Little help here, please (part 2 of 2)

[King Scooter]

Continued from part 1...

Carb #17057204 (750 cfm) with APT

Here's what came with Cliffs SR kit:
Main jets: .070
PMR’s: 44L
SMR’s DA
Orange PP spring
1.5 second choke pull off

I think my combo falls between recipe 2 and 3 so here’s where I’m at, what I’m thinking and what I need help with. Please make suggestions or let me know if I’m off in left field.
Secondary hanger “L”
Air flap open distance: @ 1.28”. Can be opened further if needed
Upper idle air bleeds: @ .068
Lower idle air bleeds: @ .076
Idle mixture screws: Drilled to .094”, no holes in throttle plates
Idle bypass air: @ .081
Secondary POE well restriction: Cleaned/drilled to .040
New Idle tubes: @ .040
Air flap spring: @ 5/8 turn

Here’s where I need help or suggestions.
MAB’s: In main body.  Restrictors are installed.  I believe Cliff suggested .050 in upper and lowers. I’m not questioning Cliff but I may have heard him wrong and they aren’t drilled yet so anything is possible yet.  Does .050 sound right?
Secondary POE restriction: @ .052. Suggestions?
Secondary tube restriction: @ .023.  Thinking .036…Suggestions?
Accelerator pump discharge holes: @ .023. Seems small…Suggestions?
Idle down channel restrictors: Installed but not drilled, thinking .040?
What’s a good starting point for my APT?  It was at 2.25 turns but I never ran this carb so

This is my second q-jet rebuild and I’m actually getting comfortable with most of the terminology and where things are. I’m just a bit confused yet how one thing affects the others. Another 30 years, I may have it.

Thanks
Scott

[novadude]

0.046-0.050 sounds right for the main air bleed restrictors. 

I am running a very similar combo...

355
9.6:1 (12cc dish, 0.045 quench)
63 cc 062 Vortec heads
GMPP low rise dual plane
Howards 270/278, 217/225 @ 0.050", 108 LSA hyd roller

I tried some "unconventional" idle mods that worked very well (17080204).

I blocked the upper idle air bleeds in the main body, and I am running 0.043" upper air bleeds in the air horn.  I peened the lower idle air bleed down to ~0.063", and I am running 0.046" IDCR, and 0.029" Idle tubes, with 0.086" bypass air.  Idle quality and off-idle performance is great on my car.  Very smooth idle with just a hint of lope, despite ~58 deg of overlap with this cam.

70 jets and 44 rods sounds like a good ballpark setting for main jets / rods with upper / lower main air bleeds at 0.046-0.052 range.

[77cruiser]

Still running like a top? Have you change anything since you've built it? Been staying out of trouble?

[novadude]

Still running like a top? Have you change anything since you've built it? Been staying out of trouble?

It's running good.  Been through several iterations of power piston springs, and secondary hangers / rods, trying to get it really dialed in.  I've also experimented a bit with secondary idle air tubes (ended up at 0.028" driven into the airhorn to 1" height like in the Roe book). 

I am currently at 0.043 on the upper airhorn idle air bleeds.  Might try going a little bigger on those (0.046 or larger), as I think it can be leaned out a bit at very light throttle cruise (~30 mph).  It's not bad where it is (low 14s A/F), but I like to play.  haha

[Cliff Ruggles]

"Timing:  17d initial at 800 rpm, 34d total at 2850. Vacuum advance limited to 10d all in at idle. Cliff feels I may have too much final so I will work with this
Vacuum: 14.5” at 850 rpm idle, 12” at 700 rpm curb."

If it takes 27 degrees initial timing to get 12" vacuum at 700rpm's the engine is telling you that it could have used another full point of compression and/or the cam moved out to a wider LSA.

For some reason that I'll never understand is running mid-9's for compression then installing a cam on a really tight LSA.  Tight LSA narrows the power curve, pulls power down in the RPM range, peak VE occurs earlier, and spikes cylinder pressure pretty high lower in the RPM range at the same time. 

A good rule of thumb for compression vs cam events is that your engine build should produce good vacuum at idle without a lot of timing in it.  I like to see around 13-14" vacuum at 750rpm's with about 10-12 degrees initial timing.  This tells me the engine is pretty happy with the cam choice and I always use higher compression and longer seat timing so the same engine has strong upper mid-range and top end power but still idles well and efficient in the normal driving range.

Anyhow, I'm not surprised in the least that a 350 engine build with 9.6 compression and a decent size cam on a 108 or 109LSA is going to like, want, need and respond well to a LOT of timing at idle plus require good idle fuel delivery to the mixture screws at relatively "low" vacuum. 

That's just the way it is with these things and why I provide "recipes" to help get carburetors up to par for what they are being used on.

If you really want to help out a 17057204 or any other Chevy Q-jet from the mid to late 1970's reduce the size of the .078" lower IAB's to .063-.067" before you do anything else.  That is the first vent point above the idle mixtures screws and once you size them down EVERYTHING gets better and you'll find you don't need large IFR's, or DCR's either.......

[novadude]

Cliff... My 9.6:1 355 with a 270/278, 217/225 @ 0.050", 108 LSA Hyd roller cam idles at 750 rpm with ~15.5" vacuum.  I am running 12 deg initial with the vacuum advance on ported.  Nice broad powerband with a ton of midrange torque (i shift it around 5500-5800 rpm).  I can leave it in 4th gear (Muncie) going around corners in the neighborhood and can easily accelerate from ~1000 rpm without downshifting with no lugging, bucking, etc.  I think the short duration is key. 

I definitely agree with you on shrinking the lower idle air bleed on these late 70s Chevy carbs.  Seems to make a nice improvement, and as you said, you can run much smaller idle tubes & DCRs.

[King Scooter]

Novadude, uncanny how similar our builds are. Makes me think there is more vacuum to be had. Thanks for the input.

Cliff, duly noted and I agree with everything you say.  Hindsight being 20/20 I would do a few things different.  Now I'm just working with what I have, to get what I can.

You sent me some good distributor/timing info and I will be doing some work on that once the car is back on the road. Maybe with some distributor and carb tuning, I can find some more vacuum.

I'll resize my  lower IABs to .063-.067 and then follow recipe 2. I can always drill bigger, but it's hard to drill smaller!

Thanks

[Cliff Ruggles]

"Cliff... My 9.6:1 355 with a 270/278, 217/225 @ 0.050", 108 LSA Hyd roller cam idles at 750 rpm with ~15.5" vacuum."

For sure short seat timing saves the day here.  Head flow and ICL are big players here as well, plus a few other things like combustion chamber shape and quench distance.

You could have but another full point of compression in that build, 15-20 degrees more seat timing, wider LSA and it would have idled equally as well and made more power at every RPM and not needed any more octane to be happy......

[novadude]

"Cliff... My 9.6:1 355 with a 270/278, 217/225 @ 0.050", 108 LSA Hyd roller cam idles at 750 rpm with ~15.5" vacuum."

For sure short seat timing saves the day here.  Head flow and ICL are big players here as well, plus a few other things like combustion chamber shape and quench distance.

You could have but another full point of compression in that build, 15-20 degrees more seat timing, wider LSA and it would have idled equally as well and made more power at every RPM and not needed any more octane to be happy......

I have a Muncie M20 and 3.36 gears.  I was interested in maximum torque in the 3000-4500 range, since that is where the engine spends most of it's time in a 1/4 mile pass.  Tight LSA cams seem to be great for pumping up the midrange torque.

It works very well (12.9x @ 108 mph with a poor 2.0 60 ft in a Chevy II), gets great gas mileage, and with the lower compression it even tolerates lower octane pump gas (tight quench).  All-in-all, I am very happy with the combination.

[Cliff Ruggles]

I'm sure it works well.

Tight LSA narrows the power curve and pushes peak torque higher and earlier.  Both recipes for detonation so for sure you have to keep static compression down to be safe.

You can take the same build, add another full point of compression, another ten degrees of duration, wider LSA, a little later intake closing and it will idle equally as good if not better, and make more power at every RPM with more upper mid-range and top end power.

I'll attach overlapping dyno sheets of how this works.  The short version of the long story is that my customer and his engine builder weren't overly happy with the power of the "new" 455 build they had on the dyno.  The were using a Comp XR276HR cam, 224/230/110LSA.  They said it had a noticeable idle, but quit early and only around 425HP.  With the professionally ported head they were expecting around 480-490hp.

Of course I got the call because EVERYONE at the dyno shop right down to the guy taking out the trash was blaming the Q-jet.  Instead of talking about carbs I supplied cam specs for a better cam instead.  289/308 @ .006" and 236/246 @ .050" on a 114LSA. 

Reluctant at first but the engine builder called Comp and order the cam and installed it.  I got a call back and they sent the dyno charts below......