My Chevelle sometimes "surges" at highway speeds. It also sometimes backfires through the carb when I give it full throttle(punch it). If I roll into the throttle, it runs fine. I have a 402 punched 30 over, comp cams 268 camshaft, stock ignition system and Q jet carb. The timing is set at 8 degrees BTDC, I replaced the plug wires with new ones, cleaned and regaped the plugs to .035, and the idle mixture screws are set at 3 full turns out after seating. The dwell is set at 28 degrees. I think the problem is in the carb. I tried to connect a vacuum gauge to the pvc port on the carb and it almost stalled. It idles fine at 800 and at highway speeds(65mph @ 3100rpm) it runs fine until I go up a hill or sometimes it just surges like its not getting fuel. The pickup in the tank is new (within 6 months). I am running 2 fuel filters on the car, the inline one and the one in the carb. Would anyone have any ideas why this is happening!! TIA.

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Mark
TC#1600
69 Chevelle SS
gotagos69 (http://www.chevelles.com/showroom/gotago/showpic1.jpg)

Sounds like a vacuum leak somewhere. The backfire is a dead givaway as is the engine almost stalling when you hook up the vacuum guage. Check around the intake manifold. Someimes the gaskets don't seat corectly..try to retorque the intake manifold bolts. Do you have stick or auto trans? If auto, check the vacuum line running to the vacuum modulator also. Look at the carb base also for leaks. Retighten the carb mounting bolts. Good luck!


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MalibuJerry350
TC Member #1279
Original owner '70 Chevelle.
593,000 miles on car.
Hey, if it's got wheels, DRIVE IT!
My Chevelle: http://hometown.aol.com/erie614/myhomepage/index.html

It's definetly running lean for some reason. Set the idle mixture screws at the point where you get the best idle quality instead of a specific number of turns from the seated position. Then start searching for a vacuum leak. If you don't uncover a vacuum leak, you may need to pop the top off the carb and take a look down inside the float bowl. Q-jets are notorious for picking up trash in the main jets. Dirt and insect parts are the most common culprits, and they usually find their way in through the bowl vents.

Thanks for the replies, its really throwing me for a loop. I'll check it out in the next couple of days and post the results. Again, thanks!!

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Mark
TC#1600
69 Chevelle SS
gotagos69 (http://www.chevelles.com/showroom/gotago/showpic1.jpg)

Here's an update: I checked the carb and manifold for vaccum leaks and could not find any, and reajusted the idle mixture screws to best idle. However, it still surges so I'll pull the top off the carb and check the bowl. When I apply the brakes, there is a hissing sound from under the dash and the idle increases about 300 rpms. Any idea what causes this?

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Mark
TC#1600
69 Chevelle SS
gotagos69 (http://www.chevelles.com/showroom/gotago/showpic1.jpg)

The hissing from under the dash could be a vacuum leak at the power booster (assuming that you have power brakes). As far a the surging at highway speeds, try swapping out the fuel pump with a cheap stocker and see if the surge goes away. I had the same surging on the highway with a Holley mech. pump. It would lose power and surge going up hills until the car was almost standing still. I replace everything in the ignition trying to troubleshoot the problem, and it ended up being the fuel pump. Good luck http://www.chevelles.com/forum/smile.gif

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aka Scott B.
1967 Malibu
406 cid / 350 TH
Under Constuction

Are you running vacuum advance? Dont rule out (although it sounds like a vac leak at the booster) too much vac advance. Only comes in to play at cruise and if there is too much being thrown in the car will surge. This also happens under load, such as up a hill.

Well, I took the carb apart and the fuel bowl looked clean. I didn't get to road test it yet so I don't know if I did any good or not. About the only thing I have not done is relash the valves. Again, thanks for all the help!!

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Mark
TC#1600
69 Chevelle SS
gotagos69 (http://www.chevelles.com/showroom/gotago/showpic1.jpg)