low vacuum of idle stall solved?

[Shakey Bender]

Have a tip from a mechanic from work. Tell me what you think.
On my motor I have made some mods. 200 with flat top piston lopey cam -(low Vacuum cause) 6/1 header ported head DS2.
Pretty much everything and the kitchen sink. A lot to list but you get the idea.
Anyhow I bolted on a 1.14 2100 autolite and started having a off idle stall, also idle was rich. I have a vacuum around 15 at idle. This had me setting the idle stop screw out a ways to get enough air thru to keep her running. This had the throttle plates open above the second slot in the throttle bore. This slot is suppose to add gas during acceleration or of idle and in essence I had bypassed it. Now to the possible solution, Drill very small holes in the plates to allow air thru and get them set lower. Anyone have experience doing this? Anyone know how small, ideal location, etc.
If you have read this and find it confusing but get the gist of what I said feel free to clean the procedure and diagnosis up for me. I'm not the best at written conversation.

 

[Lazy JW]

.....
On my motor I have made some mods. 200 with flat top piston lopey cam -(low Vacuum cause) ......

That "lopey cam" is certainly causing some grief at idle, but what you need is not drill bits but rather some additional timing advance. A lopey cam can require 15º-20º INITIAL advance to idle properly. Try cranking up the advance and see if this helps any; problem is, this will mean that you have way too much at the upper rpm range. It's Re-Curve Time, folks
Joe

 

[rbohm]

Have a tip from a mechanic from work. Tell me what you think.
On my motor I have made some mods. 200 with flat top piston lopey cam -(low Vacuum cause) 6/1 header ported head DS2.
Pretty much everything and the kitchen sink. A lot to list but you get the idea.
Anyhow I bolted on a 1.14 2100 autolite and started having a off idle stall, also idle was rich. I have a vacuum around 15 at idle. This had me setting the idle stop screw out a ways to get enough air thru to keep her running. This had the throttle plates open above the second slot in the throttle bore. This slot is suppose to add gas during acceleration or of idle and in essence I had bypassed it. Now to the possible solution, Drill very small holes in the plates to allow air thru and get them set lower. Anyone have experience doing this? Anyone know how small, ideal location, etc.
If you have read this and find it confusing but get the gist of what I said feel free to clean the procedure and diagnosis up for me. I'm not the best at written conversation.

8) the low vacuum comes from the long overlap of the cam. this usually happens when you have either excessively long duration and/or narrow lobe centers( a cam with 106 degree lobe centers will have more valve overlap than a 110 degree lobe center cam with the same specs). valve overlap is the amount of duration that both valves are open at the same time.

as for the trick of drilling small holes in the throttle plates, that is an old racers trick to get more air at idle, and to make it easier to launch the car. what size hole depends on engine displacement, cam timing, etc. that said, start with a 1/16" hole in each throttle plate, and go up 1/32" at a time until you get the smoothest idle. remember to deburr the holes each time you drill. this is a time consuming procedure, and if you go too big you can make things worse, so take your time, and drive the car for a week or two under varied conditions before you decide to enlarge the hole again. as for the location of the hole, usually it goes on the upper part of the throttle plate, meaning the side that goes up when the throttle are opened. you also want them in the center of that half of the throttle plate.

by the way, i do agree with adding more timing before you mod the carb throttle plates. make sure you have done everything you can before you mod the throttle plates.

 

[Shakey Bender]

Lets start by saying the carb I am thinking of tinkering with is a little worse for wear. The shaft is worn out and I think the power valve may not be at the right vaccum for my set up. I think my brain may be a little low on vaccum to because in the end I decided to get a brand new Holley 350. That carb won't be causing anymore problems for anyone. I just put two holes in it. My buddy said we should have put 9mm holes instead. I will post results when the carb comes in. Thanks for the help and the warning.

 

[Shakey Bender]

Well I found a gem at the junkyard. 2300 out of a 78 F250. It has a oddball model# but after a google search looks like its a 350cfm. My next stop was to pickup the holley bible since this is my first holley. The venturi boosters on the carb are specified as the economisers and are said to restrict cfm more than other boosters. Well I gave the linkages and exterior a good cleaning and bolted it on. Presto started right up. Now the fun begins time for a rebuild kit and some jets and cams for the booster, possibly a new power valve. Wish me luck. At first blush this seem to have given me some gains at WOT. The throttle response is precise and also a improvement. I have yet to comletely clear up the stumble off idle but have my nose in my book and feel good that I will overcome.