Holley Tuning. When to mess with air bleeds instead of jets
 

Im working on tuning a Holley. Im currently using a nice 850 double pumper that I inherited with the engine. Its not an off the shelf unit, has billet metering blocks.

Runs and idles fine. But holy hell is it pig rich at part throttle cruise. Once it warms up light throttle cruise is like 10.8-11.9 on the wide band. Idle I have tuned the mix screws to where it idles at like mid 13s, but the idle screws are only like 3/4 of a turn out. Any more and idle gets rich as well.

It had 78s in the primaries and a 6.5 power valve. I put in 76s, and a 4.5 (just to see on the PV) The 76s helped some, but it only got me into the solid 11s, AFR which is still rich.

This is a big engine, so Im skeptical of continuing to drop down in jet size, because I think its just going to lean out WOT. Would a 500 ci 700 HP engine really only want a primary jet size in the low 70s? Im thinking its maybe time to consider drilling the idle air bleeds to a larger size to try and lean out light throttle?

What say the group? In reality, rather than drilling on this 850 I think Im going to throw on a 930 annular booster HP body I have because it has screw in air bleeds I can play with. But I still think the answer is going to be air bleed related rather than jets.


As far as WOT, I cant fully test that, because Im fairly certain I would vaporize the bone stock unknown miles t56. But I can say that if you have your foot in it accelerating, the AFR is in a more accetable range than sustained cruise.

Does it have a rear power valve?
Also what jet in rear?


:confused:

What is your level of manifold vacuum when you see signs of it going richer then you want?

Are you running vacuum advance I hope?

Are you running a single plane intake?

I think the secondaries were 80s or maybe 84s. I would have to go back and look. I dont remember if there was a PV in the secondaries. Im going to pull it off tomorrow to put on the annular booster carb, and can check the 850. I didnt think those circuits would have an impact on the light throttle cruise issue. I like having the electric choke and fast idle the 850 has over the HP carb, but I also generally like annular boosters.

At idle the vacuum ive seen when doing the mixture screws is like 12-13 inHG (so a 6.5 PV is likely right). I dont have a real time reading so Im not sure what it is under load going down the road. Single plane with a 1 inch standard (non-tapered) 4 hole. I thought the spacer might help lean it out some, but not really.

It has a progression ignition distrubutor with the vacuum hooked up. Pretty conservative tune in it right now. Timing all in at 2800. So maybe speeding that up would help. 16-32 degrees with 8 degrees of vacuum added.

Only vaccuum ports hooked up are distributor and PCV valve.

Edit: I think maybe I can get a real time vacuum reading from my phone with the PI distributor. So I can probably come back with that information as well.

With that rear jets, probably no power valve.
The power valve would not activate until a certain vacuum reading, with it blocked all the main jet is in use even at part throttle. (secondaries start opening)

Most of those carbs were used mainly for full throttle use (racing) so cruise was not a factor.

:)

With those billet blocks I would check that all the needed passages are drilled out right ( completely) and that your gasket matches 100% right.

If that 930 came fitted with those booster then it’s likely to be better calibrated for your needs.

In terms of the 850 you might try bolting on stock non billet blocks and see what happens then.

I had much the same problem with a Demon 825. The metering blocks had 4 emulsion holes each side at .031 . I have a 3310 I took apart to use as a template to adjust the 825. 2 emulsion holes at .028 (.026) each. I modded the 825 blocks to match as best I could. I blocked 2 holes, drilled out the other two and installed set screws drilled to .028 (maybe .026). I also made adjustments to the IFR's, IAB"s, MAB's . Steady cruise at 2.5 to 3 k is in the 14's.

What sizes are your air bleeds, IFR, and so on, Basically every metering orifice air and fuel..... A red flag may be in those results..

Maybe 1 block at a time, and check AFR. Pri or Sec 1st i dunno.

I put the 930 on. It was rich too but had a little more room for adjustment in terms of the idle adjustment screws.

It has 77s - 85s in it.

Oddly the 930 is lean as hell when you bend into it. I checked and the squirters are firing but maybe they are out of adjustment and not hitting fast enough.

This was a carb I had built for a BBC maybe 4 years ago all out of QFT parts. I only used it to fire the engine up. But, admittedly it was an eBay builder. So he might have been just fine or he might have been trash. Idk. It has holes drilled in the butterflies which I don’t like.

When it comes to carbs I’m on the curve of smart/stupid right in that area where I can say I know enough to know that I don’t know very much. Which is just after the idiot who thinks he knows everything.

I did take a snapshot of my so show some top level criteria of what the cars doing when it’s running like 10.9-11.7 on the AFR

Don't know if it will affect your afr, but you could use much more timing at that high-vacuum part-throttle cruise situation, (17" vac, 1900 rpm). I'm at 40-42 degrees at that rpm/42kpa vacuum

At what RPM is the 850 pig rich? The idle system is still supplying a lot of fuel below 2500 RPM. Go down on the Idle Feed Restrictions, not the main jets. What size are the HSAB and LSABs now? Don't drill them out.

The IFRs is what is controlling part-throttle fueling before and as the mains come on. Go down about 10% on the jet area of the IFRs and see if it leans out at cruise. If that works, keep reducing the IFRs until it's almost too lean from 1300-2200 RPM. If it's still rich from 2200-3000, you'll need to install transfer slot restrictors to control the top of the transfer slot as the mains pull over. Sometimes the combination of the two is too rich. If the low speed mixture is good, and above 3500 and WOT is good, its the overlap that is the problem. That is when TSRs are required.

Set the fuel level and fuel pressure so that won't vary. Then change one thing at a time to clean it up from idle to WOT. Most of the modern holleys and billet blocks have a super-rich calibration out of the box. Reducing IFRs, relocating them to the lower position and going back to the classic holley emulsion stack usually cleans them up a lot. I learned this cleaning up a 750 doublepump QuickFuel on my 400. Tom V, Kenth and a couple other guys were big helps, as I'm a Q-jet guy.

Is it a 4-corner or 2-corner carb?

Yeah that’s on my to do list today. Fab up an alternator support bracket and put in a more aggressive timing table.

Progression Ignition has a sample table for a big block with mild cam and aluminum beads. I’ll try that one.

Both of the carbs in question are 4 corner.

OK, usually 4-corner carbs have idle circuits sized the same on primary and secondary. So the IFRs, airbleeds, mixture screw settings, everything should be sized the same. 4-corner mixture screws usually end up 1/2 to 1 turn from seated.

Except on the secondaries, the butterflies should be 1/2 turn from closed with no T-slot showing. At least as a baseline. Set the primaries to show .020-.040 T-slot and then set the curb idle by opening the secondaries just enough to get it to idle. That's usually a good starting point on a street/strip engine.

So the 850 DP I thought was a custom job. But it’s not. It’s this bad boy.

https://www.holley.com/products/fuel...arts/0-76850HB

But someone has still clearly monkeyed with it a little. Because it had 78-80 instead of 80-80 jets.

And the squirters are 32 primary/31 secondary rather than dual 31.

And has a rear power valve?


Choke hooked up?


:confused:

Yes to both.

I do think I’m going to go back to the 850 and try to lean in out at cruise a little more. Because it at least transitioned to the secondaries better I think.

The bigger carb was also rich at cruise but wanted to lean pop when you laid into it.

I have some new bowl gaskets coming. Side note. Don’t buy cheap “QFT” branded gaskets on Amazon that aren’t shipped and sold from a name you know. They are fake. I had them in that 930 for a day. Took the bowl off to change a jet size and they were disentigrating.