Q-jet throttle stuck open, backfired, and now won't idle - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-02-2008, 06:50 PM
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Q-jet throttle stuck open, backfired, and now won't idle

I have a Q-jet on a 383 SBC. I changed plugs earlier today, and then took the car out for a ride. Things went great for about 15 minutes. I took off wide open from a light, and when I went to back off around 45, the throttle stayed open. I kinda paniced and killed the motor with a couple backfires and bangs. After I got the throttle cable unbound, I started it back up but the idle is real rough, like I'm missing a cylinder. I was 5 miles out, so I limped home as is. Let it sit for 20 minutes and started again, but still doesn't want to idle. Anybody have a guess what I screwed up? Everything was going nicely before the throttle problem.

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Old 09-02-2008, 07:00 PM
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My guess; Bent push rods.
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Old 09-02-2008, 07:05 PM
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Do your basic troubleshooting. Check plugs, goo in the carb around the plates, vac leaks, etc. You might have just blown off a vac hose. After the easy stuff, do a comp check, pull the valve covers, look for mechanical reasons. Tune up stuff first, valve train stuff second.
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Old 09-02-2008, 07:22 PM
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I'm gonna hope it's not push rods...
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Old 09-02-2008, 07:28 PM
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Possible rocker arm knocked off. Easy to check.
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Old 09-02-2008, 07:53 PM
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You may have kicked the secondary fuel rods out of the jets and they have now jumped out of their jet holes. Check the secondary air plates and see if they are stuck open or bent from the backfire.

If not, I would pull the top off and see if the secondary needle rods are bent at the tips from popping out.

Long story short...this exact same thing happened to a 1 ton work truck many years ago and it got about 2 miles to the gallon after the backfire. Once I got the carb off and disassembled I noticed the rods were bent at the tip from jumping out of the jets.

Easy fix if thats all it is, very similar symptoms.
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Old 09-03-2008, 03:53 AM
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if it's a hyd.cam, wait a while while the lifters bleed back down.
like a day or so...
just a thought (without me being there)...
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Old 09-03-2008, 05:47 AM
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I gave a brief check over on all things ignition and vacuum this morning. I didn't find anything disconnected or loose. Tried starting and the same thing. It wants to idle @ about 5-600 rpm in park, where it used to be about 850 rpm in park. Not sure what my next move will be...
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Old 09-03-2008, 07:13 AM
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re: Q-jet throttle stuck open, backfired, and now won't idle

If it is missing at all speeds, idle it up to about 800rpm and carefully pull each plug wire off one at a time to see if any of them does not effect the idle, if 1 or 2 do not effect the idle then you might want to look inside the valve cover for rocker or pushrod problem on those cylinders, if all effect the idle then it might be in the carb or any area of the vacuum system.

Good Luck!

Last edited by 68 Pickup; 09-03-2008 at 07:33 AM.
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Old 09-03-2008, 09:55 AM
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Also, if you are running a vacuum advance, ck to see if it still works. A good backfire out the carb sends hundreds of pounds of pressure through every hose. Could have blown a small piece of carb base gasket or intake gasket out too, giving you a bad vacuum leak. Also ck the power brake booster, it could be blown too. Might have to unplug EVERYTHING and cap them off to ck it over. Then take some carb cleaner spray and go around the carb gaskets and intake gaskets. If you get an RPM increase, you found a leak.

Mark
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Old 09-03-2008, 10:01 AM
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Quote:
Originally Posted by Jmark
Also, if you are running a vacuum advance, ck to see if it still works. A good backfire out the carb sends hundreds of pounds of pressure through every hose. Could have blown a small piece of carb base gasket or intake gasket out too, giving you a bad vacuum leak. Also ck the power brake booster, it could be blown too. Might have to unplug EVERYTHING and cap them off to ck it over. Then take some carb cleaner spray and go around the carb gaskets and intake gaskets. If you get an RPM increase, you found a leak.

Mark
Thanks, the vacuum advance crossed my mind, as I've had the canisters go bad before. I'll be going through all the hoses and stuff thoroughly tonite, and I'll report what I can find.
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Old 09-03-2008, 06:35 PM
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Check everything connected to manifold vacuum. I had a similar problem once and it turned out to be a crack in the plastic check valve on the brake booster. Check the easy (and cheap) things first.
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Old 09-03-2008, 06:45 PM
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I didn't have much time to check things tonight. I pulled all the plugs. None looked fouled. All appeared to have some wetness (gas) to them, so I decided to leave them out over night. I checked the PCV and the vacuum advance and both are working. The choke is working appropriately and I didn't notice any binding or bent throttle shafts. Pulled the valve covers. Didn't find anything that looked or felt out of the ordinary there. I know I've got more to check, but didn't get the time to look any further tonight. I didn't hear any major vacuum leaks, but I also didn't check with any cleaner. Thanks for all the pointers, and I'll check into things more tomorrow night.
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Old 09-07-2008, 06:55 PM
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Timing? Ignition?

I checked things over again, I found zero vacuum leaks while the motor was running. I increased the idle (with the idle screw on the throttle) to ~900 to keep things running. I hooked up a timing light, which showed 16 (thought I had it at 18). Weird thing is that as I'm watching the light flash, it is intermittently going 4-6 degrees more for one flash. I don't have an idea what would cause this, perhaps something is screwed up in my distributor? Could it be related to the whole backfire thing? ...and yes, this is with the advance disconnected and plugged. Thanks!
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Old 09-08-2008, 06:28 AM
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re: Q-jet throttle stuck open, backfired, and now won't idle

Having the idle at 900 with a missing motor i have noticed that the timing will jump around a little. At 900 the mechanical weights could barely be advancing it if the idle is erratic. I have had new plugs cause alot of problems after they got fouled, i ruined a set of autolites one time after flooding the engine,,,i have flooded engines before and they always cleared up after it got running but not these so no more autolites for me. Anyhow i would take a look at the plugs, make sure they are the right ones, make sure the gaps are still set. If you get to the point that you've eliminated any mechanical, electrical, fuel problems you might want to go ahaead and change them just to eliminate the possibility.
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