Just a quickie - points dwell angle Merc 120

[libellav15]

Hi again guys

Info: 1972 Mercruiser 120, GM In Line 4.

Getting a dwell meter tomorrow to set the points up correctly.

I've got the Clymer manual for my setup - and as usual its quite vague with details and procedures - thank heavens for iBoats

Ok the question is this - when they (Clymer) describe the points gap for feeler gauges its a straight forward 0.022''

BUT - under dwell angle (degrees) it shows 28-34.

Seems like a fairly big range.

Will anywhere in that scope do?

Any recommendations appreciated as usual.

Cheers guys

Adam

 

[Bt Doctur]

Re: Just a quickie - points dwell angle Merc 120

Set the points at .018 and it will run just fine.point gap is set in thousands of an inch, .018, .022, etc.Dwell is the number of degrees the points remain closed. 22-28 deg dwell is not .022-.028 point gap.

 

[bigskiohio]

Re: Just a quickie - points dwell angle Merc 120

set it with a feeler first gets you close then the dwell meter . i think the range is because it is not easy to get dead on but shoot for the higher number as that will give you a stronger spark.
then set time with a light and then recheck dwell. i picked up 250 rpms by changing points. took me awhile to get it right on .

 

[libellav15]

Re: Just a quickie - points dwell angle Merc 120

cheers guys

i realise the dwell is in degrees Doc - just can't work out why there is such a range to set the dwell at with the digital dwell meter

and the manual says .022 - is .018 better then?

 

[EddiePetty]

Re: Just a quickie - points dwell angle Merc 120

cheers guys

i realise the dwell is in degrees Doc - just can't work out why there is such a range to set the dwell at with the digital dwell meter

and the manual says .022 - is .018 better then?

....let me try!

As dwell is an angle value, its base is 360. Each decimal place of the feeler gauge value has a base of 10.

Dad always told me (and I'm 62) to set your points wide as they wear closed, and set your plugs close as they wear open !!!!

 

[daydreamer1252]

Re: Just a quickie - points dwell angle Merc 120

As Eddie above said....and to make it a little more relatable to Dwell.....if you set your dwell on the low side it will stay in range longer. As point gap decreases the dwell will increase.

The natural wear of the rubbing block on the points is rapid at first as the "high spots get worn off".....if you set the dwell to a specific number and go back and check after 3-4 hours of running the number will be different. Will drive you nuts trying to keep it perfect. Also the normal wear of bushings in the distributor will cause some fluctuation in the dwell number and that is normal and acceptable within specifications. Within the range in the specifications you will be just fine....

Hope that helps.

 

[Silvertip]

Re: Just a quickie - points dwell angle Merc 120

It doesn't matter if you set the point gap at .018, .019, or .025 for that matter. You are going to reset them using a dwell meter anyway. If you do NOT use a dwell meter, then set the point gap in the indicated range like was done for the many decades before a dwell meter came along. It was done using a match book cover in the good old days. Granted, engines were a lot less fussy back then.

 

[Lou C]

Re: Just a quickie - points dwell angle Merc 120

Also point gap and dwell are inversly related...big gap equals small dwell and vice versa...I set them with the feeler gauge and then check with the dwell meter...ususally they are in the spec...

Years ago GM had this nice distributor design...there was a little metal window in the dist cap you could slide up...then the points were mounted to a plate with an allen socket so you could use a flex allen wrench to adjust the dwell angle while the engine was running hooked up to the dwell meter...the best thing out there till electronic ignition came along...

Of course that would not be OK for inboards due to the need to keep the dis spark protected...

 

[libellav15]

Re: Just a quickie - points dwell angle Merc 120

Thanks millions guys

As usual, info explained clearly and concisely.

I think i am obsessing a bit too much over point gap....

Its a boat that has stood for a couple of years before i got it - i have done a full ignition service and set the timing etc. Now she starts perfectly hot or cold, runs ok for half an hour then starts playing up.

Never ever fails to start - just goes lumpy, bogging etc.

Think its time to do a full fuel system renewal.

On that subject where would you tap into the fuel system to run from a remote tank? Obviously before the fuel pump i guess?