Hi Frank, At idle there will be no vacuum present for the vacuum advance. The vacuum port is located where it will only work when the carburetor is in the "off-idle" and running modes. There will be a very small amount of mechanical advance present at idle...which will increase as the speed increases. Hope this is helpful.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Are the advance diagrams in the manual (mechanical and vacuum) are computed for TDC or already with the ball bearing mark in mind?
Thanks, Frank
They are in addition to any timing set at the base of the distributor, so I guess you'd say TDC.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
The ball bearing is 5 degrees before top dead center. There is another mark for top dead center for adjusting the valves. It is likely Upper Dead Center. The mark may look like U I C. The "I" is a line between the U and the C. Some flywheels have a triangle mark also, but I don't know what that is for.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
The factory manual I have gives a procedure for checking the valve timing using that triangle mark, i.e. so many thousandths lift at that mark. The figure differs for various cams. Of course this all assumes the flywheel is indexed correctly. I have reason to believe mine is not and now that I have the head off I can check that with a dial indicator and degree wheel.
1951 3800 1-ton "Earning its keep from the get-go" In the DITY Gallery 1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
Just an added note from Pre '68 Dave's comment. Something else to consider on this topic might be the question "Are the engine/flywheel an original match set?".
My '51 currently still has the original 216 flywheel, but a later '54 235 my grandfather installed back in the early 80's.
When checking/setting timing in my case, I have had to ignore the existing "ball" marker and use my grandfathers added mark to get proper idle and performance.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
The way to check the TDC mark for accuracy with the head on is with a piston stop. You'd have to get it to just barely interfere with the piston given the small window available for observing the flywheel.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
The way to check a TDC mark for accuracy with the head on is with a piston stop. You'd have to get it to just barely interfere with the piston given the small window available for observing the flywheel.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
Just an added note from Pre '68 Dave's comment. Something else to consider on this topic might be the question "Are the engine/flywheel an original match set?".
My '51 currently still has the original 216 flywheel, but a later '54 235 my grandfather installed back in the early 80's.
When checking/setting timing in my case, I have had to ignore the existing "ball" marker and use my grandfathers added mark to get proper idle and performance.
Though I have an 261 and the rest is from a 235, the TDC and the BB are exactly where they should be - I measured it, when I rebuild the engine. But that was not my question, just wanted to know if the Diagrams in the manual are already including the 5 (or so) deg BTDC of the BB or not - which was answered by Bill.
Background: The engine is so much modified, that the stock curves don't do the trick anymore, so I'm trying to tailor my own curves, but use the diagrams in the manual as a starting point and go from there. The digital Ignition I use, works from TDC. So - if the curves would have included the advance of the BB, I would have taken that into consideration - now I don't have to.
