I'm having a difficult time with the q-jet on my '68 4-4-2. It's the stock 7028251 carb for the car and all of the innards are correct. What happens is that, at cruising speed, be it 30, 40, 50, or 60mph, the engine just starts cutting out for a second, then running strong again for 4-5 seconds, then cutting out for a second, then running strong again for 4-5 seconds, and so on. Start up is fine. Acceleration is fine. The float is set to spec. It just happens when you settle into a steady speed. Any thoughts on what might be wrong?

Right now we are thinking that maybe the spring we used under the power piston is either too weak or too short and that, at a steady speed when there is vacuum, the vacuum is pulling the power piston down enough that the primary fuel control rods temporarily shut off the fuel supply. But that's just an "educated" guess on our part.

Any thoughts on what we theorize or what you might have in mind will be greatly appreciated!

Randy C.

 

Float to low, crap in fuel bowl? It could also be an ignition issue.

 

What type of ignition? Stock, HEI, if HEI what type?

 

Sounds electrical to me.

Power valve shouldn't do this - It's fully closed most of the time. It only opens under acceleration and on hills.

- Eric

 

dirt

 

The carb is newly rebuilt and thoroughly clean. With the attention we gave to cleaning out this carb (taken completely apart, it's been in the "soup", it's been thoroughly inspected before re-assembly), I don't see (or understand) how dirt could be a problem.

Before, this same carb ran OK (after warm-up) but displayed some unusual characteristics during start-up. It started real, real hard and, when it finally started, the engine would not idle any lower than 2400rpm for 10 or so minutes before you could finally knock it down. After that, it ran OK. If you killed the engine and let it set 15-20 minutes, it would start fine. If you let it sit for an hour or so, however, you went through the hard start-up process all over again.

I originally bought this carb as a so-called fresh rebuild in 2006. Given those unusual start-up characteristics over the years, I decided to go into the carb (I've got a great friend that rebuilds carbs for a living and I was by his side all during the rebuild process, trying to learn how this baby works! Even did some of the work myself.). When we took it apart, we found several discrepancies. One, the power piston was stuck in place - it didn't move up or down. Two, there was something not right with the accelerator pump. It appeared as if the shaft had been cut/filed down to make it fit right. And three, not only were the secondary fuel control rods not correct for this carb, they didn't even match each other. I obtained all the right parts and a rebuild kit and we rebuilt the carb again with the correct internals.

I have a Pertronix ignition set-up in my distributor. With the carb in its previous condition, there was no break-down or anything of that sort.

I can't think of any other particulars right now but, if I do, I'll add them in here. As well, if any questions come my way, I'll do my best to promptly answer them.

Thank you.

Randy C.

 

Was anything else at all done to the car when the carb was rebuilt? When the carb was rebuilt were the primary jets and/or metering rods changed? Was the power piston and/or spring changed? Were the new gaskets matched to the old ones, the old and new put on top of one another and checked on both sides by turning them over to check for blocked passages by the new gasket?

Lets forget about the secondaries for now, they should be closed at cruise. Does it feel like a lean surge or a more pronounced misfire? What color are the spark plug insulators and tailpipes after a cruise?

Try disconnecting the vacuum advance and see if it goes away, if it does it could be a wire problem inside the distributor as the plate moves back and forth with vacuum fluctuations. Early HEI's did this alot when the wires on the pole piece were going bad.

 

I'm gonna say wires or plugs - how fresh are they?
Open the hood at night, with the car running and lights out.
Look around the engine wires for arcing, and listen for a faint click, maybe to a vc or another wire.

 

Randy,

I'd start with a temporary fuel pressure gauge at the carb inlet. Watch the pressure when the miss happens. If the pressure drops, check the entire fuel delivery system. Could be pump, could be crap in the tank being sucked up against the inlet screen, could be cracks in the rubber suction line hoses from the tank.

 

You can isolate the electrical with a setback timing light by cranking the rpms up and watching for fluctuations. I'm with you, I think its a fuel problem.

 

I'll answer some questions first and then lay out our plan of action.

Nothing else was done to the car when the carb was rebuilt. The primary metering rods and the jets in the carb were correct for the carb and re-installed. The power piston is the same but we aren't sure about the spring. Seems we had 3-4 springs that got mixed up in the pile and we couldn't remember which one was in the carb when we took it apart (the power piston was stuck in place when we took the carb apart - it didn't move at all). All the gaskets were matched to the old ones.

We believe it feels like a "lean" condition and not a misfire. We need to check the spark plug insulators and the exhaust - we aren't throwing any kind of smoke out the exhaust to speak of.

Things we are going to do:

1. Install a little stiffer spring under the power piston and see how that affects the engine under running conditions.

2. If (1) doesn't make any difference, try disconnecting the vacuum advance and see what happens.

3. Check the fuel pressure. We did this once before, but only at idle and we were getting a steady 6-7 lbs as I recall. But we didn't increase RPMs so, when we do this again, we'll run RPMs up to about a steady 2000 and see if we recreate the situation and, if we do, see what happens to fuel pressure.

4. We'll take a look under the hood at night as well to see if there is any arcing going on. The entire fuel system was new in 2007, hoses and all. The underhood wire harness and the spark plug wires were also new in 2007. The car has around 1900 miles on it since then.

Thanks to everyone who offered help. Again, this clearly demonstrates to me what a wonderful site this is, with the world-wide ability to share info and diagnose problems like the one I have. I'll let you know what happens as we go through our testing.

Randy C.

 

Under the circumstances, I'd recommend just calling one of the places that sells QuadraJet parts and ordering the spring that is spec'd for your engine, that way you'll know it's the right one. Any other way and you'll never be sure.


The RPMs don't matter - it's the throttle position.
Fuel consumption at 2000 RPM with no load is essentially the same as fuel consumption at idle.
You need to do what Joe suggested and connect a gauge temporarily that you can read while driving. You can't make it use enough fuel to challenge the fuel pump in the driveway.

- Eric

 

The 'power piston' always keeps the large portion of the rods in the jets for economy. The only time the piston lifts the rods out of the jets is when engine vacuum drops below the tension on the spring to richen the mixture for acceleration.
I have a saying; "90% of carburetion problems are ignition".
I would first disconnect the vacuum advance and drive the car. If it stops cutting out then I would check the breaker plate ground wire and then all the wires in the distributor for either a break or rubbing to ground.
Joe

 

I like the idea of a skinned wire in the distributor shorting out when the vacuum advance releases.

- Eric

 

Vacuum leak around the lower throttle shafts?