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Little help here, please (part 2 of 2)

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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:

--- Quote from: 77cruiser on April 22, 2024, 09:33:55 AM ---Still running like a top? Have you change anything since you've built it? Been staying out of trouble?

--- End quote ---

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.......

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