750 Holley backfiring

[hur86]

Hey guys, I'm looking for some advice about adjusting the idle mixture on a Holley 750 double pumper with mechanical secondaries. I'm running it on a '68 440 GTX with a purple cam (.484 lift - 284/284 duration) and a Holley Street Dominator manifold. The problem I'm having is that when I crack the throttle open fast, it wants to hesitate, and often times backfires thru the carb. I can make it better by adjusting out the idle mixture screws in both the primary & secondary sides, but then it seems to run very rich. (This is the way I had it set last year when the engine was stock, except for the carb. It ran rich, but didn't backfire thru the carb.) When I close off the secondary idle mixture screws, and set the primaries richer, it seems like it idles better, but wants to hesitate more and backfire more. I tried adjusting the spring tension on the accelerator pump, and that seemed to help. I also noticed that the engine seems to idle best around 1000 rpm. Slower idle makes it backfire thru the carb even more when I quickly crack open the throttle. By the way, before I made any adjustments I set the float levels so gas was even with the bottom of the threads. Which way do you guys adjust these carbs to prevent the backfiring? Thanks in advance for your help...Brian

[jelsr]

Brian,
When they spit back at you like that it usually indicates lean. Probably in the pump circuit, they need lots of fuel for a jiffy when accelerating. Make sure it pumps immediately upon throttle movement, both sections. Might need a larger nozzle. Don't try to compensate with the idle screws, they are for idle only, the pumps are for the sudden RPM change. Remote possibility of the timing being a little fast, shows somewhat the same symptoms, but usually bucks the starter when cranking.

[MY340]

With your engine mods you probably need to rejet the carb and go to a lower rated power valve. Hopefully some carb experts here can help you more. Good luck.

[SHY-PLY]

You need to look for a Holley book. See if you can find the one by Alex Walordy (sp). It's written in layman's terms and is easy to understand. You may need to change the cam on your pump shot, your squirter size etc. Walordy's book takes you through diagnosing the problem real well. It wasn't that much money and I still carry it in the trailer today. Try Summit or Jegs, I'll bet they still carry it.

ENJOY the RIDE

[Jims451]

As jelsr said, it sounds like the accelerator circuit is too lean. Holley sells accelerator pump "cams" that give a larger and quicker accelerator pump shot. Before you start changing the accelerator pump cams, double check the float levels and that the jetting is correct and you have the correct sized power valve.
The idle screws should only be used to get the best mixture at idle, not to cover up jetting or accelerator pump problems.

[kamstra]

Just a thought that you may wish to re-check your timing. Its always a good idea to ensure that timing isn't contributing to your problem.

[cuda66273]

1. Check your power valve you may have blown it out by now.
2. With the engine idling spray some carb cleaner around the manifold gaskets and the throttle shafts and see if you have a vacumm leak.
3. Messin' with the idle screws it won't help, Idle is set up on a separate curcuit and it switches over immediatly on acceleration, this switch over point is known as the transition point.
4. Determine the size of your squirter nozzles and the CC's of the accelerator pumps, they should be I believe the 30CC type which are fine, the nozzles may be too small.
5. Did this carb work before and if so what changes have you made since?
6. What size of fuel line are you running to the log and what size to the bowls?
7. What is your fuel pressure?

Your describing a lean condition, my guess is a vacumm leak or a bad or incorrect power valve.

[PRO]

Check your manifold vaccum at idle and timing,post them along with your elevation ,also have you done a compression check recently? if not do that and post those results and check your plugs to be sure none are gas fouled,then Ill give you specific jet sizes/power valve,pump cam etc thatll be in the ballpark of what you need........PRO....ps headers? 4 spd or auto? gears?

[71383bee]

This may be a dumb question, but you said that it did this before. Is it possible that you might have too much carb with the mechanical secondaries and double pump. If it was boging with the stock intake I could see it. Also it sounds kind of like a vacum leak. Did you replace the gasket when you switched to the Holley manifold? If you ask me a 750 dp with mechanical secondaries is a pretty stout carb for a stock 440.

[hur86]

I readjusted the accelerator pump like jelsr suggested. Mine wasn't correct. After engine warmed up no vaccuum leaks were found. Set rpm at 1000. Adjusted front mixture screws with back ones closed. Still backfired. Opened back adjustment screws 3/4 turn and manifold vaccuum improved 1/2 inch. No backfiring, but engine still hesitates when I quickly crack open the throttle. I guess I have to fine tune the power valve as suggested. Here are some other things that might help: No changes made to carb, but I ran it with 4 mixture screws open before. 5/16" lines from tank to carb - stock for this model. Manifold vaccuum is running between 7 to 8 inches when idling at 1000 rpm. Elevation here is 1000 ft. Compression checked before I started engine work. Most read 140 - 145. 4 & 6 read 120 (head gasket looked like it was leaking when I took it down). Heads completely reworked (new valves, seats, guides, head trued flat). Running stock exhaust manifolds, 4 speed, Dana 3.55 rearend. Note: the book SHY-PLY suggested I buy is available thru Year-One. Making progress. Thanks again to all you guys for your help...Brian

P.S. - all new gaskets used during reassembly. Car didn't backfire thru carb before I took it down. New cap, rotor, coil, wires & plugs. Vaccuum advance disconnected and port on carb is plugged. Timing is set at 10 BTDC....Brian

[cuda66273]

Vacumm seems aweful low to me....

[PRO]

(1st) check to be sure you dont have any gas fouled plugs,once their sooty black you can clean them once after that throw them away,dont be fooled they can foul out in 2 minutes of operation in a full rich situation.(2nd) slowly cover the primary side of the carb up(like your choking it)if the rpm increases more than 100rpms you have a vaccum leak.(3rd)pull the float bowls off9catch the fuel)and note the stamping on them(3.5,5.5 etc)providing you didnt have a vaccum leak youll want ones that are at least 3" below your manifold vaccum,something around 3.5",also note the jet size while your checking P.V.s,(4th) look at the accelerator pump cams,these are mounted to the linkage and actually open the accelerator pumps,what color are they? and which hole is the screw in?#1 or #2?then post your findings..............PRO.....

[djswwg]

Now I'm pi$$ed! I just spent ten minutes typing out a reply to this topic, and when I hit "submit reply" I got sent back to the "log in" page. So of course I lost everything I had typed. This isn't the first time this has happened to me. Anyone else had this happen? What the hell is going on? ....djs

[hur86]

I slowly covered the primary side 3 different times, and it never increased engine rpm. The power valve has the following stampings: 6 at the 1 o'clock position / 5 at the 3 o'clock position / 8 at the 7 o'clock position and F at the 9 o'clock position. Both accelerator cams are a dark pink or light red color and each has the screw in the #1 hole. The front jets are #71 and the rear ones are#80.

I drove it this morning and it lacked power on every start-up from a dead stop. Only backfired once, while the engine wasn't completely warmed up. I don't intend to race the car on the strip, but I want to be able to get on it without it backfiring or hesitating.

Thanks again for your help...Brian

P.S. I forgot to tell you that I checked the plugs and they were still OK.

[mr.b]

even though you have recieved excellent info. and i fell it is most likely the carb go back to some basics. set your timing at 2-5 degrees before like the factory says. really check your wires and follow them out. nest make sure your air bleeds are clean. if the carb has been sitting around uncovered they could beed dirty. and then make all butterflies are closed then look and see when they open as compareed to the squiter shot. also does this happen when only when driven or also sitting in the driveway. could be a load factor.

[pishta]

djswwg, do you have cookies disabled, or maybe a internet privacy program running? Anyway, I would try and adjust the secondaries to open a little later and maybe look into your vacuum advance, it may not be pivoting like it should, maybe gummed up? PRO, what is your Email address? I have a jetting question for you, or should I just post a new thread? Thanks

[George G. Leverette]

I have a similiar problem with a 750 Holley, first the power valve blew in leaa than 500 miles on a fresh 340 engine it would surge severly even at about 50 MPH. Replaced the Holley with a 600CFM Edelbrock and took the Holley to the shop for expert repair. Installed the Holley and lost 3" of Vacuum at idle regained the lost power at the expense of 8 miles per gallon and a very rich condition. With 9.1 pistons, 470\504 cam 2800 stall and 355-1 gears I thought the 750 CFm would be right but the 600CFM in giving excellent driveability good gas mileage and better overall performance what do you guys think?

[amoparguy]

I would try more initial advance. Even a few degrees can improve the vacuum @ idle.

[Leigh]

Good advice on going back to checking basics. Set carb idle settings in gear. When the p/valve and idle circuit is done, start going up in nozzle size. Let the engine tell how large they need to be. Good luck, I hope you get it fixed.

[jelsr]

George,
I prefer the driveability of the smaller carb. The big spreadbores have small primaries for driveability and the coffee cup sized secondaries for the power (the transition sucks at times) but the squarebores have 4 relatively large bores. The low velocity due to size hurts fuel atomization. Response is poor and economy is poor also. The bigger carbs are good at the strip but the smaller units are more friendly on a day to day basis.

[caveman]

Put bigger sqUarters in ,50cc pump on the back,Give the manifold some heat,Big mopars dont like a real cold manifold,it will Spit back at you at a idle and drive crummy this time of year,when its hot out they run better with out heat

[rumblefish360]

George G. Leverette, i'm with jelsr on a smaller carb. There better all around. I have suffered those to big blues before. A swap back to the smaller unit and all was right.

djswwg; YEA! Thats happened to me. X3 ugh!

hur86; Sounds like a bigger pumpshot is needed. The advice your getting is sound. Keep working on it. Cavemans right as well, I have never seen a big block mopar like the cold. Must have something to do with that stock air-gap design.