Setting point gap (1 Viewer)

[caz]

I have a question on setting the points gap.

The manual calls for a gap of 0.018". I have set it to that time and again, but the dwell is ONLY 20 at this setting.

This is on a 74 with a non-usa distributor.

The truck is running very rich (as verified by looking at all of the plugs), and it really breaks up at anything other than idle. Idle is not exceptionally smooth.

Is it true that the dwell effects timing?

Could the dwell being off like this cause it to run rough at idle?

Should I set the gap lower to bring the dwell into spec? It seems strange that it is off as far as it is.

Thanks,
CAZ

 

[caz]

Does EVERYONE have electronic ignition?

 

[Pin_Head]

The timing changes 1 degree for every 2 degrees of dwell. I have also found that the normal point gap does not give the correct dwell spec. The dwell isn't that important on a relatively low revving motor like the F/2F. Just set the gap at approximately .018, set the timing and go.

The dwell isn't going to cause a rough idle problem. It becomes important at RPM about 6,000. Rough idle is more likely a vacuum leak.

 

[caz]

How do I set the timing correctly if my dwell is off by half? I really don't understand how dwell can effect the timing - just the spark strength.

 

[theo]

How do I set the timing correctly if my dwell is off by half? I really don't understand how dwell can effect the timing - just the spark strength.

Physically impossible to have the gap correct and the dwell angle off by half in a healthy dizzy. Not calling you out here, just saying something doesn't add up. You're sure your dwell meter is set for 6 cylinder?

 

[caz]

Well, hmmm...

Perhaps my meter is off then. It was dropped good enough at one point to crack the glass.

I can't imagine how I am not setting it correctly! I have a 0.018" feeler gage, I turn the engine until the points are open all of the way, I futz around with it until the gage slides in with the slightest of resistance when the screws are tight, etc.

The meter says that it is 20 for a 6 cyl. I know that I have not misread that.

 

[theo]

Then I recommend going with Pin's advice and assuming your distributor is set up correctly. You sure as heck are being careful with the point gap. Set the timing (again becuase we're self doubters) and go on to looking for other causes.

Rough idle and craps out and fouled plugs - maybe too rich, look to the carb; maybe lots of blowby, look to rings.

How's compression?

 

[caz]

Compression was fine when I parked it. Haven't checked it since I got it running again.

Plugs are fouled in all cylinders. I am now leaning toward a carb problem. It was rebuild by JimC by the previous owner and then sat. I cleaned it out and got it working, then put on a rebuild kit last year - never did put fuel back in until now.

I still have ignition problem in the back of my mind though. When I first started it, it went good. Then it died and would not start. Finally narrowed it down to something with the coil since when I put another coil on it fired right up.

That coil had a resistor, the one that I have on now is older and does not have one. I followed the wiring layout in the engine manual when I wired it.

I can't help but still think it is ignition related... weak spark or something. I am pretty confident in my carb rebuild. I have had it apart a million times in the past.

 

[Pin_Head]

How do I set the timing correctly if my dwell is off by half? I really don't understand how dwell can effect the timing - just the spark strength.

You set it with a timing light as usual _AFTER_ you set the point gap. Forget about the dwell meter and just set the points at .018. I could never get the "proper" dwell spec with any reasonable point gap. I think the spec is bogus.

The narrower the point gap, the longer the points stay closed (more dwell) and the later in the cycle the points open to initiate a spark. This is why the timing varies with the point gap. Once the gap is set, the timing is set and will only change as the points wear. More dwell is useful at high RPM as it provides more time for the coil magnetic field to saturate.
This isn't an issue for normal F/2F RPMs under 4,500.

You either need a ballast resistor or not depending on what type of coil you have. It usually says what type it is on the label.

Carbs don't like to sit with fuel in them. It causes lots of problems by plugging the tiny air or fuel ports.

Oil fouled plugs are usually an oil control problem. You might try a hotter heat range (not spark) plug. As long as it makes a spark in the combustion chamber it is all good.

Running rich leads to sooty looking plugs, but it doesn't foul them like oil does.

 

[theo]

I hear you, caz. I'm about tapped out, though, since I only know a few basics about ignition. But you should get some "new blood" in here in no time at all.

I do know you can measure spark duration and strength with an oscilloscope, but not everyone has one laying around.