Author Topic: Wandering AFR at idle, bad transition on throttle tip in  (Read 6160 times)

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60sIron

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Wandering AFR at idle, bad transition on throttle tip in
« on: May 30, 2020, 08:42:59 AM »
I am trying to tune this Holley 750 converted 4150 with a vacuum secondary and I'm totally lost.

Engine is 449 (428 .035over + 4.125" stroke).  Hydraulic roller cam .556" lift  236° duration @ .050".  Single plane Port-o-sonic intake.  C6 Automatic trans

Warmed up it idles at 700 RPM with about 11 inches of vacuum with 12-12.5 AFR.  Setting the idle mixture screws can take it down to 11 AFR and up to 13 AFR without affecting the idle or the vacuum too much.

On the highway it was running about 10.5 AFR @ 2200 RPM and about 20 inches of vacuum.  Stinky and fat, so I went from a #72 primary jet to all the way down to a #68 primary jet to get the cruise AFR up in the 12-13 range. 

Now it dies at low speed when tipping in the throttle to go up a hill.  At 1200 RPM I approach a hill at 30 MPH, then tip in the gas, I see AFR drop to about 10 when the pump shot hits, then in a second or two that burns off and AFR shoots up to 15-16 and it starts to die.  I can nurse it up the hill going on and off the gas to keep pumping it, or I can floor it and downshift which bumps the RPM and draws more fuel out of the primaries.  With either one of these techniques I can keep the AFR below 14 and the engine running.

This is where I'm stuck.  I want to add more fuel at low RPM under load without adding more fuel at higher RPM which is what happens when I increase the primary jet size.  From what I've read, this 'transition' from the idle circuit to the primary circuit is controlled by "air bleeds", but how do I change those?  I could slow down the pump shot, but I don't think I could make that last all the way up the hill.  That doesn't seem like the right way to do this.

Another thing I've noticed is the idle AFR drifts upward as the engine gets hot.  I tune it fully warmed up to 12.5 AFR.  Then I get it really hot after say 10 miles on the highway and the AFR has dropped to the low 11s.  Is this a float problem, or a fuel pressure issue or a defective carb?

Please help.  Carbs are a mystery to me.  What are my options?

blykins

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #1 on: May 30, 2020, 09:13:58 AM »
Quick question first....where's your initial timing set?
Brent Lykins
Lykins Motorsports
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60sIron

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #2 on: May 30, 2020, 10:06:51 AM »
With the vacuum advance disconnected idling at 700 RPM it is at 13° BTDC

60sIron

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #3 on: May 30, 2020, 10:14:35 AM »
The fuel pump is a stock mechanical unit with the integrated fuel filter

blykins

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #4 on: May 30, 2020, 10:15:33 AM »
Total mechanical?
Brent Lykins
Lykins Motorsports
Custom FE Street, Drag Race, Road Race, and Pulling Truck Engines
Custom Roller & Flat Tappet Camshafts
www.lykinsmotorsports.com
brent@lykinsmotorsports.com
www.customfordcams.com
502-759-1431
Instagram:  brentlykinsmotorsports
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Drew Pojedinec

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #5 on: May 30, 2020, 10:21:55 AM »
Brent has her on the run..... without appropriate timing, it'll never run correctly.
A/F ratio is massively impacted by ignition timing.

Not saying your carb is perfect, just saying you shouldn't touch it until you have the timing in a reasonable place.... 13 degrees initial for your engine/cam/etc is kinda 5-6degrees retarded by my thinking.

GerryP

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #6 on: May 30, 2020, 10:29:01 AM »
Is this a 3310 or variant?  Did you buy it new?  Other than the secondary metering block and the primary jet, have you done any more fiddling with it?  If it's a used or older carburetor, is it really clean?  Does it have the proper gaskets?

A 68 primary jet is too small for a 750.  Go back to the 72.  Put the AFR meter away.  It's not helping you.  Tune the carburetor to make the engine run good regardless of what the meter says.  Once everything is well, hook up your meter to see what the engine likes, write that down, and put the meter back in the box.

Forget this being a bleed problem.  Unless they are replaceable and someone fiddled with the carburetor before you got it, the bleeds should be fine to keep you on the road.  Not perfect, but not the source of your problem.

Check your primary and secondary throttle blades to make sure you are not over-exposing the transfer slot.  What can happen in achieving your 700rpm idle is that the secondary throttle could be completely closed, and the primary open too far and exposing too much of the transfer slot.  That detail will make sure your idle and tip-in response is working correctly.

Just so you know, if you have a really stinky exhaust...you know, eyes watering, runs you out of the garage...that is raw fuel -hydrocarbons.  You get that from a lean condition where the fuel doesn't fully combust or you get a lean misfire.  It can also be an ignition misfire, but in your case, you are running too lean.  A rich mixture doesn't have that smell.  And other than really poor fuel economy and short plug life, with a rich mixture the engine will start fine and run okay.  Not perfect, but it won't be giving you a lot of problems.  This is why it's better to have an engine on the rich side, than lean.  Particularly at idle.

1964Fastback

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #7 on: May 30, 2020, 10:30:09 AM »
Would the power valve be something to look at, for the A/F spike going up hill?  Not saying it is, just asking.

Pat
1964 Galaxie 500 2 dr Fastback, 390, 4 speed, Indianapolis Indiana

blykins

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #8 on: May 30, 2020, 10:33:01 AM »
Yep.  I was trying to see how far he could advance it before needing a recurve, but that's generally where I head too.  About 18 degrees initial.  Increasing the timing will allow the idle speed to be lowered, which will make more vacuum, then you can start tuning.
Brent Lykins
Lykins Motorsports
Custom FE Street, Drag Race, Road Race, and Pulling Truck Engines
Custom Roller & Flat Tappet Camshafts
www.lykinsmotorsports.com
brent@lykinsmotorsports.com
www.customfordcams.com
502-759-1431
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YouTube:  Lykins Motorsports

60sIron

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #9 on: May 30, 2020, 11:32:33 AM »
I can't really see the degree marks with the alternator bracket in the way so I guessed wrong with the 13° BTDC.  When I did the mechanical advance measurement and I saw where 20° came in, I'd say at idle it is around 16° or 17°.  I had retarded it a little from the initial setting because sometimes it doesn't like to crank over when it is hot, but I could probably put those degrees back into it.

At 3000 RPM total advance is 42° with the vacuum advance disconnected.  I've never curved this distributor.  It is a FoMoCo piece with a Pertronix.  The mechanical advance is not stock, but I'm not familiar with it.

60sIron

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #10 on: May 30, 2020, 11:40:03 AM »
The carb is a Holley 1367 I bought from Summit a couple years ago.  When I started with the tuning I added the secondary metering block and put a removable top on the vacuum secondary pot.

Based on the idle vacuum I changed the power valve to a standard flow 5.5.

The throttle tip in problem is happening before the secondaries kick in or the PV opens up.

60sIron

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #11 on: May 30, 2020, 11:48:11 AM »
Sorry that is the date code, I guess it was made in 2017

The model 80508-6

pbf777

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #12 on: May 30, 2020, 12:01:42 PM »
Put the AFR meter away.  It's not helping you.  Tune the carburetor to make the engine run good regardless of what the meter says.  Once everything is well, hook up your meter to see what the engine likes, write that down, and put the meter back in the box.

    +1       ;)

     Scott.

60sIron

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #13 on: May 30, 2020, 02:22:32 PM »
If I knew how to tune the carb to make the engine run right I wouldn't need the oxygen sensor.


Joe-JDC

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Re: Wandering AFR at idle, bad transition on throttle tip in
« Reply #14 on: May 30, 2020, 05:22:12 PM »
What is the carb number on the choke horn?  Body number?  On Holley carbs, everyone thinks adjusting the idle screws or the throttle blade screw is adjusting the idle.  WRONG.  Start by emptying the fuel, turn the carb over and release the throttle blades adjusting screws so that they close completely.  Then open the secondary butterflies until you see the idle transfer slot uncovered ~.015-.020" with the little screw that controls the opening.  Turn the screws on the side of the metering block in gently until they seat, then back them out 1 complete turn to 1 1/4 turn.  The accelerator pump arm should have .015" clearance with the throttle blades at WOT(wide open throttle).  This should get you a clean idle if all the carb circuits are back to stock and clean.  Idle speed is adjusted by that slot in the SECONDARY throttle blades, if the engine is revving too fast, then lower the secondary butterflies a couple of thousandths.  Don't open the primary until you have the idle set.  Just open the primary enough so that it doesn't stick when the throttle is released by letting your foot up off the pedal.  A .028" primary shooter should work for a longer shot on acceleration, but if it backfires, then a .031" shooter can be installed.  Now you can check the O2 for jet changes.  Joe-JDC
Joe-JDC '70GT-500