Baseline recommendations

[Sinatra]

I'm hoping someone will be willing to help with baseline settings on my Holley 9381 carb (850DP, annular). I'm not new to carb tuning, but I'm out of my element here. My previous tuning was largely with ThermoQuads, Q-Jets, and Chrysler Six Pack setups. This particular engine is also significantly more modified than previous efforts, putting me further from my element.

The engine is a Chrysler 340, 10.5:1 compression. The heads are heavily-ported W2 with oddball "rectangle" intake ports, 2.06"/1.60" valves. The camshaft is a Lunati custom solid roller, 254°/262° @ .050" with .582" lift on both sides. Rocker arms are 1.5:1 ratio. The intake is a Victor W2 and the headers are TTi 1.75"/1.875" step. The car is a 1969 Plymouth Valiant, about 3,150 with me in it, with a 4-speed and 4.10 gears. Lunati recommended a "starting" shift point around 7,500RPM, but it might like higher... hence the potentially oversized carb. Last year, when I initially got the car running It ran unbelievably rich and the idle mixture screws had little, if any, effect. Based on an excellent thread about 4150 metering on another site, I joined there and asked jmarkaudio specifically about the kill bleeds. He suggested checking out this site after answering my primary question.

Digging into the carb I found an amalgamation of other carbs. Wrong baseplate, Demon secondary block, etc. Nothing I did seemed to have any effect, so I decided to get a new baseplate and metering blocks, with the expectation that I'd be drilling and tapping for replaceable bleeds, etc. I was pleasantly surprised to find out that the metering blocks and baseplate were "old school" calibrated (approximately OE 3310), which seemed to be the hot ticket in the thread I'd repeatedly read. Jetting was 72/80, the primary PV was a 2.5" 4-window. The secondary had no provision for a power valve.

Here's what my current metering layout looks like with the unmodified 9381 main body and new metering blocks:
IAB: .068" primary/.070" secondary.
HSAB: .028"/.026"
IFR: .032"/.033", low position
E-bleeds: .027"/.028" , only 1 pair per block in typical OE-carb low position, which I think is 2nd from bottom on a 4/5-bleed block.
Kill: .028"/.028"
PVCR: .055"/blocked (the new secondary block does have provisions for one but it's corked)
Jets: 72/80 (I never messed with them; I couldn't even make it idle last year)
If memory serves, it had a blue primary and pink secondary cam, but I never got far enough to worry about them.
The transfer slots in the baseplate also appear to be old school "production 3310" dimensions, although I haven't measured them.

The car is dual-purpose. I have every intention of flinging it down the quarter this season, but it does have plates on it and will be street-driven. I realize it'll never get 20MPG but driving around at 10:1 AFR seems daft. By the time I got the new parts and the tools to modify them, it was time to store the car for the winter so I just walked away. Now I've got to walk back, and I'd like to be better-informed and less flustered this go-round.

If anyone has any thoughts on jetting or ideas on modifications I should make while the carb's still apart, I'm all ears. By no means am I asking for a perfect tune, just a good starting point... sort of, "here's where I'd start."

Thanks in advance for any thoughts you might have.

[Shorty Medlock]

Welcome from another newbie here... I'm in about the same deal with my 850DP for alky. Got a carb in a trade and it's not quite right so I'm out to get it right.

[GTO Geoff]

Backward step with your combo getting rid of/not using the TQ.

Recent combo tested brand H [ or clone, cannot remember ], a QJ & a TQ, made just under 600 hp with the H. TQ was 5 hp behind. The TQ has numerous calibration 'tricks' that cannot be done with the H: change sec blade angle at WOT; modify air valve shape to that of Comp Series TQ; remove the choke shaft across the AV & block the hole; use a different AV dashpot; modify the sec discharge tubes to mimic those of the Comp Series TQ.

Had these mods been done, it is more than likely the missing 5 hp could be found...& exceeded.

[gntkllr]

In somewhat comparison, original 3310, 3310-1/2/etc, have #72 PMJ's and 0.063" PVCR's, and 1-3/8" primary (1-7/16" secondary) venturi diameters, and 1-11/16" bores. Your 9831, listed as 830cfm, has 1-9/16" venturi diameters and 1-11/16" bores... listed originally coming with #78 jets all around, and PV's front and rear...

From the book How To Build And Power Tune Holley Carburetors (note List 9381 about 3/4 down the list):


I think I'd (make adjustable and) go with 0.070" primary PVCR's and #78 PMJ's for starters, and no secondary PV and #88 SMJ's for starters... and at least a 6.5"Hg PV... the rest of your calibration looks good... might like 0.033" PIFR's too though with the rowdy camshaft...

As for fuel economy, stock, mild, or wild, there's no reason good numbers can't be had, with the majority of the super-tuning for that being done with primary transition tuning (ideally with (made-adjustable, 8-32 or 10-32) PIAB size)... but for deeper open throttle cruise situations, PMJ size can decrease to what the combination will tolerate, with a corresponding increase in primary PVCR size...

[Sinatra]

[Sinatra]

[Sinatra]

I've been messing with the carb today, solely trying to get the idle mixture sorted. The carb is set up exactly per my original baseline, along with gntkllr's suggestions. Engine was at 180 before I started messing with anything.
It's quite a bit better than it was last year with the mishmash carburetors, but it's still not right. Too fat. I'm pretty sure the idle screw is right on the borderline of the cruise circuit at the moment. It jumps to 1,000+ with about 1/16th turn of the idle screw; staying just below that, I can get it to idle around 750. There is no "in between". Back it off an equal amount further and the engine dies.
I wouldn't think the camshaft is so obnoxious that this engine would only pull 5" of vacuum, but that's all I can get. Idle mixture screws seem to have very little effect. Closing them kills the engine, as does opening them way up, but just about anywhere in between, the vacuum gauge--which is a little twitchy at that RPM--stays right around 5". I did try messing with the secondary-side idle screw just a scosche, maybe half a turn in from contact to avoid exposing the secondary transfer slot. It didn't seem to make a difference so I backed it off completely again. With the screws about 5/8 of a turn out I pulled the vacuum gauge off its hose. The idle didn't change. Previously it had risen, so that appears to be its happy spot... but it's not that happy. Cleaner than last year, but still quite rich. Float level is "just dribbling" with the sight plugs out F/R. Am I missing something? Does it come down to drilling holes in the throttle plates, or ?

It comes off idle cleanly and makes RPM well. On the way back down to idle, it hangs at 1K for about 10 seconds before slowly dropping back to an enormously choppy 750ish. It seems to be that, too high--enough to vacuum-advance signal--or a stall. Utimately I'd like to get a clean 850RPM idle out of this thing but I'm starting to think it's not possible. Back to the 4780 main body? Try out this ancient Bo Laws DP that came with a different (solid-cam) W2 340 I bought?

[gntkllr]

Sounds like opening the secondary throttle plates should be a help, and closing the primaries some if they're open to the "cruise" area, the upper primary transition...

There should be room to increase PIAB's up from 0.068" as well...

Both of those ^ ^ moves should allow some more idle airflow and some more turns out of the idle mixture screws and some more sensitivity to them...

Your last paragraph sounds like the effects of a sticky-in-the-bores near/at idle when decelerating to it's secondary idle setting... see sentence #1 ^...

[Sinatra]

It does the "hang at 1K" thing whether I engage the secondaries or not during revving. If I manually pull the throttle lever forward, no change--it still does it. The baseplate is brand new and I didn't notice any stiction during inspection or assembly... but I will definitely check for it again.
I'm hesitant to mess with the secondary idle screw much (even though I installed a top-side one for just that reason) because I fear engaging the secondary's T-slots and making things worse... but I do agree it seems like it needs more air.
Safe to assume I need to go "baby steps" on the PIAB, like .070" as a first step?

[Right hand drive]

What happens when you depress acc pump arm giving it a little squirt without giving it any throttle at idle?