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63chevy 08-06-2011 12:40 PM

is back fireing normal for cold motor
hi i have a 1963 pannel van(4160lbs)
i resently have a machine shop build a motor
for me ,blue print balanced,and bored

469 (454)
roller cam howard 0 lash
10.5 pistons
598 horsepower
582 tork
830 cfm
373 rear end grear
vac plugged on carb and distrib.
i had the timing at 38 degr.
now it's 34 deg runs better and cooler now
when its cold it back fires a little (very little)

driving down road when under 180' (temp) it does it
when 200deg it runs fine fine
and the oil temp is at 70 psi when temp is at 200 it drops to
40 ( in has a high per pump.)
any thoughts thanks :cool:

ericnova72 08-06-2011 12:54 PM

If the carb doesn't have a working choke, it will backfire when cold, usually when you try to accelerate(any opening change in the throttle blade position). This is quite normal.

Increasing the size of the accelerator pump shot will lessen or remove the tendancy for it to backfire. Experiment with larger pump shooters(nozzle) or/and larger pump cams on a Holley carb.

63chevy 08-06-2011 12:57 PM

i un hooked the choke simply i dont drive it when its really cold
ok so your saying bigger jets/smaller or main valve in carb??

ericnova72 08-06-2011 01:10 PM


Originally Posted by 63chevy
i un hooked the choke simply i dont drive it when its really cold
ok so your saying bigger jets/smaller or main valve in carb??

No, not bigger jets or power valve changes. Change the accelerator pump nozzles(shooters) to a larger size, and you may possibly also need to put on larger pump cams(the plastic cam that moves the accelerator pump arm when you open the throttle).

This is a Holley carb we are talking about? You said 830 cfm and that is a Holley size, so I just assumed.

63chevy 08-06-2011 01:12 PM

its a super street

ericnova72 08-06-2011 01:22 PM

Super Street what? Made by who?

63chevy 08-06-2011 01:32 PM

quick fuel (i didnt get holley ,this one was lighter just have a heavy van trying not to add alot of weight)

ericnova72 08-06-2011 01:37 PM

OK, Holley type/style carb. The advice I gave is still correct. All chanfging the jets will do is decrease fuel mileage.

This is assuming the backfire is when you are opening the throttle, not a steady cruise backfire.

You're not giving a ton of info on the exact symtoms and total conditions when it happens. Be specific.

63chevy 08-06-2011 02:05 PM

when i start it up it doesnt backfire
then when i put in d and drive out onto the street it cough and sputters
alittle when it's cold (120/140 degree).
but when it is above 200 deg its fine

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