Vacuum leak/idle issue

[butler57]

So...here's the backstory. Truck was running fairly well and idling fine. Took it out at highway speed last week and it really started idling rough after that. It's a re-built stock 350, HEI,with a Holley 650 carb.
I suspected a vacuum leak so I've sprayed carb cleaner along every gasket and can't seem to find one. I've replaced all the vacuum hoses - no change. Turning the idle mixture screws makes no difference - can't get it to bog down. Also tried a piece of cardboard over the carb and can't choke it off at all. PCV appears to be working fine. Changed all the sparkplugs and checked all the plug wires too.
Even with the idle mixture screws all the way in it seems to be running really rich and it's really burning through fuel.
Replaced the fuel pump not too long ago. Doesn't appear to be a fuel problem as I see gas when I move the throttle (unless it's getting too much?).
I'm not a mechanic so would really appreciate where to go next?

[geezer#99]

How's your float level set?
Maybe it's flooding a bit!
Look in the carb while it's idling. You shouldn't see any fuel.

[DT1]

You can also try isolating the vacuum. Plug off all the ports on the manifold vacuum manifold and see if that makes a difference.

[brad_man_72]

Blown powervalve?
Posted via Mobile Device

[PanelDeland]

Check the vac hose to the modulator on the trans.

[Toxic Orange]

I agree with brad_man_72... a blown power valve. Turn one or both of your idle mixture screws in all the way (clockwise). If it doesn't kill the truck its certainly you power valve. The newer Holley's have a power valve protection built in but the older ones did not. One backfire and they are toast!

[butler57]

Thanks for all the suggestions.....I havent checked the float level yet as I need to do a little homework on that, Nervous about burning down the truck.

I have pulled off all the vacuum lines one at a time and replaced them with new. Also checked and replaced the one to the trans modulator. My vacuum guage is running steady but really low (10 - 12hg) however I'm at 3700 ft above sea level too. That's why I thought vacuum leak.

AS for the power valve - I'll check that today. How would that blow? Truck has never backfired or even had an issue. Could it just fail?

Lastly, how much fuel should be spilling into the carb at idle? Just looking down it's really spilling down the carb on the passenger side.

Thanks for any help....

[luvbowties]

Originally Posted by butler57

Thanks for all the suggestions.....I havent checked the float level yet as I need to do a little homework on that, Nervous about burning down the truck.

I have pulled off all the vacuum lines one at a time and replaced them with new. Also checked and replaced the one to the trans modulator. My vacuum guage is running steady but really low (10 - 12hg) however I'm at 3700 ft above sea level too. That's why I thought vacuum leak.

AS for the power valve - I'll check that today. How would that blow? Truck has never backfired or even had an issue. Could it just fail?

Lastly, how much fuel should be spilling into the carb at idle? Just looking down it's really spilling down the carb on the passenger side.

Thanks for any help....

...see geezer's post#2...(none?)
Sam

[geezer#99]

If you see gas then it's not the power valve. The only time you should see raw fuel inside is when you tap the gas pedal and the acc pump shoots a little in.

[butler57]

Thanks Geezer#99. When it's idling I see a lot of gas going down the carb. It's a fairly steady stream especially on the passenger side. I'm heading out of town for a few days but I'll check that when I'm back. I need to do a little research as I'm unsure how to check the float level in this thing.

[geezer#99]

Look on youtube. Lots of vids there like this one.

How To Adjust Fuel And Float Level On Holley...

[Toxic Orange]

Those Holley videos are excellent. Apparently they have a DVD with all of them but you can get them for free on You Tube. Good call geezer#99!

[butler57]

thanks guys - I've been out of town so just getting back to thetruck tonite. I've looked thru the videos and there is some really helpful stuff there. Thanks geezer99 for pointing me in that direction.

[domeier]

Since it runs with a piece of cardboard over the carb throat, you know it's a large vacuum leak. I'd guess your carb base gasket has a problem, or maybe the brake booster circuit. You'd think you could hear where it's sucking while you choke off the carb?

[butler57]

I couldn't choke it with the cardboard so I was thinking major vacuum leak but I think it was just getting air around the top of the carb throat extension. I covered it with a rag today and it chokes right off.
My carb doesn't have any site holes (Holley 4174) so do I need to take the carb off the manifold to check the float level?

[Don Quixote]

Is the carb loose on the intake? I don't mean to insult your intelligence but half the time I would have already broken down my carb before I do a rectal head extraction and realize that I need to tighten some studs.

[50mercfan]

unplug the hose going to the brake booster. if it quits, then your booster is bad

[butler57]

Pouring rain today so I'll spend the day in the garage (gets me out of other work around the house). Don't worry Don Quixote, I'm not insulted in the least - just really appreciate all the advice. The bolts were loose and I didnt want to over tighten but didnt seem to help when I did. Manifold is tight too and doesnt seem to be the problem. I'll check the brake booster and see if that's it.
Thanks again.