Author Topic: 301 backfire  (Read 232 times)

77Formula

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301 backfire
« on: November 05, 2018, 04:33:15 PM »
I just bought a 1981 Turbo TA (with no turbo on it) and I can not find what couses of the backfire I keep having. It backfires when the trottle is opend moderate fast and fast. Only when very slow no backfire occures.
There is not much vacuum stuff on the engine anymore. Only vac advance, PVC valve and powerbrake.

I sprayed the carburetor, the spacer plate, inlet manifold with carb cleaner to find leaks. No results.

I pinched the vacuum hoses with plieers. When I pinched the vacuuum advance hose the backfire stops.
If I put vacuum on the vacuum advance canister it moves and the vacuum stays on.
If I plug the opening on the carburator where normaly the vacuum advance hose goes the engine runs very rough and stalls.

What can it be that the engine stops having backfire if the vacuum hose is pinched?

Other questions I have is: The PVC hose is on a nipple on the carburetor that (almost?) dont give any vacuum. Where should the hose go?
From the exhoust is comming very black soot. But the exhaust gas dont smoke black. What can that be. Is this carburetor to big for this engine?






b_hill_86

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Re: 301 backfire
« Reply #1 on: November 05, 2018, 05:21:07 PM »
Sounds like too much vacuum advance from the canister? Maybe a broken or weak spring on the distributor weight?
-Brian-

1977 Trans Am 400 4 speed

cyber104

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Re: 301 backfire
« Reply #2 on: November 05, 2018, 06:14:44 PM »
I'm seeing 2 black spots under each exhaust in the above picture which tells me it's running REALLY rich.

I'd be trying to figure out what is causing that issue as I suspect if that's corrected the backfire will go away.

77Formula

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Re: 301 backfire
« Reply #3 on: November 05, 2018, 08:49:07 PM »
I'm seeing 2 black spots under each exhaust in the above picture which tells me it's running REALLY rich.

I'd be trying to figure out what is causing that issue as I suspect if that's corrected the backfire will go away.

It sucks also down gas like crazy. I took it for a fast spin, like 8 miles. Almost half of the gas tank was gone.
But no black smoke.

77Formula

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Re: 301 backfire
« Reply #4 on: November 06, 2018, 05:58:18 PM »
Sounds like too much vacuum advance from the canister? Maybe a broken or weak spring on the distributor weight?

You are right. It was to much vacuum. I switched the vacuum hose with the PCV and the backfire stopped.
It still needs some work but at least the car is drivable.

Thanks

Re: 301 backfire
« Reply #4 on: November 06, 2018, 05:58:18 PM »

b_hill_86

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Re: 301 backfire
« Reply #5 on: November 06, 2018, 07:45:45 PM »
Might still check the amount of total timing. From memory, the vacuum advance cans don’t add THAT much advance so if you’re getting so much advance that it’s backfiring through the carb (plug firing with the intake valve open) you may still have quite a bit of initial timing and/or some broken springs in the carb still.

Keep in mind I’m no expert though so maybe someone else can chime in.
-Brian-

1977 Trans Am 400 4 speed

Bluebandit

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Re: 301 backfire
« Reply #6 on: November 06, 2018, 08:30:54 PM »
  I definitely see something wrong with your carb. Your carb should have a vacuum operated secondary but the vacuum canister has been removed. Chances are good someone has "converted" it to mechanical action by putting a screw in the throttle linkage that forces the rear throttle shaft to open. When that happens there isn't enough pump shot from the accelerator pump to cover the sudden vacuum drop and can cause a lean backfire. I also suspect that has put the rear throttle shaft out of proper position at idle.
  As far as running rich you should check the floats, pull the sight plugs and adjust them so they are just below the opening and will slosh some out when you bounce the fender.
  Also its very possible you have a blown power valve, backfires can rupture them on a Holley. 
« Last Edit: November 06, 2018, 08:38:23 PM by Bluebandit »

77Formula

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Re: 301 backfire
« Reply #7 on: November 08, 2018, 03:52:48 PM »
  I definitely see something wrong with your carb. Your carb should have a vacuum operated secondary but the vacuum canister has been removed. Chances are good someone has "converted" it to mechanical action by putting a screw in the throttle linkage that forces the rear throttle shaft to open. When that happens there isn't enough pump shot from the accelerator pump to cover the sudden vacuum drop and can cause a lean backfire. I also suspect that has put the rear throttle shaft out of proper position at idle.
  As far as running rich you should check the floats, pull the sight plugs and adjust them so they are just below the opening and will slosh some out when you bounce the fender.
  Also its very possible you have a blown power valve, backfires can rupture them on a Holley.


This is the first time I have a car with a Holley carb. Yesterday I noticed that the secondary valves where molested. Choke is also gone. I will not bother with this one anymore and order en new one. You think this is the right carburator for a 301? In the furure I maybe want to put headers and camschaft in it.

tajoe

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Re: 301 backfire
« Reply #8 on: November 09, 2018, 04:13:06 PM »
Sounds like too much vacuum advance from the canister? Maybe a broken or weak spring on the distributor weight?

You are right. It was to much vacuum. I switched the vacuum hose with the PCV and the backfire stopped.
It still needs some work but at least the car is drivable.

Thanks

How'ld you do that? 2 very different sized hoses.
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firebirdparts

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Re: 301 backfire
« Reply #9 on: November 10, 2018, 05:05:04 PM »
The runs-really-rich-has-no-choke Holley is pretty much normal for "car guys" everywhere that don't know anything about cars.  That is just what they do.

The Holley was never intended to do precise metering, so they just come out of the box super rich and most people leave them that way.  They have a power valve that in my limited experience won't close when it's old.  The rubber in there gets hard and they won't close.

You can change the jets in it and get it to run about right.  that is the beauty of it, it's Easy to change stuff.  But without a choke of course you'll always have a serious drawback there.  Well, maybe not in Aruba.  It's hot.

I am coming to Aruba in January.  spent my honeymoon there, too.
« Last Edit: November 10, 2018, 05:07:04 PM by firebirdparts »
I want to be like Paul Kenny when I grow up
Joe Bays
1977 Y82 4-speed (red interior)
1978 Skybird 350
1978 Redbird 305
1979 Redbird 301 4-speed
1992 Lincoln Mark VII LSC/SE (red)
1970 Datsun Pickup
1953 Chevy pickup (converted 4x4)
1988 T-bird Turbo Coupe

Bluebandit

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Re: 301 backfire
« Reply #10 on: November 10, 2018, 07:51:42 PM »
Cant tell for sure if the carb is bolted to an adapter plate or is a 4175 model that's a spreadbore design. Either way youll be better off getting a new carb for it. Holley makes a 4175 series spreadbore and there are various quadrajets that you can purchase. The Holley 650 cfm shouldn't be too much for your engine as the secondarys will only open when the load demands it.

77Formula

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Re: 301 backfire
« Reply #11 on: November 11, 2018, 10:49:53 AM »
Sounds like too much vacuum advance from the canister? Maybe a broken or weak spring on the distributor weight?

You are right. It was to much vacuum. I switched the vacuum hose with the PCV and the backfire stopped.
It still needs some work but at least the car is drivable.

Thanks

How'ld you do that? 2 very different sized hoses.

The ones wih the arrows I switched

Re: 301 backfire
« Reply #11 on: November 11, 2018, 10:49:53 AM »
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