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Old 01-25-2011, 11:00 PM   #1 (permalink)
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Backfiring throughout the carb still

So ever since I bought my truck if you punch the throttle right to the floor right away it would spit back out the carb and give me a good flame. To start with I...

Replaced spark plugs-- no difference.
Tuned air/fuel.. Which seemed to help to a point., also with the choke on over 3/4 of the time I Punch it to the floor it will take it with no backfire.
Then I did a compression check (all very close to 135 psi). If they are all Steady that should mean no burnt valve? Or am I wrong?
Next was intake gaskets just recently, and I just got it running again, what do ya know its still doing it. I just ran some seafoam ( intake/injector cleaner) through the intake to help clean up the valves also.

My next thinking is it HAS to be a warped intake.

If anyone can help that would be great. Thanks
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Old 01-25-2011, 11:14 PM   #2 (permalink)
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I'd check the timing first
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Old 01-25-2011, 11:19 PM   #3 (permalink)
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I'd check the timing first
Forgot to include that, but I did. I beleive it's at 8 or 9.
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Old 01-26-2011, 05:26 AM   #4 (permalink)
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take some carb cleaner or either and spray around the intake and carb base when it is running and see if it makes any difference while it is running and if it does it has a leak somewhere.
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Old 01-26-2011, 06:04 AM   #5 (permalink)
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Possibly a worn lobe on the cam.
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Old 01-26-2011, 06:11 AM   #6 (permalink)
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Valve lash on the intake sides may be a tad too tight.
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Old 01-26-2011, 07:16 AM   #7 (permalink)
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Good on you for checking the basics first.

I would check the throttle shaft bushings before going too much further. A badly worn bushing or two will cause a decent leak. You can try blowing smoke around the carb to find the leak easily.

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Old 01-26-2011, 08:20 AM   #8 (permalink)
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What kind of carb? Sound to me like you're too lean off the bottom end and the motor is starving for fuel. Thats why it runs better with the choke on.

The 670 truck avenger I had ran like that out of the box on a very mild 350. Too lean off the bottom. Needed to add more "pump shot" with the optional accelerator cams

http://www.summitracing.com/parts/HLY-20-12/
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Old 01-26-2011, 10:56 AM   #9 (permalink)
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What kind of carb? Sound to me like you're too lean off the bottom end and the motor is starving for fuel. Thats why it runs better with the choke on.

The 670 truck avenger I had ran like that out of the box on a very mild 350. Too lean off the bottom. Needed to add more "pump shot" with the optional accelerator cams

http://www.summitracing.com/parts/HLY-20-12/


Yeah, havent sprayed around the intake yet. But I believe I have changed th position and settin on the accelerator pump, bigger shot and smaller shot and I actually can't remember what happened but both ways it still wouldn't take it. I have adjusted valves with engine off, and tapped every nut so they shouldn't have moved.


I still think it's intKe, I'll try it later and give a update
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Old 01-26-2011, 10:59 AM   #10 (permalink)
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It's a Carter 625
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Old 01-26-2011, 12:03 PM   #11 (permalink)
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its timing and only timing.
you may have a balencer that slipped, making you think that your setting the timing correctly.

you may have to put something in either #1 or #6 hole and crank it over by hand to find TDC and verify the timing mark is correct. this is a very common problem.

after you verify that the '0' mark is good then check to see that the timing advances with rpm.

you should have a total timing (with vacuume unhooked) of 32* @ 3000 rpm

i'm assuming its a large cap hei if you have a 79 scottsdale- these are really good at seizing the mechanical advance in the distributor. be sure to pull the dist cap and remove the rotor, move the advance plate under the rotor by hand and see if it springs back easily, if not you can rebuild it easily and inexpensively.
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Old 01-26-2011, 12:40 PM   #12 (permalink)
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Angry

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its timing and only timing.
you may have a balencer that slipped, making you think that your setting the timing correctly.

you may have to put something in either #1 or #6 hole and crank it over by hand to find TDC and verify the timing mark is correct. this is a very common problem.

after you verify that the '0' mark is good then check to see that the timing advances with rpm.

you should have a total timing (with vacuume unhooked) of 32* @ 3000 rpm

i'm assuming its a large cap hei if you have a 79 scottsdale- these are really good at seizing the mechanical advance in the distributor. be sure to pull the dist cap and remove the rotor, move the advance plate under the rotor by hand and see if it springs back easily, if not you can rebuild it easily and inexpensively.
I have checked timing. But never did check to see if was at "0". I did get total advance at about 32 if I remember right tho. And see once the motor is over a certain rpm then it takes it just fine. And sometimes when choke is on.

It's irritating haha
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Old 01-26-2011, 12:42 PM   #13 (permalink)
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try advancing it about 3-4 degrees and see if that helps. then you'll know your on the right track. also, was that 32* with or without vacuume advance?
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Old 01-26-2011, 01:15 PM   #14 (permalink)
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try advancing it about 3-4 degrees and see if that helps. then you'll know your on the right track. also, was that 32* with or without vacuume advance?
I'm not sure actually
It was a while ago when I did it.
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Old 01-26-2011, 01:27 PM   #15 (permalink)
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So I sprayed around the intake and no difference in idle.
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Old 01-26-2011, 01:47 PM   #16 (permalink)
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I would say its your timing also. You should check everything like these guys said about the distributor advance, etc.

also if your carb needs set, the timing would have to be set correctly, to get everything just right.

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Old 01-26-2011, 01:53 PM   #17 (permalink)
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Sure your wires are on the right plug? Crossing your wires will cause this problem. Sounds stupid, but yooud be surprised how many times Ive seen this happen. Make sure its firing in the right order.
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Old 01-26-2011, 03:14 PM   #18 (permalink)
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I'm saying its timing too .... set your base timing to 8 btdc with the vacuum advance removed from carbuerator and plugged then rev motor to 3500 rpm and set timing to 35-38 degrees
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Old 01-26-2011, 04:34 PM   #19 (permalink)
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Okay, when I put the intake back together I did get cylinder 1 top dead. But I didint check if the mark was lined up. Then " according " to my timing tab and balancer mark it's at 9 I believe.
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Old 01-26-2011, 04:35 PM   #20 (permalink)
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So you think timing off like that will make it so with choke on it's fine, and after certain rPms it's fine also? Like a bottom end lean. Even just at idle not moving, a quick blip of the throttle it will pop out the carb also
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Old 01-26-2011, 04:40 PM   #21 (permalink)
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Quote:
Originally Posted by sjracer View Post
its timing and only timing.
you may have a balencer that slipped, making you think that your setting the timing correctly.

you may have to put something in either #1 or #6 hole and crank it over by hand to find TDC and verify the timing mark is correct. this is a very common problem.

after you verify that the '0' mark is good then check to see that the timing advances with rpm.

you should have a total timing (with vacuume unhooked) of 32* @ 3000 rpm

i'm assuming its a large cap hei if you have a 79 scottsdale- these are really good at seizing the mechanical advance in the distributor. be sure to pull the dist cap and remove the rotor, move the advance plate under the rotor by hand and see if it springs back easily, if not you can rebuild it easily and inexpensively.
The advance worked when I last checked total. But it won't hurt to try it and check it
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Old 01-26-2011, 04:45 PM   #22 (permalink)
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that could be your motor is cold and runs good with a choke, but when its warm the timing is off enough to cause it to backfire. I have adjusted my timing by ear in my 305 atm, Before i replaced the timing chain it would backfire, and i was always adjusting the timing to get it to run good enough to get me down the road. I knew the timing was bad, and got it down the road for awhile, but towards the end it got so bad it backfired trough the carb and caught fire. after i adjusted the timing once more, it quit backfiring after that but i knew the chain was getting worse.
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Old 01-26-2011, 04:49 PM   #23 (permalink)
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So you replaced chain?
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Old 01-26-2011, 04:52 PM   #24 (permalink)
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I've seen situations where on older motors, you can have a good reading off your timing mark, but your timing is still off for various reasons, whether your timing tab is jacked up or the harmonic balancer has issues.

At any rate, on older motors, I always time by:

A) vacuum gauge first if available
B) if no vacuum gauge available, ear

I have had this very problem you're describing on my '81 chevy with 350 crate motor and 600cfm edelbrock carb. Timed it with vacuum gauge and problem fixed. Not saying this is for sure your problem, but worth a try before you go digging into deeper possibilities...
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Old 01-26-2011, 05:07 PM   #25 (permalink)
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yea i replaced the chain and i have not had that issue since. If you have over 100,000 its prolly time to get it done :P
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