73 Gran Torino Sport 351W 650 Holley Backfires when under load. - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-28-2009, 10:18 PM
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73 Gran Torino Sport 351W 650 Holley Backfires when under load.

As far as I know the engine is a stock 351 Windsor. new holly 650 dbl. had a 750 on it when i got it, but did not run. I put new plugs wires points and carb on it. it runs now, but backfires out carb under load. it runs fine in park or neutral and can rev it up all day long, but in gear backfires. Also it has an electric fuel pump with gauge and regulator. It was running at 10psi until i put the regulator on now it is running at 5. It has shown improvement but still backfires. Any suggestions would be great.

-DOC

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Old 11-29-2009, 02:27 AM
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src-racing

check the power valve in carb, could be way off. warm up motor and put in gear hold brake daaa! with vacuum gauge to vacuum port in front of the carb
at the base plate.should be on the lower left side when facing carb.read vacuum and go 1 1/2 below reading with power valve number.I would also
check timing at this point to see if it's moving the wrong way! possible timing
chain???I hope this will help you! Let me know.....................also fuel pressure should be a little higher 6.5 would be nice on a stock motor.having
a fuel gauge on the cowl panel is nice to see where the fuel pressure is at
at top end if you are racing the car at the track!Most race cars I work on
which are built for racing fuel pressure should be at 8 and no lower then 7
at top end. your motor is stock 6.5 is ok!

Last edited by steve lasheff; 11-29-2009 at 02:36 AM.
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Old 11-29-2009, 06:56 AM
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Quote:
but backfires out carb under load. it runs fine in park or neutral and can rev it up all day long, but in gear backfires.
Ayuh,... That sounds like it's goin' Lean...
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Old 11-29-2009, 10:40 AM
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I'd start by checking for retarded timing and/or vacuum leak which would cause the mixture to be lean. Since you just swapped carbs my money is on a vacuum leak. Just went through the same thing on a 318, it had a double whammy. The timing was off and it had a cracked vacuum hose. I also moved the vacuum advance line from ported to full manifold vacuum which made a huge difference in performance and mileage. 5 PSI of fuel pressure on a stocker is plenty, a lot of factory carbs can't handle much more than that.

That said, I have to ask, why do you need a 650 DP on a stock '73 Torino with a 351W? Those were heavy cars and the '73 Windsor was pretty mild. I'm sure you can make the 650 "work" but I bet it would be a lot happier with a smaller vacuum secondary carb. A 600 would be plenty, a 500 even better.

I ran a 600 DP on my 383 '72 Chevelle with TH400 and 3.73 gears. It had an Erson cam with 230* Dur. @ .050", ported heads, single plane intake, headers and pulled to 6,000 RPM. I had to lean out the primary side to #60 jets, I left the secondaries as is. It was perfect, great launch, throttle response, strong top end and it even got halfway decent mileage. That was a lot more motor and gear in a slightly lighter car than your current combo and even the 600 was a tad much out of the box. If I had it to do over I would run a vacuum secondary 600.
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Old 11-29-2009, 11:12 AM
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I agree with Hippie

The carb is too big. I just sold a 351C-4V with a 650 Holley on it this year, and it did not perform well.

I would look into the distributor on yours. Check the vacuum advance for correct operation, and the point gap/dwell for proper adjustment.

This may just solve your drivability issues.
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Old 11-29-2009, 11:17 AM
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Originally Posted by carsavvycook
I agree with Hippie

The carb is too big. I just sold a 351C-4V with a 650 Holley on it this year, and it did not perform well.

I would look into the distributor on yours. Check the vacuum advance for correct operation, and the point gap/dwell for proper adjustment.

This may just solve your drivability issues.
X2 With Stephen and Hippie on this , Just what i was thinking. You said it was not running but is now how long did it sit not running?

Cole

Last edited by eloc431962; 11-29-2009 at 11:22 AM.
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Old 11-29-2009, 11:20 AM
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I agree with Hippie, 5 lbs of fuel pressure is plenty. 7 or more will usually blow the fuel right by most stock needle and seats. Since you changed everything in the ignition system I would go back and double check the firing order, plug gaps etc. If all that looks good then it may be time to look at the carb. Holley's are fairly easy to adjust or rebuild rebuild if you need to.
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Old 11-29-2009, 06:57 PM
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I was told that the car had been sitting for 10 years in a barn. I checked firing order, gaps, and vacume advance, everything is in order. I also thought that a 650 was a bit much, but am confused that it had a 750 when I got it. If the motor is not stock, would a 500 hurt it?
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Old 11-29-2009, 07:28 PM
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Quote:
Originally Posted by 73Torino
I also thought that a 650 was a bit much, but am confused that it had a 750 when I got it. If the motor is not stock, would a 500 hurt it?
Unless it had a factory Q-Jet on it ( and while Ford did use a few on some of the CJ's I never heard of a 351W having one ), whoever put the 750 on it was confused and I'm being nice.

"If" your 351 were revved to 5500 RPM (4500 is probably closer to reality) and assuming it was otherwise stock a 450cfm would be just about spot on. It would love the 500. A mild cam, headers and lower gears then it could use a 600.
Back when I was a kid '73 Torinos were slugs but now they're pretty cool. Funny how time changes things.

Let us know how it comes out.
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Old 11-29-2009, 07:28 PM
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Quote:
Originally Posted by 73Torino
I was told that the car had been sitting for 10 years in a barn. I checked firing order, gaps, and vacume advance, everything is in order. I also thought that a 650 was a bit much, but am confused that it had a 750 when I got it. If the motor is not stock, would a 500 hurt it?
No a 500 won't hurt it, I have ran as much of as a 750 3310 Holley on these motors you might have to change the jets though to get it right. But i usually go at least a 600 Holley JMO. But a 500 will do just fine if the engine is stock. IMO. But the carb size really depends on what up-grades you are going to do.
That's where the 3310 and the 600 come into play
Cole

Last edited by eloc431962; 11-29-2009 at 07:33 PM.
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Old 11-30-2009, 01:21 PM
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Quote:
Originally Posted by 73Torino
I was told that the car had been sitting for 10 years in a barn. I checked firing order, gaps, and vacume advance, everything is in order. I also thought that a 650 was a bit much, but am confused that it had a 750 when I got it. If the motor is not stock, would a 500 hurt it?
Most likely the carb needs a rebuild if it has sat for 10 years
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Old 11-30-2009, 03:15 PM
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Originally Posted by T-bucket23
Most likely the carb needs a rebuild if it has sat for 10 years

reread first post , "new 650 dbl pumper " my 68 cougar with a 302 will do this if i hit it hard , with a new 600 holley. iam thinking its choke or timeing related.
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