Guys;
This isn't on my Chevelle, but still a Chevy. The problem is with the Holley 4-bbl 750 Vacuum Secondary carburetor on my '77 K20. Last night it just started to do this out of the blue (started after I stopped somewhere and then came back out).

When I drive the truck, at a very light throttle range (say 5-15%) the engine bucks and surges. It is like it is getting pulses of fuel. At about say 5% throttle there is what seems to be a very big, rich bog. Slightly past this it begins to surge and buck. If I throttle through this range it smooths out!

Last night was a long drive home because the 5-15% throttle range is cruising speed on the highway. I had to be quick on the throttle and drive fast, then coast, throttle, coast, throttle, coast... You get the idea. Occasionally if I was easy on the throttle and moved through that small throttle angle at a slow rate it would back fire through the tailpipes. I got a couple of good blue flames out the tailpipes that I am sure surprised a few drivers! :D

Anyways...I figure it must be something to do with an off-idle circuit or maybe a diaphram that is blown, but I don't know all of the details of this carb.

Thanks in advance for any help!

I'd check your spark/timing first - did the distributor slip? Lose your vacuum advance? Bad wire?

Bad gas or water in fuel???

yeah check your timing first...sounds to me like it might have slipped...

I have the same exact problem with my Chevelle. Ive been trying to figure it out for months. I also have the 750 vacuum secondary holley. Ive replaced the plugs ajusted the timing, fuel mixture, floats anything you can think of and it still doesnt work.

I think I may have solved the problem. I talked to a friend and he said that it is probably a cracked diaphram in the power valve. I haven't checked it yet, but will let you know.

we're friends.....what about us???

So...I got out the Mity-Vac and tested the power valve. Definitely bad as it will not hold any amount of vacuum. The new power valve operates as expected. I haven't installed it yet, but it definitely looks suspicious. And Harry...YES, we are all friends here! :D

The power valve is that round thing by the choke right?

I'm having a similar problem, my carb has very few miles on it and according to holley it has power valve blowout protection so I never really thought to check the pv. What is a Mity-Vac and where can I get one?

CheZeppelinCorps, I beleive you need to remove the fuel bowls to get to the power valve, hopefully someone else will verify as I have only changed jets in my carb. When I removed the fuel bowl I thought I saw the power valve.

yes remove the fuel bowls it is screwed in to metering block i have blown em before ta the race track , if you are in a bad fix you pull it out a suck on it if it moves it`s good if it don`t it`s bad done this at track more than once when the shop was not around

Yep...rebuilt the front float-bowl and metering block. This solved the problem and also solved a few other problems. For a long-time the truck would start immediately in the morning, but then immediately die. On starting the second time it would continue to run at high-idle. After driving the truck and parking for a short while it would be hard to start and you would have to crank-open the throttle. Now she starts up without dying in the morning and upon warm restart it fires up immediately without having to touch the throttle! Now I expect to have slightly better gas mileage because the Power-valve won't be constantly leaking.

To answer Rednecks question, a Mity-vac is a hand-held vacuum/pressure pump. This tool comes in handy for almost any job! You can see it here: Mityvac (http://www.thetoolwarehouse.net/shop/MAN45.html)

Oh...also...the Power-valve is something unique to a Holley carburetor. This valve is located in the front metering block of a modular Holley carb. It is basically centered in the metering block and takes a 12-point 1" socket to tighten properly. You cannot tell if a Power-valve is bad by simply looking at it. You must use something to test it. I tried sucking on it, but you can't really tell if it is leaking or not. I was able to pull vacuum with the Mityvac, but it would not hold pressure and you could watch the gauge fall very rapidly. A new PV will hold pressure, but not forever. These valves are not perfect! If you test it, be sure that you do not pull too much vacuum or you could rupture the diaphram. Normally the stock valve is around 6.5" Hg. It is a good idea to use a vacuum gauge to measure vacuum on the engine and use this information to size the PV properly. You can read-about the procedure on many websites or purchase the Holley Carb book.

So...I thought that I solved my problems...when I first started driving the K20 again, it seemed to have disappeared, but the same problems came back again! I have rechecked the float levels, readjusted the accelerator pump, retuned the idle screws, replaced the air-cleaner, etc. The only thing I haven't had time to do yet is check the timing and vacuum advance canister. It seems that it does it only under cruise conditions. I have to either accelerate or deccelerate. This is what leads me to believe that it is either a vacuum advance issue or some clogged circuit in the carb that is responsible for cruise conditions. I do have to get into that back bowl however!

Erik,

Have you thought about a marginal fuel pump, or a clogged fuel filter?

I suppose a marginal fuel pump is possible, however it started to go-out last winter and I replaced it. I would be pretty sore if the replacement only lasted 1 year. The symptoms of that were vastly different from this problem. I have checked the fuel filters and they are fine. I got ahold of Evergreen Carburetor (whom the carb originally came from) and I think that I am going to take the carb out there and have them go through it. I still have a few more things to check however.

What about the sock in the gas tank as well? I have heard they will do wierd things.

I think I may have solved the problem. It could be a blown proportionary valve. Im not even going to worry about it and am going to get a Proform main body for it. 25 to 50 horse and no more bogging!

or a cracked distributor cap???????????????? :D

Update: I finally found time to put the carb back on the truck. I had it rebuilt by a very reputable carb shop. So...got the carb back on and drove it. The d#@$ problems still there!!! Okay, so the carb rebuild was much needed and it did improve performance and reliability greatly, but it is still surging! :mad:
I drove her into work today, but before I did that I disconnected the vacuum advance and plugged it (seeing as it is a cruise condition). No surge! I previously checked the vacuum advance and it checked-out fine. Can't figure out what would have changed with the vacuum advance over several years. Must be something with the ignition. Bad ground wire in distributor, carbon-tracked cap that only jumps on high advance conditions?... This is really a weird problem.

First guess: Broken wire in the pickup coil. (the wire breaks INSIDE the insulation-it may not look broken) As soon as the vacuum advance moves the pickup coil to a certain position, the ends of the broken wire don't touch, and your ignition doesn't work. Engine vacuum drops, vacuum advance relaxes, the wires touch, and you have spark again.

I will get out the DMM and check that. I have had coils do weird things, but I have never experienced a problem with the pickup coil. First time for everything I guess!

Thanks,