mlbinseattle

Personally, I think it's a fuel delivery issue, and here's why:

Recently rebuilt the carb. Rochester Quadrajet 17084208. I didn't trust any carb kit to get the right size seat, so I started looking around. (This is a long, ugly story of someone who never rebuilt a carb in his life and chased his tail for a while before he finally figured out what was really happening.) Anyway, found this site that stated, for this carb, it takes a .110" diameter seat. The only style they had at this size was a windowed seat. I haven't read anything good about windowed seats, and apparently, Rochester even realized these were a problem and quit using them. (Just parroting what I've read. Don't know how true it is.) The closest un-windowed seat I could find was a .115" seat. Figured it should be okay, so I installed it. Don't know how relevant all this is to the story, but I figured I'd throw it in, anyway, to see if it raises a flag with any experienced types.

The ignition system is solid. New plugs, plug wires, rotor, and cap. Ignition and pickup coils both read good, and the ICM is brand new.

Fuel filter was changed when the carb was rebuilt.

Engine idles like a champ. Smooth, no misses, and the pressure gauge shows some excellent (IMO) results.

Timing is set at 6 degrees BTDC. Carburetor was tuned using the digital dwell meter function on my DMM. It varies anywhere from around 16 - 40 but usually around 26 - 33. Seems like I need to vary the throttle to achieve this. At higher RPM's, it's up in the 30's range, and at idle, it's in the lower 20's range. I've been expecting to see it swing back and forth at idle, and it does, but not to the full extent of the 16 - 40 range. The IAB valve was turned in slightly more than 1 full turn, and I started with 4 turns out from seated. So, it's almost 3 full turns out.

The idle mixture screws are at 3 3/8 turns out. I wasn't happy with the dwell readings in the beginning, so I backed these out 1 full turn. It didn't like that too much because the secondaries never opened while taking it for a spin, and it stumbled all over itself at WOT. I put the idle mixture screws back at 3 3/8 turns out and haven't touched these, since.

The engine definitely jumps and is responsive, and the secondary is definitely kicking in while on the freeway. Overall, I am pleased with the behavior except for one thing:

Go from a dead stop to WOT and it squeals the tires, gets up and goes, and feels like it's going to fly, but then about 3000 RPM, it chokes. Sounds like it just ran out of gas. I know the bowl and surrounding chamber area that takes gas inside these carbs are small. Last time I had the carb off, I only drained maybe 1/4 cup of fuel. I'm sure I lost some while disconnecting the fuel line, but the point is, these carbs don't hold that much fuel. I'm sure with sufficient suction, what's in the bowl is getting sucked out pretty quick.

Prior to rebuilding the carb, it didn't do this, but the timing was off (way too far retarded), and it had a standard needle/seat in it. Also, the ignition was pretty crappy after sitting up for 12 years. I'm sure the ICM played into the timing being off.

What do I need to look at, now? Does it sound like I need to adjust the float level maybe higher, or should I focus attention on the IAB valve and idle mixture screws some more?

mlbinseattle

I was hoping someone would respond who experienced the same problem or similar. Oh well, seems like I get the good ones (problems, that is).

I've done more reading on the subject. I've come to the conclusion that it's not a carburetor problem, at all. However, I need to verify this.

A Quadrajet is tuned by first adjusting the primary to achieve the desired dwell reading. I've done this. Then, once the primary is adjusted, the secondary is adjusted (assuming it needs it). There are only two adjustments for the secondary, and both of these are external to the carb: The air flap spring tension and the pulloff. I could probably add the secondary lockout tang adjustment. However, if the chokes works fine, then this shouldn't be an issue. (I'm not having any issues, here.)

I haven't adjusted the spring tension or pulloff because both of these are working just fine. Secondary is definitely kicking in while driving, and even at idle, you can see the air flap begin to open at WOT once the RPM's get up above 3000 RPM.

So, either I'm not getting enough gas into the carb at a very high RPM state when the secondary is kicked in, or the spark is disappearing. It's got to be one of the two.

I'm thinking it might be a vent issue with the gas tank. I only experienced the problem when I go from dead stop to WOT. It doesn't do this on the highway when the secondary kicks in, and it doesn't do this when the car is sitting in park and I throttle it to WOT.

The ignition is solid, and a new battery was just installed. I seriously doubt it's an ignition problem, but who knows.

If I ever figure this one out, I'll report back.

mlbinseattle

Well, the problem no longer exists. Here's what I did, but I don't know if anything had a direct influence on clearing up the problem:

1) I noticed when I rebuilt the carb, the secondary metering rods were not hanging together from the secondary metering rod holder at the exact same length. In other words, the tip of one rod was hanging lower than the tip of the other rod. At the time of rebuild, I found this peculiar. Since this was the first time I ever rebuilt a carb in my life, I just took a mental note and filed it away. After reading up on secondary metering rods, I discovered that both rods should be hanging from the holder at the exact same length so that the tips are hanging the same distance into the respective jets. I also found out that the distance from the top of the choke horn to the top of the rods is 21.236/64". One of the rods was the correct distance, the other wasn't. I took a pair of needle-nose pliers and adjusted the holder for the rod that needed to be at the correct length. Now, both rods are hanging from the holder at the correct distance and are equal.

2) The battery decided to give up the ghost on me about the same time all this was happening. The engine had been difficult to start after the engine heated up to normal operating temperature and the engine was switched off. I had to let it sit for approx 20 minutes before it started, again. Then, it quit altogether the other morning. It was the first freezing day of the season, so I wasn't surprised. The car now has a new battery in it.

3) My wife reported that the SES light came on one evening, so I looked at the code which was 24 (VSS circuit). Either there was a problem with the variable speed sensor circuit, faulty wiring at the ECM, or the TPS needed adjustment. I don't know of an actual variable speed sensor by that name. Nothing shows this on the wiring diagram, and I don't know if this is an actual circuit inside the ECM. The only sensor having to do with speed is the TPS. I'm speculating that the ECM must correlate throttle position to speed. In any case, I checked the output of the TPS with my DMM, and it was slightly off. Not much. I readjusted the voltage to .48 VDC, and I haven't seen the trouble code since then. This may have been an anomaly for all I know. I haven't reinstalled the new plug in the TPS adjustment hole as of yet because I wanted to see how everything was going to perform for a few days since I have the engine where I want it to be, now. I suppose I'll check the TPS one last time and install the plug.

In any case, the engine now runs like a champ and flies like a bat out of h*ll. I can go from a dead stop to WOT, wind it up to 5000 RPM's, and it performs flawlessly. Everything now works the way it should, and I'm a happy camper, once again.

Like I said, I don't know if anything I did had a direct bearing on clearing up this issue. Maybe the ECM had to relearn a few things. I'd like to think that correcting the secondary metering rod hanger so both rods hung at the same length did have an effect.

If anyone has any thoughts on any of this, I sure would like to hear those. If I don't hear from anyone, I'll just take it that nobody had any idea why this would have happened to begin with.