Help...Backfire now = high idle..

[WVspeedfreak]

Had my boat out.....Runnin wide open, lets out big backfire. 468 chevy, 8-71 weind. Now it idles real high. I have a pop-off, and no burst panel. Still builds the same amount of boost....14 lbs. Could I have blown a power valve??? Older model 1050 dominators. I dont know much else about the motor. It was in the boat when I bought it. Thanks

[LynnsJet]

Could it be detonation that you are experiencing? That can kill a blower motor. Cut back your timing and make sure you are a little rich until you get that thing sorted out.
just my .02

[WVspeedfreak]

I don't think it is detonation. Plugs look good. First time it ever backfired, and I have had it sev times before.

[ghittner]

I once had the same problem after a backfire and the secondary flaps in my Bolley bent upward uncovering the bleed circuit just enough for high idle and rich idle as well. Stumped me, I put in power valves and diagnosed until the cows came home. It was when I pulled the carb off to replace it that I saw the bent flaps, bent 'em back and the carb still performs well....

[502 JET]

It sounds like you are sucking air. Check the simple things first such as a missing vacuum plug/cap. If your intake has a blow off plate check that reseated properly. If they are all intact I would look at your carb, intake, and supercharger gaskets.

[billet racing 1]

I experienced the same thing. After my backfire, could not keep an idle. After rebuilind the carbs, still nothing. Ended up pulling the heads, replaceing some valves and spring. Could not see any evidence of a blown gasket. Did find the boot from carb to turbo out of place.
Now I can't get her toidle below 1,500. Turn the carb udjustments all the way down. Checked the linkage, its allowing to close all the way. So where is the air and fuel coming from. If it was just an air leak, she would starve for fuel and die.
Jerry

[wideopen545]

check your intake gaskets at the head or your pop off gasket
or valve guides

[Beer-30]

Cover the tops of the carbs one at a time while idling and see if it still idles. That will let you know if it is getting air from somewhere other than the carbs.

[VDRIVERACING]

Bunch of places to check, including blower to manifold gasket.
Recently, I did the backfire thing and found the butterflys were then out of adjustment for idle. They didn't appear to be bent, but we just re-gapped them to the venturi, and it was back under control. I am not sure of the exact gap you need, but it's something like 5 to 10 thousands. This problem drove us nuts, but was easy to fix once we knew the cause.
To check, you need to disconnect the linkage, and back off the idle linkage screw all the way so the carb is closing on the butterflys. There should be an equal gap between the top and bottom of each butterfly to the venturi wall. If you find they need some massaging, remove them from the blower and do it on a bench.

[billet racing 1]

While I had the carbs off, I did notice on one, the secondaries were open a little, backed off the set screws so they were closed. While the carbs are empty of fuel, its easier to adjust. Adjusted carbs to match opening of both primaries and secondaries. Whats confusing me is the secondaries are shut tight, and I can back off the primaries all the way, and still high idle. I will check for bet or offset butterflies.
Jerry

[VDRIVERACING]

While I had the carbs off, I did notice on one, the secondaries were open a little, backed off the set screws so they were closed. While the carbs are empty of fuel, its easier to adjust. Adjusted carbs to match opening of both primaries and secondaries. Whats confusing me is the secondaries are shut tight, and I can back off the primaries all the way, and still high idle. I will check for bet or offset butterflies.
Jerry
You don't want the secondaries shut all the way, that's the trick. They need to be open just a hair so you get some fuel down those venturies. That will richen the mixture up and idle down. Sounds backward, but it works.

[ghittner]

After mine were bent, I bent them back then adjusted them as far closed as I could without them sticking when trying to open, that solved the problem.