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Old 01-25-2008, 12:03 PM
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holley carb backfires threw carb

hello im Aaron.i have a red 77 nova with a 307 turbo fire rebuilt 1200 miles ago.when i bought the motor it seemed to run perfect!but while putting it in i had to take the carb of and distibuter.well i went threw more hell getting the thing timed right but i think i wfinnaly got it. but hears the deal it a holley 4160 single pumper 600cfm looks NEW and when you punch it in drive or park it coughs/or backfires fire threw the carb but usualy cough and dies down and then usually goes back to idle.i did the the idle mixture screw trick to check the power valves and there good.float level good.timing i 10 beforo vaccum is around 10 to 13 in park or 10 to 9 in drive.i already when threw a set of plugs because they were SOOTY BLACK.running rich sounds like.no black smoke threw exhaust but some WHITE>please help i been working on this car for 9 MONTHS.

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Old 01-25-2008, 12:13 PM
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IF you're back firing thru the carb it's either timing(my guess) or plug wires mixed up. Note the more this happens the more chance of blowing the power valve in the carb too. Good Luck!
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Old 01-25-2008, 02:49 PM
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yes ive heard about that.i dont know exactly what the timing is supposed to be .and ive checked wires 2 times.i must correct myself from befor.vacum at idle in gear is 13 to 15.i drove the car with the gauge connected and found out whenever i open the throttle enough the gauge goes to ZERO and that when it backfires.i tried this in park and slowley reved it up the gauage would fall a little then rise then when reved high enough gauge would fall to about zero and cough and cough.is this NORMAL?maybe secondary opening to early?
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Old 01-25-2008, 03:02 PM
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Go to a larger delivery nozzle and up the jetting 2 #'s.

Go to Holleys site and download the free troubleshooters guide and manual.
http://www.holley.com/TechService/Library.asp
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Old 01-25-2008, 04:10 PM
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A rounded lobe on the cam, will make it pop out the carb too.
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Old 01-25-2008, 04:31 PM
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Backfiring

I think I missed the boat. He said it ran good before he pulled the carb. and dist. How'd that wear out a lobe on the cam, or change the jet in the carb?
I'd guess in the timing or wiring. I'd check the wiring again, with help if needed, then try moving the timing around and see what happens. lol
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Old 01-25-2008, 04:51 PM
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1bad80 i was thinking on doin that but whats your reasoning.everywhere ive read sais that if your plugs are black sooty that your running rich even holley.i might go down 2 sizes maybe it was overjetted by THEM
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Old 01-25-2008, 05:04 PM
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oh also i thought it was the ingition so i switched out the original whith was a little crusty with one thats almost NEW and a new coil.also checked each wire for spark and all good.same problem.it idles real nice and PART throttle is fine.only when that gauge goes all the way to about 22 on gauge and falls of to zero then FUUUCKK i bought the engine from a guy in miami who workes at a cool hotroddin shop.gave me a golden deal.i trust him that he saise its rebuilt 1200 ago.motor compresion tight.rockers seem good .CLEAN under tins.oil not even black.
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Old 01-25-2008, 06:57 PM
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Well black plugs would mean rich but popping through the carb usual means lean. I would look at what you have done so far. You say it ran good before you pulled the carb and distributor.

1 Check the wires, it is real easy to mess up 5 and 7.

2 Did you use a new base gasket when you reinstalled the carb, if not
change it.

3 if you are running a stock cam your vacuum is to low, I would expect
around 18 at idle. Try spraying some wd40 or carb cleaner around the base
of the carb to see if it changes the idle. If it does you have a vacuum leak.
This could cause the power valve to open early and soot up the plugs.

The vacuum gauge is telling you something is wrong, usually low vacuum is either late timing or a leak. Did you have to adjust the valves. A tight valve can also cause popping through the carb. My bet is you have a vacuum leak. let us know what you find

Chet
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Old 01-25-2008, 08:19 PM
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[QUOTE=lanierledford]I think I missed the boat. He said it ran good before he pulled the carb. and dist. How'd that wear out a lobe on the cam, or change the jet in the carb?
I'd guess in the timing or wiring. I'd check the wiring again, with help if needed, then try moving the timing around and see what happens. lol[/QUOT

Yes I guess you did miss your boat!!!I didn't say it was his problem.I said a cam could do it.And it CAN.Sometimes it does take a while to round a lobe.
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Old 01-25-2008, 09:09 PM
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You can run a compression check too.If you have 0 on one of the cylinders then you can back of on the rocker a little to see if your compression comes up on that cylinder.And if it does come up,then you will need to reset all of your rocker's.Check all that everyone is telling you first.

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Old 01-25-2008, 10:57 PM
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When you mash down on the throttle, your manifold vacuum will always drop to zero, then come back up when you steady out the throttle movement. Is your distributor advance hooked to Manifold or Ported vacuum? Pull the distributor cap off and #1 spark plug out, install a compression tester hose only into #1 Cylinder. Have someone else crank it over while you watch to see where #1 pop's off at. You could be 1 tooth, or 180 degrees off on your distributor timing causing that low vacuum at idle.




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Old 01-26-2008, 10:45 AM
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back firing out the carb is a sign of retarded ignition timing and/or a lean mixture.

with only around 13 inhg of vacuum, i would say you need about 20 degrees of initial timing.

you might also need a bigger accel squirter. pull the one off your holley and read the number off it, then go up 3 or 4 steps.
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Old 01-26-2008, 11:48 AM
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hey guys im new here and really like the forum,i have been coming here a while just lookin and listening {well Reading}to you guys comments and i finally joined ,,,really sounds to me like he might have a vacuum leak .
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Old 01-26-2008, 01:04 PM
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i will check wires again 5 and 7.yes i used a new base gasket.you said shoul be 18on gaugh.at idle or in drive?i will check compression with cap off.the distributer advace hose is connected to BASE OF CARB PORT.20 deg of advance i HAVE HEARD on a web site with a guy with a 307.i will try.
thanks yall.oh one thing i did change before installing engine was HEADERS and the exhaust is DYNAMAX.
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