I have a 1960 two four barrells, hi lift cam 270hp dual points disbtributor. It was runnung fine, now it isn't. I believe it is electrical in the distributor. I have the points sets at 29 degrees individually and 34 degrees together dwell wise. Exactly what the specs call for. Setting the dwell is more precise than gapping the points isn't it ?? Timing may be a little high 14 should be 12. The dwell changes the timing but not the other way around. I don't understand what is going on here. Ignition before top dead center, this measurement must be pretty small. At what RPM's should it idle at to set the timing ? I need some help to get it running correctly again. Thank You. I haven't done this in a long time, forgot more than I knew probably.
-
-
Re: timing ignition dwell
Dave, what intake vacuum are you getting? On my '62 SHP I am only getting about 12 at the 650 idle spec. The vacuum does everything it is supposed to do on this rebuilt engine but it just does not seem high enough at idle. I cannot find any leaks. I have the timing set at 10 per spec with the same 34 degree dwell.
My problem is that once it is warmed up I get a little stumble when I tip in the accelerator. I have a carb kit to replace the accelerator pump bellows. Hopefully the problem is that the bellows "dried" out after sitting in a Coney Island for 5 years (maybe it is grease saturated like the carpet floormats!!)
Chris -
Re: timing ignition dwell
I assume by SHP you are speaking of the engine with a Duntov or other high performance high reving cam. In my experience, during idle, there is a short period when the valves overlap (i.e. intake and exhaust slightly open at same time). When this occurs you lose vacuum. I am sure others could explain this better. When the engine revs up it smooths out. I know in my 57 it is mechanical advance only and you need the idle rpm set before adjusting timing. Poor vacuum is just a characteristic of this combo. My vacuum is poor also.
TedComment
-
Re: timing ignition dwell
Thanks for the explanation. When I was looking at cams I found these specs from this group and other resources. Not positive these are right but they are what I have in my notes.
Overlap
30/30 67
Duntov 097 51
LT1 53
In fact I just found a note I got from somewhere. (Maybe Duke)
"The LT-1 cam will provide very similar idle characteristics and vacuum (about 12" @ 900) as the Duntov cam because it has about the same effective overlap, and as long as you don't blab about it even the best PV judge would be very unlikely to detect that the engine doesn't have the OE Duntov cam."
ChrisComment
-
Re: timing ignition dwell
GM's idle specs for mechanical lifter cam engines is unrealistically low. Raising the idle speed to about 900 should improve off idle response. I'd also recommend increasing the initial timing to 14-16 as long as it doesn't detonate.
Be careful that you set the intial timing at a speed below where the centrifugal starts,
DukeComment
-
Re: timing ignition dwell
How do I assure I set the initial timing below the centrifugal starts? Per the st-12 the distributor starts moving the timing with any RPM.
Do I set at 14-16 at 650 RPM and then increase the idle to 900? I then assure that the timing is still at 14-16?
Thanks, ChrisComment
-
Re: timing ignition dwell
ST-12 should give the start point of the centrifugal advance. For example 0@700. This means the advance can be expected to start at about 700, so the initial timing must be set below this, and as you advance the timing engine revs will increase, so you have to keep pulling back the revs with the idle speed screw.
Once you have set the initial, you should slowly bring up engine revs and note where the centrifugal starts. If it advances immediately, it has already started and your initial timing may not be what you observed. If the centrifugal starts at low revs one way to set initial is to tie up the centrifugal with a stiff rubber band while you set the initial timing.
Once it's properly set increase the idle speed to about 900.
A final check can be accomplished to measure total WOT timing at high revs with a dial back light. Set the light at the sum of initial plus full centrifugal and rev the engine to the maximum advance speed plus about 500 revs. The balancer notch should be within a degree or two of zero on the tab.
DukeComment
Comment