WTK- dwell for 4cyl engine

[ArticiferTom]

Looking to check dwell on my Ply UT engine in my '31 Dodge truck . was doing RPM and while hooked up read dwell at 43 1/2 degrees . Any thoughts ?

                               Thanks  Tom

[cahartley]

If the points are correctly set the dwell should also be correct.

Dwell determines that the coil is energized long enough to build its peak saturation in order to produce the best spark it's capable of producing.

I think yours is getting plenty of saturation at 43 1/2º.......  .......I think I'd be shooting for something closer to 30º.

Too much dwell may also cause the coil to run hot.

[Spinneyhill]

My ten minute internet survey came up with 54 ± 5 degrees as a typical dwell for four cylinder engines. One site said the points should be closed for 55 to 65% of the time, which works out to about the same thing. But for engines of 1931, I am not sure. I have never seen it stated for engines of that period.

[ArticiferTom]

Sounds right , found site listing spark gap .022 , point gap .018-.024 , dwell 52 , timing .046" . This was for 28-31year U and U-30 engines.

                                 My manual list plugs .022 ,  point  gap .020 ,-----points to break at.050" . So all seem close . As my dwell is currently 43 1/2 degrees, something is off . Checking tomorrow . Do not know about checking timing as must be with special indicator in cylinder . Head scratching time , I have dial indicators so will work on .

[R.White]

15 hours ago, cahartley said:

If the points are correctly set the dwell should also be correct.

Dwell determines that the coil is energized long enough to build its peak saturation in order to produce the best spark it's capable of producing.

I think yours is getting plenty of saturation at 43 1/2º.......  .......I think I'd be shooting for something closer to 30º.

Too much dwell may also cause the coil to run hot.

 

I would agree.   The the dwell angle is an expression of coil saturation.  The correct value for dwell will depend on having the points gap set correctly.  You might get into trouble with an overheating coil if you try to work backwards from it's maximum potential. My Dad had an expression "you can't get a quart out of a pint pot".

 

 In the old days few garages had much in the way of testing equipment and the older hands would set ignition timing "by ear" so to speak.  Even as late as the 1960s some workshop manuals would suggest that the timing be advanced to the point of pinking then backed off a bit.  It seemed to work O.K. in those days so it should be OK now.  I think the biggest problem we have with these old engines is wear in the distributor; checking with a multimeter can sometimes show the dwell angle moving about alarmingly but somehow these old engines are very forgiving.

[Grimy]

Agree with R,White's #5.  I know y'all will correct me, but I was told decades ago that dwell angle is what makes the ignition work correctly, and point gap is a simple mechanical expression of the dwell angle ASSUMING that there's no substantial wear on the cam lobes.

A case in point:  Over 50 years ago, I was doing my first tune-up on a newly acquired 1950 Pontiac 6 with 90k miles.  The point gap was supposed to be about 0.020 but the dwell at that gap was way off spec (can't remember what the dwell was supposed to be), and the car ran terribly.  I the noticed that the distributor cam lobes seemed to be rounded off.  To get the specified dwell (which did in fact make the car run very well), I had to reduce the point gap to about 0.014.

Ever since, when dwell specs have been available (not so much on most pre-1936 cars), I've adjusted the points to render as close as possible to the specified dwell.

[ArticiferTom]

I never tuned this truck before or checked  point gap but will lean toward getting closer to dwell while keeping gap near or in spec. I agree with White on the wear issues . That's why I was searching out dwell it is so much easier to hook up two wires from a meter and test running with all backlash removed . If you get points pitting or erratic dwell you can tell in no time ,to look further . The timing sucks setting as you have to gage the 4 cylinder on exhaust stroke and watch for cam lobe ' starting to open points ' ., while holding backlash out of rotor .

    I ve read some time ago about tuning with vacuum gage also . This is something I must revisit .

                      Once again thanks all , TOM

[Spinneyhill]

I use a little light on wires with crocodile clips to tell me when the points are starting to open.

[ArticiferTom]

Was thinking of meter , was not sure of there meaning ' starting to ' . that make a lot easier . How do you tell exhaust stroke ?

[nearchoclatetown]

Tom, timing by the #4  exhaust valve is quite easy. Take the spark plugs out so you are not battling compression. Then turn engine until piston goes down and intake of #1 opens. Continue to turn until the exhaust valve on #4 opens then closes. You can turn the lifter when it is closed. At that point, coincidentally, is TDC for #1. Turn the rotor until the points break and tighten the rotor. You could use an ohm meter if you want to tell when they break. I have never used a vacuum gauge but have seen it demonstrated. Set timing to the highest vacuum setting possible at a very low idle. 

[cahartley]

Don't get dwell and timing confused.

Both are separate issues.

Get the dwell right first and forget about that.

THEN fiddle with the timing.

[JACK M]

Yes, dwell will affect timing, Timing wont affect dwell.

So get the points where you like them first before setting the timing.

[ArticiferTom]

Well ,  back from checking my dwell and points . First , I confess to miss reading the scale of the dwell meter . I read the 6 cyl scale and there is no 4 cyl scale you double 8's . I guess should read direction first , that also say's designed for 12 volt system only . Well that being said reading on this old KAL  cheapo meter,  would then translate to 64 when doubling 32 . Points gaged at about .017 . Readjusted to .020" gap making not much difference in dwell maybe 62 . Skinned Knuckle's reference in Ply33 states 50 -54 . No- way, I'm getting there and I do not put trust in this old meter . So will leave at gaged gap and set idle with vac gage. got 17.5.  Adjusting for elevation,  I think makes it 18.25 @ 1000 rpms.

Turned idle down to 900, noted 1 drop in vac and left it at that, adjusting gave no better reading . What would good idle  be ,even at 900 seems high but fear de-acceleration stalls if lowered .

                                        Thanks again,, well see how test run does . Also found a couple vac gages with some interesting info inside ,Watch for new thread about.