Holley metering blocks. Compared a few.

[Bentworker]

I am making baby steps tuning my car.  Carburetor is a 15 year old Barry Grant Speed Demon 750CFM.  I have been beating my head against the wall trying to lean it out at idle, but maintain good manners.  If it is rich at idle the manners are great, if I lean it out I get a stumble rolling into the throttle.  I used a more aggressive accelerator pump cam and it improved a bit.  I decided to try some different metering blocks, so I could have an adjustable idle feed restrictor and rule out any issues with the old Barry Grant ones (they were known for some QC issues before they closed up in 08').  Oddly enough all the metering blocks were pretty similar when it came to Idle feed restrictor and power valve channel restrictor.  Where the blocks varied (other than ability to swap jet sizes) was the emulsion bleed hole size / quantity.  Figured I'd share the as-found measurements on the AED and Proform metering blocks along with one off an OLD street demon.

Proform - Nice machining - uses 6-32 jets for the emulsion, power valve and idle feed.  The emulsion has 8 ports total, 2 blocked off as shipped, remaining 6 are .028".  Idle restriction is .035"  Power valve restriction is .055"

AED - Nice machining - uses brass 8-32 X 3/16" set screws as jets for the idle feed.  The set screws are much cheaper than the 6-32 jets which is kind of cool.  Emulsions and power valve holes are not adjustable, but the way the block is machined you could tap the emulsions and block them off with a set screw if it was desired to block or change the size of it.  The emulsion has 10 ports total, all about .027".  Idle restrictor is .035".  Power valve restriction is .052"

Crusty old zinc Barry Grant block- machining is not fantastic, idle restrictor holes are pretty rough.  Nothing is adjustable.  6 emulsions that are .031" in diameter.  Idle restrictor is .033" Power valve is .058"  

I plan on giving the AED blocks a try and will report back on what I find.  

 

[Stanglover]

Bentworker, that is some interesting info. My knowledge on carbs is somewhat limited and this helps me understand the internal function better.

As I'm sure you've read before, with the Holley Street Avenger 670 I have on my car, I had a horrible over rich situation at idle plus a stumble off idle. I took the car to a tuning specialist and he drilled a 3/32" hole in each primary throttle plate (front and center) and that cured the over rich. To stop the stumble, the "squirter" was changed from a 31 to a 35 and that fixed that issue. Combined with the best distributor curve I could achieve, the car pulls like a train and no horrible gas smell. Pre-drilled throttle plates are available from Holley, so it's nothing new.

 

[Bentworker]

Most of the Barry Grant carbs have the "Idle-eze" feature which is an adjustable air bypass.  It is like drilling holes in the butterflies, but is adjustable.  You pull out the 5/16" air cleaner stud and use a screwdriver to adjust this big needle like assembly that is in the center of the base plate.  There are 4 channels from each of the carb bores on both sides of the butterfly that are plumbed to this adjustable needle valve.  It is kind of cool, I took the carb apart to see how the heck it worked.  I was about ready to drill the butterflies as I have a 279/289 duration camshaft on my 379" Cleveland.  Luckily I figured out the Idle-Eze before I broke out the drill.  I was able get satisfactory results with about 3 turns on the "idle-eze".  There is some serious black magic to this whole carburetor thing!

 

[Stanglover]

Most of the Barry Grant carbs have the "Idle-eze" feature which is an adjustable air bypass.  It is like drilling holes in the butterflies, but is adjustable.  You pull out the 5/16" air cleaner stud and use a screwdriver to adjust this big needle like assembly that is in the center of the base plate.  There are 4 channels from each of the carb bores on both sides of the butterfly that are plumbed to this adjustable needle valve.  It is kind of cool, I took the carb apart to see how the heck it worked.  I was about ready to drill the butterflies as I have a 279/289 duration camshaft on my 379" Cleveland.  Luckily I figured out the Idle-Eze before I broke out the drill.  I was able get satisfactory results with about 3 turns on the "idle-eze".  There is some serious black magic to this whole carburetor thing!

View attachment 52964

Now, that's interesting. Someone was thinking out of the box for sure.

 

[droptop73]

On my Holley vacuum secondary "Frankencarb" (600 main body with 750 base plate and dual 750 metering blocks. I took a ton of fuel away from the idle circuit. I drilled and tapped the air bleeds and fuel restrictors for 8-32 set screws and made a fixture to hold them to drill on my drill press. The idle fuel is drilled big enough to start taking out the hex in the screw and the fuel restrictor is down about .010 from the original Holley. It's been quite a while since working on it and I don't remember specific sizes but I've acquired quite a selection of wire size drills, LOL.  Since I'm running a manual trans any stumble is minimized but I compensated for it with increasing the squirter size and changing pump cams. I pretty much left the main circuit air bleed the original size and only adjusted the main jet size. I've jetted the carb to be pretty lean at cruise as well. 

 

[Bentworker]

I'm making progress.  

I was able to get my idle leaned out a bit to where your eyes don't burn standing behind the car.  I took it out for a rip with the AED blocks.  I left them alone except for making a new set of idle fuel restrictors that were .031"  Idle air bleeds are now .076".  The idle mixture screws now function how one would expect them to.  

I was so convinced that the car was running fat at WOT that I jetted it down.  After rolling around with an AFR meter on the car I realized that it was not the case.  My vacuum is slightly better than I thought it would be, so I am going to go from a 4.5" power valve to a 5.5" power valve, along with bumping the jet sizes up.  Once the car cools down I'll play "lets spill gasoline" and swap the jets.  It is getting real close - runs great cruising at an economical AFR, idles at a reasonable AFR, and drivability is excellent.  I matted the go pedal a few times and was ecstatic with how it behaves!

 

[c9zx]

Glad you are making progress. I like the AED carbs. What kind of AFR readings are you getting? Chuck

 

[Bentworker]

Glad you are making progress. I like the AED carbs. What kind of AFR readings are you getting? Chuck

At the moment it is about 12.5:1 - 13:1 at idle.  Low speed cruise (30-40mph) varies 14:1-16:1.  Highway cruise (70mph) is pretty fat at around 12.5:1 again. WOT is about 11:1 - 12:1.

My WOT was about 14:1 and I jumped a few main jet sizes, I plan on splitting the difference between where I was and where I am.  Making small steps towards it running better.  Honestly putting an AFR meter on it was an excellent thing to do.  It is so much easier than basing decisions off of plug reads alone.

 

[boilermaster]

Bentworker,

I would want to where my secondary throttle blades are at when you get that fat 70 mph highway cruise.

And if my power valve is open or closed at 70 mph cruise..

if your power valve is still closed and secondarys  are still closed then smaller primary jet, if power valve is open then reduce power valve restriction .

wot afr should be adjusted with secondary jetting only, but bear in mind that pvcr reduction or primary jetting will have an impact at WOT.

 

[c9zx]

Some AED metering blocks do not have changeable PVCRs. The carb does appear to have changeable air bleeds. The high speed air bleeds are sensitive so make changes in small increments, one or two numbers at a time, larger makes it leaner. The idle air bleeds are not as sensitive. Chuck

 

[Bentworker]

The AED blocks I have don't have adjustable power valve restrictors.  Once I get the carb closer I do have a Proform metering block which does have adjustable PVCR if I want to mess with them.  Hopefully next weekend I'll get some time to play with the carburetor more, go for a drive, and repeat!

 

[boilermaster]

sorry Bentworker,

Forgot which of the blocky you have in there now.

I have always used primary jet for cruise A/F and power valve restriction for jetting when the power valve is open.

you may want to get jetting a little more dialed in before swapping air bleeds.