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Mysterious Idle Problem - Help Please
Posted 04 November 2007 - 03:00 PM
Well I'm still in the process of getting my '79 brat legal. I just finished installing an ignition system upgrade, then I began to tune the carb to get it within the idle speed limit and that's when this problem began.
You know how when you first start your old soob up, she revvs pretty high for a while. Then, you tap the gas pedal, and the the revvs fall down to that nice low idle? Mine's not doing that anymore.
I start it up, she revvs high and keeps revving high after its warm and I tap the accelerator pedal. I've backed the idle speed screw all the way out - no change. But here's the kicker. When I'm at a stop sign, if I keep my foot on the brake and let the clutch out to just where it begins to catch, simulating the car under heavy load, the idle speed will fall down to where it should be (900), and then stay there when I push the clutch in. As soon as I give it some gas however, the idle speed will go too high again (~2000 or so).
I've think I've got it timed right, so I don't think thats the problem. And then when I take her for a drive she runs really nice, all except for that idle problem. And of course if my idle speed is too high, I'll never pass smog.
Is there some kind of gizmo that causes the car to idle after you tap the accelerator pedal? If so, I think this may be malfunctioning.
All ideas and help are very appreciated. Thank you,
Posted 05 November 2007 - 01:22 AM
Posted 12 November 2007 - 03:45 PM
Believe it or not, it took the smallest counterclockwise turn of the adjusting screw in the vacuum control valve to get her to return to idle properly. Now she is very well tuned, and returns to idle perfectly.
The biggest clue was that during the high idle a non-stop hissing sound was coming from the carb. And I remembered reading about this hissing sound in the Idiots Guide, which pointed me to the vacuum control valve (a.k.a. decel valve).
Now I am going to speculate here, so someone out there stop me if I am way off base. But you know this idle problem didn't happen until I upgraded the ignition system with an MSD 6A, Accel Super Coil, new 8 mm wires and plugs. Basically replacing the whole ignition system. Is it reasonable for me to postulate that the engine is running so much more efficiently now that it actually creates a stronger vacuum which resulted in the need to adjust the vacuum control valve?
Damn, I'm glad I fixed this. I didn't know what was going on, I was considering rebuilding the car, or putting the old ignition system back on.
Posted 22 February 2010 - 11:11 AM
Posted 24 February 2010 - 11:22 AM
am looking into upgrade ignition of my 81 brat with 1800 motor. would i do same type of changes, msd box and 8.5 mm wires and coil? anyone with experience on this application with these upgrades and how it worked? thanks.
Biggest waste of money I can think of.It is not an "upgrade".
Posted 24 February 2010 - 02:01 PM
would appreciate knowing the why - is this your guess, experience or what? thank you.
Posted 25 February 2010 - 11:44 AM
Posted 25 February 2010 - 12:50 PM
brevity is not always illuminating.
What more do you need?
Posted 25 February 2010 - 01:52 PM
does anyone else care to comment on changes to the ignition system?
Posted 02 March 2010 - 08:25 PM
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