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#101
10-02-2021, 12:31 PM
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A backfire is a symptom a vlv that is opened when ignition is applied to the cylinder.
I would at this point remove all 8 plugs. Put the (I Think is) 15/16's socket on the crank bolt, manual trans in Neutral if have that, and rotate the engine one way then the other a little bit maybe 1/8th turn back and forth, looking at the Dist and see if there is a lag - slop . like a worn out Timing chain exhibits. 1. Drifting timing 2. Backfire thru open valves. 3. Symptoms of a fuel delivery somewhat. I am very suspicious there on those parts to verify Timing chain & Gears mounted properly. New from a previous buil, long ago doesn't mean "good" or proper!! And considering it was mounted like new, Yet NOW has worked its way loose with such few miles possibly? ? This is a easy Quick fairly simple thing to do. Usually electronic will NOT exhibit drifting, they usually fail 100%. Or maybe try another Distributor all together also.. Again, If you had a BORO-Scope could probe into the TC area and look at things. One last thing, What does the gear on the camshaft look like?? Is it intact of possibly being Ate up for some reason.? ? A failing TC setup would definitely affect the fuel delivery centric roller and cause weak fuel delivery so poor operation at higher RPM's. yada yada So if you verified fuel pressure then that Might point to what I focus on here some. Last edited by 1969400HO; 10-02-2021 at 12:36 PM. |
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#102
10-02-2021, 01:00 PM
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All engine grounds hooked back up or missing ?
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#103
10-02-2021, 01:07 PM
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Also if the Carburetor’s accelerator pump is not working right due to low fuel bowl level, passages being clogged or just plain being shot, then when you hit the throttle fast enough the motor might backfire if the jetting is on the lean side.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#104
10-02-2021, 01:41 PM
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so I swapped the coil from my TA and started the car. It basically ran the same. If you keep RPM up around 1500-2000 it runs ok (still seems like it idles too rough) but when you let the idle drop down to 800, it will idle, then spit and spurt, then idle, then spit and spurt, eventually it will just stall.
I also put the GTO coil in the TA and it runs fine. No issue with idle.
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#105
10-02-2021, 01:44 PM
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Quote:
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#106
10-02-2021, 01:52 PM
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Quote:
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#107
10-02-2021, 01:54 PM
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This may a bit of a stretch, did you put in a new wire harness in car?
So, you tried " 1. different carb -NO improvement. 2. Different Dist- NO improvement. 3. Different coil -NO Improvement. Is this correct? Also, What is the voltage measure at the POS wire? Should be 9-ish V. Last edited by 1969400HO; 10-02-2021 at 01:59 PM. |
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#108
10-02-2021, 02:07 PM
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Grab a can of aerosol carb cleaner. NOT brake cleaner!
Take the lid off the air cleaner. Allow the engine to idle, so it's running rough...toss a 1/2-second spritz of carb spray into the primary venturis. IF (big IF) the idle perks up, start looking for reasons that the air/fuel mix is too lean. Remember, half of all distributor problems are found in the carburetor. 
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#109
10-02-2021, 02:22 PM
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Here's a quick 5 second video
67 400 Running Rough
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#110
10-02-2021, 02:54 PM
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You have dirt in the carb.
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#111
10-02-2021, 03:07 PM
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Now I would say we can assume that the distributor and it’s parts are ok.
Not sure yet if the plug wires are ok but hooking up a timing light to each plug just to test for flashes can prove them good. If you do a compression test and all 8 are within even just 20% of one another then a dead cylinder issue could be ruled out. This leaves the Carb and a blocked circuit from dirt & crap to be checked out .
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#112
10-02-2021, 04:16 PM
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Does it have the factory stainless steel plate under the carb that 67 needs? Could be the fuel is getting to hot and percolating.https://www.gtoforum.com/threads/carb-gasket.34799/
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| The Following User Says Thank You to 400 Lemans For This Useful Post: | ||
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#113
10-02-2021, 07:09 PM
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Quote:
That "heat shield" along with the special high-temperature gasket, also seals an exhaust-gas passage that might otherwise leak into the fuel/air mixture in the plenum. Engine runs terrible at idle, less-bad as the throttle opens. You'd put the high-temperature gasket on top of the manifold, put the stainless head shield on top of the gasket, put the carb directly on top of the heat shield. That link makes it sound like there's two gaskets with the heat shield in-between. That's not correct. |
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#114
10-03-2021, 02:41 AM
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Your vid sounds like wrong firing order.
__________________
Triple Black 1971 GTO |
| The Following User Says Thank You to NeighborsComplaint For This Useful Post: | ||
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#115
10-03-2021, 06:36 AM
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Quote:
The wiring harness is a new M&H harness. Yes to all three (although it’s the same distributor but with an electronic module in it now instead of points). By POS wire do you mean the black wire that runs from the negative (-) side of the coil to the distributor? I will check that today. I did measure the resistance yesterday and it was 1.3 ohms, which is the same number I got when I measured the one I was going to replace it with. Sent from my iPhone using Tapatalk
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#116
10-03-2021, 06:38 AM
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Rebuilt 400 HO suddenly runs rough
Quote:
Ok. This seems straightforward. I’ll get a can of carb cleaner today and try that (adding it to the list). Thank you. Sent from my iPhone using Tapatalk
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#117
10-03-2021, 06:42 AM
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Is it possible that some of that sandy grit that I found in the fuel inlet when I took the first carb off somehow leaked back into the fuel filter (or fuel pump) and is still causing problems? I can replace the pump today (I have one from my 428) if you think that might also be damaged from the grit. Thanks Sent from my iPhone using Tapatalk
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#118
10-03-2021, 07:13 AM
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A ruff idle will not be caused by a fuel pump unless your fuel bowl level is too high and fuel at idle is just dribbling out of the boosters in the primary side.
You would see sign’s of wetness in the throttle bores in this case, and as long as the carbs accelerator pump is working good then on the other side of the coin you have plenty of fuel in the carb to idle off of. Those sintered metal type fuel filters can pass a lot of crap due to how pourus they are. So yes, you might have a partial of fully clogged metering passage that’s needed for idle.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#119
10-03-2021, 07:39 AM
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Will do a compression test today. Have to borrow (or buy) a tester, but will report back results. Should I do both a dry and wet test? Sent from my iPhone using Tapatalk
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#120
10-03-2021, 07:41 AM
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Quote:
It does have the correct gasket and stainless plate. I did not change this when I swapped carbs, I simply took the one carb off and plopped the other one down in its place. Thanks Sent from my iPhone using Tapatalk
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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