Holley 750 backfire.

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Ajp440

Ajp440

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I just recently bought a Holley 4150 hp series 750 from a local carb guy. it's been rebuilt and I was told should perform well on my mild 446. It takes of fine but as soon as I get on the throttle it starts to spit and have a back fire. Timing is set at 38* a 3000k. Running a single plane tm7, 509 cam,3000k stall. Stock Mopar electronic ignition. The ecu is actually off a 360 engine setup could that be a problem? Took the carb apart to see jetting, primary is 68 65. And secondary is both 80 jets. I'm running a 6.5 pw also. Do the primaries jets seem small for a 750dp hp series carb? Thanks in advance!!
 
the primaries are too lean....stock 750 is usually around 70 .....you can go to holley web site and check to see what the stock jetting is....

also..i would have the same jets in the primaries with a single plane intake....not 68 and 65....

also check your squirter size....at least a 31 ........

and finally check to make sure as soon as the throttle is moved that the accelerator pump squirts gas...you might need to adjust that....
 
4779 c 750 (F)75,(R)76 8.5 4150 Mechanical
4779-1 750 (F)70,(R)80 8.5 4150
4779-2 750 (F)70,(R)80 6.5 4150
4779-3 750 (F)70,(R)73 6.5 4150
4779-4 750 (F)70,(R)80 6.5 4150
4779-5 750 (F)70,(R)80 6.5 4150
4779-6 750 (F)71,(R)80 6.5 4150
 
Ok I was hoping to hear that. I have a set of 71 jets and 79 jets should I try the 79 jets first then go down or try the 71's? The bog is pretty bad and I can tell it's really starving for fuel. I have 25 shooters on it now bought a set of 28's same thing. I have a set of 31's I can throw on. Thanks for your help so far
 
With that cam, I'd lower the total advance to all be in at 2200 & get as much initial as possible, That 3000 stall is abit tight too for the cam, at least a 9.5" 3800 would work much better. Good ignition is also key, I don't use Mopar ignitions or ECUs anymore, unless its on a mild engine, Get an MSD/Mallory 6 box & a MSD Distributor like this one, VERY simple to set the timing curve where you want it. Thats what i do anyways.

http://www.jegs.com/i/MSD+Ignition/121/8546/10002/-1
 
The msd is something I plan o getting when I get some extra cash, I'll have to get the stock one going for now. So around 36 38 at 2200? How much is too much initial advance? Vacume advance or plug it off? Thanks!
 
I have a old mallroy dual point distributor too but not sure if I'd have better results with that
 
Ajp440 said:
I have a old mallroy dual point distributor too but not sure if I'd have better results with that
Click to expand...

I would'nt use it, you'll be messing with 1 set of points going wacky on you, it WILL piss you off in no time lol.

Shoot for 20* int., it'll respond great, then add the 16-18 mech. all in by 22-2300, NO vacuum advance, plug it off, just your int. & total mech.

I would definitly invest in a higher stall, but a GOOD one, no TCI junk, I like PTC, there reasonable & very good quality. For the gears, 4.10s with a 28" tire, you'll have a smile on your face. 38-4200 flash stall. 9.5"

Install the .031" squirters F&R, thats what the 750dp came with, then start with the 70/80 jets.

www.ptcrace.com

Either this one.
http://www.ptcrace.com/street-strip-torque-converter-p-1252.html

Or this one.
http://www.ptcrace.com/torque-converter-p-1253.html
 
Trust me 70 in the front not to lean and not to rich,I love Holley and great at tuning them.My 600 now has 68 jets up front but my 750 that I just rebuilt will get the 70 up front.This is on my 360.
 
Right on I'll throw those jets in and set up the dist. And see if I loose the backfire. I'll report back with results. Thanks everyone!
 
Are those ptc stalls streetable? I got a cheap b&m that will flash at 3200 but drive almost like a normal stall.
 
Ok once yo get it running ya better check to see if the power valve has blown,with back fires it can.To check if your mixture screws are not responsive chances are its blown.Have to check your vacuum reading at Idle to see what power valve ya need.
 
I did a vacum reading on it already pulls around 10 in gear. The Holley has the metal check ball built in for the power valve protection too. I have a 6.5 in it now. May need a 4.5? U can tell if the power valves are blow by sucking on them and making sure it holds and no air escapes right?
 
If I put mixture screws all the wayin it dies I heard that's a good way to test them too
 
Ajp440 said:
If I put mixture screws all the wayin it dies I heard that's a good way to test them too
Click to expand...

Thats right,also if the primary throttle is open to far @ idle that will cause a rich mixture,hence no response from ya mixture screws.Your power valve is ever so close but I think the 5.5 would be better.Also like said 31 pump nozzle is very good.U likely have a 25 in there now? When these Holleys are set up right there great.PS you have a good vacuum reading at idle for that cam.
 
Yeah I was surprised on the vacum reading I got I actually checked 3 gauges to make sure. Yup has 25's now. I'll try it out
 
Ajp440 said:
I did a vacum reading on it already pulls around 10 in gear. The Holley has the metal check ball built in for the power valve protection too. I have a 6.5 in it now. May need a 4.5? U can tell if the power valves are blow by sucking on them and making sure it holds and no air escapes right?
Click to expand...

I would pass on the B&M, that 3200 will be about what you already have now, Yes the PTCs are VERY streetable, I ran 3 different styles & all were "dead on" with the flash stall they were set up for, The plus side is now they all come with balloon plates for nitrous use.
 
Well got it all in today except the timimg issue and it does the same thing :(. I'm ruling out the carb as the problem. Can it possibly be a problem with the ecu box being from a small block vehicle? Plug gap maybe? There tapped at 35. And there autolite 85 plugs. Even through a edelbrock dual plane on it and same thing, I hope it's not something inside the motor? It will rev up in park good but it alway happens when I punch the throttle car spits throughout the carb. Good it be a fuel issue? I am running 89 octane. And the engine has the trw six pack pistons. I'm going crazy!
 
Ajp440 said:
Well got it all in today except the timimg issue and it does the same thing :(. I'm ruling out the carb as the problem. Can it possibly be a problem with the ecu box being from a small block vehicle? Plug gap maybe? There tapped at 35. And there autolite 85 plugs. Even through a edelbrock dual plane on it and same thing, I hope it's not something inside the motor? It will rev up in park good but it alway happens when I punch the throttle car spits throughout the carb. Good it be a fuel issue? I am running 89 octane. And the engine has the trw six pack pistons. I'm going crazy!
Click to expand...

If its farting back through the carb the "most" commen issue is timing, low timing, not advanced enough, or not advancing quik enough, but a lean transition will do the same thing, I'm guessing timing at this point, especially if its acting up about the same with the changes, You yourself feel at this point its not the carb, your most likely right. How does it act'd if you stab it from a 20 mph role? If it does fine once it gets going i doubt its a fuel problem, If you don't have one already, i'd mount a fuel gauge "on the cowl" so you can see it at all times, also make sure there are NO vacuum leaks. Like i said earlier, that cam needs alot of initial timing & a fast curve, & the converter at 3000 & the timing all in at the same RPM doesn't help matters, What gears are in it?

The ECU & plug gap are fine, I'm just not a big fan of the MP stuff anymore, unless its the Chrome box & the mallory style MP dist. thats fully adj., it has the gold plate with light springs & 2 hex screws with a slider plate to adj. the amount of advance you want, plus you can lock it out, Anymore before i plan a build, I make sure i have the whole fuel system up to par, A readable fuel gauge while in the car, & a VG ignition system & simple adjustability, along with good cooling. I never have aggravating issues anymore.
 
I got 4.10 gears out back. Now can I do what you suggested to my timing have it all in at 2100 with the stock Mopar distributor? I honestly don't look the way my intake sits on the heads either so I replaced the valley pan gasket today (metal one). Motor had the heads already on them already So there is a chance they could be milled. The guy I got it of said it was out of a 69 charger that would run 10.5 1/4 miles on this motor it's probaly BS but who knows what kind of head work was done or true compression of the engine. I do know that it was balanced and blueprinted though. But anyway if they are shaved I might be getting a leak from it, I did the spray bottle water test but got no change in rpm but maybe it's leaking from the underside of the intake?
 
It doesn't seem to matter what speed I'm going if I'm easy on the throttle it will go threw the gears and drive like normal. Wether at 5mph or 20 or 30 if I lay into it it spits and pops through the carb. But when I'm sitting in my garage I can snap the throttle and it's clean and crisp.
 
4.10s are good, Is it just a standard MP dist.? If so, you could tear it apart & weld up the slots & install the light springs, but its hit & miss on the slots, you may need to tear it down a few times & add more weld or file the slots till you get it right.

How off is the V pan with the port allignment on the heads? You don't want to use any silicone on the ports, while its idling, spray under the VP towards the ports & see if theres any change. Where is the int. timing at now at idle??

Are you getting a steady stream of fuel from the squirters throughout the throttle?
 
If you want, You can PM me your # & we can chat on the phone, or i can give you mine.
 
Initial is at 20 btdc. I noticed I have to push down on the center of the valley pan a bit to get it to line up with the heads. I used a little bit of sealant on the bottom side of the ports like the instructions told me too. Standard 440 distributor from a old smogged 440 I had.
 
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