Holley 670 Street Avenger: idles at 2000rpm, backfires...

[Dangerous Doug]

My Cobra has been sitting for a couple of weeks and I took it out today for a short run. The idle was really high, which I figured was the choke (electric), but after letting it warm up I could not get it to settle by reving it up. Huh! Drove a couple of miles and then stopped, and it backfired out of both side pipes.

That's when I knew something was messed up. Drove it home with the engine idling at 2000 rpm, got home and it backfired again. Checked for vacuumn leaks, sprayed carb cleaner into the four little ports on the main and secondaries. No go. Still idles at 2000 rpm.

A little help for the novice Holley-tuner?

DD

[PeteF]

Make sure timing hasn't changed due to distributor bolt being loose, and distributor rotating. That, or something preventing throttle from returning to idle. Once you set the timing, it helps to put a reference mark on the base of the distributor, so you can see at a glance whether it has rotated.

[Dangerous Doug]

Yeah, was going to check timing tomorrow... Good advice on the reference mark.

DD

[4pipes]

Backfiring is detonation up out of the carburetor. Do you mean unburned gas is detonating in the exhaust pipes? I had a stuck needle valve (float) that caused that on a Holley.

[Gaz64]

I'd bet your electric choke is stuck on.

With no other symptoms (lack of power or pinging), I doubt it is timing related.
A 2000 rpm idle has to be a large air leak, butterflies stuck open via the fast idle cam or some other issue (return spring etc).

[Dangerous Doug]

Originally Posted by 4pipes

Backfiring is detonation up out of the carburetor. Do you mean unburned gas is detonating in the exhaust pipes? I had a stuck needle valve (float) that caused that on a Holley.

Unburnt fuel detonating in the exhaust pipe is what I meant when I wrote "backfiring".

Looked at the timing, and at 2000 rpm the timing is advanced 32 degees.

Looked at the float levels through the "windows" (actually, screws on my carb) and it didn't seem too high. Unset and reset the idle/mixture screw to see if any debris could be knocked loose. Rotated the choke with no change in rpm.

Baffled. Hmmm, let's see, needle valve...

DD

[PeteF]

I'm surprised the idle/mixture screws have any effect, if you are "idling" at 2000 rpm, well above normal idle. Have you verified that both throttle plates are fully closing (looking down the bores with engine off), and that they actuate smoothly throughout the range. Check this with return springs removed, engine off. Also verify that the backfires didn't blow any rubber caps off the carb or manifold (if you have any closed/unused ports). That could cause an air leak which could raise idle rpm. I think there is a Holley troubleshooting section on their website that might have more advice.

[elmariachi]

Nothing about that carb should make it stay at a 2k idle repeatedly unless the primary throttle shaft (controlled by the gas pedal and cable/linkage assembly) is open. Look down in the front bore and see if the blades are partially open. Disconnect the linkage arm from the carb and make sure there is full travel in both the foot feed and the carb linkage and that once unhooked the front throttle blades are closed and near horizontal in the bottom of the carb. If the front butterfly is at or near vertical then its not the choke either.

The only way a carb can generally "idle" that high is if the gas being fed in is in atomized/mixture form, as raw gas would more likely drown and stall it, not rev it up. So its likely not a stuck float. The timing wouldn't make it go from 700 rpms at idle to 2k either.

.

[Michael C Henry]

I once had a piecs of choke linkage come off and hold carb open , just above idle. That was a 73 Cleveland. It was equiped with an electric solenoid that established idle then let carb go completely closed for shutoff. That had a high idle and back fired out the exhaust while engine was coasting to stop. Carb open , engine spining , carb open a little sucking in fresh gas and air ,no ingition spark so now heated gas fuel mixture in exhaust waiting fo anything in exhaust to light it off.

[Dangerous Doug]

Well, I putzed around with it all afternoon...and then, Success! (With a capital "s", no less.)

I noticed that the lever-cam for the electric choke was jamming. I could poke it loose with my finger, but it would stick again when I rev'd it up. I removed the electric choke assembly and looked at the cam teeth. They had been worn rough from the end of the high-speed idle screw, and had created a divot in one of the teeth. I filed this smooth so the teeth wouldn't catch and re-assembled it.

Back to normal. Not sure why it picked today to fail---and what an odd thing to fail---but I figured it out. Runs great, now.

And now, the rest of the story...

I took it out for a quick run to "test" it out, nice acceleration down the freeway, back through town and then as I approached the turn to my house I went straight and drove down into a redwood grove. Partially for the fun of it, and partially to test out my new PIAA driving lights. Got to a good turn around and decided to take a couple of pictures of driving lights to post here. When I got back in the car I noticed my tach had dropped to zero and needed to be reset, so I turned my Cobra off...

It wouldn't start---either all of the starting over and over again drained the battery, or my new lights eat more power than my alternator is delivering---and I was deep enough into the grove to get absolutely no cellular reception. I pushed my Cobra onto the entrance of a private bridge and luckily for me (I was at least a mile from home) caught a ride up the hill. A ride back with jumper cables and I was on my way.

My Cobra is on my trickle charger as I write.

DD