67 Engine idles rough and stalls under acceleration - Pontiac GTO Forum
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post #1 of 8 (permalink) Old 10-29-2016, 09:20 AM Thread Starter
 
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67 Engine idles rough and stalls under acceleration

First, I am brand new to GTO's and haven't worked on engines since my 65 rambler. I bought a 67 GTO 2 weeks ago with 21K original miles.Gorgeous car. I like just looking at it. The engine has been re-built and ran fantastic when I bought it on 10-17. Very strong. I drove it around gingerly for 3 days. On Thursday after filling the tank with 93 octane it started to run rough after a few miles (it had 93 in it from when I bought it) and a few minutes later the engine started to bog when I hit the accelerator. I managed to get it off the road where it stalled. I could start it with great difficulty but it ran very rough, and would stall on its own after 30 seconds but it would stall immediately if I tried to put it in gear. I had to have it towed home.

Engines don't run because of spark or fuel, right? Fuel: I checked the two filters. The one actually in the carb is clean as a whistle. I then cranked the engine with the coil wire off while inserting the fuel line AFTER the first filter into a clear jug. It pumped a quart of fuel in a dozen revolutions. The gas looks competely pure. No water, particles, nothing. I can get the car to run for about 60 seconds so it's getting spark. Since the car has only been driven a few miles in the last two years I reasoned the new fuels had done a number on the carb so I had it rebuilt. It is a Rochester 750 cfm 4 barrell. I installed it last night but it did not help.

The specific symptoms are: After a 2-3 times cranking it the engine will start and run VERY rough at idle. If I do nothing it will stalll in less than a minute. If I try to throttle it up it bogs down and beings to stall....it does not rev up. If I let go of the throttle it remains at a very rough idle and again, will stall in 30 more seconds. Last night I heard a deep engine knock a few times while trying to throttle it up. I gave up.

I have a video of it here:
. It runs so rough you will hear some front grill trim rattles so disregard that. This cannot be the first and only time this engine design has done this. I'm hoping someone will read this and say: "oh, I know exactly what this is".
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post #2 of 8 (permalink) Old 10-29-2016, 05:10 PM
 
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Over my pay grade, but carefully check each and every vacuum line, starting with the PCV grommet. Check the manifold nipples in front of the carb to be 100% sure they are snug in the holes. Doesn't look like you have power brakes. Check the base of the carb in the back as that's where the vacuum for the brakes is taken. Make sure that one is plugged. Is it an automatic? Is the vacuum line for the downshift connected? To be clear, no criticism here. I've had the manifold nipple problem, as well as a vacuum line to the a/c dashpot come off.
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post #3 of 8 (permalink) Old 10-29-2016, 07:46 PM
 
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Watching the video, it appears that the engine is not getting any fuel. I hear it sucking through the car as if it is straining for gas, but not getting any. Just for fun, while running, try closing the choke enough so it does not stall, then open the carb and see what it does.

Look down inside the carb with a flashlight (engine off/not running) and look at the primary side. Open up the throttle like you were flooring the car. You should see a nice stream of gas shooting into the carb, one stream of gas shooting into each barrel. If you accelerator pump is not doing its job, you won't see this stream of gas and this is what causes a richening of the carb when you stomp on the gas as the primaries open up. If you don't get that stream of gas, it will act as your video, fight to pick up RPM's, and stall.

You had the carb rebuilt, but the float could be set wrong and not allowing enough gas to fill the bowl. Have you adjusted your idle screws in the front of the carb? Screw them in gently until the stop - do not force these or over tighten. Then back them each out about 5 turns and see if that improves anything.

Next, have you checked your timing? Sometimes timing can be off and cause poor idle/running problems as well. This is the easiest thing to check first.

Finally, you may have taken a slug of water in that gas. The ethanol laced gas can do this and water can accumulate in the tank if it has simply been sitting around. You may want to unhook the fuel pump line at the tank side and run a length of rubber hose into a fresh can of gas to ensure you just don't have some crappy gas running through the carb.

And as pjw stated, look for any vacuum leaks. It is important to use the correct gasket for the intake/carb on the '67. This is off of a Chevy site, but applies to the 1967 intake manifold/Q-jet and explains what I am talking about. Q-Jet Carb Base Gasket Tech Info: The "Hot Slot" Manifold Problem - CorvetteForum - Chevrolet Corvette Forum Discussion

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post #4 of 8 (permalink) Old 01-01-2018, 12:26 PM
 
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tuning a original 326 engine up to get more power

i was wondering what size pistons i can go up to with out boring out my block. i was going to pull my motor install a better cam , replace the old 2 bbl intake manifold and carburator with a 4 bbl setup to improve my performance?
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post #5 of 8 (permalink) Old 01-01-2018, 02:22 PM
 
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any suggestion?
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post #6 of 8 (permalink) Old 01-01-2018, 04:39 PM
 
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Quote:
Originally Posted by mhyl2000 View Post
any suggestion?
It would be better to start a new thread rather than follow one that does not quite relate? Explain what you mean by "piston size" and give a more detailed explanation of your goals and possibly your budget - then you will get a response.
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post #7 of 8 (permalink) Old 01-30-2018, 04:27 PM
 
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I agree with the vacuum leaks suggestion and that PCV valve grommet! When changing all my other 40+ year old rubber I replaced the pcv hose and thought let's pull the valve and clean it. I tried and it took a large set of pliers to finally crush it and get it out. It finally came out with a grommet as hard as my prostate. Much better vacuum on the gauge now!
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post #8 of 8 (permalink) Old 01-31-2018, 06:36 PM
 
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Did you check timing? The symptoms sound an awful lot like that could be out of whack. What did you find? Vid is no longer available....
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