Bad powervalve?

Just broke in the new 306 bullet, it has the same carb as before. 600 holley vacuum secondaries, it did backfire a couple times tying to get it going where the carb caught fire a little. Well the problem is it doesn't want to idle. I had to crank up the idle screw to get it to idle around 750 rpms. It runs fine when I rev it but the idle screw is too high because it doesn't want to return to idle like it should. So would a bad powervalve be causing this? Or could it be the bad plugs and wires? (ordered up a new set already)

If anyone has any ideas let me know, I've never had a powervalve go bad on me but I've heard all it takes is a backfire.

Vac Leak?

most newer holley's have a check valve to keep from blowing out the power valve. On ones without it yes a backfire can take it out. If the power valve was bad your idle would be pig rich, black smoke and smell of unburn gas.

I'd agree with Dave most likely a vacuum leak.

Its running like theres a vac leak but I can't find any. Will look more...

The other thing I'm thinking about is maybe improperly adjusted rocker arms? Not allowing the valves to close. I'm using factory pedestal mounts, torqued them to 22 ft-lbs. Other than that I don't know because I cannot find a single vacuum leak. Before when it had the leak it was a vacuum port that was open without anything covering it and it didn't require the idle to be turned up like this. Cam is for sure dead on the money, timing is @16 initial like before. Maybe it wants more? It easily took more timing without any issues @3,000 rpms. It has the stock springs and advance of the pertronix distributor.

Plug all the vac connections and try it, pcv and brake booster included.

dfree383 wrote:Plug all the vac connections and try it, pcv and brake booster included.

I haven't been home to check this. But I have the pcv and transmission modulator valve T'd off of one connection at the back of the manifold (which worked perfectly before the new cam and heads). Anyways, if plugging these solves the problem, what can I do to still run this for the weekend? In the long run I'm guessing I'll have to have no pcv valve and just some breathers on the valve covers?

If they are bad replace the components or lines which ever

Check the preload on your lifters. With a cam change, new head gasket, decking, head milling, all this will change the lifter preload with the bolt down rockers.

But it does sound like a vacuum leak, just as others have said.

late response, been busy as heck! Well the powervalve was bad. I put my buddies proform 705 on it and it ran great until I revved it above 5,500 and it had to rev down. It was loading up with fuel with both carbs. Put my 600 edelbrock on it just to troubleshoot and it ran fine. I need to tune a carb to work with this engine or use the one from my 460

I can not stress the importance, if you have any doubt of the power valve's condition ..... change it before you do anything else and check the oil's condition! An over-rich air fuel mixture is a certain for component failure, especially if a flat tappet type of camshaft is in use. As to the vacuum leak, maybe it blew a vacuum line off somewhere or damaged an intake gasket.

Dave.

LivermoreDave wrote:I can not stress the importance, if you have any doubt of the power valve's condition ..... change it before you do anything else and check the oil's condition! An over-rich air fuel mixture is a certain for component failure, especially if a flat tappet type of camshaft is in use. As to the vacuum leak, maybe it blew a vacuum line off somewhere or damaged an intake gasket.

Dave.

I ran it with the other carburetor to play it safe. It ran pretty good as well, no bogging until very high rpms. Checked the oil multiple times throughout the day and looked nice and clean. Oh, and I rechecked everything and blocked everything off, vacuum leaks were not an issue. I did that before I swapped the proform on. It was a powervalve without a doubt, and now its going to have a carb on it that will be tuned for it, and blocked off powervalve or jet in its place.

Thanks for the advice guys This baby's got some kick to it as well