Whats causing this hiccup through carb?
#1
Drifting
Thread Starter
Whats causing this hiccup through carb?
About half the time now when I shut engine down, I get a rush of air through the carb and the engine actually spins backwards for half a second. :crazy:
It doesnt seem like run-on since its one big blast of air then its done. Timing is set at 10 deg and no vacuum leaks detected so far. Any thoughts?
It doesnt seem like run-on since its one big blast of air then its done. Timing is set at 10 deg and no vacuum leaks detected so far. Any thoughts?
#3
Race Director
Re: Whats causing this hiccup through carb? (Surfer69)
It may not seem like run-on, but that's what it is. When the engine has almost stopped you have spontaneous ignition of residual fuel in one or more cylinders. It happens before the piston is at TDC, so it spins backwards.
As a quick test you can remove the aircleaner and with the engine idling block the air going into the carb. I bet you it dies clean with no backfire.
As a quick test you can remove the aircleaner and with the engine idling block the air going into the carb. I bet you it dies clean with no backfire.
#4
Drifting
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Re: Whats causing this hiccup through carb? (Surfer69)
With the older higher compression engines this is a common problem. There isn't really a cure for this. An idle stop solenoid will help.
The easiest method is to slightly load the engine as you turn off the key.
In an automatic, turn off the key when in drive or reverse. The load of the automatic will cause the engine to stop without the run on.
In a standard, just slightly release the clutch as you turn off the key.
I've been doing this method for twenty-five years.
Barry
The easiest method is to slightly load the engine as you turn off the key.
In an automatic, turn off the key when in drive or reverse. The load of the automatic will cause the engine to stop without the run on.
In a standard, just slightly release the clutch as you turn off the key.
I've been doing this method for twenty-five years.
Barry
#5
Burning Brakes
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Re: Whats causing this hiccup through carb? (zwede)
It may not seem like run-on, but that's what it is. When the engine has almost stopped you have spontaneous ignition of residual fuel in one or more cylinders. It happens before the piston is at TDC, so it spins backwards.
Just be thankful it isn't like mine.... I have since made progress but when I started tuning my EFI setup it could RUN backwards for as long as 15-20 seconds after shutting off. When I say run I don't mean chuggity-chuggity it would actually run ~2000 rpm backwards. I still don't know why (or how) it did this but it sure was a pain.
#6
Drifting
Thread Starter
Re: Whats causing this hiccup through carb? (Barry's70LT1)
I was afraid there wasnt much I could do. I tried cold plugs to see if it would relieve problem but no change. Guess I'll just live with it. Thanks.
#7
Burning Brakes
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Re: Whats causing this hiccup through carb? (Surfer69)
You might try decarbonizing the engine (piston tops, head) internally with some GM Top Engine Cleaner. It's one of the best products for this. On high compression engines, the carbon particles can actually be glowing at shut off. This can ignite the air/fuel mix and cause the problem you're having. Just be prepared for smoking out the neighbors if you do this.
Steve
Steve
#9
Burning Brakes
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Re: Whats causing this hiccup through carb? (Steve's74)
I started wondering about this recently in my case as my carb went way rich before it failed and when I started working on tuning the FI I misread a part of the manual and was tuning it extremely rich.
Anyhoo... where can I find GM Top Engine Cleaner? Dealers usually? Do you know off-hand if it's cool for 02 sensors?
Anyhoo... where can I find GM Top Engine Cleaner? Dealers usually? Do you know off-hand if it's cool for 02 sensors?
#10
Safety Car
Re: Whats causing this hiccup through carb? (Cookwithvette)
where can I find GM Top Engine Cleaner? Dealers usually? Do you know off-hand if it's cool for 02 sensors?
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Re: Whats causing this hiccup through carb? (Cookwithvette)
Stickboy is right. Almost any GM dealer should carry the stuff. It comes in small cans for just a few bucks. As he said, pour it in slowly so as not to hydrolock the engine. Set the engine at a fast idle (the choke fast idle cam works well) and when you are nearly through pouring it in, let the engine return to idle and do a faster dump. Then shut the engine off and let it sit for 15 to 20 minutes. When you go to restart, you will need to crank it a bit with the throttle half open, as it is effectively "flooded". I have poured this stuff on "headless" engines before and gone back 30 minutes later and wiped off the pistons; they come out almost spotless. It is very effective at carbon removal. This was GMs official fix in the 70s for unsticking carboned up piston rings in their famous Olds 350 diesels (yeah, I know. I'm dating myself) when they had oil consumption problems. We would pull the glowplugs and pour it in throught the glowplug holes and let it sit overnight.
As I noted, you will create large clouds of stinking, smelly blue/black smoke, but it works really well. As someone else noted above, reducing the idle speed to the minimum possible will also help.
Oh, and it is safe for O2 sensors, too.
[Modified by Steve's74, 4:43 AM 4/1/2004]
As I noted, you will create large clouds of stinking, smelly blue/black smoke, but it works really well. As someone else noted above, reducing the idle speed to the minimum possible will also help.
Oh, and it is safe for O2 sensors, too.
[Modified by Steve's74, 4:43 AM 4/1/2004]