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.

All engine grounds hooked back up or missing ?

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.

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.

This is a brand new (rebuilt) 7027263 that was done by CARBGUY. Man, I hope the accelerator pump is working as it should.

Yes, I just made sure all were tight. One was a little loose, but no change in how car ran after tightening it. As soon as I touch the throttle off idle is spits and spurts, but if I holdout at let's say 1500 RPM, it doesn't do that. Is there a way to post a video here so you guys could hear it?

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.

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. :)

Here's a quick 5 second video
67 400 Running Rough

You have dirt in the carb.

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 .

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/

Thanks for the reminder.

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.

Your vid sounds like wrong firing order.

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.


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Rebuilt 400 HO suddenly runs rough
 

Ok. This seems straightforward. I’ll get a can of carb cleaner today and try that (adding it to the list).

Thank you.


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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


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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.

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?


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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


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I would first do a dry test to save some time and then if anything checks way out of normal do a wet test.

So you dropped a Carb on from another motor without adjusting the idle mixture again?!!

I will check this again just to confirm.


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Ok. I’ll grab a tester from HF this morning.


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Since I am going to pull all the plugs to check compression, I could also look for slop in the distributor.

I do have a BORO-scope that the wife bought me for Christmas. I’ve never used it (not even sure I know how) but I’m assuming that to check the TC, I would pull the fuel pump and snake the scope into the timing cover?


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Don’t waste time with the scope unless !something with the compression test goes off the rails!

The correct firing order is on the intake manifold for reference.

Rebuilt 400 HO suddenly runs rough
 

The 4 and 6 plug wires were mixed up!. It definitely made a difference in the starting/idle. The timing also didn’t jump around. Total timing is at 35. Is that correct?

Here it is with correct firing order:

https://youtu.be/Hv_kIQ7ZHI4
https://youtu.be/Hv_kIQ7ZHI4

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Guess I missed the part you said all was well running till you filled up at a gas station, "then" things went bad gradually.
How does to run "ok" then after fueling went "bad" when you were driving?
And now find plug wires mixed up?

That I don't understand at all.

Was running good, then fueled up, then degraded fairly rapidly, worsening seemingly, barely running.

Put about a 1/2 gallon from the fuel pump supply side into a clean container and let settle and see what it looks like.

That sounds fine... Maybe you've solved your problem?

No, the sequence went like this:
It was running fine, and then it felt like it started to stumble. I thought maybe the gas gauge was incorrect, so I stopped to get gas. But it only took about 5-6 gallons, so the gas gauge was fine. However, the stumbling got markedly worse as I limped home (about 5 miles). No power, backfiring, sputtering, etc.

The mixed up wires had to have happened when I pulled the cap to do the points. That one is completely on me.

I'm going to take it out for a spin now. If it runs ok, then the issue I was having was either the points, which definitely looked a bit tired for only 300 miles or so, or all the grit in the carb. I will report back shortly!

Yes, 35 degrees is safe with those 670 closed chambered heads, but for the best around town drivability they may like 38 degrees total with the vacuum advance disconnected and then 8 degrees of vacuum advance coming in at 1800.

Ruining 38 degrees will Hinge on how the motor cranks when hot, if the motor pings at part throttle it knocks at full throttle which in terms of full throttle is to be avoided 100%!

Rebuilt 400 HO suddenly runs rough
 

Car runs 1000x better! No stuttering or stalling accelerates great! Thanks so much for all the help! You guys rock!

The last annoying things are, when I come to a stop, it’s idling at 1200, but if I kick the throttle it goes back down to 800. It did not do this with the previous carb.

To Steve’s point: I drive it for about 20 miles (local). When I shut it down, it dieseled a little. And once it was warmed up, it took a bit more cranking to get it started. Should I move the timing to 38 (with vacuum advance disconnected)?

Have you ever considered getting a 1967 pontiac service manual and following the service instructions in it?
It is difficult to fail if you´d start with this.
If you follow some people's advice regarding the timing settings, it will soon be time to change the engine bearings.
If you want your engine to live a healthy life, use the factory ignition settings.
Especially with the inferior fuel we have today.

FWIW

Rebuilt 400 HO suddenly runs rough
 

Car also runs a bit hot now.
https://uploads.tapatalk-cdn.com/202...fb60cb6358.jpg

Kenth
I have the service manual. It says to set initial timing at 6 degrees BTDC. I thought with today’s gas, that this wouldn’t necessarily be applicable anymore.

Also, my balancer has two marks on it that are 6 degrees apart. I’m not sure, but I’m assuming that the engine builder added the second mark. I would think that the bottom mark is 0, and that the top one represents an additional 6 degrees BTDC.
https://uploads.tapatalk-cdn.com/202...beabc18756.jpg


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I think we have discussed your balancer hub earlier?
The conclusion was that your hub is a factory 1964-65 vintage for 1964-65 timing cover with just a pointer thus the two marks, first mark indicates 6° BTDC and the second mark 0°, instead of one mark on hub (0°? for the scale on 1966-67 vintage timing covers.
Your engine builder simply mounted the wrong hub for your damper.

And, the need for setting the timing out of the scale indicates a too lean F/A mixture. The carb needs to be recalibrated for todays fuel to let the engine give its full potential. This will not happen igniting a too lean mixture with timing set out of the scale.
Also, a too lean F/A mixture will make your engine run hotter than necessary.

Well yes, your carb needs to be set up right, but your also hitting on all 8 cylinders now which is putting more heat into the coolant!
Is your vacuum advance hooked up and working above 1600 rpm?
No vacuum advance at road cruising speeds and part throttle will make the motor run hotter then it should !

Glad you found the firing order was off. If anything will cause poor running and popping through the carb, that is it. I would get someone who knows Quadrajets go over the carb. You had dirt in the gas and I think you said you ran the car with no filter which is never a good idea. Despite what others have said the stock brass sintered filter are actually very affective at removing debris. They are small and will clog quickly if you have a lot of debris in the tank. You may want to use an inline filter after the pump rather than the stock sintered filter. I wouldn’t use both. Properly setting up the carb will be important to getting your car running really well. Also if I were you I would replace the points with a Petronix unit. It will save you a lot of aggravation, for the $80 investment they are a no brainer, especially with a GM motor with the distributor at the back of the motor.

It's probably dieseling because the idle is sticking at too high an rpm. Fix the hanging throttle and go from there. Auto car should be about 750 in park and 500 in gear, stick car should be about 650-750 at idle with stock components. 800 is too fast.

You're welcome.

My 69 RAIII std bore 400, 50K orig miles.

Was running a bit hot, a water pump rebuild and set the "Plate" to 0.050", gap, helped a little.

Month later rebuilt Rad from a 3 core to 4 core.

Now it Runs almost too cool now.

Always been satisfied with New triple gauges set, they are More concise readout to monitor things.

At the time was new to Rochester carburetors so I followed a Qualdrajet bible- PDF , set up the jets/rods for "total area" to near recommended 0.003xxxxx,.

I never had any carb perform like it dd once I was done tuning setup it as per the manual!!
Very impressive performance.

See the link for resource.

http://corvette-restoration.com/wp-c...arb_Tuning.pdf

Thank you!


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I set idle at 6 degrees BTDC. It idled fine there. I took it out for another drive and this time the choke rod came loose (I must’ve knocked that tiny little clip off) and the choke rod got jammed on what I think is the fast idle cam and held it open. Idle was way too fast and it heated up again. You could also tell it was running too lean. I didn’t realize this until I got her back to the garage. I’m calling it quits fir the day.

The brown lever below was jammed by the choke rod. .
https://uploads.tapatalk-cdn.com/202...5e6b595491.jpg

https://uploads.tapatalk-cdn.com/202...f5fa363c52.jpg

Not sure if you can see how it got twisted here.

Quadrajet Guru. Cliff Ruggles has gaskets & pieces, the alcohol durable parts too, and perhaps jets/rods, if you make adjustments in it.

https://cliffshighperformance.com

I can't recall due to how long this post is, but is that Carb on the motor now the same one this post started off with?

Wow holy chit. It blew the AIR injection galley plug out of the back of the head? That is a drilled passage that intersects with drilled passages in the valve bowl of each exhaust port. It was used with optional Cali smog pump. No wonder it's loud. You'll have to replace the threaded plug. It may be that the threaded plug was missing and only the press-fit plug was installed. Enough pressure and "pop" the pressed-in plug comes out. likely why it was threaded in the first place.

I'm seeing a lot of shotgunning parts here. You need to sort the ignition and then carb. Going back and forth will drive you nuts.

Yup!
When enough unburned fuel collects somewhere and then gets hot enough to light off it’s a big explosion!

Years ago when a plug wire came off on me at 2500 rpm I had so much fuel collect in a muffler before I could pull off the highway that when it lite off the whole body of the muffler blew off!
And these where 3 month old mufflers , not well on the way to being rusted out garbage!

Hi everyone, I'm reviving this thread because I talked to Cliff R. about where to send the carb I took off of the engine (with the bad fuel inlet cement) and as I was preparing to put it in a box to ship it out, I noticed a couple of things that I wanted to get some feedback one.
First, I noticed that the little round tag on the carb has the numbers 7027273. When I bought this carb (several years back) I was told it was the correct carb for a 1967 GTO 400 HO with a manual transmission. I did a quick carb number lookup and it says that this carb is actually for a 1967 Firebird. I just wanted to know exactly what I was dealing with here.

I also noted that the aging/discoloration on the carb body and the airhorn appears to be a bit different. Is it possible that this carb is just a collection of parts that was put together to sell? Ugh. I also attached a photo of the top and bottom, figuring the experts here could help tell me what i have. I'm just trying to decide whether I should invest in a good rebuild on an 'incorrect' carb.

Thanks!

If the throttle plate is a original from a F body then it will have a tab that limits the secondary to opening only 75%.

Need som numbers to positively ID your Qjet.

Do you have a picture of this "tab"?

No, do you?
This is how I have heard that the Hp Rating on these F body motors was restricted.

If I am wrong then please post up what is correct.