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Miss fire

New User
Posts: 8
Joined: 08/13
Posted: 09/14/13 07:35 AM

I have a 302 with a holley 600cfm on it and when driving it at a low speed it has a miss. What could be the cause of it ? I have new everything on it !! Confused  

Post Reply
Posts: 124
Joined: 03/13
Posted: 09/14/13 12:15 PM

vacuum leaks from the vacuum tree on the firewall near the power brake booster..

vacuum leak from the power brake booster..

vacuum leak from open vacuum ports..

broken/cracked spark plugs... incorrect gap.

incorrectly assembled spark plug wires.. where the metal terminal inside the boots is not at the proper depth..  either too shallow so the boot is not preventing high voltage spark leaks.. or too deep and the metal terminal is not latched on the spark plug tip..

crossfiring  on the drivers side.. because of an incorrect spark plug wire installation..

the order where the drivers side spark plug wires as they pass over the driver side valve cover have to be     7, 5, 6, 8.  



Post Reply
Posts: 124
Joined: 03/13
Posted: 09/14/13 12:20 PM

pictures of your engine bay would help...

upload them to a photobucket dot com album.. its free... paste the links here so we can see whats going on...

guess what.. you can also take a video and post it to youtube and then paste the link here and we can see how it runs...

oh.. and how is the carb jetting..

how is the idle mixture setting..

how is the idle transition circuit working...

and don't just say its a new carb... stuff happens..  there have been small debris found in several brands of carbs...  

Post Reply
Posts: 124
Joined: 03/13
Posted: 09/15/13 09:54 AM

do you still have an oxygen sensor in the exhaust????

if so.. you can use that to assist in tuning the carb...

think about picking up a air fuel ratio gauge... 50 to 75 on the low end.. and around 220 on the middle upper end price range... you might find a sunpro for 35 if you look around.. they seem to have vanished from where i looked..

this replaces pulling spark plugs out... to know what you fuel mixture is...

in the carb.. you have multiple fuel delivery circuits...

idle circuit where the fuel allowed into the motor is controlled by the idle mixture screws..

idle transition circuit this fuel comes out a slot just above the lower edge of the throttle blade... as the throttle is opened.. you expose more of the idle transition slot.  this allows the air fuel mixture to maintain a steady ratio as you are changing the throttle opening and there is not enough air flow thru the booster ventury's  the idle transition slot has a idle feed restriction to limit fuel discharged thru the slot.. bigger motors need more side.. smaller motors need less IFR...  there are idle air bleeds but those are for fine tuning the fuel curve...

then the main circuit...  this is calibrated by the main jets and the high speed air bleeds and the float levels..

lets see..

idle circuit.
_____+++++++++idle transition circuit++++++
____________________________+++++++++primary main circuit+++++++++  

Post Reply
Posts: 124
Joined: 03/13
Posted: 09/15/13 10:37 AM

here is one of several articles spread out in the many magazines on tuning the carb with a air fuel ratio display..

knowledge is power...

knowledge of how to tune carb circuits is  power and also miles per gallon..

i have worked on a lot of carbs... i worked on one for a shop owner... had him doing dry hops across his parking lot while i played with his quadrajet APT.. which is the lower part of the primary main circuit...  as i toyed with it 1/4 turn at a time.. his smile grew...  once i got it right.. he took it out for a drive around the block.. he said he had no idea that quadrajets could run so good...

i did the repop holley on his friends 70 torino cobra SCJ429... man was that friend impressed..  totally different with just a few well thought out and diagnosed changes..  but each engine/carb set up needs to be individually tested... figure which is too rich.. which is too lean...  until it becomes like goldielocks and the 3 bears... this bowl of is just right..  

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