I know there are a lot of guys who are great at fabricating on this forum, and really do some incredible stuff. I jsut ran into some issues with the '76 LTD I just brought home from Illinois and thought I would share some basic tuning tips on this carb after what I found on this one, as well as some things I found on forums that are a little bit of a dis-service through either regurgitating myth, bad info or what have you so here goes: 

Assuming that your base timing is set (factory spec will be out the window if you have anything more than a stock cam, but should be pretty close if you're running a mild cam), set your curb idle speed. Now it is time to check your float level. Even though the Holley manual says to do this adjustment with the car not running, I do it with the engine running. 

If you have a sight screw or sight glass this part of the procedure will be the same: the sight screw, or sight glass will be on the passenger side of the float bowl (left side if your are facing the front of the engine). Have a towel ready and don't stand directly in front of the sight hole, as if the float is really high you will get a face full of fuel (actually happened to me when I tuned my friend's late model stock car). on the car with the sight screw, remove the screw. If the float level is just trickling over the threads you are ready to re-install the screw and move to the back float bowl. If not it's time to adjust your float level. On a Holley with a sight glass the checking procedure is much easier. with the car running, your float level should be at the half way mark. I set mine a little higher, but not much. If on either type it is higher than trickling over the threads on a screw type or not trickling out, or above or below the halfway point on a sight screw type float bowl you need to adjust it.Also if fuel is spraying out of the float bowl like a fire hose, shut the car off immediately!

So now, again standing in front of the engine, locate your float adjusting nut and set screw. On a side hung float such as an 1850 Holley, it will be slightly to the left of center and will have a nut with a set screw (large flat blade type screw. On a center hung float such as a dual feed carb like a street avenger, or double pumper (not the same as a vacuum secondary dual feed but will have a dual feed line and center hung floats), the adjusting nut and set screw will be in the middle of the float bowl. Note:in most cases unless you have a velocity stack, or 6" air cleaner, you will not see either one without the air cleaner removed. 

Once you have located the nut take a large flat blade screw driver and loosen the set screw ever so slightly as if you loosen it too much, you will start seeing fuel run out. If you are rasing the float level you will turn the adjuster nut with a 5/8 wrench counter clockwise. turn it slowly (you may have to loosen the set screw in relation to turing the nut a little). Once you start to see fuel run out of the carb over the threads, reach over and zing the throttle to pull a little fuel out of the bowl. If it conitinues to just trickle out, you're done.If not, repeat this procedure until it is set. It is the same procedure for the sight glass type, you are just looking to get the float level to be at the halfway point on the glass or slightly above. 

Now if the float level is too high, the procedure is the same BUT you will need to loosen the set screw, and turn the adjuster nut clockwise with the added step of revving the engine like before for the reason of when you lower the float it will force the fuel level upward from the float pushing down and displacing the fuel level, so loosen the screw turn the nut clockwise, rev the engine and check the level, turn the nut, rev check, until you either have the fuel triclking over the threads, or at the halfway point or slightly above on the sight glass. 

Now it's time to check your rear float. same procedure with the engine running but revving the engine with the rear barrels is s little harder on a vacuum secondary car. So on a vacuum secondary (easily identified by standing in front of the engine a looking at the rear of the carb on the passenger side and looking just behind the choke mechanism where you will see a vacuum chamber that is flat on top, with an arm that comes down to a linkage that will operate the scondary butterflies. You can also look at the throttle lever on the driver's side and will see slot on the throttle linkage that has am arm attached to another throttle set which operates the secondaries upon the throttle returning to light throttle to stop your secondaries from hanging open when you close the throttle. you can either operate the secondarys by moving the arm from the vacuum can or the secondary throttle on the driver's side. Be gentle if you decide on operating the arm from the vacuum can. 

Again, either remove the sight plug, or check the level on the sight glass (if it is too low, you won't see fuel, or if it is way too high you might not see fuel either. If the fuel is trickling over the threads on the sight screw type, or halfway or slightly above, you are good to go. If not, repeat the procedure for the adjusting your float. Again rev the engine with the secondary throttle linkage to pull fuel out of the bowl. Operating the main throttle will do nothing to open the secondaries on a vacuum secondary carb. NOTHING. If it does with the car in neutral, then your secondaries are opening way too early (especially on a heavy car like a full size Ford like we all love)) and your will need to install a different secondary spring. 

now you can set your idle mixture. you will need either a tune up tach, a friend in the car reading your tach if you have one in the car, or a vacuum gauge. Start your car and let it get to full operating temp. Once at temp, with your base timing  and curb idle set, hook up your tuneup tach, or hook up your vacuum gauge to a full vacuum source. On a holley, that will be the port on the front of the carb base on the left hand side. The port on your metering block is timed vacuum for vacuum advance if you prefer to use ported vacuum instead of full vacuum for your distributor. 

note your rpm (tuneup tach.), or your vacuum reading. Now locate your idle mixture screws which will be on the sides of the metering block between your float bowl and carb main body. Turn both the left and right screws 2 1/2 turns out (counterclockwise). I start with the driver's side and turn it about a 1/4 turn, now move to the pass. side and do the same. Notate your rpm or vacuum reading (also let the vacuum gauge settle as it might deflect a little when you first turn the screw. repeat this one side at a time until you achieve the highest vacuum reading. or highest rpm. double check how far out each side is by turning in one screw until it lightly bottoms out and then turing it back to the highest rpm setting, and double checking the next screw. The idea is that each side is getting an equal amount of fuel. I usally end up with 1 1/2 turn out from lightly bottomed. 

Quick and dirty way? Start with both front screws 2 1/2 turns out. Start with one side and slowly turn the screw in until the car starts too idle down, and then back out 1/4 to 1/2 turns out. Do the next side, double scheck the each one is an equal amount out by turning one until lightly bottomed, taking note of how many turns you have to go until it lightly bottoms and turning it back out to your best idle, and checking the other one. Make sure both are an equal amount out. 

Now take it for a ride. If your float level was too low before the car probably nosed over off idle (this can also be a case of too slow of a pump shot from your accelerator pump, but is often a misdiagnosed low float level). If your accelerator pump is good for your requirements, you will notice that that stumble is now gone. A low float level will actually give the same stumble as not enough from your accelerator pump, so the is a good place to start if your notice an off idle stumble as the BOTH will result in not enough fuel. if your secondary float level was too low, you will now notice more power as they open as you are now not going lean when they start to open. a correct secondary float level will give you much more pull higher up in your rpm range under hard throttle.

So if those stumbles off idle are taken care of, and your car pulls in a linear fashion all the way through the rpm range with a vacuum secondary carb, congratulations. If you "feel" the secondaries kick in with a vacuum secondary carb, then you need a bit heavier spring in your vacuum chamber. 

I will be happy to go through this procedure step by step with pics, or possible a video if anyone needs.

Anyhow, I hope this helps. If anyone has any questions, feel free to message me. 


have a great weekend,

Al
 

Last edited by guitar74 (June 23, 2018 9:50 am)