will a different head change the timing?

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freddyray21

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1972 Mercruiser 120 Serial number tells me the motor is right vintage. I have owned it for two years. First time I tuned it up didn't pay attention to what plugs were in it just picked up the plugs the manual called for. Went to put them in and found they were too big for the holes. A little investigation and we determined someone has put new head on it as sometime that takes the smaller plugs. My question is if we time it with a light according to the timing for the serial number I get spark ping, but if I time it according the the head I don't. Does the head affect the timing like that? It's only 2 degrees.
 
Zero Balance

Zero Balance

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Re: will a different head change the timing?

Changing heads won't change the timing but they can change the comp. ratio. do a comp test and review the maual for conformance. Higer compression means you'll want to run a higher octaine fuel. My guess is that you'll want to time it according to the changes.
 
Haut Medoc

Haut Medoc

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Re: will a different head change the timing?


He's talking about the ratio....9.5 : 1....etc.....
I would run higher octane fuel & see what it does....
I don't think I would change the timing & dwell, unless you can't get the ping to go away.....;)​
 
Bondo

Bondo

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Re: will a different head change the timing?

It's only 2 degrees.
Click to expand...

It's entirely Possible that the New Head has smaller combustion chambers,+ has raised the Comp. Ratio abit,......
If it runs Fine by cutting the Timing back 2*s,.......
That's what I'd do,.......

That's Alot Cheaper than running High-Test Gasoline..................
 
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EricR

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Re: will a different head change the timing?

If the head has been milled, as a lot of reconditioned heads are, it will reduce the combustion chamber size and in turn this increases the compression ratio. The factory timing specs are for the engine as it left the factory. My advice is to use an advance style timing light and set the initial advance to where it idles best, and does not spark knock under load.

More compression wants and needs less initial timing.
 
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