Chevelle kicks when shutoff - Chevelle Tech
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post #1 of 20 (permalink) Old Jun 15th, 19, 1:55 PM Thread Starter
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Chevelle kicks when shutoff

Well I fixed one problem and caused another. I had posted last week that my throttle was"t closing all the way at idle due to a weak throttle return spring. I increased tension on the springs and then had to turn the idle screw up to compensate and get up to the same idle speed as before. Works good now and idles well except I've managed to introduce a new issue!

When I shut the car off, I must be getting a few extra explosions that make the engine feels like it kicks. Not dieseling in the sense that it keeps running, but just enough to jerk the whole engine. ALmost feels like the engine moves backwards for a second. It doesn't sound good for the timing chain at all!

How did this cause this issue?

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post #2 of 20 (permalink) Old Jun 15th, 19, 2:36 PM
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Re: Chevelle kicks when shutoff

apparently you increased the idle speed a little in that process. turn it down a little.

or shut it off in gear. or install a idle solenoid, adjust it to idle on the solenoid, back the real idle speed screw way out so it dies instantly when you turn it off.

Holley has a bracket and solenoid deal for that. one wire hookup to a key switched voltage source.
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post #3 of 20 (permalink) Old Jun 15th, 19, 2:38 PM
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Re: Chevelle kicks when shutoff

what's the timing, is the vacuum advance hooked to manifold vac, all the usual questions.

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post #4 of 20 (permalink) Old Jun 15th, 19, 3:03 PM Thread Starter
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Re: Chevelle kicks when shutoff

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Originally Posted by Tom Mobley View Post
what's the timing, is the vacuum advance hooked to manifold vac, all the usual questions.
Initial timing is a little low, but the car always seemed to like it. Its 14 initial, 35 total. All in by about 2900. Ported vacuum advance. I'll check the fuel pressure and mixture screws this afternoon to see if it's unnecessarily running too rich. I'm seeing about 16 inches vacuum in gear and 19 in park.

72, SB 385, Voodoo roller, 219/227; .515/,530 lift; AFR 180cc Eliminators, RPM Air-Gap, 770 Avenger, 12 bolt 3.42's, Eaton Posi, Pypes 2 1/2" X-pipe system w/Race Pro's
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post #5 of 20 (permalink) Old Jun 15th, 19, 3:24 PM
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Re: Chevelle kicks when shutoff

That sounds good.

auto or stick?

what's the idle RPM?

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post #6 of 20 (permalink) Old Jun 15th, 19, 9:24 PM
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Re: Chevelle kicks when shutoff

Thats what that anti dieseling solenoid was meant to do but now you have taken it away from its purpose

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post #7 of 20 (permalink) Old Jun 15th, 19, 11:07 PM
 
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Re: Chevelle kicks when shutoff

Colder spark plug, less gap? No need for vacuum advance with today’s gas...doesn’t really help.


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post #8 of 20 (permalink) Old Jun 15th, 19, 11:37 PM
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Re: Chevelle kicks when shutoff

IMHO, The best way to eliminate dieseling is to close the throttle blades. The only way to do that is to give it more timing at idle with vacuum advance.
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post #9 of 20 (permalink) Old Jun 16th, 19, 12:58 AM Thread Starter
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Re: Chevelle kicks when shutoff

Quote:
Originally Posted by Tom Mobley View Post
That sounds good.

auto or stick?

what's the idle RPM?
Auto. Idles about 1050 rpm in park. 850 in Drive.

72, SB 385, Voodoo roller, 219/227; .515/,530 lift; AFR 180cc Eliminators, RPM Air-Gap, 770 Avenger, 12 bolt 3.42's, Eaton Posi, Pypes 2 1/2" X-pipe system w/Race Pro's
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post #10 of 20 (permalink) Old Jun 16th, 19, 8:30 AM
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Re: Chevelle kicks when shutoff

Slow the idle down some.
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post #11 of 20 (permalink) Old Jun 16th, 19, 2:10 PM
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Re: Chevelle kicks when shutoff

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Colder spark plug, less gap? No need for vacuum advance with today’s gas...doesn’t really help.


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Waa?? I use plenty of it on my 468, on 87 octane. It helps part throttle fuel economy and generally causes the engine to run cooler.
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'71 chevelle, 468, TH400, 12 bolt posi w/3.08's, drinks a lot of 87 octane.
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post #12 of 20 (permalink) Old Jun 16th, 19, 3:04 PM
 
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Re: Chevelle kicks when shutoff

I also have a 468 in my 67...10:1, cam on the milder side, msd distributor no box, 750 double pumper. Friend is an engine builder in Bakersfield, CA who does all kinds of race and high performance street car motors. He recently tuned my motor and disconnected my v/a. Said he doesn’t even bother with that any more, just dials in as much initial advance as the motor will take, then sets the curve and final advance number based on the internals. I have to say it runs better now. He said he does this because of modern fuels, especially our “summer blend.”

Last month I had my carb rebuilt by another old school carb guy here in Santa Clarita. I asked him whether he thought I should hook up the v/a. He basically said the same thing...don’t bother, no real benefit.

I recently acquired a 57 Chevy 210 post with an old Corvette 350/350hp motor. Vacuum advance was blocked off...standard distributor with electronic module. I took it to a local shop to have them put new seals in the tranny. Two old school mechanics and I were talking and I brought up the vacuum advance topic. It turned into quite a lively discussion, with one mechanic saying NEVER and the other saying ALWAYS. It was funny, these two 60 year old guys really going at it! I’ve left it disconnected for now, but may hook it up and see how it runs.


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post #13 of 20 (permalink) Old Jun 16th, 19, 7:35 PM
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Re: Chevelle kicks when shutoff

The thing is, at moderate rpm and light load, your engine can take (and indeed, really should have) more advance. Simply cranking in as much initial as it can take won't give you that. Sure, it'll run fine without it, but fuel economy will suffer, as will part throttle response.

'71 chevelle, 468, TH400, 12 bolt posi w/3.08's, drinks a lot of 87 octane.
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post #14 of 20 (permalink) Old Jun 16th, 19, 8:12 PM
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Re: Chevelle kicks when shutoff

hjdca hit it. Ported vacuum advance.

Set INITIAL timing at 12 deg/BTDC, set a stop on the vacuum advance to only allow 12 degrees, and plug it into a full manifold vacuum advance source point on the BOTTOM of the front of the carburetor. This will give you 24 IDLE degrees of timing, just what the engine wants, and will allow further closing down of the throttle plates at idle, which should also close down the idle transfer slots to the point the engine isn't idling off them.

If you have an idle solenoid in place and functional, it needs to be set up correctly. IT will become the engine running idle stop, with one full turn of throttle plate close down when the key is turned off.

If you have no clue as to what, and how to do a vacuum advance stop for the unit, send an email, ask for the stop plate package, they are FREE, and will help.

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post #15 of 20 (permalink) Old Jul 5th, 19, 10:02 PM Thread Starter
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Re: Chevelle kicks when shutoff

Finally got a chance to give an update. The throttle return springs needed to be shortened even more. And now the problem is fixed. I checked the throttle with the engine on and the car in Park and the throttle was still not being pulled closed all the way. I shut the car off, pulled the springs off and the throttle closed and opened smoothly so there was no hangup in the linkage or anything. Shortened the spring creating more pull in the idle position and the dieseling completely went away.

The strange thing I really can't explain is why it didn't do it initially when the spring was really lax before I messed with it.

So the spring was lax, it didn't diesel. I tighten the spring. It diesels. Tighten the spring even more....dieseling goes away. I'm a little stumped on that one.

72, SB 385, Voodoo roller, 219/227; .515/,530 lift; AFR 180cc Eliminators, RPM Air-Gap, 770 Avenger, 12 bolt 3.42's, Eaton Posi, Pypes 2 1/2" X-pipe system w/Race Pro's
Vehicle weight: 3390 lbs
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