idle bypass question

[wannabemachinist]

 Hello I am new to this site and I am looking for some help. I have a 350sbc bored .030 over with flat top pistons, camel hump heads with a comp cams XE 262 with 262 deg intake and 270 deg exhaust duration. My qjet is a 1708 series 1984 year. The carb has the throttle shafts re bushed and a new kit in it. Driving it was fine but it could not idle properly with out it running fast in park. Covering the primaries with my hand made the RPMs increase and idle smoothly but no control with the idle screws. Reading Cliffs book I made some changes which helped a lot. I enlarged the idle tubes, idle down restriction, idle screw holes and the idle bypass. This is what I have now.
 jets are             72
 rods are            43x
 idle down res     .052
 idle tubes are     .040 were .032
 idle screws holes .092
 idle by pass         .075
 main air bleed      .075 lower and .120 in horn and .120 in bowel ( 2 orfice per side)
 initial timing         10 deg
 Idle is pretty good now and covering primaries with hand now drops RPM very slightly after a few moments but will not kill it. Still no control with idle screw though at least I am not seeing it
 If I remove a small vacuum line, idle improves some more and vacuum is guage is showing 15 which seems the best so far and I think there is response with the idle screws although very slight

so my questions are    do I increase the idle air bleed and how much?
                                  Am I still a bit lean?
                                   what would I change to richen it up if lean? jet and rods?
Thanks for any help you can offer 
     

[Cliff Ruggles]

I'm confused with a few of your settings.  What is the rest of the carburetor part number and is it a single main airbleed model.

Most will be in and after 1980 unless it was a CCC unit or Marine unit......Cliff

[wannabemachinist]

The complete carb number is 17084285 and I would say it is not a single air bleed. There is a .120 orfice in the air horn on each side of the screws and a .120 orfice just below in the bowl by the booster ring and the lowers are .075. I am saying there are 2 upper main air bleed per throttle bore at .120 each

[wannabemachinist]

 Forgot to mention that my idle speed is about 700 to 725 rpm now in park and drops only slightly when is put in gear. This is when all unnecessary vacuum lines are capped 

[Cliff Ruggles]

Most likely a Service Replacement for the late 1970's Chevy units with the large MAB's.

72 main jets are really lean in one of those and they didn't use 43X metering rods in them.  Are they "K" series instead?

I'm also unclear as to what the .075" main bleed is if the two other MAB's are .120".  Are you referring to the lower IAB?...

.040" idle tubes are huge but will certainly put some fuel to the mixture screws if that is what is needed to make the engine happy with that cam in it.

That is a tiny cam for a 350 engine build but the tight LSA still increases fuel requirements at idle speed.

Don't know what the quench is but flat top pistons and early Chevy double hump heads will yield pretty close to 10 to 1 even if the pistons were down in the holes some and thick head gasket used.  I build our 350 SBC's here targeting .025-.035" quench and they reward the end user very nicely with great power and manage pump fuel w/o issues even at higher compression ratios.

I've built scores of SBC engines dating clear back to the 1970's, and in the past 25 years or so have dyno'd most of them.  That cam does NOT make the grade or even come close.  The stock 350/300 hp cam had more duration than the Comp 262XE does.  The short seat timing and tight LSA make for a nice sounding engine at idle, but they don't make strong upper mid-range and top end power compared to longer duration grinds and/or wider LSA.

The best cam I know of (flat tappet) for an engine like yours is the CS-179R from Speed Pro.  It's basically a modern blueprinted version of the 327/350hp cam.  It requires 10 to 1 compression to work well, but nothing I've dyno'd against it makes more power over a broader rpm range, and it really shines if you add 1.6 rockers to it and Rhoads V-Max lifters lashed at .020".........FWIW.....Cliff

[wannabemachinist]

Thanks for the reply. I guess that could be a k on the metering rods. I read in other posts that k's are not that good for tuning purposes. Would you have a jet / combo that you would recommend?
 I am having a hard time making myself clear about the main air bleeds. There are 3 orfices in each throttle bore 2 at the top and 1 in the lower part of each throttle bore which I guess is the IAB that you refer too
I realize the cam is mild but for daily driving it seems fine at this time and the car is quite responsive as it is set up but I am looking to be able to have adjustment on the air fuel adjustment
 So dealing with cards I have could you recommend something that could help my issue

[Cliff Ruggles]

Address the idle issue first.  Take a gander into the carburetor at idle and verify that there is no fuel dribbling from the boosters.

If not you may already have enough idle fuel but lack control of it because too much transfer slot is exposed.

Remove the carburetor and turn it over (make sure the stop is against the idle speed screws) and see how much transfer slot is exposed.  If you see a lot of transfer slot exposed adding bypass air will lower the throttle angle and degrease the amount of idle fuel it's getting there and may restore some response to the idle mixture screws.

Before you do that I would however back the idle screws out quite a few turns and see if it goes way rich at idle, indicating that at least you have plenty of idle fuel right to start with.

I would also full seat the idle screws (remove the springs from them since you opened up the holes under them) and see if you have any control at all of idle fuel at or close to seated.  If you see some control doing this (engine slows down) and no nozzle drip then your idle tuning may be over.

I've tuned a good many engines brought here using that particular camshaft and not overly impressed with it.  Idle was "quirky" and although it throws some power at you right off idle and initially "feels" pretty responsive it's not overly impressive otherwise and how could it be as it's 12 degrees less seat timing that a stock replacement cam and tighter LSA......FWIW......Cliff

[wannabemachinist]

I checked  the carb today as you suggested and found no dribbling from the boosters
Backed out the adjusting screws, the idle became slightly worse
Turned the idle screws completely in and pretty much stopped the motor
Removed the carb and checked the transfer slot. It was exposed close to 3/32 or .092 and when the idle speed screw was backed off completely it was exposed slightly over 1/32
My idle screws are backed out about 2 1/2 turns and showing about 15 inches of vacuum at about 725 RPM
Initially I felt I was not getting anything when turning the screws in, even with a coil removed from the spring but today I did. Definitely not a huge change when backing them out though
When it starts cold and the choke is on and I completely close the flap it will die if left
Could all this be improved upon? (besides the cam at this point
You mentioned the jets and rod being lean also. What would you recommend
Thanks

[Cliff Ruggles]

If you have control of the idle fuel from rich to lean and no nozzle drip I'd leave that alone.

Does it work OK otherwise?  Do some testing at light part throttle and heavy part throttle to see if it needs more fuel in those areas?...Cliff