Qjet going lean at part throttle acceloration

[mertz28]

I have been doing some adjustment on my carb.  I tried to get a little more vacuum so I adjusted the idle mixture screws out and got another 1".  Problem is it now pops out the exhaust when I let off the gas.  I attached my wideband A/F gauge and it is rich at idle which I can fix.  After a test drive I noticed a lean condition when I accelerate at part throttle.  This goes away when I apply more throttle and then it really moves.  The A/F gauge reads lean under those condition and 12.5 when it starts to pick up the acceleration.  It has always been slow to pick up speed under 45mph.  When I got the carb back from Cliff, I only adjust the mixture screws a little to get max vacuum.  How can I improve the slow speed bog?  A/F seems to be fine everywhere else.  I believe the timing is at 14 initial and 36 total.  I am on manifold vacuum.  Engine runs great except the slow start and low speeds.

[Cliff Ruggles]

What is the carburetor number, and what is it being used on?....Cliff

[mertz28]

It is a 17057213 that you built for me.  It is on a 1972 Chevy C20 pickup with 350 and 350 trans.  Edelbrock manifold, headers, MSD ignition, and 1" Hambuger spacer.

[77cruiser]

How lean does it go? Might have to adjust the APT raise the rods.

[mertz28]

It goes to 14.7 and somewhat leaner.  I have a bar gauge so I don't get exact read outs.  It goes to the lean side of 14.7.  I thought about the APT and Cliff set it up so it could be adjusted.  So I would want to pull them up a little to see if that helps.  When they are modified do they get a philips screw or the stock one?

[77cruiser]

Should be a flat tip if he did it.

[mertz28]

Found the APT and opened it up 1/2 turn and now it is about dead on with good power off the line.  It still idles a little rich.  I can turn down the mixture screws about 1/2 to 3/4 turn and the engine slows and I drop about 1/2" vacuum so I left them at the highest vacuum (19").  I will hook it up to my computer and see what it does.

I tried power braking it but it just bogs down and could not get much power or rpms.

[mertz28]

I did a little more tweaking and it still goes a little lean when I first get on it but................I can now for the first time power brake it and it will spin the tires.  I melted some chip seal and left a couple of nice black strips on the road.

When I first hit the gas it goes lean then I can watch it get richer and the thing loads up and starts spinning the tires.  I could probably hold it there spinning until I go through the pavement.  When I let go it launches pretty well and picks up speed in a hurry.

I need to confirm that on the opened up APT you turn the screw in to richen.  Is that correct?  That is what is happening on mine.

[77cruiser]

CCW to richen raise the rods, correct. Sounds like your secondaries can use more tuning, maybe tighten the air valve spring a bit.

[Cliff Ruggles]

Correct, clockwise raises the metering rods and richens part throttle A/F.

Is the 1" spacer four hole, fully open, fully divided or semi-open?  I've NEVER had any luck using spacers on dual plane intakes on small cid engines or really "mild" set-ups.  They can induce a stumble at the onset of the secondaries that is difficult to tune out....FWIW.....Cliff

[mertz28]

The spacer is a Hamburger that is 4 hole but tappers at the bottom and has a little bit of an opening between the to sides of the dual plan.  Like this: http://www.jegs.com/i/Hamburger's/497/3211/10002/-1?CAWELAID=1710687312&CAGPSPN=pla&catargetid=230006180000514790&cadevice=c&gclid=Cj0KEQjw0POdBRCq3arGgYD05pMBEiQAmiUeTiV5jk--xElV8mEXCX6oH-SOkmp4hFCBHQpcXF9cT4EaAine8P8HAQ   Mine is for a spread bore.

I will try running it without and see how it does.  I did notice that it seemed to reduce performance a little when I first put it on but I was hoping to get some more torque with it.

[Cliff Ruggles]

I drove my car to the track for Friday night "test and tune" and ran 7.25 @ 96mph with a stock intake and no spacer on it.  I seriously doubt if  your mild set-up would benefit anyplace from a spacer.  I've tested all sorts of them on my set-up, and to date have ran the best ET with no spacer at all.  About all I see with spacers is at most a little more MPH and slower 60' times.  In other words more power up near the shift point, and less low speed power.....Cliff

[mertz28]

I thought I had it made when I was finally able to burn rubber with my 72 C20 truck (350/350 and 4:10). It was going a little lean before it kicked in so Cliff suggested I tighten up the spring on the secondaries. I finally got around to doing that and I did some minor adjustments to the APT and now it is back to it's old self. When I power brake it it stays lean and will rev to 1850 but it will not stand up and even attempt to brake the tires lose. I put the secondary spring back to were it was and got the APT back close and still nothing. I would appear that the secondaries aren't opening. I certainly don't hear the rush of air. What could be causing this? What could keep the secondaries from opening? I am using a narrow band a/f gauge but will hook up my Innovate to readjust the APT. When I get on it it doesn't instantly go rich but that might be the gauge. During power braking when I was able to brake the tires lose it would go to 11.6 and under but now it stays around 13. For some reason it doesn't seem to be getting fuel.

[Cliff Ruggles]

How much did you tighten up the spring?  Adjustments made there should be about 1 screwdriver blade thickness at a time, or no more than 1/8th turn until secondary transition is smooth and there is no felt transition at all.

Did you get rid of the spacer yet?  I've ran into all sorts of "issues" with secondary transition and performance trying to use spacers.  They increase plenum size, and can cause some "reversion" which effects secondary transition.  I would NEVER use one on any "mild" set-up for any reason.  They will also cause further issues in cool/cold weather due to lack of thermal efficiency.......Cliff