View Full Version : Correlation between Idle speed and timing

12-22-2004, 01:04 PM
Hello tr3 people, I have a question on the correlation between idle speed and timing. I am having difficulty getting a relatively good running tr3 to idle strong at 800rpm. First off, the engine has a valve job (replaced everything) plus new rings, good carbs (slight throttle shaft wear) 60 degree cam angle 10W/30 in carbs, needles and jets set where the lift pin makes little difference when raised, and- give or take- 4 degrees ignition timing ectů.Anyways, my question is that when setting the idle, one can either set the idle by advancing the timing slightly or by turning the idle screws. Naturally the dwell does not move. However, naturally when one raises the ignition timing the idle comes up. So, in other words how much air do I want coming in compared to ignition timing at idle.


12-22-2004, 01:46 PM
I'm not a TR3 expert by any means, but I have never heard of setting the idle speed by adjusting the timing. With MOST cars you set your ignition timing to specs, adjust your idle mixture, then adjust your idle speed. Now having said that, I have figured out most LBC's are not like most cars. Well thats my $.02 worth. /ubbthreads/images/graemlins/grin.gif

12-22-2004, 03:03 PM
As mailbox has said, there is a specific order to set things. Also, The TRactor engines are generally timed statically, meaning they are set without the engine running.

Rotate the engine clockwise until the timing mark lines up to the setting you want. I think TR's are set a 4 deg BTDC. Remove the distributor cap, and hook up a test light between the points and ground. Loosen the distributor hold down screw. Turn on the ignition, Rotate the distributor counterclockwise until the light comes on, then slowly rotate clockwise until it just goes off. Tighten the distributor hold down screw.

Anyway, I digressed. You mentioned "slight thottle shaft wear". I had what I thought was slight thottle shaft wear, and couldn't get my TR3 to idle worth a ****. Just too much of a vacuum leak. I had to rebush.

12-22-2004, 03:07 PM
I agree... Though the idle will certainly change slightly as you advance and retard the timing, that's not how you set the idle.

You *can* use the idle speed to set the timing (sort of). Usually, advancing the timing will increase the rpms (to a point) and retarding it will decrease the rpms (assuming you start out near spec). I usually set my timing by turning the distributor until I get maximum rpms. I note the rpms and then retard the timing until the rpms drop about 100. I then road test it to fine tune it (If it pings in high gear under acceleration, I retard slightly until it stops pinging. If it doesn't ping, I advance until it does, then retard slightly.). Then I worry about setting the idle.

Geo Hahn
12-22-2004, 04:03 PM
If you are using that 45-year-old Jaeger tach on the dash to get your 800 RPM know that it may not be totally accurate at the low end of the range. OTOH - you measured dwell so perhaps you have suitable instrument.

Not sure why you're measuring 'dwell' since these distributors do not allow any running adjustment to that. I just set the points, set a static advance and do the final adjustment on the road with the vernier.

12-27-2004, 08:22 PM
Thanks Kurtis and Geo for your comments. I basically tune this one on the road using the vernier scale like Geo suggested. I guess what I am wondering about mostly is how quickly do SU carbs go down to their idle speed. If I let the tr3 set, for say 30 seconds, after driving in traffic, the idle speed does go back down to about 800rpm. However, at the moment, I stop the idle is about 1200 then dropping slow to about 800. I mention moving the timing because a retarded timing seems to drop to an idle quicker but losses line takes off.