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Another Rough Idle thread! Help!
#1
Hey y’all. My 71 Mach 1, H code is not able to idle while warm. Cranks immediately when cold, choke opens up once she is hot and idle starts struggling, eventually dies. Also dies when I put her in gear. FMX tranny.

Pertronix 2 unit and coil
Rebuilt 2100
Checked TDC and dizzy alignment. Running around 10-12 BTDC to get it to run
Messed with idle screws and choke adjustment.
Removed and plugged every vacuum line from manifold.

When cold and idling around 1200 rpm, getting around 19 inches vacuum. Once heats up and choke opens, drops down around 10 inches and engine labors between 500-800rpm. Doesn’t idle steady, vacuum gauge and tach jump around

There is a good bit of smoke (exhaust?) coming out of passenger side breather.

Compression check shows 140-150 on every cylinder, except #3. She is dogging with around 90-100 psi.

This was my first solo carb rebuild... signs definitely point to a major vacuum leak. Sprayed carb cleaner around base gasket and vacuum fittings. No noticeable change in idle quality.

Did old school techniques of holding hand over carb to block air. She sped up for a second or two then a abruptly died.

Ok. So where do I go from here? Anyway to tell if blown head gasket or blown rings?

Obviously cylinder #3 having issues. Trying to avoid yanking engine if I can, I am on a bit of time deadline for when my painter can get to car and trying to finish up floor pans and body work.

Any advice welcome!

Jeff





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#2
Pulled carb to check again, and looks like throttle plates were not closing all the way. Not sure if choke linkage was bound up or if I messed up realigning them. Working on correcting that now.



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#3
Slightly better but still laboring. Going to play with choke adjustment and idle speed screws a bit more.

Was able to shift into gear without it dying. But eventually she did stumble and die out.


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#4
Made some more adjustments and moved timing to around 12 btdc, and increased hot idle speed to around 750. Got her to shift without dying. Still has a nasty miss on that #3. Any advice to tell if it is a ring, valve seal, or head gasket? She is close to being able to be test driven.


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#5
You can stop #3 at TDC and put some air pressure to that cylinder. If it's valves you should be able to hear the leak in the exhaust or intake. If it's rings it'll be coming out of the crankcase. However, if it's the head gasket it could be leaking into the crankcase or coolant. Most likely rings, as you're getting some smoke. Could be a valve that is bad or recessed, and the smoke from bad valve seals.

What did #3 plug look like when you pulled it for the compression check?

I would swap the #3 spark plug with one from the other side of the engine to see if it helps #3. Also check the #3 plug wire to make sure it isn't damaged.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#6
(01-28-2018, 06:20 PM)Don C Wrote: You can stop #3 at TDC and put some air pressure to that cylinder. If it's valves you should be able to hear the leak in the exhaust or intake. If it's rings it'll be coming out of the crankcase. However, if it's the head gasket it could be leaking into the crankcase or coolant. Most likely rings, as you're getting some smoke. Could be a valve that is bad or recessed, and the smoke from bad valve seals.

What did #3 plug look like when you pulled it for the compression check?

I would swap the #3 spark plug with one from the other side of the engine to see if it helps #3. Also check the #3 plug wire to make sure it isn't damaged.


Thanks Don. I have never tried that technique, how much PSI should I feed into that cylinder? All new spark plug wires and cap, rotor, etc. Plugs all new, but #3 had some carbon fouling as other plugs look clean. Hard to get a good read since car has been garage bound since I got her a few months ago and working on getting her going.




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#7
You can squirt a few drops of oil into cyl #3 and do a compression check. If it raises considerably then it's most likely the rings.

-john
(jbojo)
351C 4V cc heads, 10.5 : 1 CR, 290 Herbert cam, Flat top forged pistons, forged connecting rods, Atomic efi,
C6 with Gear Vendor overdrive, 3.89 Tru Trac, Hooker Super Comp with 2 1/2" Pypes Exhaust.        

Some Mod pictures can be seen at:

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#8
I should have added that it has to be on the compression stroke. It's not easy to get it at the exact top. It doesn't take a lot of pressure if it's a valve not sealing, but I like to use 100 psi, or more.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#9
(01-28-2018, 08:14 PM)Don C Wrote: I should have added that it has to be on the compression stroke. It's not easy to get it at the exact top. It doesn't take a lot of pressure if it's a valve not sealing, but I like to use 100 psi, or more.


I figured you meant compression stroke, otherwise either an intake or exhaust valve would be open. Will try that tomorrow!


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#10
Another strange thing, is that right after I shut the motor off. There is a ticking sound coming from that side of motor for maybe 60-90 seconds before it goes away. Sounds different than just heat expansion sounds... any thoughts on that?


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