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Old 03-24-2009, 01:53 PM
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How to find an elusive engine miss

I am having a tough time getting my SBC 406 to run smooth. I have tried everything that should determine where the miss is coming from. It seems its not one cylinder, but the engine miss as a whole. You can spray water on each header tube while its running and the water evaporates instantly on each. You can pull the plug wires off one at a time and the engine slows down with each one pulled off. However it slows down more with certain ones pulled off, its not equal. All of the spark plugs look the same.
I've thought it seems like a vacuum leak. I just replaced the intake gasket. Everything on the engine has been replaced. I've done a compression check with good results. Cam is not flat. I've checked timing, firing order, and advance.
I'll tell you what the engine does. At idle it shakes, but not because of the cam. When you speed it up, the engine still shakes and feels rough. The miss is there from idle on up. Going down the road it feels like a surge. It will jerk and buck, almost like its not getting enoguh gas. If thats the case, I don't know why it smells like its running rich. I've heard an incomplete burn will smell bad like a rich burn. My fuel filter is full of gas and it has good pressure.
I have suspected my Holley carb, it was rebuilt a few months ago. I've had it apart several times and all of the gaskets look like the right ones. One last thing is the idle mixture screws don't respond well. You can screw one side all the way in and it will keep running. It will only die when both are screwed in.

I haven't been able to determine if its running rich or lean. I'm thinking its more of an air/fuel issue than timing. With the rough idle its hard to tell if its a vacuum leak.
Thanks for your time and help

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Old 03-24-2009, 04:12 PM
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Quote:
Originally Posted by 4speed57
I am having a tough time getting my SBC 406 to run smooth. I have tried everything that should determine where the miss is coming from. It seems its not one cylinder, but the engine miss as a whole. You can spray water on each header tube while its running and the water evaporates instantly on each. You can pull the plug wires off one at a time and the engine slows down with each one pulled off. However it slows down more with certain ones pulled off, its not equal. All of the spark plugs look the same.
I've thought it seems like a vacuum leak. I just replaced the intake gasket. Everything on the engine has been replaced. I've done a compression check with good results. Cam is not flat. I've checked timing, firing order, and advance.
I'll tell you what the engine does. At idle it shakes, but not because of the cam. When you speed it up, the engine still shakes and feels rough. The miss is there from idle on up. Going down the road it feels like a surge. It will jerk and buck, almost like its not getting enoguh gas. If thats the case, I don't know why it smells like its running rich. I've heard an incomplete burn will smell bad like a rich burn. My fuel filter is full of gas and it has good pressure.
I have suspected my Holley carb, it was rebuilt a few months ago. I've had it apart several times and all of the gaskets look like the right ones. One last thing is the idle mixture screws don't respond well. You can screw one side all the way in and it will keep running. It will only die when both are screwed in.

I haven't been able to determine if its running rich or lean. I'm thinking its more of an air/fuel issue than timing. With the rough idle its hard to tell if its a vacuum leak.
Thanks for your time and help

Certainly the curb idle circuits aren't correct and need work or replacement. Is the Holley on a one or two plane manifold?

What is the base advance?

Bogie
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Old 03-24-2009, 05:44 PM
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Does it have power brakes? My '73 Laguna has just recently developed a vacuum leak in the booster.
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Old 03-25-2009, 01:51 AM
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The carb is on a single plane intake. This intake/carb combo have been together on this engine for a while with no problems until recently.

My car has manual brakes.

Thanks for the replies
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Old 03-25-2009, 02:24 AM
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What's the fuel pressure at the carb?
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Old 03-25-2009, 05:48 AM
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It does sound like a carb problem.

What size/model Holley and what jets are in it?

Did this problem begin right after rebuilding it?
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Old 04-06-2009, 01:52 PM
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Rough?

I ned to get in on this because I have a 97 GM 350 with the very same symptoms.
The only difference is I don't sense any "surge" at speed.
It is a 1997 with TBI.
I 'll be watching.
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Old 04-07-2009, 01:33 PM
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I went ahead and changed my head gaskets and intake gaskets. The engine runs better now, but its still not perfect. I'm thinking its either carb or timing issues.
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Old 04-07-2009, 06:35 PM
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power valve ?

A popped power valve in a Holley will do funny things.

Dan Hurst
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