blue 67 coupe

I was adjusting the idle mixtures on the carb and was going off a video I saw on youtube. So when I got the vaccum gauge on it says adjust the screw and get the highest psi or the peak. Well one of the vaccum ports reads 20 when the car is at idle. When I put the guage on the other it doesn't read at all until you push the accelerator. So now im confused is that how its suppose to be?


*The red boxes is where I had the vaccum guage*

Also made a video in case my description doesn't make much sense.

http://www.youtube.com/watch?v=j0kn1hh9Aug

kalli

that's perfectly normal.
the upper one in the metering block reads ported vacuum and this is typically 0 at idle.
The lower one is full manifold vacuum.That's the one you use for adjusting
plug it in to the lower one, have the upper one blocked like now.
Start with both screws two turns out. make sure to turn each the same amount. close one of them slowly til vacuum just start dropping, open quarter turn
do the same thing with the other. best practise is to have highest vacuum in leanest position (closing the screws leans out the mixture)

and good video. i'll keep it as reference for the next one with a holley vacuum question ...

blue 67 coupe

Thanks now it makes sense and got it adjusted within a couple minutes and it now runs around 19 or 20 psi. And now the only time the car hesitates is when you get on it a little. So not sure if this is right because its my teenager logic. But I was thinking if the secondaries open around 1500 RPM(just and example) and the spring is too stiff to open all the way could it be that they aren't opening fully causing it to get less fuel and hesitate?

Also if it is the secondary spring the carb will have to be taken apart right? I looked and the top has 4 screws but one is placed under the lip of the bowl. Is there a way to get to it without dissassembling the entire carb?

Thanks

kalli

hesitation is usually lean condition, or timing too far advanced.
The secondaries throttle plates open according to the throttle position, but the butterfly on top doesn't open at specific RPM, it is a combination of load and rpm
So if your carb opens the secondary when standing and revving then the spring is way to light
This as well you feel as hesitation. But I have a suspicion that the cause is not the secondary, but the accellerator pump. as this kicks in as soon as you tip the throttle.
See if you can increase the pump shot a bit. Some carbs do it with adjustment on the linkage and some do it with a little plastic cam.
If you have the plastic cam version make sure as well that the accellerator pump is adjusted properly. info on that on holley website somewhere. Don't know from top of my head
And yes, older Holleys don't have the quick-change thingie for secondary springs. But I would leave that for the moment anyway

Kalli

THUMPIN455

Kali he has a vacuum secondary Holley, the secondary doesnt open until there is the enough load on the engine. They open with the big round dashpot on the passenger side of the carb. There is no butterfly or air valve on top of the carb. Open too fast you get a bog, and a bog sounds like the engine just stops running for a moment then catches again and starts running. Holley sells a spring kit, and a quick change cover for the dashpot, its very handy and easy to install. The purple spring in the kit is probably going to be the one your engine likes, the lightest yellow one will open way too fast for your engine.

Agreed that a lean condition will cause a hesitation, but it could be something as simple as float level causing it. If the float is too low it takes longer for the fuel to flow to the booster. Too high and it drips from the booster at idle and burns your eyes from being too rich. The accel pump is a good place to look, larger squirters are available, those are the piece between the primary venturi where gas squirts out when you open the throttle quickly. A single screw holds them in, just dont drop them in the engine.

The cam can move the pump shot around a bit, make it longer or shorter to a certain extent,as well as come in sooner or later. Its on the main throttle shaft and the arm for the accel pump on the front bowl is resting on it. You can try moving it one hole and see what it does. They have two or three holes in them, and using a different one changes the profile of the pump shot.

Also your distributors vacuum advance should be plugged into the top port that only shows vacuum after the throttle is opened. If it is plugged into the bottom one, you have full vacuum advance at idle, and while it works well for some engines to be set up like that, it would require more changes to yours to run right like that. The idea behind vacuum advance is not to have it all there at once, but to come in over time smoothly, hence why you connect it to the top port. Your hesitation could very well be just moving the line from one to the other. Some Holleys have full manifold vacuum at that port, so its good need to check when the vacuum starts for each port. One of the 780 vacuum carbs I have has full manifold there, the single feed 650s like yours dont. Just something to be aware of, not all Holley carbs are the same.

You did set the timing with the vacuum advance disconnected right?

67mustang302

Everything Thumpin said. You also need to make sure the curb idle speed is correct. If you have the idle speed too high and are uncovering the transfer slot, then you have to lean the idle mixture, then when you start to move the throttle off idle there's not enough transfer slot fuel and you get a really quick lean stumble or hiccup. Also setting idle by vacuum can sometimes cause a slightly rich idle. I prefer to lean the idle in until the engine rpm starts to drop or it starts to run rough or have trouble running, then just ease it out a bit. You want it as lean as you can get it without having any idle issues.

Make sure the idle throttle position isn't too wide. And yes, make sure fuel pressure and float level are correct before doing anything else. What pump cam is in it right now?

blue 67 coupe

I honsetly don't know what pump cam it has. I bought this carb used from a friend a while back after he took it off his chevy. Which is why I thought the spring was too tight because he has a larger displacement motor and mine needs a softer spring. But really all I know about it is that its a Holley 600 cfm electric choke. This is my first time actually having to tune/set the carb and i'm trying to go slow and take my time and try not to mess anything up. Thats why I have had so many carb questions recently. But so far everything has been helping so thanks to everyone. Later I will go to Oreilly's and pick up a spring kit and probably a larger fuel squirter and then go from there.

kalli

i thought I had good advise and again I got educated. Now that Thumpin mentions it, ya the holley doesn't have that top butterfly on secondary. i was looking at my edelbrock too long :-) on the holley it's the secondary linkage that is blocked.
Reading the others post that#s really what I'd start with first:
- get an inline fuel pressure gauge to make sure the pressure is right.
- adjust the float levels (very easy to do, but messy)
- the accellerator pump cam is a coloured plastic piece that is sitting on driver side of carb in front of throttle linkage. at bottom of carb. once you see it you know what we are on about. what color is it.
- I would ignore the secondary springs for the moment. That's something you do later, when the rest is right. Doesn't hurt to pick up a set though.

blue 67 coupe

Well I went ahead and bought the spring kit the other day and with the instructions it was really simple to do. That was one of the problems the spring that was in it was black(stiffest) so I took it out and put in the purple spring.

The cam is yellow and has the 3 holes for adjusting.

What color cam would I need for it or is it something as simple as changing which position it was on. When I took it off it was on the number 3 position.

67mustang302

That's actually the pink cam...it must be faded, or it's REALLY old and used to be yellow and they changed colors at some point. Get the cam tuning kit, it has all the cams for the 30cc pump.

The pink cam though has the highest total fuel volume delivery, but the slowest ramp rate, so at small throttle openings it delivers only a little bit of fuel. Get the cam kit and try the orange cam. The cams deliver fuel vs throttle position, a cam like the pink one delivers a bit of fuel per throttle position over the largest range of throttle(you need to be practically at WOT to get full delivery from the pink cam). A cam like the orange delivers a bit less fuel than the pink, but total delivery is somewhere around half throttle, with a higher initial ramp(it will deliver a larger fuel shot at small throttle openings just off idle). The green and blue cams deliver the total shot of fuel almost entirely by a small throttle opening off of idle, the red and white are similar to the orange, but different enough to make a difference and let you tune it on some engines, and the black has a ramp rate about mid way between the orange and pink. They all deliver slightly differing amounts of fuel at different throttle positions.

Pump tuning is about knowing which cam does what, and trial and error. See how the engine responds and if it needs more or less fuel at a certain point, change the cam. Keep in mind too that jetting will also change the required accel pump delivery, larger jetting needs less pump, smaller jetting needs more pump.