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350 lag and pop on acceleration

  
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350 lag and pop on acceleration

 
fatty77 fatty77
New User | Posts: 40 | Joined: 03/08
Posted: 10/12/08
02:58 PM

I have a '71 Monte with 350 and the G.M. performance fast burn heads that were used in the build article about a year ago. have an edelbrock750 carb with a edelbrock  mechanical street pump. I amusing a performer intake manifold. it has a lunati voodoo cam:219"/227'.I am using the flowtech tri y headers with 2.5 dual exhaust with flowtech afterburn mufflers straight through design. It still has the points distributor in it with new points last spring, adjusted them about two months ago. Yes need a hei dist. couple guys told me could be starving when i stomp on it that is why it lags. Don't get what the pop noise is though. any help would be great.  Confused  
Montes Rule

DonScott31 DonScott31
New User | Posts: 2 | Joined: 08/08
Posted: 10/12/08
09:14 PM

Fatty77.  Same problem here.....I'm goingto switch from edelbrock to holly.  

JW454 JW454
Enthusiast | Posts: 345 | Joined: 07/08
Posted: 10/13/08
02:16 AM

Is the "pop" coming through the carb? After it "lags" does it smooth out? Where is the base ign. timing (no vaccum hooked up) set at? How about total mechanical timimg? Is the vaccum source to the distributor "manifold". The symtoms describe either fuel, ignition, or possibly both. Changes that make the engine breathe better (cam, intake, headers) will almost always call for more ign. timing. Did it have this problem before the new equipment was added? A 750 cfm carb, whether it be a Holley, Edelbrock or whoever is too big for a 350 cid street engine that is making it's peak power at 5000 rpm. Post some more details and we will help you figure it out.  JW.  

55Guy 55Guy
Guru | Posts: 895 | Joined: 08/07
Posted: 10/13/08
07:17 AM

The pop is a backfire.  

JW454 JW454
Enthusiast | Posts: 345 | Joined: 07/08
Posted: 10/13/08
07:23 AM

You think.  

SMOKESHOW SMOKESHOW
Guru | Posts: 1001 | Joined: 09/08
Posted: 10/13/08
10:32 AM

Where is your timing at? Do you know someone with a 600 CFM carb you could throw on and test with?  

Pontiacman2 Pontiacman2
Moderator | Posts: 8956 | Joined: 09/08
Posted: 10/13/08
11:40 AM

May need to adjust the accelerator pump.  
Pontiacman2
Pontiacman2

Professional Hi-performance engine builder

fatty77 fatty77
New User | Posts: 40 | Joined: 03/08
Posted: 10/14/08
04:48 PM

i had a 600cfm on it and the performance guy at burt greenwald told me to move up to a 750 because those fast burn heads like a lot of fuel. the timing was not set with a digital timing lite like it should have been.(i know big mistake) it is set on the timing marksclose to the end of them towards the pass side of motor. dont remember if that is before or after.  yes after i stomp it and it lags let out of it and it goes fine. did not do it with the old 600 carb and the original heads.
had the same intake, cam, headers, and exhaust before the heads and the 750.  
Montes Rule

JW454 JW454
Enthusiast | Posts: 345 | Joined: 07/08
Posted: 10/14/08
07:43 PM

My advise is if you have the original 600, try it and see if the problem goes away. If it does then your 750 is out of tune besides being too big. If the problem still persist's, then look into the ign. timing further. You will need a timing gun to get it just right. Start out at 10 deg. BTDC ( no vaccum ). It wouldnt hurt to get a curve kit and follow the instuctions to give your engine the correct curve throughout the entire rpm range.  JW.  

elky66mike elky66mike
New User | Posts: 3 | Joined: 10/08
Posted: 10/15/08
06:47 PM

This sounds like crossed plug wires which would explain
the popping & lagging. did the problem begin after
replacing plugs or plug wires? or when you had the
distributor out of the engine? Check your firing order
and make sure the wires are on the cap in the correct
order and that the wires are going to the correct plug.
I think the firing order is: 1-8-4-3-6-5-7-2 and the
#1 cylinder is at the driver's side front of the engine.
also check initial timing. you may need 12 + degrees
depending on your mods and altitude where you live.  

josh37083 josh37083
New User | Posts: 1 | Joined: 10/08
Posted: 10/30/08
09:11 PM

i actually have a stock 91 350 in my truck and it is popping back through the carb. Ive talked to several older mechanics in my area and they all told me to check my gaskets and my timing i timed the truck to day and it still does it. i leaning towards the float in the carb. cause when the gas moves back in fourth in the carb it thinks its not full pouring more gas into it and then burning the excess causing the popping. if anyone can tell me if that is a possibility let me know  

bbfx88 bbfx88
New User | Posts: 40 | Joined: 12/07
Posted: 10/31/08
10:58 AM

What was the condition of the heads?, who adjusted valves? what were valves adjusted too?  I agree with other posts, timing needs to be tits and carb is way to big for a street machine, I thought recent article in CHP said something about edlbrock carb issue low to mid hesitation and popping due to covered venturi nozzles?  anyone????  

bbfx88 bbfx88
New User | Posts: 40 | Joined: 12/07
Posted: 10/31/08
11:03 AM

Oh, I almost forgot....get ridd of the points set-up, get HEI there cheap and worth it!  For the most part hassle free.  

DeuceCoupe DeuceCoupe
New User | Posts: 7 | Joined: 11/08
Posted: 11/01/08
09:31 AM

I've read thing as well about the edel 750 (afb) being unpredictable and my comparisons to dyno tests back this up - sometimes the Edel 750 gives a great showing, other times it behaves like a sick 2 barrel.

Do you recall which issue of CHP? I don't quite follow the covered nozzles, they are covered on the AVS but not the AFB style....

I've experienced this popping too even on a well tuned engine, every time when I swap carbs it goes away. 750cfm is too big, but if the carb is well tuned it still shouldn't pop, it should just run soggy slow.
It sounds like about 600cfm is right anyway so I agree I'd just go back to a known good carb and try it, then maybe play with the 750 to see if it will ever run right, tune jets, rods, etc.  

fatty77 fatty77
New User | Posts: 40 | Joined: 03/08
Posted: 11/01/08
03:01 PM

thanks guys i am going to try the carb switch and this winter going to get the hei small cap but right now had to get the garage and the car ready for the winter. one car only so it is in hibernation mode now but thanks for the ideas.  
Montes Rule

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