Holley Idle Screws Not Responsive

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684mulas

684mulas

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I'm having problems setting up my new Holley 750 DP (4779). The problem is that it seems to run pretty good (maybe a bit rich) but there is no response from the idle mixture screws.

When I first installed it, the screws DID effect the idle but the primary bowl was overflowing with gas so I cleaned some grit out the needle and seat and readjusted the float levels but now the mixture screws DID NOT respond.

Next I took the carb off of the car, cleaned both bowls, metering blocks and the body completely. I blew out all of the passages with air bought new blue gaskets and put it all back together. Still running pretty much fine but no change in the mixture screws.

I took the carb off again, and re-cleaned everything and made sure the gaskets were all correct. Again, no change. Next I took out the new blue gaskets and put the original blue gaskets back in (just to be sure) and still no change.

I checked the timing to rule out any sort of issues there and its all fine. The throttle plates look to be undamaged and closing properly. I have run out of things to check. I'm past what I know about carbs. What can it be?
 
Most likely the primary throttle plates are too far open and are exposing the transfer slots. This happens when an aftermarket cam is used and the idle screw needs to be cranked down for it to idle. There are two resolutions:

A: Not enough initial timing.
B: If you have dialed in enough initial, Under the carb, there is a set screw that can be adjusted to allow additional air into intake. This set screw cracks open the secondaries. Aftermarket throttle plates like the Quick Fuel have this adjustment on top which greatly simplifies tuning.

How much initial timing are you running, what cam and what compression? This information will help us help you. :)
 
The initial timing is 12 degrees and at 2600 rpm its 28 degrees. Any more timing and it starts to ping. I think my Mopar electronic distributor vacuum advance is getting a bit weak. Its a 9.5 compression 340/416 with a xe274h Comp Cam.

Can I check the throttle plate position in comparison to the transfer slots from the top of the carb?
 
I have little knowledge with carbs but do know if idle adjustment is gone it can be a blown power valve. Might not be the case because your carb is new. A heavy backfire can cause this condition.
 
wazoo64 said:
I have little knowledge with carbs but do know if idle adjustment is gone it can be a blown power valve. Might not be the case because your carb is new. A heavy backfire can cause this condition.
Click to expand...

As long as it was apart to replace the gaskets, I replaced the power valve as well. I used the same 6.5 power valve as came with the carb.
 
Holley Idle screw (Or any carb) not being responsive is due to;

Overly rich condition
Advanced timing
Throttle paltes open to far - look into carb primaries with engine off, you should see a square.
 
What is your idle speed? Maybe you could bring it down some. Have you tried a bit more initial timing and NOT running any vacuum advance?
 
How much vacuum are you running? If its low, the PV could be putting in extra gas at idle. You need WAY MORE initial timing on that cam. I ahve the same cam and I have it at 17* for now but might bump it up more.
 
I have had to drill holes ( 1/8th) in the primary butterflies to get more air without uncovering the transfer slots, an old Holley handbook trick. Rich fuel condition=too much gas not enuff air.
 
Not enough initial... try 16-20 range and start over at the baseline carb settings, like what redfastback has in his set up. You're likely already on the transition circuit at idle, primary throttle plates too far open. More initial will allow you to close up the primary blades.

JMO, Leave the drill bits alone until you exhaust EVERY other possible solution.

Make sure the float level is proper and all the vacuum ports are either connected properly or blocked off.
 
No need to drill throttle plates on a Holley. The secondary stop screw can be adjusted to OPEN the secondaries slightly allowing you to close your primaries more at idle. This should get you out of the transfer slot and make the primary idle mixture screws effective.
 
adjust the timing first before you dink with the carb...
 
I am with ramcharger open up your rear butterflys a tad.I do that on every holley I install with a different cam this is a must lets in more air.your too rich,also check you vacume in gear if its low and you dont have the right power valve it will open and over rich it.Holleys are so easy to set up.
 
The vacuum at idle is 8.5" but it looks as if I'm being advised to put in more initial timing so I guess that might change. I'm at 12 degrees of initial timing now but when I advance it any more at idle, I have too much timing at 2600 rpm and it pings like crazy when driving. Its looking as if I need to make some adjustment to my distributor or just replace it. I don't think its worth messing with since its an old stock Mopar electronic thats been in several cars already. I don't have the cash for a complete MSD setup but I think I do need to replace the dist. I see that the Accel 59301 is a replacement for the stock unit. I think I will order that unless there are any better suggestions.

Strangely I went to the track for test and tune last night knowing the carb is not setup as well as possible. The car still went almost a second faster than it did with the Edelbrock 750.
 
I have an XE268H cam in my 360 and it likes 17 degrees of intial advance, I would think you will need that or more. If your MP disributor was built after 2002 it will have an adjustable mechanical advance, just look below the plate the pick is mounted on for two button head torx screws to confirm if it's an adjustable one. You will need to limit the total timing to prevent to much on the top end. If you have the adjustable distributor or get one there is a Mallory kit #29014 that has a selection of springs for adjusting the rate, spacers to set the mechanical and charts to help you understand what the changes will do.

Remember the first step in tuning an engine is to get the timing right, then worry about the carb.
 
Thank you very much good FABO guys for all of your suggestions. I was able to fix the problem by opening the secondary "a crack" with the set screws. I'm not sure of how but, I must have got that unit of measurement on the first try. I think "a crack" is more than "a smidgen" and less than "a bit". (thanks ramcharger for the instructions).

Along the way I did come to find that my Mopar electronic distributor had seen better days. I couldn't get the car properly timed because the timing was moving all over the place at rpm. I could not advance it past 28* total without it shaking all over. Maybe it was fixable but I splurged for an MSD Probillet dist, 6al Box, wire set and I already had a Blaster 2 coil. I put the box under the battery tray. I'm using the black bushing that allows for 18* of advance and now have the initial timing at 18* with 36* at full advance and I may be able to bump that up a degree or two. Considering the cam, 4 speed with a 3.91 gear, I set the rate of advance to the second from fastest. It seems to be a good starting point and I can fine tune it from here.

I'm very interested to see if there will be much improvement in performance because I went to test and tune last weekend (with the messed up distributor and untuned carb) and the car went a surprisingly fast 12.3 at 112. We will see. Thanks again for the help.

MSD 5-10.jpg
 
Hello 684mulas
You mentioned you purchased a MSD Probillet dist. Is that not a mechanical advance unit?
I was under the impression that unless you are all out racing a vacuum advance is recommended.
I would appreciate your comments
thank you
George Gerhard
 
You can get away with no vacuum advance if the int/total & curve is dialed in, & the carb is set up right in the curcuits, not too rich, not too lean.

684mulas, i would set up that carb for 4 corner idle, this will give you alot more adjustability, with that cam, 18 & 36 should be very close.
 
684mulas said:
Thank you very much good FABO guys for all of your suggestions. I was able to fix the problem by opening the secondary "a crack" with the set screws. I'm not sure of how but, I must have got that unit of measurement on the first try. I think "a crack" is more than "a smidgen" and less than "a bit". (thanks ramcharger for the instructions).
Click to expand...

You're welcome. :) 12.3 is no slouch, keep us updated.
 
GeorgeGerhard said:
Hello 684mulas
You mentioned you purchased a MSD Probillet dist. Is that not a mechanical advance unit?
I was under the impression that unless you are all out racing a vacuum advance is recommended.
I would appreciate your comments
thank you
George Gerhard
Click to expand...

Yes, George that is true but I know quite a few other guys that are using the straight mechanical unit and it works fine on the street if its setup well. The vacuum would probably get a little better gas mileage but I don't expect good gas mileage with this car no matter what I do.

JoeDust451 said:
You can get away with no vacuum advance if the int/total & curve is dialed in, & the carb is set up right in the curcuits, not too rich, not too lean.

684mulas, i would set up that carb for 4 corner idle, this will give you alot more adjustability, with that cam, 18 & 36 should be very close.
Click to expand...

The 750 DP that I have does have the 4 corner idle. Now that the screws actually do something, I was careful to make the best use of them (even used a vacuum gauge to get everything setup). The primary idle screws are turned out about 1 full turn and the secondaries are about 3/4th turn. The vacuum is about 12-12.5" and it has a 6.5 power valve. I didn't touch the jetting it came with 71/80.
 
Sorry George but I keep forgetting to take a picture of the engine compartment.

After all of this tinkering, I still had one idle screw that was not responding at all. I also had a terrible stumble off idle. After trying many things (like larger squirter, different accelerator pump cam and many cleanings) with no improvement I switched primary and secondary metering blocks. No more stumble and the same idle screw is not responding on the secondary side. I would have to say that there is a defect in my primary metering block and I'm going to order a replacement.
 
684mulas
are you racing your car at Englishtown this weekend?
I will be there to watch
regards
George
 
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