360 runs great one day and then poorly the next

-
TQ was rebuilt and new bushings installed by National Carb. Another carb installed runs the same way. Checking grounds on all devices.
 
Need to find someone with an oscilloscope.
In the meantime,spray bottle with water,and wet the entire ignition system down while its running. Could be spark not following its intended path.
Found a cracked spark plug that way.

Had a center electrode come loose in a spark plug. That was hard to track down. Pull plug,hold up to look at gap and electrode looks normal.point it down and gap is closed.

Rotors burn through and will still run.it is a discoloration inside where it pushed on to distributor shaft.
 
It's the motor, no lights are flickering. Motor just acts like cylinders are dropping off. Next time everything runs just fine. It comes on goes, almost like its loading up but runs great the next time.
Conceivably, could be hydraulic lifters acting up one time and not the next. Some may leak down when the engine stops at a certain angle of rotation and then not leak down when it stops at a different angle the next time. Probably not the most likely thing, but the comment that it is almost like it is dropping cylinders could come from that. (But erratic ignition would do that too.)
 
Conceivably, could be hydraulic lifters acting up one time and not the next. Some may leak down when the engine stops at a certain angle of rotation and then not leak down when it stops at a different angle the next time. Probably not the most likely thing, but the comment that it is almost like it is dropping cylinders could come from that. (But erratic ignition would do that too.)
There would be noise associated with it.
 
Trash in the tank clogging up the sock on the sender unit... maybe? Car gets moved or the engine shut down (no fuel pressure) the debris falls off the sock. No blockage then but after the engine gets ran it blocks the sock. Just guessing.
 
Trash in the tank clogging up the sock on the sender unit... maybe? Car gets moved or the engine shut down (no fuel pressure) the debris falls off the sock. No blockage then but after the engine gets ran it blocks the sock. Just guessing.
Good point. Or water in fuel.
 
I will pass along all these ideas and let you all know in the next few days. Thanks to all
 
I had something similiar happen to my dart sport 360. In the end turned out to be a bad/faulty ignition pick up in the distributor! So replaced it set reluctor gap and back in business. Just an idea.
 
They do get intermittent when they are about to go out from my experience. Good point.
 
The gremlin is still there. Fuel system has been gone through, tank has no rust in it, clean fresh non-ethanol fuel added and pressure at carb is approx 7psi (mechanical). Haven't swapped carb out yet. TQ was just overhauled by Carburetor USA, over the winter.

Coil, P/U gap is .008, cap and rotor swapped with known good. Electronic module was swapped with know good. Still scratching the head. New harness didn't change anything.
 
I would try running a fuel pressure regulator and set it to 5.5 or 6 psi, max. You might be surprised. 1 or 2 psi can make all the difference on some carbs.
 
I had a problem similar to this on a 74 dart sport 360. Ended up finding a split in the extra plug on the power brake booster, sometimes it would suck together and seal and at other times it was a huge vacuum leak, just to look at the plug it looked new.

Screenshot_2019-06-18-12-14-12.png
 
From your descriptions, I don't think this is your problem,but To check if the running fuel level is stable,
With the engine idling clamp the fuel line.
One of three things is gonna happen;
A) If the fuel level is too high, the engine will slowly smooth out and pick up rpm, then continue as at B) below
B) If the fuel level is about correct, nothing will happen for about 15 to 30 seconds, then it will slow down and continue as at C) below.
C) If the fuel level is too low, the engine will slow down, run rough, and eventually stall.

I also am wondering if you have an intermittent vacuum leak, and because of the severity, Ima thinking the PCV is cracked. I've even seen them broken and the only thing keeping it together was the engine vacuum.

If you thing it's ignition;Isolate the cylinder or cylinders. Just pull one wire off atta time (engine running of course), looking for wires that make no difference. Then you just have to prove the wire, the plug, the tower, or the valves are closing..
 
I had a problem similar to this on a 74 dart sport 360. Ended up finding a split in the extra plug on the power brake booster, sometimes it would suck together and seal and at other times it was a huge vacuum leak, just to look at the plug it looked new.

View attachment 1715351774
Booster vacuum adapter only has the one 3/8 port to manifold.
 
From your descriptions, I don't think this is your problem,but To check if the running fuel level is stable,
With the engine idling clamp the fuel line.
One of three things is gonna happen;
A) If the fuel level is too high, the engine will slowly smooth out and pick up rpm, then continue as at B) below
B) If the fuel level is about correct, nothing will happen for about 15 to 30 seconds, then it will slow down and continue as at C) below.
C) If the fuel level is too low, the engine will slow down, run rough, and eventually stall.

I also am wondering if you have an intermittent vacuum leak, and because of the severity, Ima thinking the PCV is cracked. I've even seen them broken and the only thing keeping it together was the engine vacuum.

If you thing it's ignition;Isolate the cylinder or cylinders. Just pull one wire off atta time (engine running of course), looking for wires that make no difference. Then you just have to prove the wire, the plug, the tower, or the valves are closing..
I'll let you know the findings, thanks
 
How did you make out? Were you able to resolve your issue? I have a similar issue with my 273 and it is driving me crazy.
Please let me know .
Thanks
 
After a third carb was installed, it began to run much better. It's beginning to improve with every trip. Three tanks of non-ethanol fuel and Gumout.

One thing I did find out, using the the 1/4" gasket alone with the Thermoquad is a possible issue on a OEM intake. You have to use the thin gasket. It appears the bottom TQ plate needs to be completely sealed to a four hole gasket. The thick gasket is all open between primaries and secondaries.

Perhaps someone else can shed some light on this finding.
 
interesting that it looks like it may have been an intermittent carb issue. I'll double check which gasket I put under the carb after it was rebuilt. At this time I am running the original (freshly rebuilt) Carter 2bbl ball & ball.
 
When I ran a Carter BBD on a 318, I never had an issue with the thick gasket.
 
I'm pretty sure there are some vacuum passages on the underside of the base plate on the TQ that need to be open to the manifold plenum to get a good signal. I tried running one once with a 4-hole gasket, didn't run quite right and I couldn't tune the idle, not to mention the throttle butterflies were touching the edges of the holes in the gasket so I couldn't open the throttle all the way. Swapped on an OE open-style gasket and it ran much better. It was on an Edelbrock Performer (non-RPM) intake though.

I later realized the 4-hole spread bore gaskets are meant for Quadrajets...
 
My gasket is approx. 1/8 inch thick..I'll start my own thread and describe me issue. That way I won't clutter this one and make it confusing. Thanks for the input.
 
-
Back
Top