Transfer slot and power valve question

Hello,

I am running a Holley 7448 350 carb on a OZ250 head with a comp cam 260 and DS2.
To get the best out of the engine I have a couple of questions:

1. Transfer Slots: My tslots are not exposed at all at idle. It takes a full turn at the curb idle screw to make them square.
This would mean going from 800 to over 1100 rpm at idle. In theory unexposed tslots result in hesitation off idle, which I had in the past and "cured" this with the biggest available power valve, high initial timing and a slightly rich idle mixture. What can I do better to get square tslots?

2. Power valve: In the vacuum chamber behind the power valve, in the main body, I always see some drops of fuel. Yesterday I realized that the pv thread isn't straight in the metering block. The valve sits a bit inclined in the block. I tried to mitigate this with two thick pv gaskets and high torque. But it seems to be not fully tight. Do I need to buy a new metering block or can this be ignored?

Cheers
Joerg

Ideal position of the throttle blades are no more than 1/3 up on the transfer slots.
Do you have the wrong PCV valve passing too much air into the intake?? That would lower the throttle blades. If you have a smooth idle adjusted on the lean side of the mixture range you should be ok.
Hesitation? Use the orange pump cam in the #2 position.
Have a .032" accelerator pump nozzle. Adjust the pump arm at WOT. There should be .015" gap between the arm & the adjustment screw.
The power valve apparently at one time was cross threaded in the metering block. You can try to save the metering block by taking it to a machine shop & have them re-tap the threads 90 degrees from the mating surface where the power valve seals.
I will try to get the tap size for you. It will be a US fine thread 1/2"-28. Be careful Holley used some oddball threads on their products.
You could try & put JB Weld on the existing threads if the machine shop says their is not enough meat left in the block.
You may want to find another metering block in the mean time. What are the 4 numbers stamped on the top of the block?
I use a 8.5 power valve in the 7448-350.

Hello Bill,

that is great advise to my questions. Thanks!

Regarding the PCV, I have a relatively new stock 200 valve in which seems okay. But I can try another one and see what the idle does.
I never thought about having too much air in the intake through the PCV line as a reason for too far closed throttle blades at idle. Great!

On the acc pump side I have a 31 nozzle in with orange cam but in hole #1. I experienced with different cams and nozzels in the past but I never thought about retarding the shot. I will try.

To have the pv thread re-machined in the metering block isn't easy here in Germany with this US threat size. Bonding the pv in with epoxy is a great idea, once I am clear about the final pv size. I will go back to 8.5, see how it goes and then decide.
In case I need to replace the block, it should be 134-203 according to the Holley numerical listing. However the picture on the Holley side looks different to my block with two acc pump routings instead of one. On my block I see 6387 and 7448 stamped in.

By the way, I decided to go back to manual choke after 2 years of fiddling with electric choke in our climate during the year. Is there someone who successfully adjusted an e-choke on Holley 7448 with an OZ250 head/intake? I could't find a setup that works well at 40 and 90 degr. F. combined with cold and hot engine.

Cheers
Joerg

Hi, I am also thinking about a vacuum leak because you can't adjust the idle. Have you tried to find a vacuum leak by spraying carb cleaner at the carb base and other vacuum ports?

The power valve should be closed at idle, and all other high vacuum situations. It should not be dripping at idle. Maybe there is a problem like dirt or cracking that is giving the carb an internal leak between the metering block and the body.

To check the PCV valve I pull it out of the valve cover and put my finger over it. You can also gently pinch the PCV hose closed.

You shouldn't have trouble with the electric choke. Make sure it is snapped tight closed when cold and is held fully open when warmed up. If you turn the cover rich or lean go in small steps. Also the choke needs 12 volts, not the @8 volts Ford chokes use.

Do you have the vacuum advance to the distributor on the ported vacuum at the right side of the metering block? That is where most guys like it. If you have the distributor hooked up to direct vacuum you will get vacuum advance at idle. Advancing the timing raises the idle, so maybe that is part of what you are fighting.

Also cam selection can affect the idle.

Good luck

Hello Bronco,

thanks for your points, I think I already checked the most of them - but double check never hurts.

Vacuum leak: I flattened the surfaces and put in new gaskets on mainfold and carb side this winter. But I need to re-check it plus manifold-to-block.

Power valve: Carb body and metering block look okay but I will have a closer look at next pv change.

PCV valve: It sucks air when I put the finger over it. But maybe too much at idle which then acts like a vacuum leak. I will try another valve.

Electric choke: The main problem is that I need lots of rpm to keep the cold engine running at startup (~1500). I guess this is due to the big OZ250 manifold and vaporization issues when it is stone cold. On the first stop after 1 minute driving downhill, the second step of the choke cam is reached and the engine dies. Turning the choke more to rich, the rpm keeps high at first stop and engine keeps running but the cam then never reaches the last step, which is curb idle. Neither with STA or 12V. Furthermore, driving away from a meeting with high idle due to still warm engine but choke cam on first step for a minute is annoying. I will ask in the Aussie Ford Sixes forum.

Vacuum advance: I have it on ported as full manifold vacuum makes idle even higher. 15 degr. static advance.

Cam selection: Yes but I do not know anything about my cam timing done by previous owner.

Cheers
Joerg

65Fastback":3sjyr130 said:

Hello Bill,

that is great advise to my questions. Thanks!

Regarding the PCV, I have a relatively new stock 200 valve in which seems okay. But I can try another one and see what the idle does.
I never thought about having too much air in the intake through the PCV line as a reason for too far closed throttle blades at idle. Great!

On the acc pump side I have a 31 nozzle in with orange cam but in hole #1. I experienced with different cams and nozzels in the past but I never thought about retarding the shot. I will try.

To have the pv thread re-machined in the metering block isn't easy here in Germany with this US threat size. Bonding the pv in with epoxy is a great idea, once I am clear about the final pv size. I will go back to 8.5, see how it goes and then decide.
In case I need to replace the block, it should be 134-203 according to the Holley numerical listing. However the picture on the Holley side looks different to my block with two acc pump routings instead of one. On my block I see 6387 and 7448 stamped in.

By the way, I decided to go back to manual choke after 2 years of fiddling with electric choke in our climate during the year. Is there someone who successfully adjusted an e-choke on Holley 7448 with an OZ250 head/intake? I could't find a setup that works well at 40 and 90 degr. F. combined with cold and hot engine.

Cheers
Joerg

Click to expand...

I meant to use JB weld epoxy to fix the threads, then retap the threads in the metering block.

I can't imagine it's too hard to find a machinist with an SAE tap and die set. You guys seem to really like the old muscle cars.

Econoline":2th7z1ze said:

I can't imagine it's too hard to find a machinist with an SAE tap and die set. You guys seem to really like the old muscle cars.

Click to expand...

It's a 1/2"-28 which is finer than an SAE fine tap.

Well that's goofy.