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Discussion Starter #1
Hello. :smile:

I have a 74 Challenger - 318 C.I. - HEI Distributor - Edelbrock Intake - Edelbrock Performer 1801 Carburetor - Purple Cam - Dual Exhaust.

I am having an issue with misfiring through the carburetor once accelerating above idle (750 RPM).

The troubleshooting I have done so far is:

1) Timing Chain slack check - 5 degrees - 10 degrees indicates worn timing chain

2) Compression Test - Dry - 140 to 160 PSI - 100 PSI is minimum and 40 PSI is the maximum variation.

3) Compression Test - Wet - 160 to 195 PSI - 40 PSI is the maximum variation.

4) Checked for vacuum leaks - None found.

5) Tried advancing and retarding ignition Timing - Still Misfires.

6) PCV valve checks OK.

7) Cleaned and checked carburetor primary and secondary jets.

8) Re-adjusted Idle Mixture.

9) Plugs and Plug Wires test O.K.

10) Choke is fully off.

:scratchhead: Does anyone know what else could be causing my problem? or what further testing I should be doing?
 

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did you check to make sure the chain hasn't jumped time?

Your symptoms remind me of when mine did that - low power, hard to start, etc.
 
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Discussion Starter #3
Thanks Hal. I did not remove the timing chain cover to check this as I thought if the timing chain is not worn then it could not jump the sprockets. It only showed 5 degrees timing slack and a 10 degrees timing slack by factory specs would indicate a worm timing chain. Is it still possible that it has jumped?
 

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Its possible - with mine, it jumped when I shut down the engine.

When I limped it home, checked the timing and it was retarded by 15* or more...I could just see the timing mark, which was slightly to the right of the center of the water pump.

Did this symptom suddenly appear when prior to this it ran correctly? Usually a sudden change like that is a chain that jumped.
These engines OEM chains had nylon teeth over the metal sprocket and they wear...a lot.

The replacements generally are double roller chains with steel sprockets.
 
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Which HEI distributor ? are you using.

Like Hal said use a timing light and check and see if your timing and your mechanical advance are working correctly.

I usually plug off the vacuum advance until I have the condition diagnosed.

Checking plug condition.

I am not a big fan of eddy carbs at all.
 

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Clarifying your information. When you say "misfiring through the carburetor" do you mean popping back through the carb? If so is it a rapid pop in session our just once and goes on? Misfires through the carb as you call it is caused by a few different things. A lean condition from a faulty accelerator pump in the carburetor, or if it pops more than once it could be a worn exhaust valve camshaft lobe. Most engines will not run a full tooth off on the cam chain.
 

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Clarifying your information. When you say "misfiring through the carburetor" do you mean popping back through the carb? If so is it a rapid pop in session our just once and goes on? Misfires through the carb as you call it is caused by a few different things. A lean condition from a faulty accelerator pump in the carburetor, or if it pops more than once it could be a worn exhaust valve camshaft lobe. Most engines will not run a full tooth off on the cam chain.
+1 on a potentially worn cam ("flat cam") - these older engines are susceptible to that, especially if running non zddp oils.

I had a friend's low mile '66 Mustang (only 99k original) that had badly worn cam lobes that wouldn't run correctly due to that issue.
 

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Discussion Starter #8
Thanks everyone for the replies.
The condition started after a 20 mile highway run. I was accelerating to go up a hill and that's when it started misfiring (Popping or backfiring) through the Carburetor.
It would misfire continuously when under load.
When checked later the timing was still at 10 degrees BTDC which was the initial setting.
The HEI distributor is ordered to specs by Skip White Performance. I may have cheaped out on this item but the mechanical advance seems to be working fine. It is hard to tell how much it is advancing at a higher RPM as there is very little space to view the timing marks on the 318 motor.
The accelerator pump is giving a nice full squirt of gas.
 

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Discussion Starter #9
The motor was rebuilt by the previous owner and has under 500 miles on it. He said he put in a new Mopar "Purple Cam". I found out later that he did not have the oil pump installed properly so it was run for how long I do not know without any oil pressure. The compression check seems well within limits. I think I might have to buy a cylinder Leak Down Tester to try and pinpoint exactly where the problem is. i do not want to strip down to the timing chain yet without being entirely sure that is where the problem is.
 

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Those cranking pressure numbers are quite high for a 318. Are you running premium fuel?

If not, try running premium and set your timing to 35 degrees total advance at about 2,800 to 3,000 RPM and see if that helps.

Of course check the overtly obvious stuff first such as the correct firing order for the plug wires, and make sure none of the plug wires are touching each other as you can get crossfire if they are touching.
 

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The motor was rebuilt by the previous owner and has under 500 miles on it. He said he put in a new Mopar "Purple Cam". I found out later that he did not have the oil pump installed properly so it was run for how long I do not know without any oil pressure. The compression check seems well within limits. I think I might have to buy a cylinder Leak Down Tester to try and pinpoint exactly where the problem is. i do not want to strip down to the timing chain yet without being entirely sure that is where the problem is.
I'm still thinking cam lobe. Cam lobes wear fast when things go wrong. A new cam even more so. Pull the valve covers and start it up and watch the motion of the rockers and push rods. Push rods should be spinning and the rockers should be moving all about the same. I would bet you have a flat one!
 

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I agree on the cam lobe worn but the compression test should show that if done correctly.

Pulling the valve covers might be worth a peak.
The compression check would check normal unless the lobe was worn completely off. a worn short exhaust lobe does not let the cylinder purge all the hot exhaust gas out of the cylinder in time under load. Then when the intake valve opens for a fresh fuel charge you get a pop back through the carb. That is why it does not pop at idle and does under load.
 

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Discussion Starter #14
Thanks. I am running premium fuel. I believe a carbon build up may lead to higher compression values but the rebuild has very low miles.
I can only see the ignition timing marks 10 degrees before and after TDC. Is there another way to check the timing at 2,800 to 3,000 RPM?
 

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The compression check would check normal unless the lobe was worn completely off. a worn short exhaust lobe does not let the cylinder purge all the hot exhaust gas out of the cylinder in time under load. Then when the intake valve opens for a fresh fuel charge you get a pop back through the carb. That is why it does not pop at idle and does under load.

Might be a good reason to pull the valve covers and see if all the rockers are moving.

You would need timing tape on the balancer to see how much advance you have or maybe a "dial back" timing light.

there is a formula online to make your own tape its a pia really but it tells you how much advance you have.
 

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Discussion Starter #16
Thanks. I will buy the tape as total timing advance is pretty important. Also pulling the valve covers to have a look sounds like a good idea.
 

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Should of done this earlier but I pulled off the plug wires one at a time and with number 7 off there was no longer a misfire.
Swapped plugs and wires with number 5 and still the same results.
Pulled the valve cover and definitely one of the valves has very little movement.
It is the valve closest to the firewall.

Some Questions:

Can someone tell me if it is the intake or exhaust valve?
Also which type of valve spring compression tool is the best?
Which brand of valve stem seals are the best?
 

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Should of done this earlier but I pulled off the plug wires one at a time and with number 7 off there was no longer a misfire.
Swapped plugs and wires with number 5 and still the same results.
Pulled the valve cover and definitely one of the valves has very little movement.
It is the valve closest to the firewall.

Some Questions:

Can someone tell me if it is the intake or exhaust valve?
Also which type of valve spring compression tool is the best?
Which brand of valve stem seals are the best?
The outer valve spring will be the exhaust

the layout EI IE EI IE from one end of the head to the other (looking from above)
 
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Old school Challenger tuning..............love it!

Carry on my brothers.
 
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