Engine runs bad after 30 minutes of driving

[bucket]

I'd try adding a few degrees of timing just to see if it helps or changes nothing.

 

[77 K20]

I was finally able to get a hold of JET and talk to them about the issue it is having after backing off the throttle. They feel it needs an APT adjustment. (Adjustable part throttle). After talking to them and doing some reading online I found this:

APT ADJUSTMENTS (Q-JET)
The adjustable part throttle (APT) screw can be used to fine tune the cruise (steady throttle, freeway speed) mixture. Simply turn the screw DOWN 1/2 turn at a time until you experience a "lean surge". (It will feel like someone is moving the throttle, when their not, or a hesitation on light throttle acceleration). Then turn the screw UP, until the lean surge goes away.

If you are experiencing an "off-idle stumble", you can turn the screw up at � turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble.


So I will start with backing out the APT screw 1 turn and drive it to see what happens. This should be quick and easy to do also- as it is only 16* here with wind. Don't want to spend a whole lot of time outside today.


I verified my idle mixture settings yesterday again with a vacuum gauge. I did it before adding that small restricted return fuel line, and again afterward. No changes were required to it.

 

[350runner]

i guess it does make sense. Do you experience a surge when accelerating from a stop or any other time.

Sent from the dust in front of you!

 

[Jacknife]

Hope the APT adjustment takes care of it. Alot more to really tuning a Qaudrajet than most of us relize. Got my fingeres crossed, let us know if that takes care of it. I feal you may be on the right track.

 

[77 K20]

i guess it does make sense. Do you experience a surge when accelerating from a stop or any other time.

Sent from the dust in front of you!

It wasn't from a stop, but when I would back completely out of the throttle then re-apply throttle (while driving). Then it would surge.

Before work I adjusted the APT one turn out (CCW). JET suggested 2 turns... I just wanted to try a little and see. My drive to work was only 20 minutes but I had no issues with it.

So now I'll just keep watching it and will probably take a long drive this Friday. I hope everything is good now- the whole thing was really getting to me when my commuter car was lost then a truck I've done a ton of work to started acting up.

 

[77 K20]

Just an update in case anyone ever has the same issue. The APT adjustment seems to have worked. One turn CCW wasn't enough, so I did a second turn CCW. Truck was running great when empty.
Yesterday I towed my poor totaled VW Golf 60 miles away to a friends house. It ran well on the highway and then on the drive up to his house it started acting up a bit. The road was real steep and I was going anywhere from 15-25 mph only.

So now I'm not sure what to do. Adjust the APT more? Or a mechanic friend of mine didn't like how much tension it took to open the secondaries. He says he always adjusts them so they open easier and therefor a bit earlier.

So how does one adjust for towing? If I adjust it with the APT then won't it run a bit rich when running empty? Or adjust the secondaries so they open easier. Then they should only be used if there is a need for them, right?

 

[Jacknife]

First I of all, I am glad to here that the situation is improving.
I have always felt that the best way to tune a carb was to tune it to how it performs under load. I may be doing it wrong in some peoples opinion but thats how I have had the best results so far.

 

[Jacknife]

My way may be considered a bit shade tree, but I still don't have A/F ratio meters & such tools yet.Thats just what I have found to work best so far. Someone more educated will tune in soon maybe.
I know this is the JET carb you bought for this engine. Was this carb supposed to have been biult or set up for your 383Ht application from Jet ?
Just wondering.

 

[77 K20]

My way may be considered a bit shade tree, but I still don't have A/F ratio meters & such tools yet.Thats just what I have found to work best so far. Someone more educated will tune in soon maybe.
I know this is the JET carb you bought for this engine. Was this carb supposed to have been biult or set up for your 383Ht application from Jet ?
Just wondering.

A JET Stage 2 is just kinda a generic setup for a mildly modified engine: I thought it would be a good choice. It does have excellent throttle response.


A Stage 2 has all of the features of a Stage 1 plus features fully recalibrated idle, off idle, main, accelerator pump and high speed circuits to match any modifications done to the engine such as intake manifold, headers, ignition or mild cam change. A Stage 2 features improved idle quality, instant throttle response and maximum torque and horsepower for street-strip vehicles, tow vehicles, RV's and watercraft with modified engines.
This Carburetor is intended for 305,350 & 400c.i. Chevy Passenger Car engines with mild performance upgrades such as:
Dual Plane Aluminum Intake Manifold
Headers (1 5/8" - 1 3/4")
Compression Ratio 8.5 - 9.5:1
Mild Porting on Cylinder Heads or Performance Heads
Camshaft Specifications: Duration @ .050 Min. 210 Degrees, Max. 220 Degrees
Lobe Seperation Angle 110 - 112 Degrees

 

[77 K20]

I'd try adding a few degrees of timing just to see if it helps or changes nothing.

I added 2* and didn't notice any change. I'm at 12* now.

 

[Lastwords]

It's impossible to set a carb up at the factory. They can get it close but the weight of the vehicle and it's uses will change the tuning. I highly recommend an air fuel gage. A fuel pressure gage would be helpful too. Is the intake manifold correct for vortec heads.

 

[77 K20]

It's impossible to set a carb up at the factory. They can get it close but the weight of the vehicle and it's uses will change the tuning. I highly recommend an air fuel gage. A fuel pressure gage would be helpful too. Is the intake manifold correct for vortec heads.

Yes, it is a GM performance intake manifold that is for vortec heads and a quadrajet or holley carburator.

 

[Jacknife]

It's impossible to set a carb up at the factory. They can get it close but the weight of the vehicle and it's uses will change the tuning. I highly recommend an air fuel gage. A fuel pressure gage would be helpful too. Is the intake manifold correct for vortec heads.

I agree.
My point is, That if I was buying a biult Qaudrajet for a 383 with the bottom end torque that this engine has I think the set up for the carb would be very importtant. Yes it would still need to be dail in precisely after install.
We are on the same page here, I may not word it the best thuo.

 

[77 K20]

I suppose the only way to truly do it correctly would be to just buy one of those aftermarket fuel injection units. Then it will correct for temperature, elevation and load.

I started reading up on the Holley Terminator fuel injection units the other day.

 

[MannyDantyla]

Do a "plug chop".

Get the engine all hot and bothered, then do your trick to get it to stumble. Let it stumble for 20 seconds or however long you can hold it, then turn the ignition off and pull to the side of the road. Pull some spark plugs and see what they look like. Black as coal? White as a ghost? Now you'll know your problem.

The reason you must check the plugs like this is, if you drove home when the engine was running fine then looked at the plugs, you'll get a reading from the when the engine was running fine. You don't want that, you want to get a reading from when the engine was running poorly. It's like the old reading gets erased by the current engine running conditions, and it only takes a few miles.

Now you'll know if it's a flooding issue or a lean issue.