at idle while warmed up, initial timing @ 12* advanced on a carbed 350 with a mild cam (218/218 dur .484"/.484" lift with the 1.6 rockers) I'm getting a reading of about 16-18 in Hg, but the needle kind of "seeks". I doesn't move real fast, like i've heard others describe as "twitching", but I think this may be related to my not perfect performance. The car should be much faster than it is. What could this mean? small vacuum leak? Also I noticed on top of a couple headers tubes there was a couple small sprayed spots of carbon. Does this mean a header leak? Could this cause the vacuum gauge seek? Wires and plugs are new, cap & rotor are fine. I also get a carb backfire every so often when a hit the gas in overdrive or drive under load in low rpms. What do you think?
Thanks,
John

 

The carbon is a header leak. No big deal, see if you have any header bolts loose. I use blue Permatex on my header bolts. Helps keep them tight.

As far as vacuum, pulsing can be a couple of things. You seem to have lots of vacuum at idle so that's good, now all you have to do is tune it up a little. Pulsing can be caused by fuel mixture being off a tad. The real problem is if you have a major pulse under hard acceleration. You may have a rocker that is too tight ect...

In most cases its just a small mixture problem that can be corrected on your carb. Also make sure your wires & plugs are all up to par.

Good Luck.

BTW....how many inches of vacuum does it pulse?

 

thanks for the quick reply. It only seeks within this 16-18 range. so about 2" max. a rocker too tight is possible (have had problems with them to say the least)
gonna go tighten up the header bolts and check the o2 sensor for readings.

 

ok tightened up the header bolts but no time to check mixture I will tomorrow. Runs better but not quite perfect yet

2 other things I forgot to mention:
1.) Car sometimes "diesels" after I shut it off. heard it could be mixture, temp, or octane but I use 89 octane with no knocks, car doesn't overheat but the problem does get worse the hotter it is when I shut it off, and I'm not sure about the mixture yet. Is there anything else that could cause this? It kinda sputters, but doesn't continue running normally. It is very embarrassing.
2.) The other thing is that the idle (in drive) fluctuates about a total of 100rpm and also kind of "seeks" just like the manifold vacuum does.

Could mixture alone cause all of my remaining problems (dieseling, seeking idle/vacuum, carb backfire under load, and a bit sluggish performance)?

P.S. I have DR secondary rods, K hanger, loose secondary spring tension, and modified the secondary stop so that it opens farther (all on a reman. Quadrajet from national auto which looked and felt VERY high quality BTW)