Timing down then up

B

brad.arcova

Member
Been trying to run down a low end performance issue for quite a while. Finally checked timing over a range and found that the timing tracks the performance issue. I'm running 383 with 234/244 cam,vortec heads, no vacuum advance, MSD billet distributor converted to run with an HEI ignition module. From the graphs, the timing curve is set to idle at about 15 deg and advance to 34 all in. Car idles fine and runs like a champ above 2000 rpm. At steady speed between idle and 2000 it runs fine but under load or acceleration it bogs really noticably. With a timing light run through this range it starts out at 15 deg and as rpm INCREASES, the timing RETARDS all the way down to 10 before catching back up to the curve at about 22 deg and 2000 rpm. The distributor is all the way in the hole and there's minimal play in the rotor. The distributor gear looks fine with near zero wear. the advance weights seem to work fine with no play or binding.

Mechanically it doesn't really seem to make sense but the timing retard indication is exactly tracking the performance issue. Could a weak ignition module skip a few beats and cause this issue? i'd like to be systematic about this and not just replace parts willy-nilly until I stumble across the right one.

Brad
 
The MSD Pro Billet Distributor Brad is a bit of a Hybrid Brad.
MSD Engineers took advantage of a Small Cap Points cap distributor style unit with convienent Mechanical advance on top below the Rotor.
The Magnetic Pickup is Chrysler AMC Style.
I am thinking there is Trigger issue.....
Your picking up EMI , Electromagnetic interferrence.
Best to use a MSD 6AL OR 7AL OR I THINK BETTER YET A MALLORY HYFIRE RACE IGNITION BOX.
STAY AWAY FROM MSD DIGITAL. IT SUCKS.
WANT DIGITAL USE MALLORY HYFIRE RACE.

Brian R.
 
GM HEI Modules are input polarity sensitive too.
Sawtooth Pattern input has a Vector starting point that the HEI Module reads.
Can't Oscilliscope test today & prove to You.
Have to take my word that it is.
 
Vacuum is disconnected; don't even have a vaccum canister. Could it be cam walk? How would I test for that without taking too much of the top end apart?

Brad
 
87vette81big said:
GM HEI Modules are input polarity sensitive too.
Sawtooth Pattern input has a Vector starting point that the HEI Module reads.
Can't Oscilliscope test today & prove to You.
Have to take my word that it is.
Click to expand...
Got myself an oscilloscpe this week, tektronix 465b :), wich one your using? am curious.
 
mathd said:
87vette81big said:
GM HEI Modules are input polarity sensitive too.
Sawtooth Pattern input has a Vector starting point that the HEI Module reads.
Can't Oscilliscope test today & prove to You.
Have to take my word that it is.
Click to expand...
Got myself an oscilloscpe this week, tektronix 465b :), wich one your using? am curious.
Click to expand...
Snap On Vantage .
Snap On Counselor Digital CRT Ignition Oscilloscope. About 1986- 87 Vintage. Super Accurate.
Have an Old SUN Oscilloscope that worked great but quit. 1969 Vintage. All Vacuum tube circuitry. Want to fix, short on time lately.
Textronix 100-300 MHZ scope on my want list. Not top priority right now. Maybe get lucky later this year. Retired Electrical Engineers personal gear.
 
tektronix 465b is very popular workhorse and is VERY repairable(socketed transistor, IC non-existent), 100mhz bandwidth/3.5ns rise time but i have seen them work up to 200mhz without problem triggering
http://www.youtube.com/watch?v=hXbPnY7VIes
(i love this guy channel, he do a good job at teaching things easily)
or tektronix 2467B, but the 24xx "hybrid" serie is scaring me.. older eaprom that loose the calibration data over time, and some other one with eeprom and a battery inside when it die you also loose calibration data, and its not really made to be replacable(calibration can cost as much as the x-scope itself), and its all custom tektronix IC inside wich are discontinued, the eeprom was made to have a heatsink wich they omitted and heat reduce the life span of the device). Apparently the 2467B is a nice piece of equipment if you can get agood one at good price.

I got my 465b on ebay, auction, 203$US + shipping, it came with a tektronix 350mhz 10x passive probe(p6106) and the pouch on top for accessory storage.
Very clean, and its working flawlessly.

It was calibrated 15 years ago, but am just a hobbyist, am not doing rocket science.
Been dreaming to get one of these since years :).
Click to enlarge.
 
I still dabble with electronics on occassion Mathiue.
More fascinated with Old School Tech.
Kinda how I became a Mechanic.
 
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