Missing / Runs Rough / No Power '71 402 - Chevelle Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 9 (permalink) Old Jul 21st, 19, 9:56 PM Thread Starter
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Tony
 
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Missing / Runs Rough / No Power '71 402

Hey folks,

Been a long time, hope everyone is well.

I bought a '71 Impala with an original 38,000-mile 402 and have been chasing a persistent miss since I bought it. Honestly I haven't done much to it other than replace the plugs and reset the timing which was way advanced, about 18 degrees at idle. The issues are a rough idle and general roughness at any engine speed and load (and no power). I've only had it up to 35mph driving around my neighborhood.

The engine looks 100% stock and the heads have been off for a 'valve job' according to the previous owner, although I don't know why that would have been necessary on an engine with such low miles.

I was just looking for feedback on next steps. Seeing that the heads have been off, I was going to pull the valve covers and make sure the valves were adjusted correctly, and check for a bent valve while I'm there. Anything else come to mind?

Thanks everyone.

Tony N.
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post #2 of 9 (permalink) Old Jul 21st, 19, 11:55 PM
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Re: Missing / Runs Rough / No Power '71 402

Pull the plugs and read the plugs. Keep track of which hole they came out of. Do a compression test and use the 2 to see if they offer any clues.

Check for vacuum leaks and checking the valve adjustment is a good idea. From there it's ignition or carboration.

How's the fuel and fuel filters?
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post #3 of 9 (permalink) Old Jul 22nd, 19, 12:12 AM
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David
 
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Re: Missing / Runs Rough / No Power '71 402

First thing to do is check the firing order.

later, dozer

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post #4 of 9 (permalink) Old Jul 22nd, 19, 12:21 AM
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Re: Missing / Runs Rough / No Power '71 402

Which carb are you running?

Still stock points type ignition system?
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post #5 of 9 (permalink) Old Jul 22nd, 19, 9:08 AM
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Dave
 
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Re: Missing / Runs Rough / No Power '71 402

You didn't mention how it ran before you adjusted the timing? If it seemed to run okay, I'd go back and revisit your timing. How did you measure that 18 initial timing? Does your distributor have vacuum advance and was it connected to full manifold vacuum or to a ported source? I would start at 12 initial with the vacuum advance plugged/disconnected, reconnect it and see how it performs.

Dave
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post #6 of 9 (permalink) Old Jul 22nd, 19, 9:43 AM
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Gene
 
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Re: Missing / Runs Rough / No Power '71 402

I recall coming home from college on the weekend to blast out a valve job on a 350 Nova. Brought an EE home with me to check it all out. Well we did the damn valve job ( mostly just lapping frankly, they werent bad) - finished bolting it all on at dusk and fired the mill up in the dark.

Had a sparkshow going on. A valve job was done to fix a bad $10 set of wires. Oh well! Me and the EE learned about proper diagnostic procedures. Should have done a compression or leak down. But I wanted to show my frat buddy the shop I worked in , and a little engine machining lesson. The poli sci major teaching the engineer. Fool ! But I taught him well!

I always test wires with a VOM now. New or not, ALWAYS. In fact one of "tune up " procedures now. I test the wires when I RnR the plugs. I had another incident with a new set of wires on a daily. Bought from "Natiopnal Auto" to date that incident. Found a bad one with my multitester ( as made or I horsed on it and broke it upon install).

CHeck the resistance in the wires. Performance spiral cores should be from 800- 2000 ohms depending on length. ( although I have checked some NGK's that register near 10K ohms). Eliminate them as an issue. I'd then pop the cap and inspect the rotor carefully. PS, if you decide to pull the plugs again, certainly check the valve lash.

That would be my step 3 - wires - rotor- lash. ( you could also do a compression test afterwards to eliminate sealing as an issue)

If its a mid throttle miss, than its very likely the carb. Airbleeds specifically. Step 4......

You'll find it! Process of elimination.....
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post #7 of 9 (permalink) Old Aug 10th, 19, 9:58 PM Thread Starter
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Tony
 
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Re: Missing / Runs Rough / No Power '71 402

Thanks guys. Got some time to look at this thing today and try some of the recommendations.

The car has never run right, before or after my timing adjustment and plug replacement. It feels like it's running on 7 and I confirmed that with a compression test:

Cyl 1: 150 psi
Cyl 2: 150 psi
Cyl 3: 140 psi
Cyl 4: 60 psi
Cyl 5: 138 psi
Cyl 6: 140 psi
Cyl 7: 140 psi
Cyl 8: 138 psi

So, #4 has a big problem. I pulled the valve cover and both rockers on that cylinder seem to be lashed correctly. Guessing it is a burnt valve but I'm going to try the compression test again after squirting some oil in that cylinder. Gas is also old, not sure how old, but definitely smells old, but I've got a bigger issue than gas at the moment....

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post #8 of 9 (permalink) Old Aug 10th, 19, 11:05 PM
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Re: Missing / Runs Rough / No Power '71 402

Quote:
Originally Posted by 70L34 View Post
Thanks guys. Got some time to look at this thing today and try some of the recommendations.

The car has never run right, before or after my timing adjustment and plug replacement. It feels like it's running on 7 and I confirmed that with a compression test:

Cyl 1: 150 psi
Cyl 2: 150 psi
Cyl 3: 140 psi
Cyl 4: 60 psi
Cyl 5: 138 psi
Cyl 6: 140 psi
Cyl 7: 140 psi
Cyl 8: 138 psi

So, #4 has a big problem. I pulled the valve cover and both rockers on that cylinder seem to be lashed correctly. Guessing it is a burnt valve but I'm going to try the compression test again after squirting some oil in that cylinder. Gas is also old, not sure how old, but definitely smells old, but I've got a bigger issue than gas at the moment....
Borrow a leak down tester from the auto parts store. When you get it hooked up, put the tested cylinder at it's TDC. Listen for the location of leaking air. If you hear it from the valve covers it's a bad set of rings on that cylinder. Air leaks sound from the carb indicates a bad intake valve and from the exhaust, it's exhaust valve. I sort of suspect since the last owner said it had a valve job, they knew it had an issue and attempted to repair it. But missed the actual issue.

You can make a quasi leak down tester. It won't show % of leakage but will help you locate the source of the leakage. With only 60psi on 4, we already know it has a substantial leakage somewhere. Take your compression tester hose and adapt some fittings to the end so you can hook up your air compressor. Install a regulator in line and regulate it to about 120 psi. Charge up the cylinder when it is at it's TDC and listen for the air to escape.
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post #9 of 9 (permalink) Old Aug 11th, 19, 9:34 PM Thread Starter
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Tony
 
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Re: Missing / Runs Rough / No Power '71 402

Thanks - made up an adapter. Going to try it tomorrow.

Tony N.
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