure out because the detent is larger than the other marks.

 

[bruceb58]

The timing mark is very consistent, but the degrees indicator on the block is rusted all to hell. 0 is about the only thing I can figure out because the detent is larger than the other marks.

Will be fine if you have an advance timing light. I couldn't imagine not having one.

 

[vinnie1234]

Yes, well I don't have one....yet. The dwell meter I have is also for an 8 or 6 cylinder, so when checking dwell I have to set it to 8 and double the reading. Not overwhelmingly accurate. I am thinking that I may pick up a better gun. Matco has a nice one that offers dwell from 2 to 12 cylinders and digital tach too.

I will need to deal with some other issues that are higher priority as I'm sure you know from my other threads there are other issues with my boat. Once I get this one figured out I will post if I am successful and what the fix was.

I have to say as a noob to boats, being able to come on here and spitball ideas with you guys is awesome. I am glad I found this site. Hopefullly I can put some of my auto knowledge to good use and help out some others on these forums while I am here.
Cheers,

 

[vinnie1234]

Our Matco tools guy has this weird ability to show up whenever we mention needing or wanting tools. Today was no different as he shows up out of the blue to say hi. So, I have ordered the 500 series timing light. Besides, it's "on special". lol. It will be here next week. I guess now I will have an easy way to verify all of this information.

 

[bruceb58]

Wow...nice light! Those are expensive but you get a dwell meter, timing light and tach all in one.

What did it set you back if I may ask?

If I was going to buy a new light, it would be this one. Looks a lot like Matco.
http://www.amazon.com/INNOVA-5568-P...404949132&sr=1-1&keywords=innova+timing+light

 

[vinnie1234]

168 BUCKS.

 

[vinnie1234]

Canadian

 

[bruceb58]

Not bad!

 

[vinnie1234]

I figured it was worth at least that for a good service tach. All the other goodies just make me smile

 

[Maclin]

Good deal! That will really help in getting the tuning spot on,and that will be a difference you can feel.