Hello,

New to this forum, hoping to get some ideas on how to solve this issue.

I have dialed in my Quickfuel carb so it idles great (A/F 14.2) but I have a very lean condition (according to my A/F digital gauge) at 1400 rpm (goes to 18-19); which just happens to be right before tip-in where the main circuit kicks in (1550 rpm). After that everything is fine again.

I Notice it most during city driving when maintaining slow speed, you can feather the throttle and it will bog; I can duplicate the problem by setting the carb at high idle, which happens to be 1400 rpm, and it will sit there at an AF of 18-19.

I have tried a number of things with no luck (smaller idle air bleeds and enriching fuel mixture screws, lower secondaries transfer slot and raise it on primaries, etc) so I need a second set of eyes/ears/brains that can help me with ideas.

Today I talked with Quickfuel tech support who suggested I go to smaller high speed bleeds to make the main circuit come in quicker but I'm holding that one in reserve as I have never touched those unless I have access to a company with the ability to do fuel mapping.

Following are what I have sofar and my specs:
Timing starts at 20 and mechanical all in by 2800, mechanically limited to 36
With vacuum attached to timing, at idle, sits at 31 degrees
Idles at 880.

1984 SBC 350
Cam: 450 lift, 224 duration @ 50, 106 LSA
high output 416 heads unshrouded with large valves
Compression 9.5:1
Intake – Edelbrock 2701 dual plane performer EPS.
Ignition – Pertronix (PTX) Flamethrower Distributor w/Ignitor 3
Spark Plugs: AC Delco R43TS gapped at 0.43

Carburator: Quickfuel SS680VS
o Primary Main Jet 68
o Secondary Jet 70
o With 67 size outer idle air bleeds (from 74; stock is 70 )
o 28 high speed air bleed
o Primary nozzle – 35 (accelerator)
o Secondary nozzle – n/a (no accelerator)
o Needle and seat – 110
o Power valve – 7.5

Carb Spacer: 1” total, ½” 4 hole spacer with 2 ¼” aramid fibre gaskets
Fuel feed: 3/8” fuel line with high flow 40 mic fuel filter through Holley 110 GPH pump
Tires: 31x10.5R15
rear axle: 3.73 on an Auburn posi, 28 spline, 10 bolt GM rear end
Tranny; THM-375 rebuilt with stage 2 shift kit; 16 spline yoke
Torque converter – new 2500 stall 10 inch GM unit
Drive Shaft was balanced
Approx 3800 lbs curb weight

Any Help is appreciated.

 

I know this doesn't answer your question but are you sure your LSA is 106 degrees? I'm thinking that the 106 degree number is for intake valve location not LSA. Your lift and duration numbers are not that big to have that small of a LSA.

The flat tappet cam I have in my car has a larger lift, similar duration and my LSA is 112* and the smallest LSA I've seen in that range is 110*

One thing is you really can't tune your idle using an AFR gauge if you have a cam that has a lot of overlap on the valves. The raw fuel and air going from the intake straight through to the exhaust throws the numbers off a little bit.

 

Put a vacuum gage on it and see what that is reading when this happens. I would be inclined to put a 10.5 PV in it. Also, it may want some pump shot when you feared the throttle...especially if this is a vacuum secondary carb.

 

Bestracing, No I am not sure. the engine/tranny was passed to me from an uncle who passed away; the handwritten specs sticker on it was mostly illegible; the only items we could clearly make out is the duration and lift; everything else we tried to interpret as best as we can.
Is there a way to figure out LSA without pulling the cam out? Then again, is that information needed during carb/timing setup; I was under the impression that duration and lift are adequate?

Thanks for the information on the AFR, didn't realize that. I do all my tuning using old school methods then look at the AFR to see if it is in the ballpark of where I think it should be. I am using a vacuum gauge and my timing light RPM setting to dial in the idle to best vacuum.

Pancherj, will look to see what the gauge says at that RPM when lean.

Thanks for continued help!

 

The duration and lift are just fine. You don't need the LSA for anything really at this point. Most of the time on the front of the cam there is a part/grind number and if you contact the manufacture you can get the cam specs from them. I believe Crane Cams had CC etched in front of the number. I've installed so many cams I can't fully recall.

Follow what pancherj posted as I think that is the way to go.

Also, if you haven't set your accelerator pump using the following procedure. Open the throttle all the way. Manually depress the accelerator lever under the bowl till it stops. The gap between the lever and the adjustment screw should be approximately 0.063, 1/16". This sets the lever firmly against the screw so that there's no lag from the accelerator pump and it insures that the lever doesn't over travel causing damage to the diaphragm. You can adjust the gap a little smaller if the lever isn't firmly up against the adjusting screw at idle

 

Bestracing, back to your AFR comment; this only applies to using AFR being off a bit at idle correct and not at WOT? I went to the track yesterday to work on jetting and my AFR was reading 10.4, which is a tad on the rich side in my mind; I want to make sure it is reading correctly at WOT before I make decisions.

Also, I did set my accelerator as you described but i used a 0.015 feeler gauge.

Also, pancherj, I do have a 37 shot with a brown cam on the accelerator.

Thank You

 

Ok, been reading up a lot on this and I decided to start by going back to basics; so I took out the carb and reset the idle air bleeds; jets; throttle blades opening, accelerator shot etc. I really feel this has to do with a tuning/idle bleed setup so I will concentrate on working/understanding that and how the idle bleed size affects the transition circuit.

I did what Pancherj recommended with the vacuum gauge and at first I thought I had a pattern of vacuum dropping but then I realized that, since the RPM is increased, I didn't know what it was supposed to be at that RPM to determine if it changed. I think it did drop but cannot prove it. Any thoughts/comments?

I'm going to go back to basics and set each circuit in turn and see how I make out.

Thank You for your ongoing support/comments

Gary

 

BTW Bestracing, I have been looking into how to determine cam specs like LSA while it is still in the engine and while I did find some sites telling how to do that with the cam in the engine it doesn't look easy and, more importantly, accurate. So, I am looking into either pulling the cam during next months work slowdown (I don't think/hope it is a custom grind without markings) or having a local engine shop do the procedure.

As noted previously, it is not that important in the current situation but I would like to have a complete cam spec sheet in case I do decide to go to forced induction (if my wife permits me to spend the money that is )

Cheers

 

Mostly at idle but I'm sure your cam doesn't have enough overlap to throw it off by much.

 

Thought I'd say that I'm curious to see what you discover. I seem to be dealing with the opposite issue that my carb goes rich as the mains come in. I do plan on changing the high speed bleeds to address this.

 

Thanks Bestracing, that's good to know as its a useful tool.
I'm out of town during the week so I will experiment on the weekend and update this thread.