Added: May 4, 2009

From: Larry W

Duration: 6:7

Did some more troubleshooting on the carb. Found a few major issues. 1. The primary float bowl float was not adjusted correctly and was not shutting off the fuel. The tab on the float was bent down allowing the float to travel all the way to the top of the bowl without seating the needle. This allowed fuel to continually flow into the carb and richen it! 2. The power valve was the wrong size. According to Holley, the valve should be 1/2 the measurement of vacuum at idle in drive. According to my vacuum measurements, the power valve installed was opening too soon and dumping raw fuel in the engine when it was not ready for it, again richening the mixture! 3. The idle mixture screws were not set correctly. They were at 2-1/4 turns out. That is too rich for this particular engine setup. Using a vacuum gauge, I adjusted the idle mixture for maximum vacuum at idle. In my case, that was 1-1/2 turns out which is also the recommended Holley starting point. 4. My vacuum advance was not connected to the right port. It was connected to manifold vacuum instead of ported vacuum off the carb. This caused the timing to always be advanced at idle and then retarded under load as manifold vacuum drops under load! Totally backwards! Ported vacuum on a Holley can be found above the idle mixture screw on the passenger side of the carb at the metering block. I bought this carb about 10 years ago as a "store return". I didn't do much to it then, assuming it was fine . I just bolted it on and drove the car, seemed fine then. I realize now that the temporary owner messed with the float and bent the tab too far and he also changed the power valve. I am checking online to see what the stock main jets should be as he might have changed those too! Word to the wise, never take anyone's word for it that a carb is "all set to go!" Each application is different depending on the vehicle and the amount that the engine is built. Use the proper tools like vacuum gauges and look up tables to adjust for the proper values for your application. There is no one size fits all! Final word about power valves. Holley has built in an anti-backfire check valve circuit in all Holley carbs since 93. Blowing out a power valve by backfire is nearly impossible. However, the diaphragm can get old, cracked, and brittle so it is best to pull it and examine it if you suspect it to be a problem. To test it, turn all idle screws in all the way while the engine running at idle. If the engine stalls, the power valve is working properly. (keep track of the number of turns so you can put them back where they were). I did complete rebuilding the carb and reinstalled it. I will have more video of making adjustments and test drives soon...Thanks!!

Channel: Autos


Rating: 4.518072' max='5' min='1' numRaters='83' rel=' ( ratings)    Views: 134202    Comments: 81

Larry W Says:

Oct 23, 2010 - @Chevymen350SS A vacuum advance? You mean the advance in the distributer? Vacuum advance only applies after timing is set (with it unhooked) The vac hose should be connected to the metered vacuum port on the carb above the idle mixture screw. If this is not what you are referring to just let me know. Thanks

Larry W Says:

Jul 14, 2009 - Remember, vacuum keeps the valve CLOSED, the spring on the other side pushes it open. As engine RPM increases, manifold vacuum decreases so a 10 would open sooner than a 6. Avg idle vacuum on a stock engine is 18 to 20. As you accelerate, the vacuum would reach 10 before you reach 6. Picture the needle on the gauge dropping from 20 to 5. Built engines idle vacuum at 12 or less needing a smaller valve like a 6 or 4.5. Hope this helps explain a little. PM me if you need to. Thanks


Jun 11, 2010 - dude you may have saved the fate of my carb. i instaled a trick kit on mine as well, and also some new brass floats, but the back feul bowl stayed either flooded or dry. so in a rage i maxed out the needle in respons to the high feul levels. i have given up hope until now, but with this new found info i with once agean rip out that damn holley and kick its ass and either make it work or turn it into a useles pile of smashed metal.

Larry W Says:

Jul 22, 2009 - Hey thanks for the input. I did actually have to bend the tab...the needles are now new from the kit and the float would go to the top of the bowl and still just barely touch the needle. I adjusted the level after install & seems fine. I did use a screwdriver on the back screws but did not use vasoline on re-assembly. Left the vaccum sec intact. Its runs great but still stumbles pulling hard in third almost like a smooths out if I back out of the pedal..insufficient fuel supply? Thx!

terrance andrews Says:

Mar 25, 2012 - i got a problem with my sbc its a 406 with stock heads. got a thumper cam 513/498 lift cam headers an 2.5 exhaust it pops out the exhaust i reset the timing it also has new wires an plugs an ing system. i think its the carb its pops even at idel need help please com on my page

arguello1990 Says:

May 15, 2013 - Yes it has a choke but always keep it open and was bought brand new on jegs. Its been tuned at a local speed shop but im sure they messed with it, even after the timing and tuning it does the same on the first start up. It hasent been open everything is as it is when I bought it off jegs. I have the carb at 1 1/2 turns out, the float havent messed with it, the jets and power valves are what it came with.

caballo9871 Says:

Jul 17, 2009 - thanks for the info.

Rinke Kortenbach Says:

Jul 14, 2009 - oh yeah.. :). engine vacuum works the other way around.

Simpdine24 Says:

Nov 24, 2010 - my holly double pump four barrel bogs and shuts down after i floor it i was thinking bad jets or something can u give some advice....

greendogish Says:

Nov 12, 2009 - why do we work on the damn things? i love looking inside them and wish i knew everything about just one model... but so much can go wrong.. anything goes wrong may as well rebuild it completely

arguello1990 Says:

May 15, 2013 - Great video, learnt a few new things. My question is I have a holley 650 double pumper on a 88 irocz with a 305, idk if its cause the carb is cold but everytime I do my first start up it hesitates to stay on have to be on the peddle for about 5 min till i can finally let it off and usually always have to adjust the idle cause it never stays on where i left it, after its warmed up the car drives fine. What could it be?


Dec 28, 2010 - Nice i got a 72 firebird

Larry W Says:

Feb 15, 2011 - @screwopenborders Oil will get contaminated quickly if your PCV valve is not working or clogged. Excess piston backpressure will build up in the crankcase and the exhaust blowby gases will mix with the oil. If severe enough, this can also cause the engine to stall. Replace the PCV valve or spray it out with carb cleaner. Shake it with it out of the intake and you should feel and hear the ball rattle inside.

John Smith Says:

Jun 4, 2009 - A classic "Holley Leaker"

kamphwagon1 Says:

Feb 26, 2011 - @NinjaNeglector If those edelbrock carbs were so good how come NASCAR racers don't use them?........Edelbrock makes a good intake manifold but that carb is nothing but a old carter AVS a shity carb to tune ,Holley is the easyest to set float levels, make jet changes with the most versitility in tuneing options period the reason it's the racers choice !

Larry W Says:

Dec 7, 2009 - Its not the acc pump. This sounds like a major vacuum leak problem, could be leaking intake gaskets. Could also be a pinched gas line or plugged filter. Your not getting enough gas under load which also could be a wrong size power valve. White smoke is usually water or anti-freeze, blue smoke is too much fuel. Does the exhaust smell "sweet"? You could have a blown head gasket filling the cylinders with water and fouling the mixture. Do you have video of your setup?

vidchic18 Says:

Sep 10, 2009 - thanks u helped me and my holley out big time!!!

Larry W Says:

May 5, 2010 - @71bee383 All good points. Im actually swapping out the cam as we speak and will post a tear-down vid shortly. Going bigger on the cam and installed timing gears instead of chain. Im hoping the damn carb will be big enough for the new flow demand. If not, Im dumping it for an Edlebrock with no choke plate and dual inlet. Im tired of efing with this carb! Haha

Larry W Says:

Sep 28, 2010 - @1966nova2 I would first check the timing, with a light. Set valve lash where it needs to be. Double check the spark plug wires are on correct plugs. Is the distributer cap and rotor new? Is the engine built or stock? What recent work have you done to the engine? Does it backfire when you attempt WOT or just at idle or both? What is your experience level with working with engines? Do you own a vacuum gauge?


Feb 9, 2010 - Very funny comments about the number of parts.

Rinke Kortenbach Says:

Jul 14, 2009 - wait a minute.. powervalve 10-ish will open later than the 6-ish one, won't it? I mean: more "Hg it pulls up, the more vacuum it has.

71bee383 Says:

May 5, 2010 - @AllThingzVideo You'll definitely be a helluvalot happier. Holley's are great carbs for running down the track...that's about it. if your Poncho 400 is basically stock, do yourself a huge favor & snag a nice Edelbrock Thunder Series 650 CFM AVS carb. also, those Thunder Series carbs will give any Holley a run for it's money. plus, they are far easier to tune, run EXCELLENT on any street machine & the reliabilty is 2nd to none.

johncartelli Says:

Jun 15, 2014 - power valve size is half the vacuum reading on the engine...ex...19 " vac = 95 valve, 18"vac=85 (if even number vac, go one size lower to 85 not 9...)

redman58fla Says:

Feb 27, 2014 - Hey can someone help me on this I have a holley carb 600 also and when I drive for a min a shoots off every time I stop I have the holley were there's no float at the top I tried taking the front of the carb and adjusting it but it still doing it...


Sep 19, 2014 - good job