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Old 08-21-2006, 09:05 PM
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600 holley problem

I just recently rebuilt a 600 cfm carb for my 85' 305 chevy truck.The problem I am having is that the air fuel mixture screws don't do anything.You can turn them all the way in,and the motor won't quit,you can almost unscrew them from the metering block and nothing changes.I ended up screwing them all the way in then backed them out 1 1/2 turns not that it did anything.I have a fuel pressure gauge installed and the needle fluctuates like crazy at idle.Is this normal for a mechanical fuel pump?As far as I know this is the orginal fuel pump,so maybe it needs to be replaced.This may all lead up to the other problem I'm having ,when driving normal the truck accelerates as usual and drives along just find.But around say 65 mph(Idon't have a tach)if you jump on it hard it almost sounds like its running out of gas,it doesn't pop or sputter it almost sounds like your decelerating.If you let off the gas just a little then it runs fine and speed begins to pick back up.So if you get on the gas then back off then back on it will slowly pickup speed.Now if you drop it into second gear it acts normal(I don't have the kickdown hooked up right now).If you are sitting still and drop the hammer on it ,it takes off barks 2nd and keeps on accelerating.The carb is your every day 600cfm vac secondary Holley,it has 55jets in it.Could the jets be to small,I have some 64,s,could it be that the secondaries aren't opening? I thought about changing the spring on them!Any help is appreciated,I,ll talk back at ya'll tomorrow I've got to get some sleep. THANKS ALOT for the help!!

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Old 08-22-2006, 09:30 AM
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what is the carb list number? holley does make emmision carbs with idle mixture screws where in is rich and out is lean. on these carbs the idle mixture screws have very little effect


henry @ oles carb
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Old 08-22-2006, 10:18 AM
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You probably have the throttle blade open tooo far, uncovering the transfer slot. Raise the timing and lower the idle back down. If you still don't geta response from the idle screws then you may have to do some drilling. What camshaft?
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Old 08-22-2006, 10:50 AM
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What is your idle speed and quality like?
Do you have this thing idling real fast to compensate for a really rough idle?
Raunchy cam?
Does the engine "run on" when you turn the ignition off?

Here are a couple of things that I can suggest that you check:
Holey Tuning tips More

#1.) Take the carb off, leaving the idle speed screw as it is currently adjusted. Have a look at the relationship between the fuel transfer slots and the butterflies. If the slots are completely exposed then adjusting the mixture screws will not change ANYTHING.

Quote:
Originally Posted by Holley
QUESTION Why is my engine running rich and my mixture screws do not have effect?
ANSWER The first thing you need to check is your float level. No fuel should run from the sight hole unless you shake the vehicle. Next check the engine vacuum at idle. (in gear if A/T) if it is 12" or more a 6.5" powervalve will usually be fine. Anything below 12" divide in half. For example 9" vacuum will use a 4.5" power valve. Another possible cause associated with performance cam shafts are exposed transfer slots. You should not see more than .025"-.030" of the transfer slot exposed past the throttle plate at idle.
#2.) If #1 is true ... you'll have to discover WHY you need to have the idle speed screw adjusted up so high. It's almost certainly due to a vaccuum leak somewhere.

Can you get your hands on a vaccuum guage and supply us with a reading (in/Hg)

PCV hose & valve in place?
Vaccuum hose and check valve to power brake booster in good shape?
Vaccuum hose to trans modulator valve? (if equipped)
Rubber caps on all unused ports on both the carb and the vaccuum "tree" which screws into the intake?

#3.) If all of the above check out OK, have a GOOD look at the carb-to-intake gasket(s) to make sure that they are correct ... and cover up all vaccuum passageways.

#4.) EGR valve (if equipped) ... is it clean of soot/carbon/corrosion and does it seem to operate freely. Use a little carb or combustion-chamber cleaner to help loosen / dissolve any of the crap that may be causing the valve to stick open.

#5.) Back the car out of the garage (!) and fire it up. (You won't want to breathe the exhaust fumes from this test as there will be a lot of chlorine present if there is a leak.) Spray a little brake cleaner along the edges of the intake-to-cylinder head area and listen for a change in idle speed/quality. If you do, it's an indication of a gasket leak there.
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Old 08-22-2006, 09:31 PM
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600 holley problem

Thanks to everybody for your help.The motor is a stock 305,with a Performer intake,Pertronix HEI kitand dual exhaust.The Holley is a 6619(part#),4160(model#)600cfm carb I just rebuilt it.The cam is stock.I put the 600 on it because I had a 750 dp on it that I had jetted down,believe it or not it ran pretty good,had a slight bog in it off idle,just used alot of gas.The reason this was on there was because I had rebuilt the Q-jet and never could make it run right and I did not have another carb at that time.The carb idles fine and has no other problems other than the ones described.Ihave never tuned a carb with a vacum gauge how do i go about that.I have been reading a book called Super tuning Holley carbs and they do not show how to do this.Once again thank you for your help!! Michael
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Old 08-22-2006, 10:00 PM
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Quote:
Originally Posted by mupton
Ihave never tuned a carb with a vacum gauge how do i go about that.
1.) Go back to my original reply, click on the "More" link.
2.) go to page 10:

IDLE MIXTURE NEEDLES: Idle mixture needles control the air/fuel mixture at idle. These have been preset at the factory and SHOULD NOT need any adjustments. However, if you feel that adjustment is necessary, you can use the following procedure to do so. When tuning the idle mixture, you’re actually tuning for the best manifold vacuum. Idle mixture needles are found on the primary metering blocks. If you change one idle mixture needle, you must change the other idle mixture needle by the same amount. Here are the proper steps for setting the idle mixture needles. 1. Attach the vacuum gauge to a manifold vacuum port on the throttle body (Figure 3). 2. Adjust each idle mixture screw (Figures 10 & 11) 1/8 turn at a time, alternating between each screw. Turn them equally, until you achieve the highest possible vacuum reading without adjusting the curb idle speed screw. Turn screws in to lean the mixture. Turn them out to richen the mixture. Figure 9 3. Now that the idle mixture is set, it may be necessary to go back and reset the idle speed using the curb idle speed screw, as shown in Figure 9."

Re-reading YOUR original post, it does sound like you may have a fuel delivery problem. I'd try pulling the coil wire off (or unplugging the pigtail to the HEI)
Disconnect the fuel line hose from the carb, and extend it so that you can collect the fuel in a remote container ... I like to use a 1-qt glass sealer for this. have a buddy crank the engine (it won't start because you've disable the ignition, right?)

After a few strokes, the fuel should be coming in "gushes" of considerable volume and force, not bubbling or dribbling out, or getting sucked back in.

I'm thinking that the erratic guage readings that you have described may indicate that the fuel pump check valve or diaphragm may be allowing reversion of fuel in the "upstroke" of the pump?

HTH,
Don

Last edited by 66GMC; 08-22-2006 at 10:16 PM.
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Old 08-23-2006, 01:46 PM
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Put a vacuum gage on the ported vacuum source (vacuum port above the throttle blades) and if there is vacuum on that port then the front throttle blades are open past the transition slot which will make the idle mixture screws worthless and the engine idle very rich.

If this is the case, pull the carb and crack open the secondary blades with the set screw under the carb. Then turn down the idle speed on the front blades. Then adjust the mixture screw with the engine hot for the smoothest idle.

Install a new set of plugs and read the mixture on the plug to set the front jet size. The bottom ring of the plug thread should have soot 1/2 way around. Jet the rear down until no black smoke is seen coming out of the exhaust pipes at WOT.

A 305 in a truck will cause the vacuum to drop a lot even during light acceleration with a stock cam. A 6.5 or 5.5 power valve should work well for good mpg.
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Old 08-23-2006, 04:24 PM
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Quote:
Originally Posted by 454C10
If this is the case, pull the carb and crack open the secondary blades with the set screw under the carb. Then turn down the idle speed on the front blades.
Neat!
I'd never heard / thought of that. I can't think of any reason why that wouldn't work ... and it's a much less "permanent" solution than drilling holes in the primary butterflies.

Thanx for posting that, dude!
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Old 08-24-2006, 01:03 PM
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600 holley problem

Hey everybody thanks for the help.I think I figured out the problem.1)Its a reverse idle carb.2)My fuel pump must have been bad.I put a new one on there and doesn't do what it was doing anymore. Once again thanks for all your help. Michael
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Old 08-24-2006, 01:19 PM
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Quote:
Originally Posted by mupton
Hey everybody thanks for the help.I think I figured out the problem.1)Its a reverse idle carb.2)My fuel pump must have been bad.I put a new one on there and doesn't do what it was doing anymore. Once again thanks for all your help. Michael

What is a "reverse idle carb" ??
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Old 08-24-2006, 02:35 PM
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I'll bet he means it's one of those holleys that is turn the idle screws in to fatten the idle mixture, turn 'em out to make it leaner.
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