weird shaking when i shut the engine off

[JeremyL]

so every time i shut the engine off it shakes pretty hard for a second or so after i kill the ignition...what could cause this? My 85 4bbl RX7 did the same thing too...is this just something you accept with a carb?

[BMS]

Sounds like dieseling. This can be caused by having your mixture adjustment off, or having an engine that's out of tune. It's not something that you just accept, even the carb should shut off smooth and jolt-free.

[A20A1]

what rpm is your idle?

[JeremyL]

it's pretty high usually around 1200rpm when warmed up, sometimes comes back down to 1k. when i start it up cold tho it will stay at like 2-2500 until i blip the throttle and then it will settle back down

how do i adjust the mixture? I assume a tune up is the usual plugs, wires, cap, rotor and air and fuel filters...anything else?

[MIK3]

Yes, it sounds like diesling. I'm not to sure adjusting the fuel mixture will do the job, but it's worth a try. I would diesle for about 1-4 seconds before I got a new carbeurator. If so, you might want to look into just purchasing a webber carbeurator. Try adjusting the fuel mixture screw on the back of the carbeurator and see what happens. I'm not too sure on how much you want to adjust it though, that's a question for A20A1.

[BMS]

No, furst you should try to turn the idle down a bit. If it's idling high to start with, then what's happening is that the engine just doesn't want to quit putting away.

[A20A1]

the fuel mix wont make you diesel... you can adjust the screw all you want and it wont help you. It could be that the idle was improperly set or it was set too high to compensate for a leaky carb or vacuum line, or some other faulty carb part...
or the idle is raising on it's own cause of a vaccum leak that is causing the idle diaphragm to open the throttle... The idle diaphragm needs vacuum in order to pull the throttle opener away from the throttle, thus keeping th idle in check. You can adjust the screw to lower the idle... it should help... if the problem goes away and nothing else is wrong then it was simply too high an idle setting... if you turn down the idle and you now have stalling problems it probably means you turned the idle down too much or there is a vacuum leak or faulty, worn, or plugged carb / fuel / or ignition part.

Idle adjustment is made at the screw on the left front side of the carb... that is the idle screw... all adjustments need to be made when the car is fully warm.

[JeremyL]

the fuel mix wont make you diesel... you can adjust the screw all you want and it wont help you. It could be that the idle was improperly set or it was set too high to compensate for a leaky carb or vacuum line, or some other faulty carb part...
or the idle is raising on it's own cause of a vaccum leak that is causing the idle diaphragm to open the throttle... The idle diaphragm needs vacuum in order to pull the throttle opener away from the throttle, thus keeping th idle in check. You can adjust the screw to lower the idle... it should help... if the problem goes away and nothing else is wrong then it was simply too high an idle setting... if you turn down the idle and you now have stalling problems it probably means you turned the idle down too much or there is a vacuum leak or faulty carb part or valve somewhere.

Idle adjustment is made at the screw on the left front side of the carb... that is the idle screw... all adjustments need to be made when the car is fully warm.

So as you are looking at the engine from the front how do you get to the screw? Do you have to pull the air cleaner cover? Also how do you adjust and check the mixture to make sure you arent running rich or lean?

[BMS]

I pull the air cleaner housing off to get to the idle screw. If you're standing in front of your car, it's a big black plastic screw that sticks out of the rear passenger side just below the choke. It's the only big black knob on the carb. Forget about your mixture.

[A20A1]

that big black screw is not the normal idle screw... it is for idle but with the car running it isn't going to affect the idle if the idle diaphragm idle screw is pressing against the throttle.

its difficult for the black screw to fall out of adjustment. it's under quite a lot of spring tension... if you must lower the idle using the black screw... only do so if you have tried with the one on the front of the carb first... and find out you can no longer turn out the front idle screw to lower the idle... and have ruled out vacuum leaks. Then go ahead and lower the idle with the black screw.

use the black screw with the car fully warm.. set the best and lowest idle speed. thins should be around 800 rpm... if you are automatic you may have to up the black screw to 900 rpm so that with the car in gear you run about 800rpm... then you can set the idle screw at the front of the carb to raise the idle to 1000.

read this... it will help you remove the air box and know which vacuum lines to plug so that your car will run properly without the air box on... this is so you can make adjustments fast nd whithout anything in your way.
http://www.3geez.com/showthread.php?t=38108
Just make sure you reinstall everything when you're done.
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[A20A1]

Oh also make sure your ignition timing is set right. Cause it can cause increased idle speed... etc. :)

[BMS]

I didn't even know there was a second idle screw. Learn somethin new every day.

[HondaBoy]

mine used to do that. it was caused by over advanced timing.

[JeremyL]

mine used to do that. it was caused by over advanced timing.

how does one chekc the timing? Anothwer weird thing it was doing today is that if i shut it off when its been driven awhile and fully warm it doesnt really do it at all but when its still a little cold it will do it pretty badly

I must be blind cuz i cant find any idle screws. I almost just wish I could swap in a B16

[lostforawhile]

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.you have it all backwards, you got me so confused i had to go out and look at an extra carb, and i've been working on these for years. The big black screw IS the idle screw it pushes directly on the throttle lever and does nothing else. if you screw it in where the spring is compressed the primary throttle plate will open the other way it closes. what you are looking at is an idle controler that the computer uses to control idle if it needs to raise it up. it also keps the throttle from closing to fast and causing other problems. :uh:

[A20A1]

But the computer one (idle diaphragm) is for the most part always engaged... thats why the throttle stop screw (idle screw) wont seem to affect idle if you adjust it.