Carburetor setup for an newly installed HT383

[mkilmer402]

Good evening

This is my first post with this site.  I have read through a large number of the postings, but haven't been able to find an answer.  I recently installed a HT383 GM Performance Crate motor in my '85 C10 Long bed.  Previously, it had the stock 305 that was just tired after 31 years and 140k.  It had a 17085226 that was rebuilt about 3 years ago, and I transferred it over to the HT383.  It has 0.070 main jets, 0.049B primary metering rods, DP secondary rods, and a "N" hangar.

The HT383 is a "truck" engine with a mild cam:
288/208 duration at 0.050",
intake/exhaust lift 196/206
max lift (intake/exhaust) 0.431/0.451

I kept carburetor's configuration the same when I transplanted it over to the HT383.  I reset the idle mixture using about 4.5 turns on each mixture screw and no issues with the idle or transition from the idle circuit to the off idle and primary circuits. The 383 has plenty of vacuum (20in Hg) so didn't require drilling out the idle passages.  The combination ran well (no bogs, lean backfiring or anything), and transitioned over to the secondaries very smoothly.  However, I had a hunch that I was leaving a lot of performance on the table, so I bought Cliff's book, which has been an absolute wealth of knowledge, and changed out the "DP" secondary metering rods with some "DA" that I had from a salvaged carburetor.  I also ground down the secondary air valve stop to allow them to travel to approx 90 degrees (1.27 in from leading edge to back of the secondary bore) and loosened the secondary air horn spring from 7/8 to 1/8.  Holy smokes, this really woke up the truck!!  It has just a puff of black smoke when I floor it and immediately clears up, but the performance is much better than with the previous "DP" metering rods.

I am now thinking that I can probably optimize the primary circuit for snappier performance before the secondaries are activated and was looking for some experienced guidance.  While there are no bogs or hesitation, I am guessing that the 383 will be able to capitalize on more fuel early on than my old, tired 305.

A couple of questions:

1.  What would be your recommendation for a jet/rod combo for the primary side? 

2.  I also have a set of "DU" secondary rods, which are 0.0337" at the tip vice the "DA"'s 0.044" in tip.  Do you think my combination would benefit by switching to these or would that provide too much fuel. 

Appreciate any knowledgeable advice that y'all have.  It goes without saying, after seeing the wealth of information on this site, I would purchase any parts from Cliff.

Thanks again,
Mike

1985 Chevrolet C10
HT383 installed in Sept 2016
3:42 rear end with gov-loc
700R4 with Transgo JR Shift kit, Corvette 2nd gear servo, Bowtie overdrive 40% larger 4th gear billet servo, 2000rpm stall torque converter

[Cliff Ruggles]

"B" series metering rods will not work in that carb number, they used "M" series with .036" tips in the later carbs and they are shorter.

We have much better parts available than 3 years ago, and most likely it doesn't have the correct rebuild parts in it anyhow.

At a minimum I would install one of our kits to get the carb up to par for this new fuel.  We can spec out the correct tuning parts at the same time to set the carb up exactly for what you are doing.....Cliff

[mkilmer402]

Thanks for the correction, Cliff.  The primary metering rods were original to the carburetor, so they must have been the "M" rods. I was reading through Doug Roe's Rochester Carburetors book last, and it had the "B" rods as the latest and greatest.  I think it was written before they came out with the "M" rods.......

I will give you a call here shortly to determine the best path forward.

Thanks!

[Cliff Ruggles]

"B" series were used in early style carburetors and are too long for the later units.

In the 1975 and later two MAB carburetors they used several different style primary rods including K, J, L, and P with .026" tips.

The when the single MAB design came out most used "M" style metering rods, which are also shorter but had .036" tips instead of .026".

None of the early metering rods will work in the later units, but the later SMAB models can be effectively built and set-up with the K, J, L or P shorter primary metering rods.

When you call the shop we'll spec out the best parts to set the carb up exactly for what you are doing.......Cliff

[mkilmer402]

Cliff,
It was great talking to you last week.  It is always good to find a common background when you least expect it!  I received the basic rebuild kit, new accelerator pump, air horn cam and spring, the two PP springs, 74 jets, and threaded APT cap. 

I got some time this weekend to pull the carburetor apart and put in the new parts.  The new parts (I am guessing the longer PP spring made the most), along with the ability to tune the APT without taking the carburetor apart, made a big difference.  I ran out of time for fine tuning, but ended up changing my APT from 3.5 turns out to 4.25.  It runs great, more responsive down low.  I may be able to open the APT a bit more, but will play around with it when I get a little time.

One other question, I currently have a "N" secondary rod hangar with the DA rods.  Would it be beneficial to change to a "G" hangar to or would that provide too much fuel too quickly for my combo?  I am running long tube headers and a dual 2.25in exhaust (forgot to put that in my original post).

[Cliff Ruggles]

DA rods should be fine with the lower hanger.  Do some testing and if you don't get any hesitation/stumble/bog after the tension spring is correctly set, leave the N hanger in place. 

I use an "L" hanger in my carb with the same DA metering rods and it is flawless going into the secondaries.  If I try taller hangers I get a little "burble" out of it on hot summer days from a momentary rich condition. 

Hope this helps some?.....Cliff

[mkilmer402]

It definitely does help, Cliff.  Appreciate the advice.  As an engineer, I like to fiddle with stuff until I break it, then go back and fix it .  I think I'll play with the apt settings a bit more first before changing anything on the secondary side. 
Thanks again!!