If that doesn t cure it, I think you will find the Q-Jet and a big cam don t mix without modification

 

These numbers indicate that your mechanical advance can advance the timing up to 26*. The stock springs don't allow that to happen until 5000 rpm. weaker springs would allow it to come in earlier (by 3000 rpm).
This would mean that the stock can will advance the timing 15* at 12" of vacuum. so your stock can cannot fully advance the timing until 12" of vacuum. This is going to give you an unstable idle trying to use the stock can and not enough advance to clean it up sufficiently.
If you use a can that allows allows full travel by 8" of vac then your going to get the advantage of full vac advance at idle and a stable idle.
However with you mechanical advance having a travel of 26* you will need to limit the travel in order to allow more initial timing and not exceed 52* of total advance at cruise.

So if your vac can advances 16* and you set lets say 18* initial, that is a total of 34* at idle. add to that the 26* of mechanical advance and you get 60* of total advance at cruise.
That is too much, so you'll need to reduce your mechanical to something around 17* or 18* so as not to exceed 52* at cruise or 36* all in at WOT and not have to reduce your initial timing to achieve these numbers.

Then you'll need to get the carb adjusted. I had to mod my q-jet to get it to idle well after a cam install due to lower vacuum as well. If your handy you can use this book to do it.

If not send it out to have it done. The difference is night and day.

 

Speaking of #s on vac canister mine has 436 20. Can anyone tell me what it means?

 

Lars papers had the numbers and the specs associated with them

 

The Q-jet carb has nothing to do with this issue. Any carb you place on the car will need to have the same A/F mixture at idle, so the air bleed holes (or throttle openings) will need to supply the same amount of air for the engine. If anything, the Q-jet carb would be better, because it can be "tuned"/adjusted for optimum settings.

 

Which paper is that? I don't have that one.
Thanks
Fred

 

... you seem confused on what idle bleeds do.. perhaps a refresher course.. AIR BLEEDS

You've heard of them, but what do they do? Air bleeds, sometimes referred to as "air jets" or "air bleeders" play a vital role in the operation of your carburetor. Air bleeds are responsible for determining the amount of air that will mix with each circuit in the metering block. Virtually every carburetor you come across will have these which make this a universal discussion. The amount of air bleeds a carburetor will have is dependent on the number of throttle bores and circuits the carburetor has. In the context of racing carburetors, it helps to think of them as (4) one barrels. Most racing carburetors will have either 8 or 12 air bleeds depending on whether they are 2 or 3 circuit. Each barrel will have one bleed per circuit.

Idle Air Bleed: The idle air bleed could be the hardest working one of them all. Air to be mixed with idle fuel is provided by the idle air bleed. The idle mixture screws rely on air provided by this bleed. Often racers have complained of poor idle quality and no response to adjustment of their mixture screws, this is due an incorrect idle bleed. Many idle issues can be addressed by simply adding or taking away air.

Intermediate Bleed: The intermediate bleed is found on 3 circuit carburetors ONLY. The intermediate bleed provides air for the 3rd circuit. The intermediate circuit is only adjustable externally by the air bleed and to tune it otherwise would require you to take the fuel bowl and metering block off of the carburetor. On most large flange carburetors this would be the bleed found in the middle.

High Speed Bleed: The high speed air bleed or also referred to as the "main bleed" correlates to the main system. The high speed air bleed controls how much air is fed to the emulsion channels of the metering block. To explain further, the emulsion channels distribute that air further to different parts of the main well where it mixes with fuel and ultimately goes to the booster. The high speed air bleed is generally located closest to the squirter when looking at most race carburetors.

Now that we know what each air bleed does, how do we tune them? Tuning with air bleeds is often easier than anything else on a carburetor. Most modern race-style carburetors have screw-in air bleeds that can be exchanged externally with a simple hand tool. Gone are the days of pin drills and dial calipers. Air bleeds often resemble a main jet only slightly smaller. Tuning is simple in that you just need to remember the size of the hole determines how much air is coming into the carburetor. If you want to richen up the idle simply replace your idle air bleed with a smaller one (less air in the system = more fuel = richer idle), this will help you get your mixtures screws within the 1-2 turn range. What if you want to lean out the intermediate so you can get your car to come off of the transbrake a little cleaner? Increasing the size of the intermediate bleed will lean out that circuit. Just remember that too much of a good thing can get you into trouble - before fine tuning a carburetor you should take note of the stock specs , to return if necessary

 

Sweet70,
The anomaly WAS the MSD dizzy. I didn't know that the B28 can would fit the MSD dizzy. It did, but will definitely fit the factory dizzy too!
Good luck.

 

I'm gonna take a gues at what those numbers might mean.
The 20 I'm gonna guess is 20* of advance. This can be confirmed with a timing light.
Disconnect vac to the can, look at timing and record that number.
Then reconnect vac to can and idle engine up to 1000 rpm with mechanical advance locked so that it cannot advance. Wire it in place or something. Then read your total advance If 20 * then that number is confirmed.

Now for the first batch of numbers, it looks to me that maybe they could be pressure (or vacuum in this case) in millibars.
If I convert 436 millibars to Inches of mercury ( what were are measuring on the vacuum gauge) I get 13 inches of mercury.
So this could represent where it starts to pull in advance or where the advance is all in. Since yours is not working at 11 inches of mercury I'd lean towards the former of the two.
Either could be verified with a mighty vac. Just a guess.

 

Sounds reasonable to me. Will check it out as you suggest when I get a minute. Thanks for your thoughts.
Fred

 

Ftf396,

Do you see fuel dripping into the primaries while it's idling?

I have a 71 LS5, stock 454 with 63000 miles. Mine has same issue as yours but I do have leaking intake, just need time to fix it.

Have you unhooked all vacuum hoses from intake to see if it makes a difference? Sorry if you already tried that and I missed it.

Roger

 

My thanks to all that have assisted in my education in solving this problem ... I have learned a great deal. In looking for the cause of my problem I believe that my lack of understanding and the persuasion of others I installed a CompCams XE274H cam when my engine was rebuilt. Although I'm sure this is a fine product it was more radical than I understood it to be, with the higher lift than the stock cam for an L46 motor it resulted in the lower vacuum readings and the ineffective operation of the stock vacuum canister on the distributor. Hence the inability of the motor to idle. This is all spelled out in the Technical Bulletin regarding Distributor Vacuum Advance written by Lars, my thanks to the person that sent the link to this resource. Based on this information and the -2" rule, I see why I now need to replace my stock vacuum advance for one operating at a lower vacuum. Looking through the available options the B28 (VC1810)looks to be the right choice. This is no longer available from NAPA but can be purchased from Advance Auto Parts under part # of DV1810, price $11.99. A small price for such a big head ache ... or changing the cam again. Ordered one today so should know in a few days! Thanks to Bob for his coaching.

 

[QUOTE=roger3;1585841807]Ftf396,

Do you see fuel dripping into the primaries while it's idling?

I have a 71 LS5, stock 454 with 63000 miles. Mine has same issue as yours but I do have leaking intake, just need time to fix it.

Have you unhooked all vacuum hoses from intake to see if it makes a difference? Sorry if you already tried that and I missed it.

Roger[/QUOTE

No I don't see any drips. Haven't had time to disconnect everything and check hoses yet but have checked intake and carb no apparent leaks.
Fred

 

Sorry, but I know I'm piggy backing on Fred's thread but I see my next problem being the setting of idle speed without the B28 vac can being connected. You mentioned modifying the Q-Jet to allow for this. I have the book by Doug Roe but not the one you mentioned. Can you be more specific or send me a PM and I will share my e-mail address then you could send a copy of the page?

 

There is much to understand in order to make the mods and make adjustments down the road. The mods are more than a few pages long in order to get the most from them.
Buy the book read it cover to cover... several times. Know how the q-jet works then make your adjustments. You'll be miles ahead of the average joe doing this. It's that or send it off to be tuned by Lars or Cliff.