holley power circuit

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boilermaster

Well-known member
Joined
Feb 19, 2015
Messages
540
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Location
michigan
My Car
71 fastback
Hey gearhead gang,

looking for some information on a 750 street avenger and possibly a quick fuel metering block.

Engine idles perfectly and accelerator pump circuit is good and @ 60 mph in high gear cruise I am at 14.0 afr.

changed primary main jet from 72 to 70 to get to that point and I know it can go a little leaner

But when I mash the pedal it goes quite rich (11.5 to 12.0)

Was thinking of getting a quick fuel front metering block with replaceable pvcr to dial my richness down to about 12.5 or whatever this motor likes best.

I cannot find what pvcr comes with this unit (QFT-34-8) nor do I know what my existing pvcr is.

Measuring my stock sizes are is not a big deal, but I would like to get everything at the same time.

 still don't know if their bleeds are 6/32 or 10/32 and whose bleeds to buy.

will also the that same dilemma with the idle feed restrictors and emulsion bleeds.

Someone has to have done this before me, please help.

                                                                               boilermaster

 
The metering block you have now is closer to the calibration you need than a new QF block will be. You can easily modify your current block and it will be a lot cheaper and less work than a new metering block. All you need is a bottoming tap with the corresponding drill bit, some brass set screws and some pin drills. The brass set screws for the ifr will want to be 6-32 x 1/8" or 3/32". The PVCR can be either 6-32 or 8-32 depending on how big the hole needs to be. Since you are wanting to reduce the size of the PVCR the 6-32 will probably do the trick.

The tap, bit and set screws can be had from McMaster Carr. The pin drills you can get individually at a good hardware store or you can buy a set off ebay.

 
Tommyk,

So you are thinking that since I am already good on my calibrations until the power valve opens (6.5)

that it would be much less hassle than taking three steps backwards ?

it would only cost $ 75 if I ruined my holley block.

will have to figure out some kind of jig table to do the drillings.

Thanks Tommyk, I knew nothing of the calibrations of the quick fuel stuff.

                                                                Boilermaster

 
Two things are happening when you mash on it. The power valve is opening and the accelerator pump is going.

Might try backing off the accelerator pump some and see if that helps

 
Tommyk,

So you are thinking that since I am already good on my calibrations until the power valve opens (6.5)

that it would be much less hassle than taking three steps backwards ?

it would only cost $ 75 if I ruined my holley block.

will have to figure out some kind of jig table to do the drillings.

Thanks Tommyk, I knew nothing of the calibrations of the quick fuel stuff.

                                                                Boilermaster
No jig needed. I clamp the pin drill bit in the vise and chuck the set screw in a cordless drill. I use a tiny bit of silicone grease as lube. Go easy and you shouldn't have a problem. For the metering block you have the existing holes to act as a pilot. The bit will follow. When tapping you want to tap a little and then screw in the set screw to check depth. Rinse and repeat. You don't want to over thread. The set screw needs to "fetch up" on the bottom to hold it securely.

Use the pin drills to qualify the size of the existing pvcr orifice. Drill the new set screw about .010 smaller. That should make a significant difference in WOT AFR and will probably be too lean. Go up .002 at a time until you like what you see on the gauge. It takes some time but you can dial it in perfectly this way.

 
Tommyk,

Had just dropped the accelerator pump from a # 32 to a # 30 , but for a different reason.

I had just replaced my 370 gearset with 350's

when I was running 370's it required the #32 nozzle.

Will drop to a #28 and see what gives, cam in #1 position.

I do have some junk metering blocks to do a practice job on.

Thanks again

                       Boilermaster

 
Well,

I may have put the cart before the horse here, but I had to give it a try.

I came up with 4 scrap metering blocks, got a # 29 drill bit (for 8-32 threads) and got out a 8-32 tap.

I drilled out a plastic sleeve to put over my bit and cut to length, as to not over drill on the metering block.

drill did follow very well, material also tapped very well.

Will have to wait until I get my bottom tap from McMaster carr , along with the brass set screws.

I did slip a steel set screw in there just to see how it was going to fit = very nice.

Sizes ?

of the 4 metering blocks I had, I came up with at least 3 different sizes, just did a go/ no go  with a torch tip cleaner.

the largest was .062'' and smallest was about .042''

I can come up with at least 4 drills within this range, should have that covered.

Will play around with accelerator pumps, nozzles and cams/ positions today to gain some more data

                                                        Boilermaster

 
Tommyk,

So you are thinking that since I am already good on my calibrations until the power valve opens (6.5)

that it would be much less hassle than taking three steps backwards ?

it would only cost $ 75 if I ruined my holley block.

will have to figure out some kind of jig table to do the drillings.

Thanks Tommyk, I knew nothing of the calibrations of the quick fuel stuff.

                                                                Boilermaster
No jig needed. I clamp the pin drill bit in the vise and chuck the set screw in a cordless drill. I use a tiny bit of silicone grease as lube. Go easy and you shouldn't have a problem. For the metering block you have the existing holes to act as a pilot. The bit will follow. When tapping you want to tap a little and then screw in the set screw to check depth. Rinse and repeat. You don't want to over thread. The set screw needs to "fetch up" on the bottom to hold it securely.

Use the pin drills to qualify the size of the existing pvcr orifice. Drill the new set screw about .010 smaller. That should make a significant difference in WOT AFR and will probably be too lean. Go up .002 at a time until you like what you see on the gauge. It takes some time but you can dial it in perfectly this way.
Finally got to do this modification

The drilling and tapping of the power valve orifices were a breeze, just as you stated.

I ended up with a pvcr of .046'', I was astonished to say the least.

Started @ .060 and worked my way down, almost no change in the a/f with the first two sizes and then she started coming around.

Now when the pedal is mashed in high gear, I am reading 12.4-12.6 afr until the secondary's start to open.

Going to look at the quick fuel adjustable housing for this project.

My car pulls so much better now ! crisper and cleaner for sure.

the carb I am using is 80508 s/ sa and is rather popular for our 351 C engines but in my opinion the pvcr is WAY too large for a MODERATE street build, of course Holley actually recommends a 600 cfm for the 351.

Wonder what the pvcr for their smaller carbs are ?

Completely satisfied with this mod.

               thanks tommyk      

                                     Boilermaster

 
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