Daren67

My stock L-79 vert m21 370 posi has a stumble around 2500-2800. Engine has never being rebuilt or anything monkeyed with badly (at least what I don't know yet). Car runs decent all around but I feel like it is not running smooth for some reason and started to fine tune it with my limited skills.

1) On top of the stock Holley carb, I see the top rubber circle on top of the front fuel bowl leaking. I think its the accelerator pump, but wanted to clarify and I assume its an easy fix?

2) I purchased a used Snap On digital advanced Timing Light and came up with the following charts (I know I am doing something wrong)

With vacuum adv plugged and carb side plugged:
30 degrees adv @ 1000 rpm.I shot the gun at the mark and adjusted it upward till it read 0 on the timing cover. Same for all below:

30 degrees adv @ 1000 rpm.
36 degrees adv @ 2000 rpm.
43 degrees adv @ 3000 rpm

With vacuum advance hooked up: .I shot the gun at the mark and adjusted it upward till it read 0 on the timing cover. Same for all below:

50 degrees @1000 rpm

52 degrees @ 2000 rpm

61 degrees @ 3000 rpm.

I know I am using the timing light wrong, The dwell was 30, and I believe I hooked up the timing wire to the #1 wire.

I know its should have 8-10 degrees at idle and be all in advance at 2500 but I obviously don't know what the hell I am doing.

midyearvette

sounds like too much timing to me....look up the tune up specs for initial timing with vac hose plugged and try that for starters
you want to loosen the timing bolt with an special wrench and turn the dist clockwise to retard the thing.....a good starting point

as far as you not knowing what you are doing, don't be so hard on yourself!
sounds to me as if you know more than most about the dial back lights and you will learn more as you tune it.......

JohnZ

For item #1, are you talking about the flat-head lock screw and hex nut for the float adjustment, or the little rubber "tit" on the rod that seals the bowl vent? What year is your car, and what's the LIST number of the carburetor stamped on the front side of the choke air horn?

For the rest, you may have a problem with your used dial-back timing light. Read the "Advance Mapping" article at the link below, compare that procedure with what you're doing, and note any differences we can help you with.


http://www.lbfun.com/warehouse/tech_...%20ADVANCE.pdf

mikem350

If the car starts and runs now, DO NOT play with the timing until you verify a few things!!!

Make sure you are looking at the correct line on the balancer..some have other marks
Make sure the balancer outside ring has not slipped..hard to tell, check archives
Make sure the timing pointer is correct one too

Best is to verify TDC at #1 firing, then you can know where to start tuning!! Crank the engine w #1 plug out, get the piston at TDC, then check the balancer mark at the pointer..should be close

If you verify the balancer marks are good, maybe the advance weights in the dizzy are stuck or binding..easy to look at once the cap is removed.

Daren67

its the tit. Its a 67 L-79 M21 370post, Carb says 3906631-DA, LIST-3810, 0193

I have a vacuum gauge like the one pictured in your article and it says motor late timing, not normal. Can my car ever run with that much advance. Maybe I will try to check the mark with TDC and my fingernail in the sparkplug hole.

JohnZ

OK, you have a 3810 (made in '73), so you don't have adjustable floats (unless someone changed the float bowls), only a bowl vent on the front bowl. Should look like the photo below.

No, it can't run with that much advance - you need to establish a baseline so you know what you're working from; use a piston stop tool (photo below) to establish EXACT TDC and verify the accuracy of the timing index line on the balancer and the "0" mark on the timing tab. Then you can proceed with confidence to analyze what you find as you trouble-shoot the timing issue.

Frankie the Fink

I can tell you that when I installed my electronic (Mallory Unilite) distributor in the 396ci Chevelle I got whacko timing readings for a while, even though the car ran fine, until I discovered the distributor was one tooth off. Readings were like 36* to 38* initial with vac advance plugged when they should have been 12*-14*.

Daren67

My timing light may be shzzed up, I will check with another timing light. With respect to the distributor, I unbolt it and it comes up around 1 inch and I twist and turn and cannot get out. I am afraid to pry it up, I move it up and down but cant get it out . I ponder the dist shaft is fused to the oil pump pickup or am I overthinking things.

hope2

easy top dead center, pull right valve cover and when valves "rock" on number 6, that is top dead center. in other word the point where intake or exhaust open when crank turned either direction. taking distributor out, dumb question did you disconnect tach cable? if so probably just gummed up, don't pry it just keep working it, maybe some WD40. and on that tooth off, doesn't matter at all tooth off, just different position of distributor. just find TDC, make sure damper lines up and set it, done.