1975 Cj5- AMC 304 - Stage 2 Cam - Edlebrock Performer intake - DUI Distributor - Long tube Hedman Headers - (Holley) Brawler Carburetor 600 cfm (vacuum advance) - everything else is stock.

Upon initial startup from complete rebuild I could never get the timing mark to line up in and consistent manner with a timing light.  I timed it by ear and by how the engine performed and it was almost perfect. Easy to start and good drivability. It did have some lag when accelerating, this is when it all went down hill. 

I tried to do a little carb tuning and when I put the vacuum gage on it held steady at "wrong valve timing" at idle but it I revved slightly it would be in the green. (I figured this was ok since the cam is slightly bigger and the carburetor might be a tad too big for this motor. 

Once I started making small adjustments with the mixture screws and the timing again things went south... I now have a popping in the carburetor and it will not go away, nor will it get even close to starting easily or idling nicely as before. (my spark plugs foul in a matter of minutes, even when I try to lean the carb out as far as possible)  

I had checked, rechecked and checked again that the motor was set at TDC correctly and every time I had myself convinced I screwed up and set it wrong... Each time it check out correct...

Yesterday I took the timing cover off to check yet again that TDC is correct. When I clock the cam gear at 3 o'clock I have 21 pins from the dot on the cam gear to the dot on the crank gear.  The manual says this is acceptable but is it really??

I did not complete a full inspection of all the push rods yet but felt of them and there does not seem to be any anomalies there yet. 

The only other thing I have yet to think of was my fuel delivery, I have a small Holley electric fuel pump that I tested and without the engine running it is pushing close to 7 psi.  I have the Holley Red fuel pump coming in today and plan to switch those out...

It really feels like a timing issue to me but from what I can tell everything is set in accordance with the manual... I'm at a loss.... 

Also are you checking the timing with the vac advance vacuum line disconnected?

Also Re verify firing order just for kicks.

On the carb side of things verify the float level first

Then run those mixture screws all the way in softly till they stop then back them out 2 turns . Now I suspect you may be checking the eng vacuum at a ported pick up point. You want to be checking the vacuum off the manifold vacuum. All vacuum is not created equal.

If your manifold vacuum is too painful to acquire you can adjust the mixture screws to highest idle as you back the throttle stop out.

My guess is it is something simple . Float level and mech advance. Good luck

It is a new DUI sir.

Are you connected to the stock power feed with the resistance wire or are you on a full 12 volts?. 

Brand new chinese parts doesn't always mean fully functional either.

I have a new wire harness (Painless) and used the coil power wire that shows to have 12V with the ignition on as the power feed.

 One other thing that came to Mind Is.what did your distributor gear look like when you pulled it?

Which harness did you put in the car? If you have a voltmeter check to verify 12 volts at coil.

1.5 to two turns out is a standard starting point for mixture screws on every carb I have ever messed with.

-chris

Secondly the HEI distributors are notorious for failing distributor gears.  I just bought a 304 car that had bad distributor gears and some of your symptoms are familar.  Goes from running pretty good, to popping and backfiring.  My problem was the distributor would jump a tooth or two, and I chased the timing for a while before I pulled the distributor to rebuild and recurve.  The gear damage was then obvious.  Hope that isn't your problem.  

My distributor gear failed because the timing sprocket bled off all of the oil that is supposed to lubricate the distributor gears.  As my dear friend Phat said, "of course its the timing gear (bleeding off oil) they haven't made one right in 20 years".  So if you take the timing cover off again, delete the slots on teh timing sprocket.

good luck.

bass boat mentioned the power valve. this may be a problem, but most newer (as of 1995) have have  power valve protection built in them.

        once you get the motor to idle,  you can time the motor to total mechanical advance. assuming you have a timing light with an adjustment for advance.. to do this you would dial in 32 deg (thats what I would do for your set up) ) on the timing light, run the rpms up until the mark in the harmonic balancer stops moving and then rotate the dist to line up the harmonic balancer mark  to the zero mark on the timing chain cover . basically what you are doing is opening up the flyweights in the dist via centrifugal force and setting the timing there. then take not of where you started timing at idle.

on another note how much are the timing marks moving around at idle?

when the engine idles it pops through the carb, if I am able to bring up the RPM's the popping will reduce and subside until it is brought back to idle.

#2 fires just before #1 and the timing light would indicate a lot of advance when the motor is running smoothly like you said was happening. 

https://youtu.be/dRA4_7N7rSc" rel="nofollow - https://youtu.be/dRA4_7N7rSc  

distributor cap / front 

Like this  https://static.summitracing.com/global/images/instructions/edl-7800series-performer-linktim.pdf" rel="nofollow - https://static.summitracing.com/global/images/instructions/edl-7800series-performer-linktim.pdf

I replaced the power valve and still have the exact same result. 

Spits and pops in carb and now back firing through exhaust... 

The exhaust backfire is the only thing new.

-chris

Will do.

Problem identified. I am pretty sure the valve is locked up. Lots of work to do now...