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Author
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Topic: Holley carb question
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cobravenom71
Gearhead Posts: 927
From: Poinciana, Fl USA
Registered: Aug 2002
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posted 03-14-2004 12:05 PM
   
My 71 429 has a Holley 750 dual-feed. I am not very knowledgable about carbs, so be easy on me please.
At idle, the car has always run a little rough and there is always a strong gas smell.
Today, after putting in new plugs, I adjusted the idle mixture screws.(one on each side of the primary metering block) The farther I turned them in(clockwise) the smoother the idle. The smoothest idle is when they are turned completely in! Does this make sense?
I thought that if they were turned all the way in, there would be no fuel for idling, and therefore the engine would stall.
Out on the road, it runs good, actually better than before.
What gives? Can someone post a detailed explanation of how to set the carb?
Thanks to all!------------------
1971 'J' code 429 Mach 1
1976 'Starsky & Hutch' Torino (for sale AFTER March 5th!)
1988 Lincoln Mark VII LSC***New Purchase!***
1973 Convertible***Sold to Lisa H. and Clint B!***
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V8 Thumper
Gearhead Posts: 4413
From: Phoenix, Arizona
Registered: Dec 2001
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posted 03-14-2004 12:56 PM
Is the choke opening up completely?------------------
1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9" All Blue Oval, no blue bottle
http://mustangsandmore.50megs.com/V8Thumper.html
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cobravenom71
Gearhead Posts: 927
From: Poinciana, Fl USA
Registered: Aug 2002
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posted 03-14-2004 01:19 PM
Yeah, the choke is working fine. I live in Florida, so I set it pretty lean. It is fully open within 2 minutes or less of starting.
The car runs great, but there is always a little gas smell from the exhaust, with a little black smoke upon start-up. There is black soot around the inside of the tail pipes.
Once warmed up, there is no smoking, and it continues to run well.
The engine was completely rebuilt about 8,000 miles ago, and the carb was brand new then.
I keep the carb and engine clean, with fresh oil, and all fresh tune-up parts.
The carb does not have a lot of varnish in it when I clean it, which is about every 6 months(3,000 miles or so).
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Fastymz
Moderator Posts: 19583
From: Reno Nv USA MEM#1240
Registered: Apr 2001
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posted 03-14-2004 01:21 PM
This should help,https://mustangsandmore.com/ubb/Forum12/HTML/005903.html ------------------
SCOOP oddly obsessed with big scoops on little Mustangs 65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc. My Pics
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Hans olsson
Gearhead Posts: 670
From: Sweden
Registered: Apr 2000
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posted 03-14-2004 01:47 PM
Have you tried adjusting the float level?
Is your carb vacuum secondary with a front power valve?
------------------
- 71 Mach 1 Ram-Air
- 351C 4V 285 HP
- F-G/Stock Aut
- Stockers are way cool!
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allenman
Journeyman Posts: 17
From: Allen, TX, USA
Registered: Oct 2003
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posted 03-14-2004 02:10 PM
In almost all cases when the condition is as you reported it's due to fuel leaking into the intake for one or both of the follwing reasons:1) improper angle of throttle blades at idle causing fuel to pull from the mains
2) improper gasket seating (warpage maybe) of the main body to metering block. First pull the carb and drain it. Flip it over and look at the throttle blades. You'll see a small thin rectangular slot that the primaries will be exposing (in the main venturis). Make sure that the throttle blades don't expose more than the first .040" of the transfer slot while the throttle blades are resting at idle. Turn back the main idle screw until you acheive this. Next remove the fuel bowls and metering blocks. Examine the metering block/main body gasket. Is it wet? If so, you either have a warped metering block/body or you didn't tighten down the block properly and now it's leaking. I would install a new gasket and retighten the bowls using a nice even criss-cross pattern. Next, screw in the idle mixture screws all the way (lightly) and then back them out 1 1/2 turns each. You now are re-set to test your carb. Install the carb and start the engine. Set the timing such that the car stays running without touching the idle mixtures or main idle. Where is the timing sitting? Is it way advanced? Start backing the timing down towards ~14 degrees (your setting may vary). Your engine should start loping a bit. Now you should be able to hear a change in the engine speed if you turn the idle mixture screws in. Always turn each side evenly! If you don't get any response from turning the screws in 1/2 turn you may still be puking gas down the carb and all bets are off. If the carb started adjusting as stated it's now time to hook up a vacuum gauge and get started balancing the two sides against each other and adjusting the advance (again, leaving the throttle setting alone). For more on this do some searches as the how-to of balancing comes with lots of practice...
This link from Edelbrock gives some sense of how-to:http://www.edelbrock.com/automotive/eps_sect2.html
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ted
Journeyman Posts: 70
From: Central Texas
Registered: May 2003
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posted 03-14-2004 08:18 PM
If this carb has always done this, then checking either the power valve or the throttle blade positioning would be at the top of the list. A vacuum check with the engine idling and in gear will first tell you if the power valve has the correct opening point. Ideally, the power valve needs to open at least 2 in/Hg below what ever vacuum reading you obtained. If you've got 8?" Hg, then a #65 power valve would fit the bill. If the carb is old or has been several years since a thorough servicing, then the power valve diaphragm can be getting stiff which in turn be arbitrarily raising it's opening point by lieu of being harder to close. On an engine with marginal vacuum to begin with, then the older power valve may not be closing fully at idle although it was originally sized okay.The other item is the throttle blades. If they are open too much at idle, then too much idle transfer slot is exposed and you could be pulling fuel from the main circuit instead of the idle circuit. This is fixed by either opening the secondary blades a little bit more so that the primary blades can be alternately closed that amount or...
(last case scenario) drilling a pair of holes in the throttle blades on the idle transfer slot side of them so that additional air can be pulled in while closing the blades to restore idle mixture adjustment. A good starting point are a pair of 1/8" holes and working up in 1/16th's from there. Granted this may not be your problem, but it's just another avenue to go down. ------------------
Ted E.
Fe's are plenty fast, but "Y"'s are fun when they run in the nines.
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cobravenom71
Gearhead Posts: 927
From: Poinciana, Fl USA
Registered: Aug 2002
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posted 03-15-2004 05:58 PM
OK, so let me see if I have this straight:If the idle speed adjustment screw(drivers side of the carb) is set too fast, then the car will get its idle fuel from the main circuit instead of the idle circuit. And this will cause the idle circuit-supplied fuel to basically have no effect on the idle quality. And this may also be the reason that turning the idle-mixture screws all the way in does not cause the car to stall when idling. Is this correct? If so, then tell me if this is what I should do: Turn the idle speed adjustment screw down until the engine stalls, then open up the idle mixture screws 1 1/2 turns each. Set the idle screws to get the highest vacuum at idle, then set the idle speed. Yes?? This may sound ignorant, but is '1 turn' of the idle mixture screw a full 360 degrees(12:00 o'clock to 12:00 o'clock)?
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Fastymz
Moderator Posts: 19583
From: Reno Nv USA MEM#1240
Registered: Apr 2001
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posted 03-15-2004 06:01 PM
Yes I'd try it and yes one turn = 360.  ------------------
SCOOP oddly obsessed with big scoops on little Mustangs 65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc. My Pics
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cobravenom71
Gearhead Posts: 927
From: Poinciana, Fl USA
Registered: Aug 2002
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posted 03-24-2004 02:50 PM
Here's the latest: I made sure that the throttle linkage was closing all the way (it is), and that the throttle blades were closing all the way (they are). With the idle-screws both turned all the way in, the car started and ran good. I drove about 15 miles, got a tank of hi-test, and upon restarting, it would not idle.
I opened up each idle mixture screw about 1/4 of a turn only, and the engine started easily, idled good and ran fine.
I then drove the car for a total of about 75 more miles with nary a problem, no gas smell, and it ran as good as it ever did. I can't remember the specs on the cam I have installed, but it is a 'Comp Cams Magnum 280', if that means anything to anybody.
Someone told me that with a 'healthier' than stock cam, low-rpm vacuum is very slight, sometimes almost non-existant, and this may be some of the cause of the problem.
His explanantion was that the low vacuum situation could not utilize the full amount of fuel supplied with the 'regular' 1 1/2 turns of the screw's adjustments, and that the very low setting I have could very well be 'normal' for this particular engine. Does this sound right to any of the more-knowledgable experts out there? ------------------
1971 'J' code 429 Mach 1
1976 'Starsky & Hutch' Torino (for sale AFTER March 5th!)
1988 Lincoln Mark VII LSC***New Purchase!***
1973 Convertible***Sold to Lisa H. and Clint B!***
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Hans olsson
Gearhead Posts: 670
From: Sweden
Registered: Apr 2000
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posted 03-24-2004 04:32 PM
The motor should not idle with the idle mixture screws turned all the way in!
The fuel must enter the carb somewhere else.
How far from the seated position are the throttle plates. The correct position shows only a small portion of the transfer slot below the plate.
There are something called "Reverse" Idle System. Some Holleys have that. If yours have it then you richen the mixture by turning the screws in and back them out to lean the mixture.
There should be a label next to the screws if that is the case.
Hans ------------------
- 71 Mach 1 Ram-Air
- 351C 4V 285 HP
- F-G/Stock Aut
- Stockers are way cool!
[This message has been edited by Hans olsson (edited 03-24-2004).]
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cobravenom71
Gearhead Posts: 927
From: Poinciana, Fl USA
Registered: Aug 2002
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posted 03-24-2004 07:28 PM
Hans, re-read all the posts on this topic and you will see that all of your observations have already been discussed and considered.
Thanks for your help, though.
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