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1969Fstback

Accelerator pump tuning guidance

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Car has original 302 2v but with 2 year old holley 570 street avenger. Not sure on specs of motor.  By ear sounds like small cam.   Car runs great except for kicking it down from idle. It bogs a bit.  You can smell fuel when it does for split second. I adjusted the arm and it seems correct.   You touch the throttle and it starts to squirt.  The nozzle shoots out 2 thick streams fast.  I haven't pulled the nozzle yet to get the size, but it almost seems to much too fast. 

Next steps?

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 Ha that's what my wife says when It try to explain something to her. Bog probably not the best word. stumble is more appropriate.

Ok, so, idles great, put it in gear take off normal no issues runs good.  At 40 or so if you are accelerating and floor it it picks up and goes fine.  If you are running 40 and let off of it and let it coast for a few seconds and get into it hard it will stumble for a second then pick up and take off fine.  if you are sitting park and floor it from dead idle it stumbles for a split second and takes off fine.  If in park and you have the throttle down a bit around 1200 rpms and floor it it picks up fine.   

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When you adjusted the pump was the throttle cable disconnected? You need to adjust it at WOT to a snug fitting .015, if it's adjusted to tight it won't come back to idle like it should & you'll smell gas. 

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did you adjust your fuel mix screws?

how old is the power valve?

is the power valve the stock one?

have you changed any jets?

do you know how to set ignition timing by ear or by a vacuum gauge and tachometer?

if you remove the float bowl inspection screws immediately after turning the engine off does any gas come out?

 

 

 

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I set the fuel mix screws with vacuum gauge.

Power valve is not stock but is a 6.5.  It's probably year or so old.

Yes I did change the jets.  It was running 64's up front and 68's in back out of the box.  and was extremely rich.  I am now running, if memory serves me, 60's(maybe 58's) think I still have the package, up front 64's in back.

I set ignition using light.  at 10 degrees.  with advance unplugged.  But yeah mostly by ear. distributor is using metered vacuum.   explain the process using vacuum gauge and tach and I'll reset it how every you say.

Carb has site glasses and primary and secondary are both right at bottom.  

thanks

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Adjusting the accelerator pump linkage is very simple.  At idle there should be no clearance at all between the lever and pump arm.  The nozzles should start shooting as soon as the throttle starts moving.  At wide open throttle you should be able to move the accelerator pump arm a little more.  Another words, at wide open throttle the pump diaphragm should not be bottomed out.  Very common on Holley carbs to have to increase the pump nozzle one or two sizes from stock to eliminate a stumble during hard acceleration.  I would probably try a #28 or maybe a #31 nozzle and a pink accelerator pump cam.  #31 nozzles are commonly needed on 650-750 CFM carbs.  But you are using a smaller carb so I think the #28 should work okay.  Old tube style nozzles are believed to work better because they help direct the fuel towards the center of the venturies.

Be certain your throttle plates are in the correct position at idle.  At idle about 0.020" to 0.060" of the two transfer slots should be exposed below the primary throttle plates.  Outside of that range and issues arise such as stumbles during light accelerations or idle is too rich and the A/F screws are not affective.  If you have any vacuum at the ported vacuum port at idle you can be certain the throttle plates are open too far.  Holley has videos on this topic on their website.  It's best to set the primary throttle plate position with the carb off the car and upside down.  Then you'll get an idea how much adjustment range there is with the idle speed screw to maintain proper position with the transfer slots.  If you have to turn the idle speed screw beyond that range, it's time to adjust the secondary throttle plate position.  To adjust the secondary throttle plate idle position there is a small slotted set screw accessed through the bottom of the carb base plate on the right side. Typically for mild street motors 1/4 to 1/2 turn off seat is all that is needed.  Many people ignore these details when adjusting Holley carbs then complain about driveability issues. 

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It is NOT common to have to need to change the pump nozzle size on any of the  holley's that I have jetted and I have jetted literally hundreds of them in the last 45 years and NEVER once had to change a nozzle, however, I haven't jetted a lot of the new avenger style carbs. If someone changes a pump nozzle and it improves a particular condition, it still does not mean that the pump nozzle was the problem.

If you change the pump nozzle, I will leave it for others to try and help you.

 

 

 

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17 minutes ago, 1969_Mach1 said:

Be certain your throttle plates are in the correct position at idle.  At idle about 0.020" to 0.060" of the two transfer slots should be exposed below the primary throttle plates.  

I actually did check those when I was fighting it being rich.  I should probably pull the carb and verify the slot is still correct.  From what I gathered the slot should look square when set properly.  

My cam looks more red then pink right now.  I was gonna pull the nozzle tomorrow and verify which size it is once I got some good guidance from this thread.

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3 minutes ago, barnett468 said:

It is NOT common to have to need to change the pump nozzle size on a holley . i have jetted hundreds of holley carbs in the last 45 years and NEVER once had to change a nozzle.

If you change the pump nozzle, i will leave it for others to try and help you.

 

 

 

I was thinking the same thing on the jets.  From what I read on holley's website and other places they 570 normally came with 54 primary and 65 secondary.  Mine had 64 primary and 68 secondary.  I know my secondarys are 64 cause I moved the front to the back.  and fronts I am pretty sure is 58's.  It's still a tad rich, but not near what it was.  I was pretty familiar with how to adjust the cams, but wasn't aware of the nozzle til recent research.  But, having it come with the larger jets then what was specified on the site I am starting to wonder what the nozzle size actually is.  I have no idea what stream size of volume a nozzle should spray, but I was surprised at how thick the stream was that came out.  But like I said, nothing to compare that too.

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I'm insanely busy right now but one simple test you can try which won't cure the problem but it might help a little is the following.

IGNITION TIMING

1. remove the vacuum advance and leave it off until further notice.

2. get the engine up to operating temp.

3. see if there is any vacuum at the ported vacuum port on the carb.

4. advance the timing 3 degrees and see if the rpm increases and if the engine still runs smooth.

5. lower the timing back to where it was.

6. rev the engine to around 2000 - 2500 rpm then rotate the distributor clockwise slightly and see if the rpm increases.

7. lower the rpm to the previous setting and post results.

 

FUEL MIX SCREWS

Turn the idle up to around 1400 rpm.

Turn 1 screw out 1/2 turn and see if the rpm changes.

Turn the same screw in 1 turn and see if the rpm changes.

Turn the same screw out 1/2 turn and see if the rpm changes.

Do the same thing to the other screw then post results.

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3 minutes ago, barnett468 said:

I'm insanely busy right now but one simple test you can try which won't cure the problem but it might help a little is the following.

IGNITION TIMING

1. remove the vacuum advance and leave it off until further notice.

2. get the engine up to operating temp.

3. see if there is any vacuum at the ported vacuum port on the carb.

4. advance the timing 3 degrees and see if the rpm increases and if the engine still runs smooth.

5. lower the timing back to where it was.

6. rev the engine to around 2000 - 2500 rpm then rotate the distributor clockwise slightly and see if the rpm increases.

7. lower the rpm to the previous setting and post results.

 

FUEL MIX SCREWS

Turn the idle up to around 1400 rpm.

Turn 1 screw out 1/2 turn and see if the rpm changes.

Turn the same screw in 1 turn and see if the rpm changes.

Turn the same screw out 1/2 turn and see if the rpm changes.

Do the same thing to the other screw then post results.

Will do.  Thanks

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These specs are for SA570 out of the box.

Primary Main Jet 54
Primary Power Valve 8.5
Primary Pump Nozzle Size 31
Product Type 4150 Model Carburetor
Secondaries Vacuum
Secondary Main Jet 65
Supercharged Application No
 

 

 

 

Mine is an older carb it came with 58 primary 66 secondaries & 31 pump nozzle. I found the 54 & 65 gave much better gas mileage & throttle response with one size down on the  power valve. You can pull the primary bowl without removing the carb just disconnect the fuel line & 4 bowl screws.

You can download the Holley doc's from their site with the list on your carb.

https://www.holley.com/support/carburetor/ 

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On 5/25/2018 at 12:29 AM, barnett468 said:

I'm insanely busy right now but one simple test you can try which won't cure the problem but it might help a little is the following.

IGNITION TIMING

1. remove the vacuum advance and leave it off until further notice.

2. get the engine up to operating temp.

3. see if there is any vacuum at the ported vacuum port on the carb.

No vacuum on meter at idle

Quote

4. advance the timing 3 degrees and see if the rpm increases and if the engine still runs smooth.

Increased decent amount and ran smooth and started easily.

 

5. lower the timing back to where it was.

6. rev the engine to around 2000 - 2500 rpm then rotate the distributor clockwise slightly and see if the rpm increases.

Yes some.

7. lower the rpm to the previous setting and post results.

 

FUEL MIX SCREWS

Turn the idle up to around 1400 rpm.

Turn 1 screw out 1/2 turn and see if the rpm changes.

Turn the same screw in 1 turn and see if the rpm changes.

Turn the same screw out 1/2 turn and see if the rpm changes.

Do the same thing to the other screw then post results.

My answer for timing are in twined in your post.

 

Primary idle mix screws made no difference screwing in or out.  Secondary turning either in increased idle.

 

Thanks 

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ok just saw your post . will try to get back to you tonight or tomorrow morning but it is obvious by your results that you will benefit from changing from your current timing spec. more advance can in fact reduce hesitation. i have done it countless times. this is not to say that more timing will fix your problem but it is guaranteed to improve performance and mileage at least a tiny bit.

for the time being, i would advance the timing 3 degrees from your original setting and plug the vacuum advance port and test drive it.

fuel level at the bottom of the windows when idling is correct.

 

 

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