[Nsgrn]

My carb has the pink cams. I am trying to resolve off idle stumble issues. In and out of gear. To me it seems like plenty of air not enough fuel.
One question I have is, On the primary throttle linkage were the pump cam sits it has 2 screws holes marked 1 and 2. On the cam itself has 3 holes. What hole should I use the screw in the linkage to what hole in the cam? The primary was set using linkage hole 2 into pump cam 3.

I am confident my timing is good and I have no vacuum leaks. Thanks for any input.

[Nsgrn]

No, none at all.

[Tom Vaught]

Originally Posted by Nsgrn

My carb has the pink cams. I am trying to resolve off idle stumble issues. In and out of gear. To me it seems like plenty of air not enough fuel.
One question I have is, On the primary throttle linkage were the pump cam sits it has 2 screws holes marked 1 and 2. On the cam itself has 3 holes. What hole should I use the screw in the linkage to what hole in the cam? The primary was set using linkage hole 2 into pump cam 3.

I am confident my timing is good and I have no vacuum leaks. Thanks for any input.

Pump cams tailor the duration of the pump shot and where it starts.
More lift, more duration thru the same sized shooter holes.

What size are your squirter holes?

Typically you fix off idle stumbles with more pump shot hole diameter.

Stock on a 750 is around a #31 shooter, sope have had the shooters swapped for a #35 shooter. The best fix is to move the rear 50cc pump, lever arm, and shooter to the front and put the 30cc pump stuff on the rear.

You will like the change when you start opening up the shooter with the 50cc pump on the front.

Tom V.

[BruceWilkie]

Off idle stumble just barely moving the pedal??? Sometimes its not the pump or pump settings. Can sometimes be primary open too far at idle combined with secondary not open far enough.

Agree swappin the 50cc accell pumps and stuff from the secondary to the primary is usually a good thing.

[4dblnkldude]

I am getting that book! Good luck!

[Tom Vaught]

Originally Posted by Pontirag

Tom...

while your advice is good, there is not enough information in his post to draw any specific remidial conclusions yet.

Nsgrn....

"No, None at all"

I would spend about 40 bucks and 5 minutes on amazon.com looking for a few good pubs on holleys. One or two really good ones come emidieatly to mind and will answer more questions than you can think of.

i would not buy a 50cc pump yet untill you know if the stumble is a lean condition or a rich condition, and there are other ways of resolving the issue. several of them cheaper than a new pump. If I recall between the two holes in the arm and the three in the cam there are five different settings you can explore without spending any money. those books will also come in very handy when it comes time to instal a 30 or 50 cc pump, squirters or shooters and cams

Have someone stand behind the car while you hit the gas, Black smoke ? its already too rich.

the book below is a good one, I searched Amazon and found it in about 13 seconds, most of that time was spent dounloding its image here for you. There are a few others I would highly recommend to but I dont have here.

The book you mentioned is a Good Book.

The book, my Boss (Mike Urich), at Holley Carburetor wrote is better.

Always read the book written by the guy who actually designed the carbs first.

Then you can hot rod them and play all you want but at least you will understand how to get back to the factory calibration again.

Agree that the primary blade vs transfer slot position (if we knew what the rest of the combination was) would be helpful:
Engine, camshaft idle vacuum, stall in converter, manual or auto trans, actual ignition timing, etc but the OP did not provide that info. Did say no vacuum leaks.

He wanted to talk about accelerator pump cams and positions and what they would do for him.

By the way Pontirag, Double Pumper Carbs typically will have a 30cc pump and a 50cc pump. Don't have to buy anything, just swap positions of the pumps and related linkage.

You take it from her Pontirag, I am done.

Tom Vaught.

[ta man]

Set the transfer slot square or as close to square as you can...

[Nsgrn]

I have set the transition slots accordingly. Idle warm is 900 out 750 in gear. All 4corners are 1.5 turns out. I bumped open the secondary throttle blades a little to get the idle where its at. When hook my vacuum gauge up it reads 7-10, it bounces around a lot. Timing is 14 initial all in around 2500-3000 around 34 on the tape. I did step up on discharge nozzle to a 33, I believe.

The setup is a 400ci with 48 heads/comp springs and rockers, 10.1 comp, comp XE284 240-246@50 .507-.510
110LS, Holley Street Dom single plain, HEI, 2800 stall, 4.11 gear

I found this on you tube explaining the mounting configs on pump cams helpful. According to the information I should be using hole 1, mine was at 3 which states more a delayed shot. If this has no effect I will go after the pump swap.

http://www.youtube.com/watch?v=eiRIfcrc2yA

[Nsgrn]

Couple things I forgot to mention about fuel delivery and settings. Float levels are good.
Running a holley red pump regulated at 5psi. It is dead headed with no return, which I read is not to good. The car is a 68 Bird, so the tank has no return or vent line on the tank. Since vented gas caps only breathe out and not in, I drilled a 1/16 hole in the center of my gas cap.

[olescarb]

Many of the later Holley carbs have a lean off idle problem that was caused by the idle well being too small for todays gasoline, the original design of a 750 double pumper used a idle well that was about 0.106" but they used a taper core plug to form the idle well on most of the carbs built after the 4779-2 production. This tapered core plug leaves the hole at the bottom of the well right by the idle jet in the 0.080" range which will cause a lean problem but the new production now uses a one size idle well. We have had good luck drilling the idle well to 0.106" plus you need to be sure the idle jet is pressed in all the way so it is not blocking the passage from the main jet to the idle jet. the following link has a pic of the idle well with arrows to the area plus a picture that shows a stronger pump duration spring we use to bring the duration spring tension back to what Holley used back in the 1960s so the pump squirt will be as strong as the original design.
http://www.enginebuildermag.com/2013...ttle-response/
Holley also used a "kill bleed" in the 4779 that they did not use in the later production unit that we drill when tuning these carbs, they drilled it in the secondary side but not in the primary block, this bleed seems to help the mains come in quicker.
I hope this helps Henry @ oles carb

[ta man]

Is your timing steady at idle or is it bouncing around?

[Nsgrn]

It's been awhile since I had a light on it. I would say it was steady.

[Nsgrn]

Henry, Thanks for the good read.
Unfortunately I may be fighting with something I cannot win. Instead of buying new I bought this basket case 750dp AED. It was complete but I am not qualified to tell what internal mods have been done. It had an odd configuration, to me anyway. When I took it apart for cleaning, gaskets and pumps. It had 70 pri main and 80 rear with a plugged PV in the primary block..

Currently its running 74/80 for jets. And 3.5 PV in front and plugged secondary PV.
What gives the carb a more rich condition 3.5 or 6.5 PV? 3.5 correct?

[olescarb]

a 3.5 power valve comes in at lower vacuum a 6.5 comes in sooner. Most of the current production 4779 Holleys have the original design idle well sizes but if your carb has a tapered idle well you may need to resize the idle well. The new Ultra series carbs also have the correct calibration. If you are able to read the a/f mixture you can watch the a/f mixture shift lean off-idle the you will know if it is the problem or you can speed the engine up to about 2000 rpm, then enrichen the carb by closing the choke. if it runs better or speeds up when you make it richer you know it needs some tuning attention.

Henry @ oles

[HWYSTR455]

If your vacuum gauge is bouncing, you have other things to deal with before you start on the carb. Pointless to try to dial it in before correcting that.

1.5 turns on all 4 sounds fat to me, but every engine is different.

I believe Holleys also like about 6.5-7 psi too, I set mine there, but maybe others can chime in.

.

[Nsgrn]

This setup will not idle, unless all 4 corners are at least 1.5 turns out. Turn them all in 1/4 it shuts down

I set the pump cams to hole 1, adjusted the pump arms, changed front nozzle to 37 from 33, made the transition slots just a little square on pri and sec throttle blades. Drove the car 4 miles, it was a little better. But still stumbled. Pulled in driveway, it stumble a lot in park. Engine temp reading 185-190. I left it idling, removed the air cleaner, wicked the throttle about an inch and nothing but spit came out the front discharge nozzle, not a nice firm stream.

I have had a couple warm start problems, but never investigated. thought the starter was getting old and tired when hot. But after todays test, I am leaning more towards vapor lock. Turned the engine off at 190deg, turned over again sounded like timing too far advance. Under the hood you can hear sizzling/gurgling in the carb. The only heat source under fuel line is the intake.

Can this cause pump squirt issues? What is making the fuel so hot if the engine temp is 190?

[Nsgrn]

Fairly sure heat soak-vapor lock is my problem. I did not block the crossover on the heads, is the only thing I can think of.
I just let it cool down for 20 min, fired right up, front nozzle shot great.

[Nsgrn]

I have a 1" carb spacer on its way. Thanks everyone for your responses.

[RamAirIV28]

Originally Posted by Tom Vaught

The book you mentioned is a Good Book.

The book, my Boss (Mike Urich), at Holley Carburetor wrote is better.

Always read the book written by the guy who actually designed the carbs first.

Then you can hot rod them and play all you want but at least you will understand how to get back to the factory calibration again.

Agree that the primary blade vs transfer slot position (if we knew what the rest of the combination was) would be helpful:
Engine, camshaft idle vacuum, stall in converter, manual or auto trans, actual ignition timing, etc but the OP did not provide that info. Did say no vacuum leaks.

He wanted to talk about accelerator pump cams and positions and what they would do for him.



By the way Pontirag, Double Pumper Carbs typically will have a 30cc pump and a 50cc pump. Don't have to buy anything, just swap positions of the pumps and related linkage.

You take it from her Pontirag, I am done.

Tom Vaught.

what ever Tom.....

each book and a few others have both strengths and weakness's and where's it written that there is an order to which book to read first? I got Ulrich's book and he covers things but other books cover other things too. yea its a good read but its not the do all end all. get over your self.

why cant other people offer help here?

And Rag is right ; there really wasn't enough info initially to make anything but assumptions

"Typically" doesn't always apply in every case. You make a lot of assumptions and then go off all butt hurt.

the nice things about books is that if you don't get it the first time they don't all storm off in a tizzy.

play nice

[ta man]

Check for vacuum leaks..