quadrajet 7042210 idle question

[gclark]

With my engine idling at 800 rpm, if I pull the cap off of the vacuum port on the side of the carburetor, the engine speed increases by about 75 rpm.

Does this mean that my engine wants more bypass air? 

Is opening the vacuum port a valid diagnostic for the idle circuit?

Thanks in advance for your help.



My engine and carburetor specs:
355 SBC, 10.4:1 CR   
TFS 23 Degree Aluminum Heads   
Hyd Roller Cam: 110 LSA; Timing @ 0.050: 214 Intake, 220 Exhaust   
1.5:1 Rocker Arm Ratio   
MSD HEI Ignition   
TH350 Automatic Transmission   
1972 Chevy C-10 Pickup   
   
Quadrajet 7042210   
Idle Tube   0.036
Idle Channel   0.047
Lower Idle Air Bleed   0.070
Upper Idle Air Bleed(airhorn)   0.052
Accel Pump Discharge Holes   0.024
Main Air Bleed(main body)   0.070
Main Air Bleed(air horn)   0.070
Main Jet   0.074
Primary Metering Rod   43B
Fuel Inlet Seat   0.135
Float Level   1/4"
Bypass Air     0.084

[omaha]

It is possible yes or maybe the idle is just a tad bit rich. Of course if you try to lean out the idle and the rpm's drop off then yes, I would think the carb might need more bypass air. It depends if you can open up the primary butterflys without any nozzle drip, this is a hard thing to tell as far as I'm concerened. (when it's just on the edge of needing bypass air) I guess adding a bit of bypass air would not hurt because it is easily plugged.  I guess bypass air is better than the butterflies in any case but go small and "creep up on it".  Getting an engine to idle correctly can be a real PITA sometimes, there's the timing, the throttle opening, vacuum reading, bypass air and A/F ratio. I remember the carb in my old VW was bypass air only. The throttle was totally closed during idle. {PICT 34 Solex carb}. Just my .02.

[Cliff Ruggles]

Did you move the upper idle airbleeds to the airhorn? 

Were the upper idle airbleeds blocked off in the main casting?

Did you install the main airbleeds as well?

Were you able to free up the APT screw in the baseplate?.....Cliff

[gclark]

All- Thanks for the responses.

Cliff- yes to all of the above.  I rebuilt this carburetor after discussions with you on how to set up the airbleeds and free up the APT screw.  I used your kit with the replacement APT screw. 

The engine smells a little rich at idle.  Just wondering if the test with the vacuum hose means the engine wants more air/less fuel at idle.

Thanks,
Geoff Clark

[Cliff Ruggles]

Do you have control of the idle fuel with the mixtures screws?

A lean setting will stink as much as, if not more than a rich setting.

Often with tight LSA (110 or narrower) cams, a rich idle mixture is required to get a smooth idle, or decent idle.  I don't like or use tight LSA cams for most street engines for this reason.  They sound great, lots of "attitude" etc, but allow a lot of unburnt fuel out the tailpipes due to the additional overlap and poorer scavenging at idle speed(s)......Cliff

[gclark]

Cliff,

I currently have the mixture screws set about 3 turns out.  This is where I get the highest vacuum reading (about 14").  I can kill the engine by turning either one or both screws all the way in. 

Thanks for the help.

[Cliff Ruggles]

Geoff, if you have full control of the idle fuel, and no "nozzle drip", engine speed doesn't fall off a lot when you place the trans in gear, etc, that may be about as good as it gets with your particular combination of parts.

I really LOVE the rumpity/rumpity sound from aftermarket camshafts ground on tight LSA's, but they comprimise idle quality in many cases.  You can take the same basic cam and grind it on a 112 or even a 114LSA and the engine will idle much smoother, with a broader/flatter power curve. 

However, not nearly as much "bling" as the "performance" oriented grind........Cliff

[73ss]

My combo stunk quite bad as well from the exhaust. The engine seemed like it had enough idle fuel. I pulled the tubes and replaced them with new ones opened up to .038". Stock measured .035". I got more control of the mixture screws & the exhaust does not stink nearly as bad although it does still smell. Also picked up about an inch of vacuum. Cam is 110 degree centerline. My 2 cents.

[gclark]

All- Thanks for the replies.

Cliff-  My idle speed is 800 rpm in park, 600 rpm in drive. 

I don't know about the nozzle drip- how can I determine if I have this?

Also-  if I pull the cap off of the vacuum port on the side of the carburetor at idle speed(800 rpm), the engine speed increases by about 75 rpm.  Does this mean anything at all?

Thanks for the help.

[Cliff Ruggles]

When you add more air, the idle speed is going to increase a tad.  If the mixture is slightly rich it will also increase and smooth out some.  If you are right on the edge of being lean, it usually invokes a slight miss into the exhaust note.

To look for nozzle drip, glance into the carb at idle speed and see if any fuel is flowing from the boosters.  If not, the carb has sufficient idle bypass air and the plates are closed enough to keep you on the idle curcuit.

200 rpm is a pretty big drop when placed in gear, is the timing rock stead when this is happening?.....Cliff

[gclark]

Cliff-  timing is steady @ 12 degrees.  Vacuum advance is connected to a ported source and not in play at idle.

How much drop would you expect going from park to drive?

Thanks for your help.

[Cliff Ruggles]

It really depends on the idle speed and how tight the converter is.  Typically, we see about 50-100 rpms or so drop with most set-ups, provided the idle speed is down around 650-750rpm's.  A faster idle speed and tight converter will see quite a bit more than that in some set-ups.....Cliff

[gclark]

Cliff-  I am using  a stock torque converter. 
what would cause excessive rpm drop?  Not enough fuel?
What would you change to affect this?
Thanks for the help.

[Cliff Ruggles]

A stock converter pulls pretty hard on the engine at 800rpm idle speed.  Have you tried lowering the idle speed closer to factory specs?.......Cliff

[gclark]

Cliff,

I believe the factory spec is 600rpm in drive. 

I have the idle set at 800rpm in park.  It drops from 800 to 600 when I put it in drive.