idle issues

[wcumagic]

Alrighty then!  Its nice to have a quadrajet only forum with experts about!  I have recently purchased and read Cliffs book, with a few questions yet unanswered!  Here is my setup, more to understand!  (make sure I didnt screw up somewhere)

1996 Lt1 350 with Aluminum heads “Gone through” by Barnes and Reese Racing (everything done) breathing through headers and suckin fuel through a good ol' quadrajet and GM performance intake.  3:73 rearend with a 5 speed totin' around a 1989 camaro Iroc-z.  I have a Comp cam  with duration at .050 @ .218 and .224 with valve lift of .495 and .502, but I think that changed since I have put 1.6 ratio rockers on it since.  I have the factory fuel pump with a nice mallory regulator holdin' it to 6 psi.  With Cliffs book, I have taken my quadrajet ( number 17057230) and done the following mods

idle mixture holes   .090
idle bypass air    .080
idle tube      .037     (pita!!!)
idle down channel    .052   
upper idle air bleed    .070  not in air horn
lower idle air bleed    .077
idle by pass air    .080
no plate holes

fuel inlet       .145   
smaller float installed
main jet       .072
rod.         .042 with .025 tapered tip
secondary hanger   G
secondary rods   CE
longer power piston spring installed (yellow in edelbrock kit)

Questionably, I didnt see mention of actual float height or how to measure it – have since found info and set it at 5/16”

My main question is my idle – it idles well at 1000ish, but every little bit it jumps to 1200 and my vacuum stays about 10 until it jumps and then it jumps to about 12 or 15.  I cannot seem to get the idle screws set right because there doesnt seem to be a sweet spot where the vacuum stops gaining when adjusting it. 

Its possible its because of the throttle shaft bushings, but I had a hard time finding any info on what are acceptable tolerances.  I have enough vertical movement to hear a rattle when I shake it, and enough side to side movement to go about 1/16 or possibly a little more.  Is there a way to test to see if that is indeed what the problem is, or do you see another issue?  I didnt have a problem with finding idle before the rebuild, so thats my reasoning for not replacing the bushings this time through. 

I test drove it today and its nasty, I have a lot more midrange torque and I no longer have traction in first gear.  The secondaries open up much earlier now with no bog, only when I rev the poo out of it.   In other words, previously they opened up near 6000 rpm only, but now with a slow pull from 1k to 6k they didnt open, but a quick jab at 3k made them open and sing their evil song.   I'm concerned about the idle though, and although I love the smell of rich carburetion, I dont want to damage anything by running it too rich or even too lean at idle.  I have the screws set now at 2 turns out per.   Thoughts?

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forgot to mention heads shaved to raise compression as much as Mr Barnes could, so I have to run 93 or better. 

[Cliff Ruggles]

Check the distributor.  It sounds like some of the mechanical advance is coming in, causing idle speed to fluctuate.  The timing needs to be rock solid at idle speed.

I would also think with that engine/cam combo you could easily idle it down to 700-750rpm's......Cliff

[wcumagic]

The mechanical timing doesnt start advancing until 2k to 3k.  12 degrees advance idling (up to 2kish) and upon revving it goes to 36 degrees. 

I could idle down before the carb build, I just havent gotten that far yet.       

So what do you think about acceptable tolerances on the throttle shaft?  Can it run too lean at idle and would that cause this?  Would a small vacuum leak cause fluctuation? 

[omaha]

   I'm thinking that the power piston spring that you put in there is too stiff to keep the rods down wile idling. (it might be overcoming the low vacuum during idle). you might try leaving it out and see how it idles just for a test. or find a really light spring to put in it. (maybe the orig?)
bushings could cause some fluctuation true but, you said that this prob started afdter the build so that kinda negates that theory. I'd still put the bushings in it though, cant hurt. I'd advance it just a bit on the initial timing and see if that has any good effects. If it dont, just put it back where it was. Float level seems ok, Cliff recommend 1/4 but I dont think where you got it will affect the idle much. Small float;did you check it (for leaks) {i am sure you did} Just a possibility (small one). I am not fond of the FI fuel pump of the original z-28. Weren't they like putting out 44 psi? Kinda alot to regulate down to 6 psi (unless you got a real good bypass on it). I think 15 psi pump would be better. Something happened between when it was idiling good and now. so it must be something that was changed or some thing that went bad from then to now. Oh well, just my 2 cents worth. I am not the tuning guru like some of these drag racers like Cliff.      Hope you figure it out.  Until then; have fun anyway!

[Cliff Ruggles]

The distributor advance curve should be steady at idle speed, and start advancing right off idle.  2000 is pretty late and will hurt engine power and throttle response.

Any side play at the primary shaft is NOT a good thing, as it is a vacuum leak, and adds additional air at idle speed, which requires additonal fuel to compensate.  The additional air also increases engine rpm's, all else being equal......Cliff

[wcumagic]

Cliff, I thought about what you said for a few days and decided to throw a timing light on it.  At idle, it was about 20 degrees, which meant you were correct.     I looked back over my paperwork for the dist, and it starts advancing at around 1000 or 1250.  I set the timing back just a hair (from 12ish to 10 degrees advanced), and set the idle down to around 750.  Vacuum, advance (at 10), and idle are now steady.  I also double checked my idle screws and gave them a full 2 turns out, I had them at 1.25 or 1.5 previously.  I appreciate you suggesting I look at the timing.     I could put lighter springs in the mechanical advance for the distributor if I backed the timing back down a little bit at idle, are there any advantages to that? 

omaha, it is the original fuel pump (originally at 45 psi or so) but I have a good mallory regulator on it with the factory fuel return. 

I drove it today and it seems great, but there is a slight bog before it digs and goes.  My choke pulloff is around a second or a little plus, I guess I should tighten the tension spring on the secondaries a little to compensate? 

also, on a side note, if I drove the 70 miles down the road to get a 5 gallon container of race gas, assuming I'd never driven ANYthing on race gas, with the higher compression, would I be happy I did it?  I'd seen something where a hot rod had a 5 gallon fuel cell with race gas in it for when he wanted EXtra on the street.  Are we talkin 3 hp difference or more?   

[Cliff Ruggles]

Good news!  I would NOT use lighter springs.  The timing curve should be rock solid at idle speed, and start right off idle, or apprx 900-1000rpm's for most set-ups.  If all the timing is in by 3000rpm's or so, the curve is fast enough.....Cliff

[kelley555]

Cliff,this is Kelley,I'm going to be ordering my Hei vacumn advance 7000 rpm (with a rev limiter module for my column shifter.(No accidents can happen to good engine encase I have a mishap) Probably the distributor going to come from distributor from Performance Distributor soon for my 415ci. With the engine having 9.5 to 1,a Crane cam hyd . roller .585/.595 lift (with the 1.6 ratio rockers) & 234/242 dur. (maybe a tad  bigger cause the rockers),Qjet 800cfm,3000 stall,350 turbo,3.73 gear,pump gas around a 89 oct.(maybe even premium some times Eventually I'm gonna buy a taller tire than the street M/T Indy Profile's(25")they suppose to have a softer compound,probably a 28" tall tire that will make the engine turn around 2665 rpm at 60mph will be out cruize range of the engine.Do you think that will be to low rpm to cruize around or stay with the shorter tires around + 320 rpm more. If the car idles at around 900-1000 would you have the distributor advancing soon from idle & all in by__rpm ? How much max timing with pump gas? The timing's a little different on your car due to the cr. & your setup but,What  your opinion about timing with these crap gases of today?,I don't want to have a detonation problem so when I order I want to speak with a little knowledge.Later kelley

[Cliff Ruggles]

At 9.5 to 1 compression with aluminum heads and that much cam, I'd expect the power curve to start coming in really strong right around 3000rpms.  It will probably have low vacuum at idle speed, and may be a little "soft" right off idle. 

Crusing rpm's should be OK if over 2500rpm's or so, but it will like to be up around 3000rpm's or a tad higher, IMHO.

The engine will also like a fast timing curve, and all in as soon as possible.  The curve still can't start till the engine is just off idle, or about 1000rpm's or so.  All in by 2500rpms would be a good thing.

Total timing is always on a case by case basis.  I recomend the dyno or dragstrip to find the best total timing setting.

Start out around 28-30 degrees, and make some pulls or runs and watch the power (ET/MPH).  When it stops improving, you have found the best total timing setting.

My 455 at 11 to 1 compression easily manages currently available 89 octane fuel, and actually runs fine on 87 octane.  I still mix in 92 or 93 octane for track outings, makes me sleep better at night!.......Cliff

[wcumagic]

Cliff, it does have a vacuum of around 10 at idle. 

Ben Barnes (who did the heads) suggested the best timing should be around 36 degrees advanced with that cam.  I had backed it down a tad, maybe 34 ish and its happy. 

[Cliff Ruggles]

36 degrees total timing is a "standard" answer for SBC engines.  It really depends on the final quench distance, combustion chamber efficiency, dynamic compression, octane rating(s), etc.

We shoot for about .035" quench distance with engines built here.  To date, with only one exception, all of them have wanted right at 30 degrees total timing for best power, and best ET/MPH at the track.

The one exception was a Pontiac 455 we built with 87cc Edelbrock aluminum heads.  They have "bathtub" chambers, and due to less efficiency that engine wanted right at 37 degrees total timing for best power numbers.

I like to see lower numbers, 28-32 degrees, as it's less negative work and frees up some power.  It also reduces octane requirments, and tendencies for detonation as well........Cliff

[wcumagic]

I'll try your suggestion as soon as it gets above freezing around here, and post my results.  Thanks for your suggestions and help!