Backfire in carby

[Phantom]

G'day all, having just fitted a worked 202 and 350 holley carb, the car is hard to keep running when cold (they tell me all hot motors dont like the cold)but the carby does backfire a bit , is it the mixture screws,timing or are the jets the wrong size maybe?
                                Cheers Rusty

[Brad]

Backfiring through the carby you say.
       Are the spark plug leads on the correct plugs?
      If so , then check your ignition timing ?
             Brad.

[2door350]

Could be garbage in the carb....holleys are particular with fuel...grab and efi style filter, blow out the carb and give it another go....

IF your sure its the carb also worth checking are the powervalve and the possibility of an air leak.......

Powervalves can blow out and do cause very irregular running.....


Also if you have a monster cam check to see that your powervalve is not open on idle with a vacuum guage -   Fouled plugs and a poor mixture will be the result.  You may need to run around a 3.5 and jet up mains a couple to find a happy medium

[2door350]

oh...and wrong size jets wont affect idle much in a holley............

[Phantom]

Thanks guys will check it all out
                   Rusty

[Dave_EH]

I'm with 2door350.

Having been through similar problems a few months back with a Holley 350, I might have a few points to help.

Do you know your cam grind?  You said you bought the Holley new - do you know the power valve and main jet sizes?

If it is an XU1 type cam (long duration), you will prob need to run a 4.5 power valve.

I'd also guess 63 - 65 jet sizes would suit your worked 202.

Are you running a good filter?  You should run two with a holley - one before the fuel pump and one after it.

I like to use the Speco glass type.  These are about $25 each and have a reusable/replacable filter.  They make it very easy to assess muck in the filter.

If muck has got into your metering block (possible) do the following.

Take out the four long bolts that hold the float chamber and metering block to the carb.  This is the big square part that faces you when you look from the front of the car.

Remove metering block and chech powervalve.  Push the valve in and put the other side on your tongue and make sure the valve stays in if so its ok.  It should be marked what size it is.  If its above 6.5 its def too big and will be opening up at idle.

Buy a can of Carby cleaner (Nulon is good) and thoroughly spray the metering block in all the holes, then use an air gun if you have one to blow all the holes out.  Repeat this process a few times.

Unscrew the jets from the metering block and check their size as well and give them a good spray.

Give the float bowl a good spary as well as the needle and seat.

Reassemble.  once the float bowl is full of fuel again (after cranking) check the flaot level.  Undo the big screw on the front - fuel should just trickle out.

It can't hurt to give the throat of the carb a good squirt with carb cleaner either.

With mixture screws, have them out 1 and a half turns out to begin with.  To adjust turn in until the revs drop and it runs rough then give it a half turn back out, or about 3/4 if you drive it a little harder.  You might like to go between each mixture screw 2 or 3 times each just to get it right.  

Fire up and see how you go.  Good luck

Dave