Mercruiser 470 tune up

noralogdog

Cadet
Joined
May 31, 2012
Messages
9
I am doing a tune up on my 83 470 Mercruiser. I will be using a dwell meter. I'm trying to understand the relationship between the gap and the dwell. If the dwell reads less than what it should be, should I apply more or less gap. And if the dwell reads more than what it should be, should I apply more or less gap. I have to remove the dist cap to make adjustments (no slide window). So I'd like to have an idea of what adjustments to make instead of just guessing. Need some help from all you engine techs out there.
 

sharkin95

Petty Officer 2nd Class
Joined
Jun 2, 2007
Messages
106
Re: Mercruiser 470 tune up

junk that delco or prestolite point system and buy a petronix electronic ignition and coil. there is no guessing or tinkering.faster starts and quicker throttle response...I had a 1989 3.7 224 4 cyl best investment I made, beside buying a 4.3 v6 replacment engine
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
Re: Mercruiser 470 tune up

I am doing a tune up on my 83 470 Mercruiser. I will be using a dwell meter. I'm trying to understand the relationship between the gap and the dwell. If the dwell reads less than what it should be, should I apply more or less gap. And if the dwell reads more than what it should be, should I apply more or less gap. I have to remove the dist cap to make adjustments (no slide window). So I'd like to have an idea of what adjustments to make instead of just guessing. Need some help from all you engine techs out there.

Pretty easy, the dwell is the number of degrees of rotation of the distributor shaft that the points are closed (in your case 28-34 degrees out of 360 for each time they close). That translates into time for current (12V) to flow through the primary windings of the coil to build an optimal magnetic field. When the points open the current stops and the magnetic field collapses and generates a high voltage (20,000 or so) in the secondary. So setting the gap should set the proper dwell too, but factors like wear can change the gap/dwell relationship. So to answer your question to increase the dwell you decrease the point gap. I have a Petronix too, but there is nothing wrong with points if you maintain them properly (changing them every spring as part of sumerization is a good idea). Have fun.
 

DaveG55

Petty Officer 2nd Class
Joined
Jun 17, 2012
Messages
144
Re: Mercruiser 470 tune up

junk that delco or prestolite point system and buy a petronix electronic ignition and coil. there is no guessing or tinkering.faster starts and quicker throttle response...I had a 1989 3.7 224 4 cyl best investment I made, beside buying a 4.3 v6 replacment engine

I'm sorry, I find that kind of non answer irksom...
Sure, going electronic is a nice way to go with many advantages. BUT, not everyone can, or wants to, spend $200 ish on the conversion. So, really, that answer is no answer at all...

To the OP, I believe that the dwell can be adjusted with the rotor cap off. Hook the Dwell meter up normally, remove the rotor cap and button and have a helper crank the engine over. You should be able to read ane set the dwell by adjusting the points open or closed. You can then button it back up and recheck the dwell while running. But that should get you pretty close.
 
Joined
Mar 27, 2010
Messages
3,008
Re: Mercruiser 470 tune up

I'm sorry, I find that kind of non answer irksom...
Sure, going electronic is a nice way to go with many advantages. BUT, not everyone can, or wants to, spend $200 ish on the conversion. So, really, that answer is no answer at all...

To the OP, I believe that the dwell can be adjusted with the rotor cap off. Hook the Dwell meter up normally, remove the rotor cap and button and have a helper crank the engine over. You should be able to read ane set the dwell by adjusting the points open or closed. You can then button it back up and recheck the dwell while running. But that should get you pretty close.

I don't let a comment like that get to me.... you know the smart ones carry a spare set of points in their tool box. ;) :D
 

noralogdog

Cadet
Joined
May 31, 2012
Messages
9
Re: Mercruiser 470 tune up

Pretty easy, the dwell is the number of degrees of rotation of the distributor shaft that the points are closed (in your case 28-34 degrees out of 360 for each time they close). That translates into time for current (12V) to flow through the primary windings of the coil to build an optimal magnetic field. When the points open the current stops and the magnetic field collapses and generates a high voltage (20,000 or so) in the secondary. So setting the gap should set the proper dwell too, but factors like wear can change the gap/dwell relationship. So to answer your question to increase the dwell you decrease the point gap. I have a Petronix too, but there is nothing wrong with points if you maintain them properly (changing them every spring as part of sumerization is a good idea). Have fun.
Hey, great job explaining this. This makes the job a lot easier. Thank you for taking the time to explain this clearly. And yes, I will have fun.
 

noralogdog

Cadet
Joined
May 31, 2012
Messages
9
Re: Mercruiser 470 tune up

I'm sorry, I find that kind of non answer irksom...
Sure, going electronic is a nice way to go with many advantages. BUT, not everyone can, or wants to, spend $200 ish on the conversion. So, really, that answer is no answer at all...

To the OP, I believe that the dwell can be adjusted with the rotor cap off. Hook the Dwell meter up normally, remove the rotor cap and button and have a helper crank the engine over. You should be able to read ane set the dwell by adjusting the points open or closed. You can then button it back up and recheck the dwell while running. But that should get you pretty close.

Ok, so someone once told me that a boat is nothing more than a floating hole in the water that you dump money into. Can anyone agree with me on that? I know, an electronic ignition would probably be better and I probably will convert at some point. But I don't have an endless budget with boat improvements. And I just installed an alternator conversion kit. Which wasn't cheap. I have to pick and choose what I want to do, otherwise I won't be able to put gas in my tank and really have fun with my "hole in the water". Thanks everyone for their input.
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
Re: Mercruiser 470 tune up

I just installed an alternator conversion kit. Which wasn't cheap. I have to pick and choose what I want to do, otherwise I won't be able to put gas in my tank and really have fun with my "hole in the water". Thanks everyone for their input.

You have made the wise choice Grasshopper.:D
 
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