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  #1  
Old 05-26-2009, 10:50 PM
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Default Backfire through carb!!!

Background info: Stock engine, stock th350, new 3.45 gears, K&N X-stream air filter.

Just after my 500 mile break-in for the new gears, there was a 'rattle-like' sound coming from the engine bay when the RPMs reached 20-2600. Starting this weekend, there's backfire coming from the carb during quick acceleration. We tried to set the time and we noticed at idle it was 12 degrees before top dead center (btdc). We then changed it to the factory setting which was 4 degrees btdc, but there was A LOT of backfire during quick throttle. We adjusted it to 6 degrees btdc and it seemed fine. I drove on the freeway and as I reached 2000 RPMs it started to back fire constantly. And every once in a while (at 2600 RPMs) I lost power for a split second and then regained it (like it lost vacuum).

My father and I will be checking the valve springs this weekend....Does anyone else have a suggestion on what to do. I'd appreciate it. Thank you.
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Old 05-26-2009, 11:06 PM
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Could be bad news like a timing chain gone bad or something as simple as a vacuum advance unit malfunctioning.
Check the hose on the advance unit for cracks etc.
Pull the dizzy cap and put a socket on the crank to turn it back and forth and see if the rotor is turning like it should,if it takes a long time to move from one direction to the other you may need a new timing set.
You could also put a timing light on it,if the mark is jumping all over the place that could also indicate a bad timing set.
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Old 05-27-2009, 12:13 AM
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A good rule of thumb (but not necessarily law) is: popping out the exhaust = valves. Backfiring out the carb = timing.
What vacuum source is your distributor advance hooked up to?
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Old 05-27-2009, 09:27 PM
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Hey guys!

Y2K: Thanks for the info, we'll check all that on Friday when I'm off of work
Camaro69: The distributor advance is hooked up to the Carburetor....
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Old 05-29-2009, 04:09 PM
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Hey guys! Just worked on the car....checked all the hose; one looked cracked so we replaced it. My father and I did a compression test for each cylinder. They read:

Cylinder #1: 100
Cylinder #3: 115
Cylinder #5: 118
Cylinder #7: 110

Cylinder #2: 95
Cylinder #4: 100
Cylinder #6: 104
Cylinder #8: 130

I tried to look up what is recommended and could find anything
We freshen up each spark-plug, put them back in, and took her for a test drive.
She doesn't backfire until I get her up to 2500 RPM's.

When working on the car we also noticed the secondary butterflies never opened when we brought up the RPMs. We had to manually open them up, stick a screw driver down to open up the internals and then.....brrrrrrrrrrrrrap brrrrrrap braap.... it stalled! Is there a problem when the secondaries never opened up, and when we forced them opened, the car stalled?

Any help would be amazing!

OH, there's also a knock coming from the engine when I let on the gas and then when I let off the gas. Would this mean a busted cam? or worse, crank??
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Old 05-29-2009, 07:43 PM
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Boy,hard to tell what's up without seeing it in person.
What carb are you working with? they don't all work the same.
Safe bet it's not a Holley manual secondary carb like a double pumper.
Where does the knock sound like it's coming from? up top or down low?
Could be a lot of things,from a rod or main bearing to a bad wrsitpin or cracked piston.
First place to look is under the valve covers and see whats going on with it running,is every rocker moving like it should,any sign of a broken valve spring or bent pushrod,that sort of thing.
Bent pushrods could be caused by a bad timing chain set.
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Old 05-29-2009, 07:47 PM
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As far as your compression readings,well you look for consistency more than a particular number.
Yours show some signs of wear but depending on how well you did the test, I've seen a lot worse,it should be a runner.
Did you pull all the spark plugs when you did the test? if not you could be getting some false readings doing one at a time.
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Old 05-30-2009, 01:56 AM
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Hey Y2K! Thanks for writing back! Yeah, I hear ya....maybe I should buy you a plane tix to SF so you can come hear her for yourself My carb is a quadrajet by Carter. Apparently they use to build carbs for GM...who knew. The knock sounds like (to me) from the driver side valve cover. My dad insists it's coming from dead center of the engine. I'll check again tomorrow and see where exactly the sound is coming from.

We're opening up the valve covers bright and early tomorrow morning....hope we find the problem!
Is the timing chain easy to replace? If I do replace it, is there a certain one I need to buy for a stock engine with 3.42 gears?

As for the compression, we pulled all the spark plugs first before we did the test I'll let you know how everything goes tomorrow morning.

Oh, and about the secondary butterflies not opening when revving the engine...do you know anything about that?
Thanks
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Old 05-30-2009, 04:01 AM
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You've got a bit of wear on that engine, judging by the numbers. Do you know how many miles are on it? Does it blow any smoke when you start it? Or while driving it, backing off the throttle a few seconds, and then hitting the gas again?
Compression for a fairly tight engine should be in the 150-160 range. And the lowest reading wants to be no less than 75% of the highest reading (25% less). Obviously, the higher and closer to each other they are, the better.
You can also test the engine by first getting your reading, then squirt some oil in the cylinder and read it again. If the number goes up, you'll know your rings are worn since the oil will seal them momentarily as you take your reading.
On the carb secondaries, check that your choke is opening all the way, and that the linkage is clear. There is an interlock pawl on the linkage that keeps the secondaries from opening until the engine warms up (choke opens). Or, the secondaries wind up spring could be too tight, it's adjustable.
If you find your timing chain is worn, getting a name brand double roller chain would be a good choice. You need to lower the oil pan in the front to clear the corner tabs of the timing chain cover. Or you might be able to force it off without lowering the pan. There's a trick where you cut the covers inner upper corners of the gasket lip on an angle to make it easier to get back on. Just use a little extra silicone schmootz in those corners when you put it back together.
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Last edited by Camaro 69; 05-30-2009 at 04:05 AM.
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Old 05-30-2009, 10:41 AM
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Quote:
Originally Posted by 78LT350 View Post
Hey Y2K! Thanks for writing back! Yeah, I hear ya....maybe I should buy you a plane tix to SF so you can come hear her for yourself My carb is a quadrajet by Carter. Apparently they use to build carbs for GM.
Most of those Carter Q-jets were replacement carbs but they are the same as a Rochester.

Cam69 has some good info above,if the oil in the rings test makes little or no dif it could be the valves just need touched up but he's right, that engine looks a little tired.
Still it should run ok even if it smokes a little,unless of course that knock is something major like a bearing.
Good luck with it and keep us posted.
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Old 05-30-2009, 10:41 AM
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