Odd /6 problem

I have a /6 installed in a rat truck we are building for my son. The engine has a fresh overhaul and ran sweet as could be until about a month ago. It started running like the timing had jumped a tooth on the timing chain. We pulled the timing cover and it was all ok and timed correct on the chain. The pin was still intact on the cam and was solid. We pulled it off to double check. We changed the carb to a different one. We switched the plug wires, coil, rotor and cap for new ones. We installed a petronics electronic ignition. We have check for vacuum leaks and found none. It still backfires through the carb and is hard to start. The wires are installed correct with #1 cylinder one top dead center with the rotor at #1. The firing order is correct 153624. The compression is good on all cylinders and the cam seems to be operating correct as the rockers are all going up and down. I'm stumped and a couple of friends have all been stumped as well. A /6 should be a no brainer to make run but it has me scratching my head. Any ideas?

IGN! Put the points or MP electronic dizzy back in & see what happens

Problem started with the old points dist in (it had new points which had run great before). That was why I changed to the Petronics thinking that was the problem.

Pull the distributor.Ill bet the gear is going or gone.I bought a /6 Volare wagon for 25 bucks a few years ago for pretty much the same thing.The gear on the dist was shot.Quick trip to a buddys junk yard and a free dist and it was like new.Rocky

Pulled it and checked it. Plastic gear was intact.

Its not so much they are intact but they wear.Like a speedo gear that wont turn.The teeth wear donw and dont engage right.They used to sell a repair kit.It was cheap if still avaliable.Is this an older model with solids?Id pull the valve cover and check lift and do a compression check.Rocky

I've seen the plastic dist gears in the parts stores 'HELP' brand section. If you want something other than plastic, NAPA sells (or used to) a metal gear.

Did you lose any cam lobes?

When you checked for vac leaks you sprayed all around the intake top to bottom/end to end & no change?

Cam lobes look ok. Lifters show no odd wear and rocker shaft was opening the valves ok. Sprayed starting fluid all over the intake and carb base with no sign of vacuum leaks. Put a new carb gasket under the carb. Just pulled the head and the gasket looked fine. The valves all look good and open and close when tapped with a small hammer on the stem. I ordered a new head gasket to reinstall the head. 5 cylinders showed 125 psi and 1 showed 115. Back fires through the carb like timing is way off. Distributor was a new point one with a brand new gear.

Check your coil wire, had a customer come in with issues similar to yours, after he mangled up the distributor drive gear by "adjusting the the point" on his 78 pickup (he had electronic ignition) and taking out his distributor drive gear I found his original problem, the center core of the coil wire had eroded back about 1/2" into the insulator.

Changed the coil wire with a different one. Same results.

What a toughie . It has to be something we've already covered/mentioned. I'd reiterate either: vac leak/ECU module/cam timing. I keep coming back to vac leak since it first acted up with the points & I THINK the cam timing has to be in the ballpark since the long block compression numbers are normal.

How does it run after getting it started? Will it rev freely or start to rev then seem to lose power? Been a while but I worked on a /6 Duster w/ almost the same symptoms, IIRC. Turned out the muffler had come apart and was blocking the tail pipe.

Tim

I agree. I'm stumped. A /6 should be the most simple to figure out and make run. Vacuum leak would have to be pretty big to act like it does and therefore should be no problem to find but it evades me. Like I said the head is now off and waiting on a new gasket. Napa says the closest is in Indiana. Who would have thought a head gasket for a six would be that scarce. when it does start it is really low on power and won't pull it's self like a jumped timing chain. The exhaust is a straight shot to just behind the cab.

Do you have the correct voltage going to the ignition system? maybe it's a fuel problem? weak pump,clogged line? hydraulic lifters or mechanical? too tight of a valve adjustment will do strange things. check engine vacuum when running,should be somewhat steady at 18-21.

Hydraulic lifters. Fuel pump might be something to look at. Voltage with points was 9 volts (ballast resistor) with Petronics was 12.

Did the head have the valves redone? On a hydraulic motor the stems have to be very close to the same height and you have to have the correct preload or they can hang valves open after the lifters pump up.

The iron gear from NAPA is discontinued. I just had a buddy do a national search. The part number is still good, but all 6 boxes we got had nylon gears in them.

FWIW, I have destroyed a lot of stuff racing Slants, but the only time I ever broke a distributor gear was when I got in a hurry putting the distributor back in and jammed it onto a tooth on tbe cam gear. They really are fairly tough.

Valves were all new and height was set by the machine shop. Like I said it ran great until about a month ago when I backed it out for a test run. Barely got it back into the shop. Once I get the head back on I'll look further into a vacuum leak and try another carb again. I'm just not seeing anything that would cause these problems. Yesterday before I pulled the head I took the Petronics ignition out and put in the points. No change in how it ran. Talked to the machine shop and they were mentioning to check if the valves were sticking due to the fine quality gas we get today but they seem to be ok.

Sorry, nothing to add, just watching this thread with curiosity as I had a slant sick from hell about 20 yrs ago.

In my case, it was a low mileage '79 Dodge PU, stone stock. Ran great....for the first two days I owned it. After that, it wouldn't run/drive more than 50 ft at a time. Hot or cold, engine had good, even compression, good vacuum. I basically ended up swapping EVERYTHING either on or around the engine, and it made no difference. With the help of a friend, I even ran the engine totally separate from ANY of the truck's control (as if the engine were on a test stand).

NOTHING made a difference.

Ended up scrapping the truck out of disgust.

I hope you have better luck.

What carb/intake are you using? You don't happen to have one of the 2 piece welded intakes do you?

Was the head or block decked much? If so the intake may not be sealing against the head. When you deck the block there is a casting block that will hold the intake away from the head.

From that pic with the head off it looks pig rich. I have seen that caused by vacuum leaks before.

Carb is a Holley 1945. Intake is stock. Deck and head were milled just enough to make it flat. Probably less than 20 thou. It is running really rich but mostly due to the amount of cold starts. It hasn't been warmed up properly for a while. I'm leaning towards a vacuum leak being the issue but I'll change the carb again and see if I get better results.

Just a thought, but did you check for TDC? Maybe the crank gear key sheared.

Fouled plugs don't "unfoul" themselves, and they also create carbon hotspots which screw up cylinder firing. so if you are doing all of this diagnostic work with fouled plugs, you're wasting your time

the other thing I notice is the exhaust pipe flange. it looks like the entire thing has been leaking. slant sixes are very sensitive to exhaust leaks because they are small engines and already dialed in to be very lean. that could certainly cause popping.

Make sure the heat riser valve in the exhaust manifold isn't frozen shut. that'd make it run like a dog

Installed a brand new set of plugs 2 days ago. Exhaust is only tack welded together so a leak at the manifold flange shouldn't have changed how it runs just recently. The riser is free and even pulled the EGR and cleaned it.

You have fuel, compression on all 6, spark, it should run if the fuel and spark are right. You have no vacuum leaks. Compression rules out bad cam lobes, valves, timing chains and head gaskets. Two different carbs rule out fuel, unless they are both bad.
Must be an ignition problem of when it ignites. Is the mechanical advance working? Try timing it by ear instead of a light. Could be a bad light or slipped damper ring giving a bad reading.

My bet is on the valve lash. Maybe an exhaust seat is pounded out or if too much material was taken off of the head ?

The head is off and I can see the valves and seats. They look good. Can't get it to run well enough to even tune it by ear. Like I say it has me stumped. The compression test should have shown a bad valve. I'll give it another carb change and see if that does it when I get the head back on.

I doubt if you have one laying around but I'd wanna swap the intake manifolds. After all this I would have to know what it was all along no matter what the cost

You don't know the kind of pack rat I am. I do have another intake laying around and I think I'll try it when I put the head back on. That is one thing I haven't tried yet.

Installed the new head gasket and blocked off the EGR port. It runs like a champ again. Was it the head gasket? Was it the EGR? I don't know but it runs good again. Feel much better about it. The head gasket looked good enough to reuse so I don't think that was it but the new one I torqued tighter than the specs ask for thinking it might have been leaking across.

If the EGR was stuck open it would run like crap.

Glad to hear all is well again.

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Was it the EGR?

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If it was leaking (I had one that did) it'd be a massive vac leak