I have been having troubles getting my 79' to run well with either issues with no bottom end and all the top or vise versa. I used Lars's "How to set your Timing for Peak Performance" Tech article to adjust my timing. After I got the advance to top out at 36 deg it seemed to run like a champ, but I had rough start off the line due to a bigger cam and running ported vacuum. I put a vacuum pump on the distributor diaphragm and added a little vacuum and it seemed to do the trick. From this I went by the recommendation and moved the vacuum to the manifold to get vacuum at all times to alleviate this issue. All was good for a bit and I revved it several times, then after I let off the gas I started to get a nasty knock or crack sound periodically from the intake manifold and carb area. Kinda like someone was hitting it with a large wrench. I tried to retard the timing to see if this was the knock the article was referring to and it would not go away, even if rev it back up again, the sound is still there every so often. I ran down and got premium fuel to see if this was the issue and it got a bit quieter but it is still there. the sound is nasty and I haven't wanted to run it too long to figure it out without getting some advice first. I tried emailing Lars but I think he's busy or I got a bad address. Any help anyone could give me, would be very much appreciated.
Thanks in advance,
:banghead:

 

You may be getting too much advance with the full manifold vacuum coming in when letting go of the throttle. Only an advance unit that makes less vac-advance could help you there.
Other than that- are you sure it's knock and not just a backfire?
More typically you get knocking at WOT operation. Does it go away if you go back to ported vacuum with the advance?

Stefan

 

I'm not sure, but I think it is backfire since I also get a kick back out of the carb when it hits. As far as the Vacuum, I even went as far as to remove all vacuum and retard the timing and it still persists every so often. I even get a bit of it when I have the throttle down and am running about 2500 to 3000 rpm. I did notice this last time that once I removed all the vacuum and let it idle, it almost all went away till i kicked the throttle again. I am overall at a loss since this never did this before and it has a lot more of a crack sound than I have experienced with backfire before.
I think my carb needs a rebuild though. Could this be a culprit and maybe I just choked something lose recently and it all went to hell?


[Modified by Jerr79Shark, 4:28 PM 4/12/2003]

 

uuuuh. I'm really not the carb speialist here. Could be a vacuum leak I suspect- but not at ALL sure...

Stefan

 

i had the same problem as you did . it turned out to be a flat spot on the exhaust cam lobe worn .250 of an inch. just finished putting the cam in an hour ago. hope to get it started on sun. Take your valve covers off remove coil wire have someone turn the eng. over and watch the rockers one should be moving up less than the others. that's how i found out it was the cam. good luck

 

Was your's doing that over time or did it happen all of a sudden. I have adjusted my valves earlier last month, however this started happening right after I adjusted the timing and added the manifold vacuum two days ago. Before that, it was running fine without issue? A worn cam lobe just seems a little off given the situation, but then again I could be wrong.
Thanks for the help.

 

I just went through the distributer recurve from hell,and found the vacuum advance to be the biggest PIA.I went with a Crane adjustable vacuum advance,installed the limiter plate set to two notches from maximum lockout,and set the rate to six turns out from fully closed. This limits the vacuum advance to 10 degrees.I run 16 initial,and 36 total mechanical in by 3000. No pinging what so ever.
Hope this helps. :cheers: