Idle Circuit Recipie Problems

[Novarolla05]

Hi all,

I'm pretty new to Quadrajets, but not carbs in general. I have a 17059201 Quad that was on a chevy 350 I bought from a friend. It ran perfect, but the cam was a bit much for a daily driver (my friend can't find the specs on the cam he put in it unfortunately). I swapped cams for a milder one, but now I can't get the carb dialed in. The idle is horrible even after rebuilding and modifying the idle circuit per Cliff's first recipe. It seems the idle is way too lean. Here's what I have:

79 chevy truck 350
stock heads/rockers, about 8.5:1 compression
Edelbrock Performer intake
Comp XE256H cam 212/218 @ 0.50   LSA 110

Right now, I have the carb idle circuit for recipe #2 and it still seems lean, and has bad off idle hesitation.

Idle Tube .038
Idle Down Channel .055
Upper Idle Bleed .067
Lower Idle Bleed .070
Single Main Bleed .050
72/52M

I'm just confused on what to try next. Any help would be greatly appreciated.

Thanks,
Richie

[Shark Racer]

FWIW, I built a 17058228 for my 78 Corvette with a 216/216-110 cam. I'm pretty sure I had less static CR than you. I had to go even heavier on the idle DCR. I ended up doing 038/059, this got me just a bit too rich(mid 13s AFR). If I did it again, I'd do 038/057. (there's a metric size, (1.45mm, that ends up at .0570).

[Novarolla05]

Thanks for the reply.

I'll try opening up the restriction more tonight and see what happens. I'm trying to get the best mpg possible so maybe I've been playing on the lean side too much. Would making one or both of the idle bleeds smaller give the same results?

Thanks,
Richie

[Novarolla05]

No luck so far. I've been battling this for several months now, and honestly its getting hard to not just put on the Holley I have laying around  I don't really want to admit defeat though. I set the idle mixture for highest vacuum, and did the tip in procedure for the APT. The hesitation is much less noticable but still there. Here's what I changed today

Idle Restriction .059
Upper Idle Bleed .070 (I was wrong on my first post  )
Unplugged idle bypass air .055 (Just something else I felt like trying)

It only stumbles if I ease away from a dead stop. If I hammer on it, it doesn't miss a beat.

Any thoughts?
Richie

[TommyK]

No luck so far. I've been battling this for several months now, and honestly its getting hard to not just put on the Holley I have laying around  I don't really want to admit defeat though. I set the idle mixture for highest vacuum, and did the tip in procedure for the APT. The hesitation is much less noticable but still there. Here's what I changed today

Idle Restriction .059
Upper Idle Bleed .070 (I was wrong on my first post  )
Unplugged idle bypass air .055 (Just something else I felt like trying)

It only stumbles if I ease away from a dead stop. If I hammer on it, it doesn't miss a beat.

Any thoughts?
Richie

What size are the idle mixture screw holes?

[Novarolla05]

What size are the idle mixture screw holes?

Mixture screw holes are .089

[TommyK]

Mixture screw holes are .089

The 72 jet and 52M rod equal a very lean .00195  metering area. Does this carb have APT? If it does have you tried adjusting it to see if the idle improves?

[Novarolla05]

The 72 jet and 52M rod equal a very lean .00195  metering area. Does this carb have APT? If it does have you tried adjusting it to see if the idle improves?

It has an APT. I guess I forgot to mention when I increased the main bleed size, I also put in 74/52M. My bad. I can screw the APT out almost all the way, and there's no difference.

Today I went up to .064 on the idle channel which didn't do anything noticable. I tried driving with the accel pump diconnected, and there was no change in behavior. Seems to me that would mean its too rich because the pump definately squirts fuel instantly and strong. I feel like I need to start over, but I'm not sure what a good baseline would be. I'm open to any suggestions beacause I'm definately no expert on this 

[Shark Racer]

What are the symptoms?

My car would shake and stutter at low RPM events, like moving in and out of a parking spot. (RPMS a few hundred Rs over idle)

Going down the road, passing, etc was all fine. Idle(no throttle) was smooth as well.

[Novarolla05]

At low RPMs and idling in gear, it sounds like there is a misfire. Out of gear, the idle is really smooth. Sometimes when I'm driving and pull up to a stop sign, it idles fine for a few seconds then starts to get rough. When I slowly pull away from the stop, the engine sputters like crazy. Harder accelerations from a stop behave perfectly. Also. like you said Shark Racer, all other driving conditions seem fine.

I got to thinking yesterday (usually pretty dangerous  ) I'm running the stock torque converter, so could it be a bit too tight?

[Shark Racer]

I don't think it could be the T/C. Mine is a stick shift so these maneuvers would be around 1000 RPM at most slipping the clutch - your T/C is likely at *least* 1500 RPM, probably closer to 2000.

When you set the idle mixture did you set it in drive or in neutral?

With an auto, I believe the mixture should be set while the car is in gear. Obviously you'll need a helper and some sturdy chocks for this.

[Novarolla05]

I always have the car in drive when I set the mixture. I've never officially heard if that's the right way or not, but it makes sense to do it that way. Makes me feel better to know I'm not the only one doing it that way 

Something else I thought of. Would running an electric choke conversion help out? Right now I have a hot air choke which as I understand it is basically a vacuum leak. Could I go electric and plug the hole that normally has the small plastic tube in it?

[Shark Racer]

I like electric chokes, but I don't think it would help much in this regard. The amount of vacuum "leaking" is very small. If it's hooked up correctly it should be next to none, as the air gets circulated back into the air cleaner (above the secondaries)

[TommyK]

What is your initial timing?

How much does the rpm drop when put in gear?

I know everyone says to set the idle screws for best vacuum but I like to set them to where the carb performs the best whatever the vacuum reading may be.

Please bear in mind that I am a Ford guy. I don't know jack about SBC.

[Novarolla05]

What is your initial timing?

How much does the rpm drop when put in gear?

Initial timing at 18*, total 38* all in by 3500. My RPMs usually drop 150 to 200 RPM.

Please bear in mind that I am a Ford guy. I don't know jack about SBC.

That's OK, I don't know jack about Fords except my back likes the distributor up front much better than by the firewall .

Here's an update on what's going on. I gave up on the 17059201 quad, but I have another one that I just finished cleaning/rebuilding. I swapped them out, and there was a night a day difference in the low RPMs! Much cleaner throttle response. The new carb is a 17056228, which is off a Chevy 400. Here's the specs.

Idle Tube              .036
Idle Channel          .050
Lower Idle Bleed    .073
Upper Idle Bleed    .070
Main Bleed (Horn)  .125
Main Bleed (Body)  .125
Idle Mixture Holes  .089
Idle Air Bypass       .055
Float Level             .25
Jet/Rod     76/48K  APT 3.5 Turns

The only thing I changed on this carb was the idle tube (was .034ish). I have no idea what made the difference between the two carbs idle circuits. The major difference casting wise is the different main air bleed setup, which I would have thought had no impact on the idle circuit. It wouldn't be the first time I was wrong though  . I still have some tweaking to do on the primary and secondary side so we'll see what new gremlins pop up. I'm sure I'll have more questions down the road.

Thanks everyone for all the help and great suggestions!!

Richie