TR2/3/3A Dwell angle on TR3A? Meaningful?

[Jim Lee]

Hi all,
I am having a hard time starting my TR3A cold first first time
in the morning. After I get it started with the help of starting fluid, it starts up fine for the rest of the day.
Is there a specific dwell angle that I should be seeing at 800 rpm. I see about 49 degrees right now. I have a combination dwell/tach and have never really understood what the dwell represented. Some research on the net seems to indicate that it could indicate the points gap which I have measured and is as close to the .016 inches as I can make it. I read more than once that looking at the dwell on an older distributor can also be a waste of time but if it helps me to verify the gap I have set I am all for it.

Another thing I am wondering about is if it is possible to set the advance too far when the engine is warmed up. Making it harder to start when dead cold. Does more advance
equal harder to start cold?

Thanks very much,
Jim Lee

 

[YankeeTR]

Dwell angle will change with point gap. If your point gap is dead on your dwell angle will be correct....

 

[Dave Russell]

Hi Jim,

The cold start problem is likely caused by the carbs not being at full choke. It's common to have the chokes adjusted so that they are not fully on when the choke if fully pulled.

Dwell angle is simply the number of distributor rotation degrees that the points are closed. Dwell is a close representation of point gap. Eg. The closer the point gap the later in the cams rotation that the cam will contact the point rubbing block, the later the points will start to open & the earlier they will close. This will decrease the time that the points are closed. (dwell) Also a closer point gap will cause the points to start opening later in the rotation which retards the timing.

This is a bit counter intuitive because the fixed point is moved away from the cam to decrease gap while the movable point remains in the same relative location. At zero gap, the fixed point would be so far away from the cam that the movable point would never touch the cam.

Advanced timing would be likely to cause easier cold starting & harder hot starting.
D

 

[martx-5]

Dwell is the amount of time (degrees of rotation of the distributor shaft) that the points are closed. This time allows the coil to build up enough magnetic flux in it's core. Since on a four cylinder engine, the cam opens the points every 90 deg., point dwell is typically around 45-50 deg. Point dwell on a four cylinder engine is less critical then a 6 or 8 cyl. engine, because the points are opening and closing less times per engine revolution.

Every degree of point dwell that changes, will affect the timing. As the fiber wears on the points, dwell increases, the contacts will open later and timing will retard.

Your points are adjusted correctly, as they fall into the TR specs of .014"-.016", and the dwell is between the 45-50 deg. that I always see for 4 cyl. engines.

 

[Twosheds]

[ QUOTE ]

Advanced timing would be likely to cause easier cold starting & harder hot starting.
D

[/ QUOTE ]

I agree with your post except that I would think that advanced timing would cause harder starting whether hot or cold. Dunno about cars but most airplane engines have systems that retard the spark to TDC during starting to make the engine easier to start. Is something different about car engines?

 

[Adrio]

I agree with John. Advance timing makes starting harder. Not only airplanes but motorcycles and cars have retarding features to make the engine start easier. Even the TR3 TRactor engine has one in a manner of speaking. If you look at the plot for the mechanical advance on that distributor, the first "knee" of the curve is below idle speed, so in fact the timing is held retarded until the engine starts.

 

[Dave Russell]

I based the statement on the following:
A COLD mixture is less homogeneous (not well mixed) thus slower burning. It is also richer which makes it slower burning. Thus it may tolerate more spark advance without kicking back due to early ignition.

OTOH, a WARM or hot mixture is well mixed, less rich, easier to ignite, & faster burning. Because of this, the engine will tolerate less ignition advance before it kicks back. Note the difference between "warm" & "cold".

I think in the case of hand propped airplanes & kick started motorcycles, it would be safest to have retarded timing along with a rich mixture, even if it wasn't absolutely needed.

To me, not a big issue either way on a car engine.
D

 

[Twosheds]

The cold mixture is richer because the choke is on? Thanks for the explanation!

 

[Dave Russell]

Yes - Forgive me for no so stating.
D