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PostPosted: Mon May 17, 2021 3:28 am 
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Mildly built 408C, nothing crazy, and I cannot prevent this thing from fouling plugs almost instantly; it won't idle unless I turn the idle adjustment screw in, which exposes the transfer slots and introduces way too much fuel.
Engine Specs:
Hydraulic flat tappet 230/236 and .562 lift.
Carb is QFT SS-780-VS.
CHI 190 2V heads.
Pulls decent vacuum if plugs are fresh; about 14"
It likes 15-20° of initial timing, but even then I have to turn the idle screw in to make that happen. I've got about 500 miles on this engine, but I've never been able to dial it in. Just today I cracked open the secondary butterflies to introduce more air, but still was unable to get it to idle. Idle air mixture screws are at the stock setting of 1.25 turns out and the engine responds properly to their adjustment. I've tested extensively for vacuum leaks and have found nothing. Again, it will idle if I turn the screw in/idle up, but it's eye-burning, tail pipe smoking, pig rich and the oil/dipstick smells like fuel. I've tuned several engines, all with Carter/AFB/Edelbrock carburetors and went with a Holley/QFT on this one because I thought it would be a performance step-up. This is not making sense anymore and I'm ready to throw it away! Any further insight would be appreciated!


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PostPosted: Mon May 17, 2021 5:59 am 
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Couple of solutions:

1. Spray some carb cleaner down each air bleed up top.

2. Check for blown power valve. Quick Fuel/Holley power valves are really junky these days. Easy to get a bad one out of the box.

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PostPosted: Mon May 17, 2021 6:51 pm 
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Joined: Tue Nov 06, 2012 10:23 am
Posts: 1496
Location: Texas
Nothing worse than burning your eyes from raw gas.

You did not mention if your QF carb is new or used. If its brand new then box it up and ship it back to QF.

If its used then:

1st - What fuel pump are you running? What is the fuel pressure at?

2nd - If your fuel pressure is between 5-6 psi then what is your float level? You most likely have float sites on your bowls. With the car on a level floor, ou want the level in the middle. To high and the vacuum will suck fuel through the boosters.

3rd - if you can not get the float levels under control then your (needle and seats) o-rings might be trashed. Thats what happened to my Mighty Demon carb. When I bought my MD it was before this junk ass pump fuel. I try to only run 110 octane but at $10.50 a gallon that is just not economical all the time. So the pump fuel tore up my o-rings and would flood in the secondaries. Took out needle and seats and had a hell of a time cause the o-rings were catching in the threads. Rebuilt carb and good as new.

4th - could be a power valve issue. what power valve do you have? with the vacuum you say you have , in theory you should have a 6.5 or 7.5 Shoot, my engine has around 10" vacuum and I still have a 6.5 power valve. Check for cracks or rips. If you run pump fuel alot - sorry but this is just the trade off we have to deal with because the EPA are stupid.

5th - not most likely but is possible there is a restriction in you air circuit. I like to periodicaly spray a little WD-40 in the air ports while engine is running - with safety glasses on of coarse - just to make sure things are clear.


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PostPosted: Mon May 17, 2021 9:08 pm 
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Joined: Sun May 16, 2021 10:51 pm
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Carb was new when I bought it, but that's been a few years. Starting over again; carb is off the engine and I've removed bowls and metering blocks. Have been inspecting and cleaning/spraying carb cleaner through every hole I can find. Power valve is the stock 6.5 and looks new upon inspection. I've set the primary and secondary butterflies to where the transfer slots are square (.020-.025). I'm hoping the secondaries being slightly open will introduce enough air to help the issue. If not, I may consider drilling 1/8" holes in the primaries.

Stock mechanical fuel pump and I haven't tested fuel pressure. I'm gonna load her up with fresh spark plugs, again, and I'll have my fuel pressure tester hooked up before I do. Ordered a pressure regulator today from Holley to keep it consistent moving forward.

Float are working properly, I've had them set half way up per the manufacturer specs. I may lower them a touch just to see if there's any change.

I've never had a carburetor/engine give me this much trouble. Again, I've never tuned a Holley, though I do understand basic tuning procedures. Every other carb I've had, new or used, has performed good enough out of the box with some additional/ongoing changes required to fully get them dialed. It's not rocket surgery, but sure is aggravating!


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PostPosted: Tue May 18, 2021 5:18 am 
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The fuel pressure being high is one that I forgot to mention. If you're over 5-6 psi, it could be causing problems for you.

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brent@lykinsmotorsports.com


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PostPosted: Tue May 18, 2021 12:06 pm 
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Joined: Mon Oct 29, 2012 2:39 pm
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Also watch the power valve gaskets - they tend to deteriorate quick with ethanol blends. They also need to be "just so" when tightening the valve or they will seep.

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http://raceabilene.com/kelly/hotrod
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PostPosted: Tue May 18, 2021 1:00 pm 
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If you pull the primary bowl and metering block off and there's gas in the power valve well, you've found your culprit.

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