VETTE077

I got the car back on the road after winter project and runs great except for 2 concerns.Afterburn popping in exhaust when letting off throttle and hesitation when rolling in 1st gear and mashing the pedal.Confused is at lean condition? Timing is correct 36deg by 3200rpm
14deg initial.Edelbrock has a chart to richen carb 2 stages with jets and rods.Do I change jets and rods,Just jets, and just primarys or both. 1977 4spd 3.55gear,Edelbrock 600cfm,mild cam,headers and dual exhaust,Edelbrock performer EPS MANIFOLD.

cherrybomb78

OH OH, another Guy with an Edelbrock #1406 600 cfm carb. I have the same set up with the EPS also and the same carb, still can't get it right. I have been working with the techs at Edelbrock, but so far I'm still chasing my tail. There are many threads on here with Guys and the same carb. BUMMER

olescarb

the poping in the exhaust is fuel not burning in the cylinder that is buring in the exhaust. if you are too rich at idle it will pop in the exhaust when you back off

if you only problem is hesitation on acceleration but the cruise and power mixtures are correct then you can use a edelbrock accelerator pump # 1468 to give a stronger accelerator pump squirt. you could also go richer to cover up the hesitation problem but if you go too rich you are just wasting this $3.00 a gallon stuff that sells today for gasoline

lars

That's not correct. Popping in the exhaust when you let off the accelerator is a lean condition - not rich. What happens is that you get a lean misfire condition in the cylinders under deceleration when you're running a tad lean. With a lean misfire, you throw unburnt fuel into the exhaust, which then explodes and pops. A rich condition will not cause a misfire, and a rich condition will not throw unburnt fuel into the exhaust system - just high CO content exhaust. But no "raw fuel" goes out the exhaust when you're rich, so you don't pop.

If it's popping, richen it up. This will also solve your stumble. Often, backing the idle mixture screws out a tad will solve the problem. If not, you need to swap rods/jets.

olescarb

i disagree if the idle system is rich the fuel will ignite in the exhaust on deceleration (look at the flames that come out from the exhaust on a nascar car on decel on a short track) but try what you want. a engine that is rich will pop in the exhaust expecially if there is a air pump involved that is adding air on decel (bad diverter valve)

lars

You can't ignite the fuel in the exhaust system on a car running rich because there is no oxygen to make it light. It's running rich because there is inadequate oxygen to start with. If it's rich, it has consumed all the oxygen, and all you have in the exhaust is carbon monoxide, which will not cause the fuel to ignite.

On some racecars running very rich, you will get a flame out the exhaust tip. The fuel is burning after it exits the exhaust and is exposed to air. It's not exploding inside the exhaust pipe. Fuel exploding inside the exhaust system is doing so because there is both oxygen and fuel in a ratio to make it explode. That oxygen is coming from cylinders that have not fired off their air/fuel charge due to a lean condition (lean misfire). Once enough fuel gathers in the exhaust, it lights off and causes the exhaust popping.

On the carbs I test, if I get any exhaust popping at all on deceleration, I richen the carbs up. This will always solve the problem. Every time. Popping on decel is an absolute sure-fire sign of a lean running carb. I've never been able to run a carb rich enough to make a pop occur in the exhaust or out the tail. I doubt you could make it happen on a car with a full length exhaust system.

olescarb

in my experiance a rich mixture made even richer by the high vacuum of decelerating a car in gear will make the mixture so rich it will not compleatly burn in the cylinder so it will pop in the exhaust full length exhaust or open headers. if there is any extra oxygen from a exhast leak or the air pump system it will make the problem more obvious

lars i hope you do not mind my opinions on this forum, i do a lot of r&d work working with exhaust gas analyzers and wide band o2 based air/fuel meter tuning race and performance engines. if my opinions which may not always be the same as yours bother you i will stay off this site. as in the fact i am limiting the vaccum advance to 10 degrees to cure part throttle ping and also the high HC (hydrocarbons/misfire) we see when the Total timing w/vac advance added in are above 48 degrees of advance with the reformulated fuel we are having to use.

lars

I'm certainly not wanting to get you off the site - not my intent at all. Differing opinions and different observations are a good thing, and it's not my intent to offend or hurt any feelings here.

Like you, I do a heck of a lot of tuning and carb setups. If you're running an A.I.R. system, you could probably induce exhaust popping with a rich condition. But most of these cars don't have air injection systems, and you'd have to run the mixture awfully rich in order to induce a rich misfire condition. Although a rich pop is probably possible, I've never been able to induce that condition in the testing I've done. But lean popping is very common. Not to rule out a rich condition as a possibility, the most probable condition that I observe with these carbs is that there is a lean condition when popping occurs.

It's very easy to test for this: If a lean condition is suspect, simply crank the idle mixture screws out a turn-or-so. If the condition on decel gets better, it's lean. If it gets worse, it's rich. You can also induce the lean misfire condition to exaggerate the lean popping: Simply crank the idle mixture screws in far enough to cause a poor idle (lean condition). Then go out and do a high-speed deceleration in gear... it'll pop pretty darned good..!

olescarb

yes anything that causes the fuel to not burn in the cylinder can cause a poping in the exhaust. a lean mixture would be more likely but add in air from the air pump and anything can happen, i am here in California the land of smog pumps and i still see the nigtmares they can cause (1966 and 1967 are the worst).
you could not hurt my feelings they have been challlenged by the best sometimes i win sometimes i lose but i am always learning, i just do not want to step on any toes. this new world of reformulated fuel is changing the game and so i will have a few my hands full trying to keep up the this fuel that is blended differently for each season to reduce fuel emmisions. if we are not carefull the cars from 1967 and up could come back into the smog so i always try to not go too rich or too lean.
any comments on total timing including vacuum advance like i said we are seeing ping problems on light cruise when the total exceeds 48 degrees and as i just saw on another post even the chevy dealers are suggesting not to hook up the vac on a ZZ502 with the factory HEI (my cure is a a-c delco #d1370a and i wish i could find a non adjustable unit that comes with 10 degrees (crankshaft) for a points dist but i am haveing to limit the travel on a D1383a delco unit

have you tried the #1467 or #1468 edelbrock pump for hesitation problems? i am having custom duration springs wound for us to solve the hesitation issue which is even more common on air-gap manifolds

RW68RSConv

Understand this is a really old thread, but it popped up in a google search - so I thought I would add to the debate....

Have exact same problem. Exact same carburetor. After reading several threads, I decided to try the easiest thing first... I adjusted the idle mix MORE RICH (counter clockwise) almost 3/4 of a turn on left and right - set idle screw down to lower RPM (caused by enriching the idle circuit) -- problem gone. NO OTHER CHANGES TO THE CAR ANYWHERE! Note: I did about 1/4 turns, then test drove, repeat, until the problem was totally gone. 3/4 was the "all in" adjustment to eliminate the problem completely.

So everyone can talk theories - real world, enrich the idle circuit.

blue427

old thread or not,it just might help me with my tri-power problem. thanks

windemere

i'm enjoying reading the different opinions. i have a question. i have a 72 with a rebuilt rochester, been stumbling very badly. timing adjustment not help. seems likes its missiong on one or two cylinders. last week, it "backfired" twice, and the second one blew one of the mufflers wide open. still stumbling tho. too rich or too lean? or any other suggestions. tried replacing the coil, no difference. tested the fuel pump, seems ok in volume and pressure. thanks.