This thing is kicking my butt! I've been driving it around for years and it always has had a transition stumble. I used Lars directions and set the X-fer slots to .020 and actually had to turn up the Pri and sec screw a little to get my 900 rpm idle hot. I have both Pri and sec "Four Corner" idle air fuel ratio screws I set them with a vacuum guage all the same. About 14 inches.

I have the idle nice and everything is great, unless I punch it to get the squirters to give it a shot it goes into stumble mode when you ease into it. I thought that since it has a 6.5 power valve maybe it needs an 8 so it richens faster. That didn't help so I went up one jet primary and that didn't help either. My plugs have been a nice tan for years so the one jet up would be even more rich.

I'm starting the hate this Holley.

 

What color is the cam on the primaries? If any color other than white, try the white and then move the teh cam on the various holes...think the std ;ocation is #2 hole. Also make sure you have .015 clearance between the doohickie and the thingamajig...daaaah...can't think of the names...the squirt lever and the carb linkeage

EDIT:

Ooops meant to say hole 2....labeled #1

 

LT1er, I have the same problem on my 72LT-1. I spent today trying to chase it down. Correcting a vacum leak on the fuel canister improved it considerably but its still there. The 72 is almost totaly stock with A.I.R system connected and missing only the Transmission controled vacuum valve that controls spark advance. I have the vacuum spark advance connected to the Holly vacum port that applies vacum off idle. I am now wondering if I have the correct carb base to intake gasket. I still suspect a vacuum leak!! Those have given me problems in the past on the 70 LT-1. The 70 runs great but it doesnt have all the A.I.R and vapor canister junk on it. Let me know if and how you solve your problem. I will do the same.



Bullshark

 

My cam is pink. I have the excellerator pump arms set to near "Zero" to give a fast squirt. You said .015. I have vacuum advance on the ported vacuum. I have no emission equipment. As for vacuum leaks, Is it possible for the Power brake booster to have a leak. I'll try plugging off it's line and driving after work today.

 

I had that issue too. I palyed with the pump cams and the ignition timing and the exhaust and it went away. I wish I could tell you exactly what did it for me but here are a few things I did.

I went from pink cam to green cam and this was good. I advise you to try that first. I know the green is supposed to be one step down from Pink but there is a pretty big difference between the pink cam and all the rest. The rest are quite close in shape/rate but the Pink is pretty fat. There are special cams for the Reo pump but forget about them for now. Just try the green cam.

The adjustment is like this...You want .015" ADDITIONAL travel available in the pump arm while carb is at WOT. You want it to be able to go an additional .015" so put a .015" feeler gauge in there and take it to WOT and see if the arm is binding on the feeler gauge. You do not want slack in the accelerator linkage at idle or you WILL have a flat spot off idle.

If you are running like 16° initial timing, try dropping back to 12° or so and see if it helps. This is easy to change back if it does not work. If this actualy does fix it, you might want to add the 4 degrees to the centrifugal advance permanantly.

I also pulled off my 1-3/4" Hooker Super Comps and installed the cast iron manifolds. I doubt this is what fixed it for me and I doubt you want to pull your headers off. I have never had a stumble with 1-5/8" primaries on any car but this one had an issue.

Let us know how you made out.

-Mark.

 

Moving the pump cam to the #1 hole will prolly help. Also, just for poops and giggles, adjust the primary transition slot to the required .020". Then adjust the idle using only the secondary idle screw. This helped iron out the stumble on my 850, it might work for you too.

Brett

 

Symptom - Remedy

Fuel comes out of vent tube or runs out
A) Lower float level
B) Decrease fuel pressure
C) Remove, clean/replace needle-&-seat
D) Inspect float to ensure it's moving freely

Backfires or Pops through carburetor
A) Open idle-mixture screws
B) Increase squirter size
C) Increase jet size
D) Raise float level

Backfires or Pops through exhaust
A) Lower float level
B) Decrease fuel pressure
C) Decrease jet size
D) Readjust butterfly position, primary & secondary

Engine won't start
A) Prime carburetor with fuel
B) Pump squirters to get fuel flowing
C) Check ignition timing

Fuel leaks from throttle shafts
A) Lower float level
B) Decrease fuel pressure
C) Readjust butterfly position, primary & secondary

Is rich at idle
A) Increase initial timing
B) Go in on mixture screws
C) Readjust butterfly positions
D) Lower float level
E) Decrease fuel pressure

Stumbles under light acceleration
A) Open mixture screws
B) Readjust butterfly positions
C) Raise float level

Stumbles under hard acceleration
A) Readjust butterfly position
B) Increase squirter size

Won't return to idle
A) Increase initial timing
B) Readjust butterflies

Surges at part throttle
A) Open idle-mixture screws
B) Open primary butterflies & close secondary
C) Raise float level
D) Increase primary jet size

Emits black smoke under hard acceleration
A) Lower float level
B) Decrease fuel pressure
C) Decrease secondary jet size

 

Don't screw with the pump cam just leave it alone for right now. The hesitation is caused by the lack of fuel when transistioning from the idel to main fuel circuits. Put the power valve back in. To correctly size the power valve take the manifold vacuum reading at Idle and divde by 2. That will give you the correct power valve size you will need.

To correct you stumble or hesitation you need to fix the lack of fuel void during the transition. IF you will open up the choke horn and look down inside there you will see a little tower that the fuel squirts from. On a double pumper there are two of them. You just need to keep going up on the squirter size until it goes away. Do the primary side first and give it a test drive. If still there then go up on the secondary side. Once its gone your there. The size of the quirter is stamped on its body. I wouldn't mess with jetting. Sounds like your dead on.

 

I was amazed at how sensitive my eng is to having any slack what so ever, if fuel doesn't come out immediately my eng will stumble. A slight crack in the throttle plate will not drop the vac level enough to open the power valve. Also check for fuel boil, I noticed the last time I changed my carb gasket I started to get fuel perculating out the squirters with the eng hot at idle.

 

After a logical process of troubleshooting and elimination I finally determined my stumble problem must be internal to the carb. So I just removed it and did a visual......I think I found the problem and now I am really scared :shocked: :shocked: Do you see what I see??




Please pray for me

Bullshark

 

Pray that someone took those screws out and forgot to reinstall them?

 

When I first installed a 750DP , I had the same problem.

If you drove it hard, (hard launch), you didn't notice the problem.
If you start easy and as soon as you are off idle you get the stumble.

I tried all the things mentioned in all the replies. You could make it better or worse, however not completely cure it.

I did however fix the problem completely, and the carb has given me many years of reliable service.

I DON'T recommend you do this unless you have tried everything else.
There may be a better way.

However, in my case, I could not get the transfer slot/idle/throttle plates to work in harmony.
Fix one thing and screw up something else.

Now, to the point, I increased the length of the transfer slots.
I lowered them slightly to have them become active earlier.
The stumble went away completely.

The problem I had was the idle circuit ran out before the transfer slots flowed enough.

The problem with trying this, is that you can't go back once you cut the slots lower.

All I can say is that it worked for me.

Barry

 

I hope you are right!!! I don't have any evidence that the screws fell out into the engine. I would think that if they went thru the valves I would know it

Bullshark

 

There probably not hiding like little hand granades. I guess it depends on the spacer/manifold you are using also. Are you running a single plane?

 

No its the LT-1 stock dual plane. I stuck a flexable magnet down each port to see if anything came up. Nothing!!! I plan to take out the plugs tomorrow and see it there is any evidence of anything bouncing around. A potential positive sign was that all the other screws were not loose.

Bullshark

 

Stop wasting your time! The screws were NEVER there! The only time I have found screws there is when a complete IDIOT has rebuilt a Holley from more than one junk core. The screws would have made themselves quite evident by now if they fell into your motor (and they would do considerable damage!) DO NOT! put any screws in those holes! Holley left them out to prevent just the nightmare you are worrying about. I think your jetting is LEAN! You should try going up 2-3 jet sizes and see if it helps the problem. There is also a secondary power valve on your factory Holley in the rear metering block (should be an 8.5" one witha 6.5" primary) if some IDIOT has taken it out and plugged it off you could have an EXTREME lean condition (or a very rich condtion if it is "blown" from a backfire.) Check the power valve by sucking on the diapragm side of the valve, it shold hold a vacuum (this may be cancer causing but, I have sucessfully done this for over 30 years now.) Holley sells a "Trick Kit" rebuild kit for your carb with extra jets, accelerator cams/shooters, they are pricey but, you may want to start there. You carb is a 800 CFM Vacuum secondary "FACTORY" Holley by the way NOT a 750 CFM "double pumper" but, on the positive side, I think your car is probably a "legit" LT-1 because it has this carb on it. Email me if you have any other questions.

 

I have the same slight stumble with my 650 dbl pumper. I have been doing other stuff to my car up till now, brakes, exhaust, plugs, bent pushrod and on an on.

This car has set for over 10 yrs and needed attention to just get it going. I'm at the point where I'm going after this slight stumble. I have also noticed that it seems my carb is running a lil rich but after the stumble it seems to pull hard.

I got a feeling its going to take some time to get that stumble out but I'll get it right or I'll be getting a new 850 dbl pumper. I'm building up my engine and when I'm finished I'll probly need an 850 anyway.

 

SolidLT1 is right: Those 2 screws are not installed on Holley carbs - they should not be there, so that is not the problem.

As if you haven't received enough advice already, let me throw a few things at you:

First, make darned sure you're running enough timing. Check and set total timing to 36, and curve the distributor so you have at least 18 degrees initial timing at idle. Switch the vacuum advance over to manifold vacuum and make sure the vacuum advance control unit (the "can") is soft enough so that it sucks its full advance in at idle - try a NAPA VC1810 if you have a bit of a cam.

Once done, bump your primary jetting 2 sizes up from the stock jetting for your carb list number. Let me know if you need the stock jet size to verify baseline. Set and verify float levels and adjust idle mixtures just slightly on the rich side (like 1/8 turn out) from best idle.

If you still have a stumble, bump your squirter size up. You can also install the 50cc accel pump for better tunability of the accel circuit.

 

BTW. . .
Lars, you are the man!!!
My '79 purrs like a kitten thanks to your advice a couple of months ago. Thanks!.
Timing and carbs can be a pain, but; just have patience.

Part of my troubles was timing, and part carb. (650DP)

If you've already messed with the jets and power valve, I'm with lars, check your timing. Make sure that your distributor weights are not rusty or gummed up. (mine were) So, I wasn't getting proper advance and was getting stumbling.
If not that, then check to make sure that you are getting that 36* he is talking about.

Good Luck!!

 

79 -
Glad to hear you've had some successful tuning out there! As you note, the key to overall tuning is a systems-level approach that addresses timing as well as carburation. Since the systems have to work together, both have to be correct in order to avoid performance problems. It's fun to do this kind of tweaking, and quite rewarding when it all works!