More Holley carb tuning..

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More Holley carb tuning..

Postby marco_1978_spyder » Fri Jan 22, 2010 1:04 pm

In my quest for perfect all around driveability I'm down to 2 last niggling issues with my newley rebuilt Holley 600 Vac secondary carb.

I would think both issues are related, but one is showing improvement while the other is not.

1st Issue:

This is the one that I've almost worked out...
A hesitation when going from cruise speed to WOT. It started as a major flat spot...not a bog... just a delay followed by a sharp burst of smooth power.

The first adjustment I made was to the Pump arm Lever...I found it was too loose and set it for .010 clearance at wot as per holley instructions.

That helped shorten the delay, but completely eliminate it.

I then tried to richen the idle mixture a bit... but that made no major change...

I then readjusted both floats so the gas tricked out vs. just being level with the lower threads of the sight hole.

That's almost cured the problem, it's down to just a split second delay now. Although if I'm just cruising at 40 mph, in 3rd gear at about 2500 rpm, if I mash the pedal, the minor delay sends a quick shockload through the drivetrain which I don't care for. I'm sure my universal joints weren't happy with it.

I'd like to try a different pump cam and see if that helps...I have an orange cam on setting 1 right now.

Some folks are saying that the Blue cam helped cure a similar issue.

If that dont work, maybe I need to step up to a larger squirter? I'm getting instant fuel when I move the throttle lever barely by hand.

See next post for problem 2..
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1980 Sunbird Hatchback 4cyl, 4spd.
2006 Chevrolet Aveo 5 speed Hatchback
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Re: More Holley carb tuning..

Postby marco_1978_spyder » Fri Jan 22, 2010 1:26 pm

2:
Violent drivetrain jerking when trying to roll forward in gear, no gas.
I'm trying to run at the factory recomended Idle speed of 650 rpm.

After the above adjustments, I have it so that if I tip into the throttle ever so slighlty I can roll forward smoothly at about 7-8mph but if I try to ease off the gas, forget it, the bucking comes back.

It disappears if you accellerate reasonably, although you can feel a mild jerkyness if you also run the rpms up in gear gradually and just hold it there for a while.

I hooked a vacuum gauge up to the car to try and tune the idle mixture screws one more time.

I noticed something interesting, there's an up and down surge at idle that the vac needle follows exactly.

At worst its worth 2 -3 lbs of manifold vac... at best adjustment I got it to drift between 16 and 17 HG.

Not a flutter, the needle slowly drifts back and forth with no rhythm.

Starting at 1-1/4 turns out, it idled at 14/15hg. With a few 1/4 turns in I got to 15/16 Hg. So the screws are at about a 1/2 turn out.

I tightened the carb down. no major change. I'm at a loss as to what is causing the idle flutter. But I believe that its this intermittent drop thats causing the bucking at idle.

I've been through the distributor. Not sure what else to try. According to this website... http://www.secondchancegarage.com/public/186.cfm
My problem looks most like scenario 7 or 11. I'll recheck the timing...maybe my balancer has slipped?
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1980 Sunbird Hatchback 4cyl, 4spd.
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
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Re: More Holley carb tuning..

Postby chevyart » Fri Jan 22, 2010 4:30 pm

marco the acelerator pump adjustment is more for assuring that the accelerator pump dont bottom out. usually there is never slack in the linkage at idle or any other time. what i would do is first try one jet size larger in the primary side only(your carb may only have a metering plate on secondayr side)this is a tried and true first step. the next thing i would do is check the power valve. probably your carb has a 6.5 up front and that may not give you the fuel quick enough. try an 8.5 and see what happens. these are 2 steps that alot of guys use when experiencing a little flat spot when going from low to higher speeds. hope this helps chevy art
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Re: More Holley carb tuning..

Postby marco_1978_spyder » Fri Jan 22, 2010 9:21 pm

chevyart wrote:marco the acelerator pump adjustment is more for assuring that the accelerator pump dont bottom out. usually there is never slack in the linkage at idle or any other time. what i would do is first try one jet size larger in the primary side only(your carb may only have a metering plate on secondayr side)this is a tried and true first step. the next thing i would do is check the power valve. probably your carb has a 6.5 up front and that may not give you the fuel quick enough. try an 8.5 and see what happens. these are 2 steps that alot of guys use when experiencing a little flat spot when going from low to higher speeds. hope this helps chevy art


Thanks Art.

When rebuilding the carb....I thought of this and went with an 8.5 when I was putting it back together.

You dont happen to have a blue cam anywhere do ya?

I'm making sure there isn't anything I'm missing before getting into upping the jet sizes. I don't want to mask an issue by running too rich.

The 64's do seem a little small compared to what lots of guys are running tho...


I think my 600dp has 67's up front...I could try those
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1980 Sunbird Hatchback 4cyl, 4spd.
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
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Re: More Holley carb tuning..

Postby 72 SS PNL » Fri Jan 22, 2010 9:39 pm

pick the rpm up to 750-800 and see how the vacuum is , variations up and down are usually not good signs on a stock cam , it could have a flat lobe , bad timing chain , sticking valve , burnt valve

on jetting what color are your plugs ? you may want to get a wideband o2 gauge to help set up the afr(air fuel ratio)
first and best car owned 72 GT(changed to SS) panel wagon

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Re: More Holley carb tuning..

Postby marco_1978_spyder » Sat Jan 23, 2010 12:50 am

A WB O2 sensor would be the shizzle.

But I can't afford that right now.

According to the vac gauge reading, valve sealing may be an issue. Based on the way #3 looked I can only imagine what kind of crud has built up on the valves.

I'm hoping its retarded timing, but if that were the case the engine should smooth back out once I hook the manifold vac bac up to the advance can, but it makes no change...(You can see the timing advance with a timing light) I'm gonna try running a 700/800 rpm idle and see how that goes.

I guess I should also going to pull the carb and see if the idle transfer slot is correctly exposed.
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1980 Sunbird Hatchback 4cyl, 4spd.
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
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Re: More Holley carb tuning..

Postby marco_1978_spyder » Sun Jan 24, 2010 4:14 pm

Ok here's the latest,

For problem #1 Flat spot, I believe I have this one worked out...

I found that the secondaires, were sligthly sticky when completely shut. So I sprayed the throttle linkage down with WD40 on both sides and afterworking it rapidly, it had a smoother operation. On a test drive there seems to be no more delay, and a smoother transfer of power. It's easier on the drivetrain now, but at the expense of some seat of the pants feeling too.

As for #2. There's still a problem, but I did find a band-aid solution....

Cranking up the idle to 900/950rpm, seems to tame keep the jerking under control.

It's not what I like...but in the end, I found it wasn't to bad...I would just be done with it then, but I still feel there's a gremlin somewhere...

I noticed if I ran the rpm's up in gear,say 3000 and then cruise steady, it stumbles a bit.

I rechecked the timing and it's ok. I think I might dive into the distributor 1 more time...1 want better bushings on the weights, for less slop, and I recall the pink wire on the pole piece looking kinda frayed...I'm now wondering if it could be the cause.

As far as valve's not seating well...I do think the valves could be dirty enough to be causing some issues. I'm hoping not...
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1980 Sunbird Hatchback 4cyl, 4spd.
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
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Re: More Holley carb tuning..

Postby 72 SS PNL » Sun Jan 24, 2010 4:28 pm

when checking the timing with a light , is the mark steady or does it move a little bit ?
first and best car owned 72 GT(changed to SS) panel wagon

pictures of the car from 86 - http://www.myrideisme.com/Garage/06_IBM_GOAT/955
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Re: More Holley carb tuning..

Postby marco_1978_spyder » Sun Jan 24, 2010 5:21 pm

For the most part rock steady. At one point I did notice the timing mark drift a tiny bit.

I lowered the idle way down just to see if I was "in the curve" at idle rpm , but it wouldn't go any lower than the 12*btdc I found it at. I adjusted it up to 14*.

Seems like the curve is normal...ie; doesn't start untill 1,100 rpm, but I did see it drift away from 12*to 14* on its own at one point..

I shimmed the dist. recently so excessive play on the shaft shouldnt be an issue. Thats why I was thinking about the weights in the distributor, and how there a little worn at the pivot point.
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1980 Sunbird Hatchback 4cyl, 4spd.
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
User avatar
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Re: More Holley carb tuning..

Postby 72 SS PNL » Sun Jan 24, 2010 5:22 pm

how many mils on the motor and how old is it ?

i was kinda thinking timing chain , they are known for stretching
first and best car owned 72 GT(changed to SS) panel wagon

pictures of the car from 86 - http://www.myrideisme.com/Garage/06_IBM_GOAT/955
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Re: More Holley carb tuning..

Postby marco_1978_spyder » Sun Jan 24, 2010 7:12 pm

24,800 miles since a rebuild in 1995. A Clyoes double roller chain was used. It's probably stretched some by now, Due to my hard driving, but probably isnt out of tolerance yet. At least I would hope.

The Balancer has 80k on it...I wonder if it could have slipped some?
1978 Monza Spyder V8 4speed, posi
1978 Sunbird Formula V6, 5 Speed, Hatchback
1980 Sunbird Hatchback 4cyl, 4spd.
2006 Chevrolet Aveo 5 speed Hatchback
Yes, you can refer to me as Mark
Please visit my blog... http://chevymonza.blogspot.com/ Follow if you like!
Featuring special guests; Carl Beraytor and Ray D'atore

My Red spyder now has a youtube channel please Like and Subscribe!!!
https://www.youtube.com/channel/UCOzzSR ... ISuing7KLA
User avatar
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Re: More Holley carb tuning..

Postby 72 SS PNL » Sun Jan 24, 2010 9:19 pm

24k it should still be good , possible on the balancer but slim
first and best car owned 72 GT(changed to SS) panel wagon

pictures of the car from 86 - http://www.myrideisme.com/Garage/06_IBM_GOAT/955
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