Timing Question whats the point

[gnj588d]

Just checking my timing while the bonnets {hood} off, my books say to check it at 800rpm and timing set 8 degrees btdc another book says check at 3000rpm and timing at 30 degrees.my question is if one is right so should the other should so why rev an unloaded engine to 3000 rpm I did rev it to 3000rpm and the timing went to 35 degrees is that ok? the engine runs great apart from a slightly lumpy tickover any thoughts on that. many thanks. Cheers Mike. P.S the air screw for the tickover does not adjust to  more than 600rpm

[Larry & Norma]

I guess checking at the different rpm's checks that the vacuum alters the distributor advance and retard correctly. I would suggest that the 3000rpm measurement
should be spot on for best tuning.

[stickandrudderman]

3000 rpm is the best one to use but you should also check it at idle with and without vacuum and then again just off idle to be sure that the mechanical and vacuum advance are working correctly.

If you can't get more than 600rpm with the idle air screw and you're sure that all of your linkages are correctly adjusted and that the timing is correct, then it's likely that your mixture is too rich.

[JimVillers]

You use the 3000 RPM method because the timing curve is flat above 3000.  If your tach is off by 100 RPM, your timing can vary by five or more degrees.  Likewise, you need to disconnect the vacuum advance before checking the timing.  Again, it is hard to tell what exactly it is doing at any specific time.  so keep it simple and eliminate as many variables as possible.