How to check Dwell on a MK I 3000?

[Swiss Toni]

Just checking things before replacing the points and condenser and thought I'd check the Dwell.

The manual says 35 degrees +\- 3 deg.

At idle (925, it's a bit high but it likes it there with the cam and Webers) I have 40 deg and when rev it up say to between 1500 and 2300 the dwell drops to 34 to 32 and back to 40 to 41 when it settles back down to idle.

Any thoughts?

 

[Keoke]

AS Long as the dwell is steady at Idle you are probably OK

 

[Swiss Toni]

Thanks Keoke, I think I'm going to owe you a beer or two if you're ever in town for all the gems of info

The figure of 40° is rock stead at idle, it only changes when the RPM changes.

I've read else where that you should check the dwell at 2000 to 2500. What difference does it make and why does the dwell change with RPM anyway?

 

[Keith_M]

I don't think dwell should change with RPM. A substantial change (more than a few degrees) probably indicates a problem with your distributor. I'm not sure what that problem would be, though.

I suppose it could also indicate a problem with your dwell meter...

 

[Swiss Toni]

I did read somewhere a change in dwell could indicate a worn disrtibutor but I just don't know if that's the case.

The dwell meter is incorporated in a brand new all sing and dancing digital Snap On timing light, coil energy meter, dwell meter etc, etc so very unlikely its that.

 

[Keoke]

I did read somewhere a change in dwell could indicate a worn disrtibutor but I just don't know if that's the case.

.

YES:

Variations in dwell can indicate a worn dizzy shaft. But this would occur at Idle too

However, you might back it down to 35

Note:
Point gap and dwell are inversely proportional.

 

[Swiss Toni]

Great, I'll put the new points and condenser in and set it to 35° at idle

 

[johnea]

Variations in dwell indicates a problem with the distributor. Could be a worn shaft or a worn or loose breaker plate.

 

[Swiss Toni]

Perhaps I'll put a overhauled distributor on the to do / parts list.

That said I just replaced the points and condenser and have the dwell spot on at 35 deg at idle and the max advance at 35 deg as well and she's running crisper and clearer and the seat of the pants Dino says a few more ponies may have been found.

I could probably put a bit more advance in as I'm running Vpower but if I put in 98 or go to high altitude there won't be any margin.

 

[Michael Oritt]

I'm sure that someone will correct me but I am of the impression that dwell has nothing to do with WHEN the ignition fires in relation to TDC (which is "advance") but rather with the DURATION of the firing, as determined by points gap or the equivalent for solid state, plus the condition of the distributor itself.

 

[LarryK]

Dwell moves with timing set and idle. That is why you set dwell at specific rpm and timing degree.

 

[Michael Oritt]

I guess it is a matter of terminology but when I saw people referring to 35 degrees of dwell I assumed they were referring to advance. I have never used a dwell meter and still set points the old fashioned way with a feeler gauge, etc.

 

[johnea]

Michael, Dwell is the time the contacts are closed. For our six cylinders that is 35 degrees and this is the time needed to charge the coil. As soon as the contacts open the magnetic field in the coil collapses and generates the high voltage needed to ignite the mixture. It has nothing to do with the duration of the firing.

 

[Bob_Spidell]

I'm sure that someone will correct me but I am of the impression that dwell has nothing to do with WHEN the ignition fires in relation to TDC (which is "advance") but rather with the DURATION of the firing, as determined by points gap or the equivalent for solid state, plus the condition of the distributor itself.

Dwell determines the amount of time that current is allowed to flow through the primary windings in the coil. Too little time and the electromagnetic field induced in the secondary winding will be weaker, resulting in a weaker spark; too much time and the coil will eventually overheat. Spark is caused by the induced field collapsing when the points open, pushing current through the secondary wires and resulting in spark at the plug. Pretty sure--but not positive--that the duration of the spark event will be the same no matter, but of course if dwell is too short there may be no spark at all, which can happen at very high revs. This is the advantage of electronic ignitions; the dwell--in time--remains constant (although dwell is measured in degrees of cam rotation, it's really a measure of time).

 

[steveg]

Ultimately you'll wind up setting the points with a feeler gauge then checking it on a dwell meter (if desired).
Index marks can be helpful helpful between the distributor base plate and the block - for removing the distributor and gapping the points on the bench.

 

[Michael Oritt]

Looks like I should have heeded Abraham Lincoln's advice: "Better to remain silent and be thought a fool than to speak out and remove all doubt."

 

[Swiss Toni]

Just for reference, I very carefully set the the points gap to 0.015", which is mid way of the spec and due to skill (highly likely luck was more significant) the dwell at idle as 35 deg.

 

[Bob_Spidell]

My dad told me some makes--he mentioned GM--had a means to adjust the dwell from outside the distributor while the engine is running (of course, modern electronic ignitions control it with computers).

 

[DerekJ]

The best way to set the advance is the old fashioned way. Advance until you get pinking then back it off by degrees until you don't. That way the advance is set to the particularities of your own car and not to a shop manual.

 

[Swiss Toni]

That's the next step, though previous with 35 degrees it started to detonate a bit under heavy load if I put 95 in it, especially at altitude.