Groggy acceleration and stalling - Impala Tech
Troubleshooting Diagnosing problems

 
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post #1 of 11 (permalink) Old 09-05-2014, 11:18 PM Thread Starter
 
 
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Groggy acceleration and stalling

i have a 1969 impala with engine completely redone. I have issues with poor idle and stalling and also at times the engine bucks slightly while throttling as if it isn't getting enough fuel. Here is a list of what has been done

New carb 4 barrel
new fuel pump
new fuel filter
new fuel sending unit

at one time there was dirt in the engine but it has gone. The only thing that hasn't been is flushing the tank. Frustrated and I am on a family trip.
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post #2 of 11 (permalink) Old 09-06-2014, 07:11 AM
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Flushing the tank wouldnt be a bad idea as they absorb moisture when sitting but some berrymans gas treatment might help with that. Tell us about your mew carb.
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post #3 of 11 (permalink) Old 09-06-2014, 09:21 AM
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The poor performance doesn't have to be all the carbs fault. The ignition curve and initial timing are another consideration. Chevy had maybe twenty or so distributors sitting in bins all color coded to reflect the different weights and springs (or a mechanical tach drive if going in a Corvette) to fit each individual motor that they made that year. Changing from a two barrel to a four barrel would change the weight and spring combination to change the ignition curve. It isn't a curve at all: but a line that rises at an angle determined by the spring strength; and as to when the "curve" started up was determined by the weights being held in place by the different strength springs.

The carb if out of gas would pop and back fire going down the road. Too much gas and it will stumble and lag behind you stomping on the pedal. There are two different circuits that have to be tuned in the carb to make it behave; the accelerator pump circuit and the main metering which is controlled by holes in the emulsion tubes and the main jets. Idle quality is determined by manifold vacuum and your turning in or out the idle mixture screw that adjusts air, not gas.

Carb size is also a factor if you have a carb too large for your displacement or engine RPM. You can put an 850 cfm Holley double pumper on top of a five liter SBC engine. If you put it on top of a 302 with a 30/30 Z/28 cam and use the factory 4.10 gear set and a close ratio Muncie it will be a dog on the street but deliver nearly 370 horsepower at ten thousand RPM. Put the same carb on top of a 305 and it probably won't even run. This is an example of too much carb for engine displacement (keep in mind the 454 had a 720 cfm Holley from the factory on the hot engine, and the higher reving solid lifter 427 got a 780 cfm Holley).

A 327 for the street should have a 500 to 650 cfm sized carb. You choose the 500 cfm carburetor for a true street driven automatic PowerGlide or TH350 with a high (low two series gear) rear end, and you pick the 650 cfm carb if you have a manual and a lower three series gear in the rear end. In every case you chose a vacuum operated secondary with a heavy car like the Impala.

Add to the mix a cam chosen to make that rumpitty-rump noise and you are going to have issues. This is because that rumpitty-rump cam steals all of your low RPM power and shifts it up higher in the RPM where you can not get it for driving on the street.

Big Dave
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post #4 of 11 (permalink) Old 09-06-2014, 07:01 PM Thread Starter
 
 
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Location: Annapolis, MD
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yep here we go 4 brl Holley and over sized cam. How ever I notice particles in the replacable air filter. where shoud i start gents? I am frustrated. Im going to bring it to a shop this wee. what should i tell the mechanic at first?
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post #5 of 11 (permalink) Old 09-06-2014, 07:28 PM
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It may not be broke. The way it is driving now may well be your new normal.

The main thing to have the shop look at is tuning the carb to lowest safe air fuel ratio (it is set fat to protect Holley from law suits when you buy it), so a lot of the carb experts enlarge the main jet size making it go even fatter. If you have had an expert look at it I would set the jets back to the factory settings, and see if that helps performance any.

Hopefully your shop will have a portable gas analyzer (tail pipe sniffer) and an get your air fuel ratio down to the 12.9 range for the street (12.5-13.3). He can also check your ignition curve; and if you have an MSD or similar aftermarket HEI distributor he can change the weights and springs to reflect your cars gearing. He can also advance your initial ignition timing as needed to reflect the changes you have made. Finally he can tune the idle mixture screws to obtain maxim intake manifold vacuum; not for better power brake operation, but because that means your idle is set correctly (wherever it happens to fall in terms of idle speed which can also be adjusted if your converter will allow it).

Big Dave
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post #6 of 11 (permalink) Old 09-09-2014, 03:40 PM
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What cam, converter (if automatic) and the rear gears are in the car?

Miserable performance can be achieved if you have a big cam, wrong stall converter (automatics) and for example stock 3.08 gears and of course the carb needs to be adjusted properly as well.

Hope this is helpful!
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post #7 of 11 (permalink) Old 05-21-2015, 11:57 PM Thread Starter
 
 
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Turns out there were perforations on the fuel line allowing air to mix in causing the slow pick up
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post #8 of 11 (permalink) Old 05-22-2015, 08:15 AM
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Joe those holes in the fuel line were caused by alcohol in the gas. The fuel lines on cars made before Ronnie Ray Gun was in the white house were not even aware that alcohol existed except for a Martini (wrong alcohol I know).

If you replace the fuel ine it has to be plastic lined to resist the corrsive affects of alcohol.

The carburetor and all of the other parts are at risk of damage from alcohol but there isn't much you can do other than replace the fuel lines and gas tank with stainless steel parts and the carb with one designed to operate with alcohol (or at least have the gaskets upgraded).

Proper Fuel line:


Why you want to change it:


Standard Holley carburetor gaskets:


Compared to the Alcohol resistant gaskets:


Big Dave
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post #9 of 11 (permalink) Old 05-22-2015, 08:32 PM Thread Starter
 
 
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Yep Dave that was it!!!!
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post #10 of 11 (permalink) Old 06-12-2015, 10:54 PM Thread Starter
 
 
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Now it stalls out when warmed up on idle at a stop light and I smell gas. 4 brls edelbrock carb. This is driving me nuts!
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post #11 of 11 (permalink) Old 06-13-2015, 06:18 AM
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Quote:
Originally Posted by impalajoe1968 View Post
Now it stalls out when warmed up on idle at a stop light and I smell gas. 4 brls edelbrock carb. This is driving me nuts!

Could be flooding out if:
1.) floats are not set correctly,
2.) if there is trash under the needle and seat,or
3.) if you have an electric fuel pump and you are running more than five PSI.

Flooding out results in poor idle to stalling and a smell of gas. Of course you could be starved for fuel due to a massive fuel leak and have similar symptoms. This is why diagnosis over the internet is next to impossible.

You need to have some one that is familar with these old cars and carburetors (a whole generation of mechanics have grown up without ever seeing a carburetor) look it over as a gas leak can burn your car to the ground; and the house it is parked in.

Big Dave

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