Replaced the carb on my LG4 today with a rebuilt quadrajet, tuned it, put on a new open element cleaner, and it runs much better than before. I also put in DR secondary metering rods as per suggestion from users here. The car now has much more low end torque than before and better throttle response, below ~2500 rpm.

At any point accelerating past 2500-3000 rpm (either WOT or holding it in a gear and trying to accelerate at half-throttle) it barely goes, no matter how long I hold the gas. Its not bogging down, it just has no power at all. Holding it at WOT it barely makes it above 3200 rpm after about 5-10 seconds (climbing very slowly).

Any ideas whats happening here? If it was a problem with the air valve or secondaries, would it still lack power at half-throttle?

 

I can't help you out with your Q jet problem, BUT I know someone who can:

http://www.cliffshighperformance.com/

 

Cliff built my Qjet, but even so, it still needed to be "tuned" to the car. When you get a new Qjet, or any carb for that matter, it has a very basic tune, usually good enough to keep the car running. I would suggest looking over the carb section here on "tuning" and making sure that you have it dialed in to your car. So yes, while sending it off to Cliff would be great, if you just "bought" a new rebuilt carb, you'd basically be doing the same thing.

Verify your fuel pressure. should be around 5-6 PSI. I'm assuming you have a mechanical fuel pump? Was that replaced as well? is your return line hooked up?

Check your dwell, make sure you're at 30% on the 6-cylinder scale and that it's deflecting 10* or so while running. Replace your fuel filter.

Since it falls on it's face, I'm tempted to say you're running out of fuel, but more diagnosis are needed before jumping to conclusions.

Obviously, the car runs, so you have spark, fuel and air. since you replaced the carb, did you also do a tune up? Cap, coil, rotor, plugs and plug wires? does the car stutter at high rpm (Like hitting a rev limiter?) or does it just fall on it's face? Stuttering can be an indication of spark jump, meaning its possible your plug wires or plugs are arcing and you're losing spark in the system. Alternatively, your cap and have spark scatter if it's going bad, causing power to arc to the wrong plugs internally to it.

Beyond that, read, read, read, and read more about tuning these carbs. 90% of all car shops out there can't tune carb's anymore. Not these Qjets anyway. There is a huge thread, myself, NAF, and Aperion went round and round while I was learning to tune my carb. It was a long process, but it was also invaluable as a learning experience.

 

If part of "tuned it" involved loosening the air valve spring, go back and undo that mistake.

 

Most likely the air valve tension. Start with 7/8 of a turn tighter than the point where it 'just' falls open.

Check your choke pull off (the vacuum can in front of the choke). Disconnect the vac line, hold it in and cover the vac nipple. It should stay in until you release your finger.

If it 'ran' and then died, I might suspect fuel starvation.

 

In my case, my fuel pressure was too low, the car would run great when I hammered on the gas...3000 rpm, then just call on it's face, stutter, and would die, unless i let my foot off the gas. Then it would "catchup" as the fuel was replenished.

 

Wow you guys are so helpful. From what I can tell its not fuel or spark issues. At high RPMs it just loses power. No stuttering, and it doesn't "run great and then fall flat on its face" or "catchup if I let my foot off" like Ozz1967 suggests.

I know the choke-pull off is working fine.

Sofa and naf both suggested it could be the air valve tension, which I did tune following the tuning section in the tech articles here on TGO, so I will definitely revist that and hope for the best.

If not, I will look closer into what Ozz suggested earlier.

 

I tried to adjust the AV tension, and it helped a bit but the problem is still quite bad. Now that its a little better its more clear to diagnose. Starting at around 2500 rpm I start to SLOWLY lose power at WOT, and by 3200 rpm or so, there is basically no power at all.

Is this right around where the vacuum should start to advance the timing? Does the loss of power mean that it is not advancing the timing at all, is it advancing it too much, or is it impossible to tell? I have a timing light but will not be able to look at it until the weekend, so any insight is helpful.

 

The timing starts advancing at idle, and reaches full advance by around 3000 RPM.

Probably not the problem.

That type of behavior is fuel related.

If the air valve is now back where it belongs (7/8 turn is about right for a 305) and it still does it, then it's most likely a fuel delivery issue.

Remember, the engine is busy DRAINING the carb, in proportion to (a) the amount of power it's making at the moment, and (b) the RPM it's running at. That is to say, at any given RPM it's consuming about twice as much fuel, or maybe a little more than that, at full power than at no-load; and if load stays equal, then if you double the RPM, you double the fuel burn rate. The fuel system has to be able to flow enough fuel to replenish the carb fuel bowl at an adequate rate to keep it full no matter how much the engine is using. If it can't, the motor acts just like it's running out of gas ... because that's EXACTLY what's happening.

Very easy to see if that's the issue: drive the car, and in a place where you can pull over and stop safely, force it to exhibit the problem. Once it gets to acting up pretty good, shut off the key (to "off", NOT to "lock"!!!) and take it out of gear at the same time, WITHOUT letting the motor idle or using it as a brake; pull over; and pop the top off the carb, and look in and see how much fuel is in the bowl. If it's empty, then you found the problem. You can work on the carb from now til Doomsday and NEVER accomplish ANYTHING as long as the fuel system can't keep it full of fuel.

 

Do the above test and if it's starving, first check the fuel filter. I've seen the little spring loaded check ***** in some filters cause enough restriction to limit fuel flow. Remove the check ball assembly and try again.

You could next blow out the fuel lines with air, replace the rubber pieces to the fuel pump and visually inspect the metal lines back to the tank for crimps or damage. If none of that fixes the issue (assuming it's fuel), replace the fuel pump with a new one. The Carter 6626 is an excellent choice and around $15 at Rock Auto.

 

The reason I was hesitant to blame the fuel system is high-rpm performance was fine with my old carb on the car. Replaced it with this one mainly to fix the choke and a funky idle. Even has the old filter, as it was pretty new. Would a new carb really suck this much more fuel than the old one? Or could a float setting be to blame.

The only way to check if theres fuel in the bowl is to take the air horn off right? Kinda hesitant to do this on the side of the road somewhere and chance losing a bolt or something, I have only taken a quadrajet apart a couple times so I'm by no means an expert.

 

Tinker with the air valve tension first. Try tightening it up some. See if it makes a difference?

 

Make sure your oil pressure switch is connected because the same happened on y 305 and the choke will stay on and allow it to idle really high without turning off because it cannot read the oil pressure. The oil pressure switch is located on the driver side next to the distributed. Mine came loose and was giving me hell. Or you might want to replace the little switch as well

 

Oil pressure switch would control the in-tank pump, which some 86 LG4s and all L69s should have had. A failing in-tank pump can cause fuel starvation.

Your 85 probably doesn't have an in-tank pump, although you can check for the oil pressure switch, which has a two wire connection to the brass fitting connected to the port at the china wall by the dist. The two wire fitting will be IN ADDITION to the single wire connection for the oil pressure guage. Since an in-tank pump could have been installed after the factory, there's no certainty in how it could have been wired or where the switch may be. Again this is unlikely to be your issue if it ran fine the day before the new carb was installed.

The OPS does not control power to the choke. The choke is powered directly from the alternator circuit so that it only gets voltage when the alternator is spinning (which means the engine must be running).

 

I've only heard of it once, but maybe you're the second.

With the engine off, open the secondary air valve by hand. Do the hanger and metering rods come up as the air valve opens? If not, then there's something wrong with the plastic cam that is on the air valve shaft. Since your previous carb didn't have that problem, your rebuilt carb could be the culprit.

Like I said, rare, but nothing else seems to be the problem.

 

five7kid, my hanger and rods come up properly, but that seems along the line of the problems I have - when I step on it, it feels like I have no power from the secondaries and the primaries are just giving me a little bit of power.

Any more insight into what could be causing a fuel shortage? I never checked the float setting before putting it on so could that be the issue? I'm hesitant to blame anything outside the carb or the fuel filter, as they remained unchanged between the old carb and this one.

 

I rebuilt a carb once and used a different base plate and experienced fuel starvation problems with it. Never figured out why, but I bushed the original plate, installed it and it ran fine.

And I've had the secondary cam fail like above.