Ignition Question: Timing at Idle Wanders

[eldplanko]

Hopefully this is a simple question. I have a Mercruiser 888, with a 1974 Ford 302.

It has a points ingition with a mechanical (springs) advance. I'm trying to set the timing at idle (~800 rpm) to the specified 10 deg BTDC. I'm getting about 5-10 degrees in wander with the timing mark, which doesn not seem right. Any ideas why its not staying steady?

Its not supposed to advance until ~1000 to 1200 rpm, so I was thinking maybe the advance springs are not as tight as they should be.

Thanks for any suggestions.

 

[EddiePetty]

Re: Ignition Question: Timing at Idle Wanders

....I would suspect slack in the timing chain.

With the distributor cap removed, bar the engine over to TDC on the timing tab. Slowly bar the engine in the reverse direction until resistance is felt and the rotor just starts to move. Note the variance in the timing tab....close to the variance with the timing light?

 

[libellav15]

Re: Ignition Question: Timing at Idle Wanders

brilliant advice - brilliantly explained

does the engine have a chain tensioner on it? might need replacing.

have you looked at the mech advance springs - are they both connected?

saying that - your question reads as if the timing is fluttering about over a 10 degree range. - if the centrifugal advance was weak springed the timing mark would be steady - just in an advanced position.

also, could be play in the distibutor arm - that would show as a jittery timing mark

hope any of the above helps

cheers and good luck

adam

 

[eldplanko]

Re: Ignition Question: Timing at Idle Wanders

Thanks for the good suggestion.

Luckily, I had the dist cap and points plate off to pull the springs and check the weights.

I tried it checking the timing chain for slack, and it doesn't seem like there is any slack. As soon as I bar it backwards, the rotor turns backwards.

The springs look ok (I guess), a little rusty though. I don't have a way to check the tension. I did notice that the post (not on the weight) that the lighter spring was attached to was bent a little out.

Also, before I ripped the dist apart, I lowered the idle to 600 rpm, and reset the base advance to 10 deg BTDC. It still wobbled a little (not as much as before). As I revved the engine, it started to advance around 800 rpm.

 

[eldplanko]

Re: Ignition Question: Timing at Idle Wanders

libellav15:

Agreed on the advice from EddiePetty.

Attached is a pic of the springs. Like I said before, the post for the lighter spring was bent out a bit, perhaps by the PO to keep it from advancing to early.

It seems to me like a slightly weakened spring would allow it to begin to advance early. Am I right on this? I see you point though that at any constant rpm, it shouldn't fluctuate, so perhaps I have two problems: 1) weak spring causing early advance and 2) something else causing jittery timing mark.

Not sure about the chain tensioner, its a 1974 Ford 302. From what I can tell, the timing chain is behind a cover, and covered by the water pump and balencer.

Thanks for you suggestions. I appreciate it.

 

[libellav15]

Re: Ignition Question: Timing at Idle Wanders

no sweat

have you got a dwell meter? try hooking it up - does the dwell range alter up and down as well?

if so its possible that the points spring is weak or they are binding ever so slightly causing the dwell to be less than constant

this would then obviously reflect in the timing fluttering accordingly

 

[eldplanko]

Re: Ignition Question: Timing at Idle Wanders

dwell was constant at 27 deg.

 

[eldplanko]

Re: Ignition Question: Timing at Idle Wanders

Update... put the old spring back in and checked the advance curve. I could get it to idle down to ~600 rpm. Anything lower, and it surges (don't know why).

Base is 10 deg BTDC, and the advance way quicker than it should be (see pic). Is this bad enough that I need to woork about it, or can I get away leaving it as is.

Thanks for any advice.

 

[jtmarten]

Re: Ignition Question: Timing at Idle Wanders

Update... put the old spring back in and checked the advance curve. I could get it to idle down to ~600 rpm. Anything lower, and it surges (don't know why).

Base is 10 deg BTDC, and the advance way quicker than it should be (see pic). Is this bad enough that I need to woork about it, or can I get away leaving it as is.

Thanks for any advice.

Uhhh, because it's ready to stall .... it's an ancient 302, not a modern BMW masterpiece powerplant (and yep, I'm a die-hard Ford guy!)

The advance isn't coming in too early. No such thing. As long as you're at or below 36* its fine. Many HP boaters lock the dizzy at 36*; just have to run a separate switch to fire the coil once its cranking to start it.

 

[eldplanko]

Re: Ignition Question: Timing at Idle Wanders

Thats good advice... I will not worry.

 

[cr2k]

Re: Ignition Question: Timing at Idle Wanders

Did you wiggle the dist shaft to see if the bushings are worn?

 

[bruceb58]

Re: Ignition Question: Timing at Idle Wanders

Did you wiggle the dist shaft to see if the bushings are worn?

He said dwell was constant.

 

[eldplanko]

Re: Ignition Question: Timing at Idle Wanders

Let me clarify on the dwell...

Was constant at 27 deg from 600 to ~ 2500 rpm, from 2500 to 3500 rpm it decreased around 24 deg. I didnt try reving it higher than 3500. Is this drop something I should worry about. In the water I get to the around the correct rpm at WOT (~3800 to 3900, it's on the low end, but the engine is old and compression is not great).

After putting the springs back in, the base timing stayed pretty solid around 600. Let me remind that I bent the spring post for the weaker of the two springs inward, as it appeard to have been bent outward by the PO.

As I said on the previous post, the initial advance is really steep, so I'm thinking the wandering of the timing mark may just be because the curve is so steep.

 

[bruceb58]

Re: Ignition Question: Timing at Idle Wanders

3 degrees is not enough to worry about.

You should have your full advance at around 3500 RPM. That will be around 25 degrees above your starting timing.