Hello Fellas,

I am trying to get my 468 to idle (with the vac guage on) and it does not want to happen.
I am thinking it is a vacuum leak but perhaps it is the carb. The carb is a 750 double pumper.

I CAN get it to idle and am able to set the timing at 12 degress when I unhook the manifold vacuum port. I heard that this speeds up the idle but obviously I need the vac for my modulator and brakes.

Here is what I tried.

1. Put duct tape over valve cover holes (pvc and breather). Pushes out tape a bit. Does not suck tape in.

2.Removed heat shield from under carb suspecting a leak. Installed carb right on manifold. No change. Also tightened down carb bolts (not too much to avoid cracking baseplate)

3.Put rag over top of carb while running. No change.

4.Tightened all manifold bolts.

5.Turned idle screws counter clockwise 2 turns. It is probably really rich now.

6. At 2000 rpm vac guage reading is 10 pounds and at 1000 vac is 5 rpm.

I heard a what sounded like a huge hissing sound coming the ventri area of the carb. Is this normal. I tried to pin point the location using a rubber hose as a listening device, but I need a assistant. Should I focus my attention in the carb area. If so is there a typical area that it would leak.

Thanks in advance,

Kris

Might look at the rear of the baseplate. Most Holleys have an open vacuum port there. Easily missed on installation.

Hello d1_bradley,

Are you referring to the outlet that is commonly used for the PCV valve? Is there another one on the baseplate in the rear other than that?
I guess I should post the part # it is a O4779C.

Normally, its used for power brakes and the 'tube' at the front center is used for PCV valve, but yeah it would be a pipe threaded hole centered in the rear of the baseplate. I've never seen one out of the box with a plug in it.

Dave

Hello guys,

Well I am revisiting this problem now that I have all the stock electrical system running.

I am very sure it is the carb because I installed another carb (my friends) and I was able to get it to idle.

I am thinking that it could be the idle circuit or something is really richening the system. I took apart the carb and put it back together and it seemed to idle but then after awhile it reverted back to the original state. When I was able to get it idle vacuum readings were low (8 inch) and turning the idle mixture screws had no effect.
The powervalves didn't seem to be reptured when I re-assembled it.

The 2 vacuum outlets for the carb seem to be fine. One is connected to the PCV valve and the other one is blockled off.

I backed out the idle mixture screws and squirted some carb cleaner in the holes but it didn't change anything.

This is some previous info...

I am thinking it is a vacuum leak but perhaps it is the carb. The carb is a 750 double pumper.


I CAN get it to idle and am able to set the timing at 12 degress when I unhook the manifold vacuum port. I heard that this speeds up the idle but obviously I need the vac for my modulator and brakes.


1. Put duct tape over valve cover holes (pvc and breather). Pushes out tape a bit. Does not suck tape in.

2.Removed heat shield from under carb suspecting a leak. Installed carb right on manifold. No change. Also tightened down carb bolts (not too much to avoid cracking baseplate)

3.Put rag over top of carb while running. No change.

4.Tightened all manifold bolts.

5.Turned idle screws counter clockwise 2 turns. It is probably really rich now.

6. At 2000 rpm vac guage reading is 10 pounds and at 1000 vac is 5 rpm.

I heard a what sounded like a huge hissing sound coming the ventri area of the carb. Is this normal???
I tried to pin point the location using a rubber hose as a listening device, but I need a assistant. Should I focus my attention in the carb area. If so is there a typical area that it would leak.


Any tips/ideas would be appreciated. I would really like to get this monkey off my back!!!

Kris

KRIS,you state your running 12 deg inital,that not enough timing if your runing an aftermarket perf cam esp if its a failry hot cam.

Most aftermarket perf cams require approx 18 deg btdc initial timing so in that case your running approx 6 deg retartded . Retarded inital timing wastes fuel,looses power(esp low end),and will cause the motor to run a little hotter too.

So if your running an aftermarket perf cam crank up the initial timing to 18 deg but make sure the total doesnt go over 36-38 deg. If it goes over 38 deg total then you need to have the mechanical advance in the dist recurved/limited to 18-20 deg max(all in by 2,800-3k rpms) to acieve 36-38 deg total. This is important to watch out for to avoid harmfull detonation/ping.

The 6 deg additional initial timing should help the motor idle better too.

You can also try running the vac adv off the full on vacuum port insted of the ported port to get a few more deg timing at idle. This is esp helpful if the cam is large enough where it needs the additional timing from the vac adv at idle.

BTY,what type cam are you running in the 468 & what are the specs?

Scott

Kris:

Two questions

What intake manifold are you using with that Holley DP ?

What type was the different carb that worked OK for you ?

Hello guys,

The cam&intake specs are

Intake lift .560 Exhaust lift .560 Based on 1.75 Rockers
Duration at .050 231 degrees
hydr flat tappet lifters

Eldelbrock Performer RPM manifold EDE–7161


The borrowed carb is a Holley 750 vac secondary.

When the double dumper carb was on this same motor in my old Beaumont it worked well. This is before I sent it out to be rebuilt and after the rebuild it didn't work properly. I have since rebuilt it myself and the problem still persists.

I can't even really set the timing because the motor will die below 1000 rpm. I think the timing is at least in the "ball park" because it idled okay with the borrowed carb. I have being trying to borrow the carb again just to get the timing set right to totally elimate that problem.

Kris

I ran into the same problem with my Holley/Edelbrock performer combo
This problem drove me nuts for about ten days.

I suspect you have a vacuum leak at the rear of the carb baseplate where it meets the intake manifold. -- If you pull out the carb/manifold gasket and examine it you may see signs of this.

The reason for this is the performer intake is designed for a spread bore carb maybe a qjet most likely the Edelbrock version.

If you look at the underside of the Holley baseplate there is like a recessed passage other than the venturis that may be hanging over the back edge of that contoured part of the manifold. Trust me it doesnt overlap by much but it causes big problems.

If this is the case, good news - There is an 1/8" thick adaptor that fits under your baseplate. You can buy it from your local speed shop for your Holley for cheap.

This should solve your problem

Let me know how you make out

Oh yeah I forgot to mention a heat shield will not do the job. Not rigid enough.

Thanks Mike,

That is a good suggestion but unfortunately it isn't possible. The manifold is a Performer RPM#7161 which is meant for a square bore carb and the borrowed carb (which I was able to get idle) was a 4150 series which has a simillar baseplate as the 750 double pumper.

Kris

Kris
Is this an older model Holley?
Did changing the idle screws do anything?
Are you sure idle circuits are functioning ( idle screw set too high to kick in the idle circuits ??)
What size power valve are you running now?
What size before it was rebuilt?
Can you post a pic of your carb looking at it dead on from the drivers side?

Hello Mike,
It is a Holley “Double Pumper” 750 CFM 1450 Carburetor 4779–0 produced in 1994.

When you say changing the idle screws do you mean switching positions on each screw? I took all of them out and examined them and there didn't seem to be much of a difference. The ends seemed okay but maybe I could replace them with new ones...
I also blasted some carb spray through the holes and no change.

I wanted to check the transfer screws to make sure the idle circuit is functioning. I looked in the Holley book and it doesn't give the exact location of this adjustment. I think there is 2 adjustments one for the primary blades and one the secodnary. I know the adjustment screws are in the baseplate area. I searched and found some sort of procedure to adjust them something like put the blades the same level as the holes and then a 1/4 turn????
I also wanted to remove the metering blocks and blow compressed air through all the passages.

I think the new power valve is HLY-125-65 6.5 hg. This was the same powervalve size before the rebuild.

I will try to get a drivers side pic soon.

Kris

As you adjust the idle screws in an out there should be some change in idle.
Ideally you should use a vacuum guage to determine the optimal setting with the highest vacuum reading indicating the desired setting. If there is no noticeable change when adjusting the screws then theres a good chance the idle adjustment is not low enough for the idle circuits to kick in.

First thing to do is try advancing the timing a little so you can back off on the idle screw and close those throttle blades up in the primaries so the idle circuits will kick in.

Your not pulling a ton of vacuum at 1000 rpm but I dont think the 6.5 power valves are the problem.

You also have a newer model Holley which is euipped with power valve blow out protection, so its unlikely that these were damaged by, say, a backfire through the carb during initial startup where maybe the timing was off. Its hard to tell if theyre damaged by just looking but if the little membrane in them is wrinkled they may be shot.

Its always a good idea to "dunk" the metering blocks overnight as well as the main body assembly. Blow out all orifices with compressed air especially the idle circuits (which are those slots in the venturis front of the throttle blades) in the main body and the metering blocks. Holleys dont like dirt!

Thanks Mike,
I am hoping tonite to remove the carb and start to examine it. We have a lot of snow/ice falling right now and it isn't supposed to stop until tommorow. So I wouldn't be able to try to start it. I will keep you updated.

Kris

Hello,
Last night I I got it idle. I adjusted the fast idle screw to expose more of either the idle transfer slot or an additional idle hole these adjustments seemed to let it idle.

I was able able to set initial timing at around 13 degrees.
After around 5 minutes it started to idle rough and die. Upon further examination it seemed like the primary venturi area was flooding with fuel. I thought the next thing to do was to adjust the float level. I couldn't seem to get them adjusted and I think that the primary side is way out of adjustment. I will take off the primary float bowl and set it back to a baseline adjustment and then start it up again and see what happens.

I tried to start it up after the "flooding" and it didn't want to fire.

Kris

Kris while youve got that primary fuel bowl off there , remove the float itself and check for gas in the float by shaking it. Obviously if you hear anything sloshing around in there it should be replaced. Check the inlet valve and seat assembly as well to make sure its not damaged or contaminated.

UPDATE:

Hello guys,

Well I finally got it idle for a extended period of time which is great. I was able to set the float levels and one needle (primary) was contaminated with debris. Which is probably why it was flooding. I cleaned the needle seats of course.
I made up some note to remind me how to adjust them.Maybe someone can use them one day.

TOO MUCH fuel-Float level too high-turn nut CLOCKWISE to lower
TOO LITTLE fuel-Float level too low-turn nut COUNTER-CLOCKWISE to raise

Okay next up set the idle mixture screws. There isn't much change when I adjust them in and out. I only have around 8 inches hg at approx 900 rpm.
My cam can't be that radical????
Intake lift .560 Exhaust lift .560 Based on 1.75 Rockers
Duration at .050 231 degrees
So one thing I can assume is a vac leak. So the next step is to run the engine and spray some carb cleaner and note if the rpms increase in a specific location. I noticed too that when a unplugged the manifold vac source the idle speed up?? I wonder why this happened??

Okay I hoped I don't seem like I am rambling like Smokey Yunick in his biography.

Kris

Get it to run, put a air cleaner on it, and spray carb cleaner around the base of the carb, then around the intake where it meets the head, and any other ports, when the idle changes " and it will when you hit the leak " you'll have found your problem, WD-40 works too, but carb cleaner is a bigger change

Update2
Hello guys,
I sprayed carb cleaner while ideling and no noticable increase in RPM. THERE was a noticable INCREASE in RPM when I disconnected the vac line feeding into the manifold. I noticed that the front 2 idle mixture screws didn't seem to have much of effect when turned in and out but the back 2 seemed to respond. I can get the motor to stall when the back 2 are fully seated.

Perhaps the 2 front idle mixture screws are "bad" and therefore richening the idle circuit?? Perhaps when a vac leak is introduced it leans out the mixture??? I recall one idle mixture screw being a bit out of shape....

What do you think guys??

Thanks
Kris

Hello guys,
I swapped carbs (his is a 750 vac seondary Holley) for my friends which ran fine on his Olds 442. No change in vac levels. Infact my carb ran better and the vac levels were slightly higher. I couldn't hardly get it idle with my his carb. So now I can assume that the problem is not carb related.

When I re-installed my carb again and got it to idle I tried to spary carb cleaner in all the intake manifold areas and no change in idle speed.
Okay so maybe this is a intake vac leak which is very hard to find?? A few years ago I had the motor out (on a stand) and turned it upside down so maybe it ruined the intake gaskets due to oil infection???

One thing too is that these are small oval port 236 casting heads and the intake manifold is meant for large oval port. Maybe this is part of the problem?
I seem to remember an old post where the guy couldn't get good vac because of using oval heads and rec intake ports....

Kris

Try taking a pencil eraser and plug the idle vents while running. You should have a definite idle speed change; up if its to lean. They are the outer bleeds in the air horn. Check both sidesand if no change you have an idle circuit problem. The power valve has nothing to do with the idle circuit.

Hello Gary,
I guess you are referring to the idle air bleeds located in the choke area? Should I put the eraser on all of them or just the primaries? I don't have adjustable air bleeds.
Yes the power valve is a main circuit system.

It is too bad I never took any vac readings 7 years ago when this motor was in another car. I wonder what levels it was back then? I wonder if it is worth changing the gaskets? They are likely a $20 item and around a 3or4 hr install time.
Thanks
Kris