Hello again everybody. It's been a long time since I've been on here with some problems. This really isn't a problem but I just need some advice before I take this car into a shop. I became a mechanic for a reason, and that was to not spend a fortune on putting the car into a shop!

I have a 70 Stang with a 351W (completely stock internally) with an auto trans. It has a 670cfm Holley, Street Avenger carb (P/N: 80670-1/1402), an Edelbrock Performer Aluminum intake, Flowmaster Exhaust and Headers. Sadly to say...still has a points distributor.

Everything is brand new. Motor has been rebuilt. The carb has been rebuilt as well. The car has been in my garage as I've been trying to tune it. It seems that building the motor was easier than tuning it!

I've adjusted the F&A mixture screws as the instructions say. I've set the curb idle speed at roughly 600RPM (in drive) as instructions say. I've timed the motor to what I think is the correct timing. (Maybe I can get a little help in that area such as what timing degree)

Here's the issue: The car sounds great! When I rev it up, it sounds clean and stronger than ever. HOWEVER, if I "romp" on the gas quick and hard (while sitting in park), the carb back fires. The car then dies down a little and then comes back up, rough idling for a little bit on it's way up.

Really, that's all I need to say. Obviously somethings not tuned properly and it's very frustrating!

Please help!

Thank you

 

Advance the timing a bit and see if it goes away. You can limit the full advance in the distributor to keep it from surging at cruise rpm if it wants more initial.

Could also be a leaking intake gasket, they pop through the carb under hard acceleration if there is a vacuum leak at the gasket surface. It wont do it if the leak is at the carb, only the manifold to head gasket will do it.

 

You may have to tune the accelerator pump shot for a little more fuel.

 

THUMPIN455,

Thank you for your response. Well, I tried that but then when starting the car up, it gives that starting struggle as if the battery is low (which it's not the battery). It has a hard time starting because the timing is too high. When I have the timing exactly where it needs to be, it starts fine. I'll check to see if the intake is leaking.

hiboostwoody,

Thank you as well for the response. Where would the accelerator pump be located? And, you saying this because you think that the motor is starving for fuel?

THank you again.

-TNT

 

This is kind of hard to read with the run-on words but it explains how the accelerator pump system works.

The accelerator-pumpsystem consists of three main components: the pump diaphragm, the pumpcam, and the pump nozzle. This is the carburetor system that is mostresponsible for having good, crisp, off-idle throttle response. Itspurpose is to inject a certain amount of fuel down the throttle boreswhen the throttle is opened to smooth the transition between the idleand main circuits so that no stumble, hesitation, or sluggishness willbe evident during this transition phase.

The first adjustment to check is the clearance between the pumpoperating lever and the pump diaphragm cover's arm, at wide-openthrottle. This clearance should be around 0.015 inch. This ensures thatthe pump diaphragm is never stretched to its maximum limit at wide-openthrottle, which will cause premature pump failure. Next, make sure theaccelerator-pump arm is activated the moment that the throttle begins tomove, which ensures instantaneous pump response. These adjustments canbe made by simply turning the accelerator-pump adjusting screw locatedon the accelerator-pump arm together with the pump override spring andlocknut.

The amount of fuel delivered by one accelerator-pump stroke isdetermined by the pump's capacity and the profile of the pump cam. Thetime it takes for that fuel to be delivered is controlled by thepump-nozzle size. A larger pump nozzle delivers the fuel much soonerthan a smaller pump nozzle. During acceleration tests, if you noticethat the car first hesitates and then picks up, it's a sure bet thepump-nozzle size should be increased. A backfire (lean condition) onacceleration also calls for a larger pump-nozzle size. Conversely, ifoff-idle acceleration does not feel crisp or clean, the pump-nozzle sizemay need to be decreased. Holley accelerator-pump nozzles are stampedwith a number that indicates the drilled pump hole size. For example, apump nozzle stamped "35" has a 0.035-inch orifice. Pump nozzle sizes areavailable from 0.025 to 0.052 inch. Whenever a 0.040-inch or largeraccelerator-pump nozzle is installed, the "hollow" pump-nozzle screw (PN26-12) should also be used. This screw allows extra fuel to flow to thepump nozzle, assuring that the pump nozzle itself will be the limitingrestriction in the accelerator-pump fuel-supply system.

When changing the pump nozzle, it's best to jump three sizes. Forexample, if there's an offline hesitation with a No. 28 pump nozzle, trya No. 31 pump nozzle. If you must use a No. 37 (0.037-inch) or largerpump nozzle, then also use a 50cc pump. A 50cc accelerator-pumpconversion kit is available under Holley PN 20-11 when maximum pumpcapacity is desired.

Once nozzle size has been selected, the accelerator-pump system can befurther tailored with the pump cam. Holley offers an assortment ofdifferent pump cams, each with uniquely different lift and durationprofiles that are available under Holley PN 20-12. The cam profileaffects the movement of the accelerator-pump lever and, subsequently,the amount of fuel delivered by the pump nozzle.

Installing a pump cam is a simple matter of loosening one screw, placingthe new pump cam next to the throttle lever, and tightening it up. Thereare two or three numbered holes in each pump cam. Placing the screw inposition number 1 activates the accelerator pump a little early,allowing full use of the pump's capacity. Generally, vehicles thatnormally run at lower idle speeds (600 or 700 rpm) find this positionmore useful because they can have a good pump shot available comingright off this relatively low idle. Positions number 2 or 3 delay thepump action, relatively speaking. These two cam positions are good forengines that idle at 1,000 rpm or above. Repositioning the camcompensates for the extra throttle rotation required to maintain therelatively higher idle setting. Pump-arm adjustment and clearance shouldbe checked and verified each and every time the pump cam or pump-camposition is changed.

Check out this link. The pump nozzles are referred to as squirters in it: http://www.automedia.com/Holley_Carb...ht20000718hm/1

 

This is an EXCELLENT book to have if you are trying to tune a Holley carb.
http://search.barnesandnoble.com/Sup.../9781884089282

 

While you're at it, dump the points and get a petronix elec ignition system for about $80 and say goodbye to timing issues forever.

CP

 

Wow...thank you guys. I appreciate this information.

hiboostwoody,

That's a lot of useful information. I truly appreciate you gathering that information for me. I'm going to look into those new nozzles and a cam kit. I'll have to look into that book as well.

Carlos Pineiro,

Dude...believe me...I know. I've been wanting to dump those points for a long time. I probably should just get the petronix today.

Thank you all again. I'll let you know how it goes.

-TNT

 

+1 for this idea
+1 for pertronix over mallory
+100 for no points/ballast resistors!