benjob

installed a rebuilt Holley 750 CFM 3310 carburetor the other day. when I mash the gas from idle it will hesitate and backfire out the carburetor. I adjusted the idle air mix screws. and idle rpms. took the electric choke off an Avenger carburetor as well as the secondary vacuum took and installed on this one. everything else is stock with the power valve jets and squirters. did any of the changes I made have any effect on did. it runs fine with steady acceleration except from idle to wide open throttle. I'm running the PCV valve at the rear vacuum line for the power booster my vacuum advance distributor line is running on the passenger side port on the metering block running no vacuum at the front of base plate.

toobroketoretire

What is your initial timing set to? If its too early and the accelerator pump isn't delivering quite enough fuel it will hesitate and backfire. Try running the recommended 8 degrees of initial advance and maybe a stronger accelerator pump cam (you do have an assortment, don't you?). And also a lot of well-meaning fools put a screw and nut into the secondary linkage to make their carburetor a "mechanical secondary" and that's a big mistake for a street engine.

7t9l82

It's probably lean, what intake do you have.

benjob

I have a weiland intake. I have total timing set to 36 degrees.

7t9l82

Is it an air gap type of manifold a single or dual plane,it makes a big difference.

toobroketoretire

I'd try a more aggressive accelerator pump cam first. Summit Racing stocks the cam kits that contain (I believe) 7 different cams along with the instructions for choosing the correct cam.

augiedoggy

without knowing what your engine is (stock, built? 350, 454?) is this a 180hp smog engine in a late 70s car?
Everyone is just guessing but if it is a stock 350 with just a few bolt ons I really doubt to need a bigger throttle cam... I just went through some similar issues with an avenger 670 carb on my mild 355 build and it turns out those are emissions carbs and are jetted lean from the factory so I had to increase all my jets (like 4-6 sizes up) as well as squirter AND change my vacuum secondary spring so they opened at the correct time otherwise it would hesitate bad when accelerating. thise should not be your case with a 350 and a 750 carb.

I watched a lot of videos on how the whole thing worked and googled a lot of forum threads as well.... In my opinion a 750cfm carb is not ideal for a stock 350. It will have very poor throttle response compared to something more correctly sized to the application... but you can get more top end out of it at the cost of everything else... quadrajet carbs are an exception since they are more of an auto adjusting "on demand" carb from what I have read of them...

you might just have some garbage plugging a passage they missed when rebuilding the carb.... did you rebuild it? are you absolutly sure the correct gaskets were used?? holley has many different versions that slightly differe and are meant for different carbs... its easy to install the wrong one or do it backwards.. trust me I've been there...

diehrd

I would first make sure your idling off the idle circuit and not the transition circuit . Usually a cam has 2 or 3 locations on it so you can adjust the time of the squirt from the accel pump by locating the cam differently.

I would also check timing .. Set it to 34 or 36 max all in by 2500 rpm and initial is going to be whatever it ends up at once you set max at 2500 rpm .. Do this with vac off and plugged.

augiedoggy

Yes you can also flip the cams around as well which changes the cam profile completely.

doorgunner

I had a similar problem last week.....using a digital multimeter I found wildly fluctuating voltage at the HEI positive connection (the needle-type meter didn't show the fluctuation)

Test the connection with key on....also test with engine running...(just to eliminate low voltage as the problem)

benjob

Engine is a stock 350 with dual plane intake. Timing is set to 36 degrees. Going to pull the carb and check the power valve and other gaskets to see if they are all good, no I didn't rebuild it. It came that way.

7t9l82

I agree it's lean. These carbs come set up pretty lean.

augiedoggy

he has a 750 carb not an avenger.... theres no way its setup too lean for a stock smogged sub 200hp 350.. just the opposite actually.

Unless someone messed with the jetting it they come setup for larger engines like a 396 big block... Holley will tell you if its a stock 350 making less than 350hp than the 570cfm or 600cfm carb is thier recommendation and if it makes from 350 to 400hp they recommend the 670... but those are avenger carbs and are setup for emissions... the regular 750 vacum secondary carb is not and has fatter stock jetting... as you can see below,

http://www.mortec.com/carbs.htm

unless somethings wrong with the adjustment or powervalve its a pretty sure bet its not lean from the size of the valve or jetting.
that carb is likely going to have pretty poor throttle response and waste a lot of fuel only to gain maybe 2-5hp at the very top rpm limit of the engine.... 750 holleys are ok for a built up 350 thats sees track time when setup for it but there are simply better choices for a 180hp chocked down 350...

If you bought it used someone else may have already realized this and tried to jet it down to make it perform more like a 600cfm carb.

it is possible your powervalve had been blown out if this is an older carb... this might help.

https://jetsrus.com/FAQs/FAQ_rejetting_holley_carb.html

Barry's70LT1

I assume the accelerator pump is actually working ?

Jason Staley

Or adjusted correctly. If there is slop in the linkage you might not be getting a full stroke of the pump.

benjob

I am Not sure what I'm looking for when adjusting the accelerator pump I understand that there is a bolt with a nut and a spring that applies tension to the accelerator pump lever however no matter how much I tighten it up or loosen it it'll still make contact with the accelerator pump lever and I can put any type of feeling a gauge in between the Bulls and the pump lever handle still pull out is there supposed to be a gap between the pump lever and this bolt

7t9l82

I have a quick fuel 750 double pumper Tha was waay lean out of the box, I can't see a 33 10 being fat. It may need a step up on the squirter.

toobroketoretire

Google "Adjusting Holley Accelerator Pump". With the throttle opened all the way you want about .015" more travel available in the pump arm. In other words the arm isn't supposed to get bottomed out but rather be about .015" shy of bottoming out. That prevents damage to the pump arm and diaphragm.

augiedoggy

Is your engine stock like the OP's??? we dont even know if its a L48 or L82?? an l48 station wagon cam is not going to let the engine breath enough to utilize as much fuel as an L82 cam might...

Again He has a stock 350 the 750 holley carb is already jetted with a 6.5 power valve and #72 primary jets... Ideally if anything he would want to go smaller to say a #66 or 68 jet for his setup and any stock squirter that comes on a holley 750 carb is going to be big enough for stock engine of that year with stock heads and exhaust . Its already over carbed..
the avenger carb was a better carb setup for that engine but unfortunatly not very many people understand how to tune them and they usually have them set up where they are idling off the primary circuit and run like crap...
The avenger I bought used was also setup all wrong this way which was why the guy I bought it from swapped it for an older holley that was setup from another engine already..

What color spring do you have in the vac secondary?? it may be opening too early like mine was which was causing my bog...

chevymans 77

what did you have before the Holley and how was it running then?

Neal