How to adjust dwell and Point gap?

DFW Miles

Seaman Apprentice
Joined
Jul 29, 2011
Messages
32
I have a 1988 Celebrity 190V with a 4.3 Mercruiser 175 that was having some issues with surging and a little lack of power. I just rebuilt the carb (2barrel mercarb), replaced the cap, rotor, plugs, wires and coil. I just got off the water from doing a preliminary test and it's runnign much stronger that before but I want to make sure that everything is tuned properly. I know how to adjust the timing and the carb idle and mixture but I am clueless about the dwell and point gap. I have read many posts about the importance but for the life of me I cannot figure out how it is done. I do not see pionts inside of the distributor just the rotor and sensor. The rotor is different from any other I've seen as it has a metal plate on the bottom with six "tangs" (?) hanging underneath. Sorry for the ignorance on the topic but if anyone could tell me step by step what do to I would really appreciate it. Thanks as always.
 

Rocky_Road

Lieutenant Commander
Joined
Oct 8, 2008
Messages
1,798
Re: How to adjust dwell and Point gap?

I have a 1988 Celebrity 190V with a 4.3 Mercruiser 175 that was having some issues with surging and a little lack of power. I just rebuilt the carb (2barrel mercarb), replaced the cap, rotor, plugs, wires and coil. I just got off the water from doing a preliminary test and it's runnign much stronger that before but I want to make sure that everything is tuned properly. I know how to adjust the timing and the carb idle and mixture but I am clueless about the dwell and point gap. I have read many posts about the importance but for the life of me I cannot figure out how it is done. I do not see any way to make adjustments inside of the distributor (I'm assuming that is where it's done). Sorry for the ignorance on the topic but if anyone could tell me step by step what do to I would really appreciate it. Thanks as always.

You would need a dwell meter to do this...do you have one? Setting the dwell is the same as setting the points gap, but far more accurate...since it measures exactly when your points are closed in relation to your piston travel.

Read this:

"11. What is "dwell angle", and how do I measure it?

Dwell angle is a measure of the duration of time that the primary circuit of the ignition system is closed to energize the primary windings of the coil. It expressed (and measured) in degrees of rotation of the distributor rotor, hence the use of the term "angle".

In actual operation, as the distributor mechanism rotates, the points (or electronic module in electronic ignition systems) are closed for a certain number of degrees of rotation, and open between these points. Simply as a matter of interest, this means that the total number of degrees during which the points are closed, plus the total number of degrees that they are open, will equal 360 degrees.

In four cylinder engines, there is usually ample time for the primary circuit to be open and closed (to energize the coil) four times during each revolution of the distributor, which makes the dwell value less critical than in 6 and 8 cylinder engines. In four cylinder engines, there is more of a concern over having too much dwell time (during which time the coil is energized) which can result in high coil temperature and premature failure.

In electronic ignition systems, the dwell value is fixed at approximately 50 degrees. In conventional systems, (considering the difficulty involved in checking and adjusting for dwell) most folks simply rely on the setting of the point gap to provide the proper dwell time. In the event that you want to check your dwell, here is a reprint from the ignition chapter of our service and overhaul manual:

CHECKING FOR PROPER DWELL ANGLE

The dwell angle specified for the Atomic 4 is 31 to 34 degrees for late models (Delco), and 38 degrees for early models (Prestolite). To check for dwell angle, proceed as follows:

1) Connect the dwell meter as per its manufacturer's instructions: Generally one of two leads is connected to the positive terminal of the battery and the other lead to the negative terminal of the coil.

2) Start the engine and read the dwell angle (making sure meter is set on 4 cylinder).

3) Adjusting dwell on the Atomic 4 is somewhat of a trial and error process. If dwell angle is too high (points closed for too many degree of rotation), you will have to carefully reset your points to a slightly larger gap. If the dwell angle is too low, the points will need to be set slightly closer. Adjust the gap .002" or .003" at a time for best control.

4) Restart the engine and re-measure dwell angle. Repeat as above until dwell angle is correct. - Updated: November 4, 2003"

As you can see, not a fun project. Years ago GM had a 'peek-a-boo' distributor cap that allowed you to change the dwell while the engine was running. Now that was the best thing since sliced bread...!

Happy boating!
 

NYBo

Admiral
Joined
Oct 23, 2008
Messages
7,107
Re: How to adjust dwell and Point gap?

Changing dwell/point gap also changes timing, so recheck the timing if you have to adjust the points.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: How to adjust dwell and Point gap?

Did everyone miss the fact that he has "electronic ignition" and that there are no points -- therefore no dwell/gap adjustment (which accomplishes the same thing by the way). Point gap adjustement uses a feeler gauge to set the point gap which ultimately is the same as using a dwell meter to set the dwell angle. Same result either way. It is just that the dwell meter provides a more precise setting. In other words, if you set the dwell angle with a dwell meter, you could check the point gap on the high point of the cam lobe and the gap would be within spec. Dwell angle (point gap) was set for decades with a feeler gauge until someone developed the dwell meter.
 

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: How to adjust dwell and Point gap?

Did everyone miss the fact that he has "electronic ignition" and that there are no points -- therefore no dwell/gap adjustment (which accomplishes the same thing by the way). Point gap adjustement uses a feeler gauge to set the point gap which ultimately is the same as using a dwell meter to set the dwell angle. Same result either way. It is just that the dwell meter provides a more precise setting. In other words, if you set the dwell angle with a dwell meter, you could check the point gap on the high point of the cam lobe and the gap would be within spec. Dwell angle (point gap) was set for decades with a feeler gauge until someone developed the dwell meter.

Goes to show you, sometimes advice is worth what you pay for it.

Agreed with ST, point system only. Setting the gap by dwell angle can compensate for cam and shaft bushing wear, within reason. On the OP's engine, you just set the timing.
 

NYBo

Admiral
Joined
Oct 23, 2008
Messages
7,107
Re: How to adjust dwell and Point gap?

Did everyone miss the fact that he has "electronic ignition" and that there are no points -- therefore no dwell/gap adjustment (which accomplishes the same thing by the way).
Oops...:redface:

Anyone remember the old GM V8 distributor cap with the metal door through which you adjusted the point gap with the engine running (automotive, not marine)? Needed a dwell meter to get those right.
 
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