Carburetor Jet size

[JimNolan]

Got a question on jet size. I just installed a 390 with C1AE heads, GT intake and 268H comp cam. It's running 9.6 CR and 7.9 DCR. The carburetor I used was a Holley Avenger 570cfm  that I used for years behind a 270HP 390 I had. But, I changed the primary jets to 62 instead of the 54's that came with it. The secondary metering block is a 65 jet equivalent. The question I've got is this: Why does a 600cfm Holley Classic carb come with 65 primary jets and 65 secondary jets and the 570cfm Holley Avenger come with 54 primary jets and 65 secondary equivalency when there is only 30cfm difference between the two carbs.

[My427stang]

Main circuit fuel (boosters) is provided by the main jets and the power valve circuit restrictors (PVCR).  When the power valve is open, both feed fuel.

Think of it as X + Y = total main well fuel.  X is from the jets and Y is from the PVCRs

So, in two carbs the same size, you could have smaller jets and a larger PVCR, or larger jets and smaller PVCR depending on how lean you want cruise mixture.

Now add a smaller venturi on the primary side with a 570, you get more signal, (more airflow around the booster) so THAT requires less jet because it pulls harder on the booster, then potentially combine it with a later carb that uses a leaner cruise and fatter PVCR, jet sizes can be significantly different

Being a little rich on the mains isn't bad though, might take a little power and mileage but helps with octane tolerance.  Not saying it is the right way to do it, but it is a conservative way to tune

[JimNolan]

Thanks Ross, I'll pull the plugs after the parts use to one another and see what the color of them are.

[My427stang]

Jim, it is very rare to be far off the stock setup, especially as far as you were from 54 to 62

When you pull the plugs, realize most driving is a combination of idle, transition and main circuit, combined with the accel pump slamming extra in there.

To read plugs to determine jet size, you almost have to run a full throttle down the quarter, and then shut it down and tow it back.  otherwise all the other circuits influence what you see.

My opinion, if you have to go more than a couple jet sizes fat, you either have a PV timing issue (too low a number delaying it too far), you are bandaiding another problem, either in the carb or otherwise, or you are just overfueling

Keep in mind, that is entirely possible that you would end up bandaiding the carb to make it run decent with Street Avengers though, I have had nothing but disdain for those carbs and how they handle the transition and main circuits, especially in hot environments.  Keep in mind, this is of zero importance on the drag strip or dyno, but in traffic, they can be annoying.

In the end, the issues with Holley SAs and some HP series are best fixed with an earlier style metering block, in fact, if I bolted an SA on now and experienced the common transition stumble, I'd likely just swap without spending too much time on it. 

[e philpott]

agree with Ross , that's way to big of a jump unless you plug off the power valve circuit

[JimNolan]

As you can tell I don't know much about carbs. But, when I first got this carb I ended up making my own accelerator pump cam out of a larger one until I got some stumbling issues fixed. It's got a 8.5 power valve in it and since I'm running 17-18 inches of manifold pressure I figure it is more correct than the 6.5 power valve that comes with the 600 cfm classic. And, since I'll never see 8.5 inches vacuum unless I put my foot to the floor, regular driving and cruising will never use the power valve. (BTW, I have run through the gears up to 4750 rpm and see no hesitations or problems.) If I'm going to have a problem with that big of a jet the spark plugs would just show up as black and sooty telling me to go down in jet size for normal driving wouldn't it.
I'm apparently not getting the difference between a 54 jet and a 65 jet when either carb would be correct for my engine (cfm wise)  other than supplying too much gas during normal driving when the power valve isn't kicked in during normal driving anyway.

[chris401]

Ross,
On a Street Advenger. What metering block numbers do I look for? I have a pair of 7255 43201 blocks from a 10 year old 600. Are these ideal for a 2 to 3 year old Avenger? I don't know anything other than its a 570 and a couple years old.

[machoneman]

As you can tell I don't know much about carbs. But, when I first got this carb I ended up making my own accelerator pump cam out of a larger one until I got some stumbling issues fixed. It's got a 8.5 power valve in it and since I'm running 17-18 inches of manifold pressure I figure it is more correct than the 6.5 power valve that comes with the 600 cfm classic. And, since I'll never see 8.5 inches vacuum unless I put my foot to the floor, regular driving and cruising will never use the power valve. (BTW, I have run through the gears up to 4750 rpm and see no hesitations or problems.) If I'm going to have a problem with that big of a jet the spark plugs would just show up as black and sooty telling me to go down in jet size for normal driving wouldn't it.
I'm apparently not getting the difference between a 54 jet and a 65 jet when either carb would be correct for my engine (cfm wise)  other than supplying too much gas during normal driving when the power valve isn't kicked in during normal driving anyway.

Well, My427Stang actually answered the 'why' question already:

So, in two carbs the same size, you could have smaller jets and a larger PVCR, or larger jets and smaller PVCR depending on how lean you want cruise mixture.

It's the size of the PVCR's that directly affects one carbs ability to run smaller or larger main jets. Likely your Avenger has larger primary side PVCR's than a comparable Holley of the same cfm.

Go back to you original post where you have to rig the accelerator pump to eliminate some transition issues. I'll suspect that out of the box the Avenger obviously did not work well. Having read lots of complaints on other sites of the Avenger's issues, perhaps you can borrow a know good and running Holley carb to merely test how it runs in comparison. I think you'd be pleasantly surprised.   

[Katz427]

Ross is correct about reading plugs. One really needs to run it/shut it off and pull the plugs and read them looking at the base of the insulator / these days I need a magnifying glass! I instrumented mine with a FAST air/fuel O2 sensor and a vacuum gage. It helps with tip in and setting up the cruise. I questioned a Holley rebuilder and he felt at that time that the " classics" tend to be a little richer than the street avenger series the way Holley was calibrating them. If this holds true for everyone ??
The hard part for me was getting the power valve right for the jets size so that it was not over rich at wide open and still have a 14.2-14.5 afr at light cruise. I don't consider myself skilled at tuning air bleeds, I leave that to those with experience.
From your comments on driving you're probably not that far off.

[JimNolan]

I just got through putting the 54 jets back in the carb and installing new 8.5 power valve it came with. One of the reasons I liked this carb is the 21 mpg. I will not go against experience. Seems to idle good at 750rpm. 17 inches manifold pressure.

[JimNolan]

Well, I took it cruising and found it acted like it was hesitating constantly (just felt rough, if you know what I mean).  This was going about 55 mph constant in 5th gear. This carb was bought for a 270 HP 390 completely stock except for a 4 barrel instead of the 2 barrel and back then it didn't have headers either.
I tried a Holley classic 600cfm on that same motor with the 65 primary jets and it left my plugs black and a sooty tailpipe. That's why I bought this street avenger 570 carb. From what I'm seeing now I think Katz427 is right, the 62's felt better in this application, ran good, no hesitations etc. I've got some 56 and 59 jets also. If it doesn't smooth out soon I'll go with the 59's and see how it feels.
BTW: I wish I could put a video of the engine and sound coming out the tailpipes. It idles really smooth at 750 rpm and the pipes sound like some of these big cam Chevy's with the expensive mufflers. I put the initial timing at 14 degrees.

[My427stang]

Jim, rule 1, do what runs well.

However, you are focusing on jets like they match the engine.  Jets match the airflow of the carb

Think air + fuel...a smaller carb provides less air so therefore it needs less fuel, additionally, as the air gets faster, it sucks harder (more pressure drop) under the booster, so a smaller carb may need even LESS jet than would be expected.

My hunch is that you are fighting the common Avenger lean emulsion circuit based on the accel pump and jetting needs.  If you are happy with the way you had it, run it, it likely won't hurt it

Chris - Look for any of the metering blocks with a low IFR restrictor.  The later ones have a higher one and usually are the culprits.  See the horrible pic below.  The top block is the bad style, notice the IFR location, as well as the different style main well design.  I think the issue is with both, the IFR location, especially when hot, seems to affect transition circuit as it climbs ONTO the main, and the main well bleeds seem to delay the mains or let them stutter at low flow. 

Keep in mind, when swapping, you have to match IFR size to the original carb and the same with PVCRs (either that or calculate total PVCR + jet area and then compensate one for the other)  often easier just to match the original

[chris401]

I do have older carbs apart in a box. The 600 is from 94 and I think I have a 390 with a metering block from February 96. I think I'll stick to the traditional Holley that I halfway undetstand. It seems to be common as you said for Street Avenger to have a rough transition and lean issues. You think the boosters too weak to pull hard with low air flow? Seems if the restrictor worked with some preasure against them all those years they must have had a good reason or theory to move them up.

[JimNolan]

Ross,
 Don't know what I did today, but reset idle, adjusted accelerator pump gap again, took it out on the highway and it is smooth now. When you jump on the gas from a slight roll it lights up the tires immediately. Cruises smoothly now and you can't feel the secondary kick in. I'm done. 54 jets seem OK. I'll keep a check the plugs soon, as long as they aren't white or black I'll be happy.

[66FAIRLANE]

Hmmm....I reckon my street HP has the IFR's down the bottom.