Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Marx3 on July 29, 2016, 12:42:55 PM
-
stock 170 hp 8.2:1CR chevy 350.
Qjet 17059276
All airbleeds .070
71 jets, 44 rods, tip-in perfect.
.036 tubes and .050 DCR.
acc.discharge .027
Car ran perfectly. Perfect response, no hesitation anywhere.
Then I put on better iron heads ( should be hitting about 280-300 hp at the crank ), a 218/224@050 flat tappet cam, an edelbrock dual plane and built a 2.5" dual exhaust.
CR is 10:1
Car still had great response everywhere.
Timing was mid to high 30´s ( cant remember exact number ), so I backed it down a bit because it would ping from the the higher CR.
Backed it down about 5 degrees to low 30's.
The timing is now 15 initial, 32 total plus 14 degrees vacuum.
Now that I have backed the timing down about 5 degrees, the engine has developed an annoying off-idle stumble.
I dont understand ?
I have been runnign proted vacuum and tried full, just to see if it would help. It didn't.
Figured the engine might needed a bigger shot from the acc. pump so I upped the discharge holes from .027 to .031. Didn't change a thing.
I can always put on another airhorn, if I need the discharge to get smaller.
-
Can you shorten your advance curve so you can put your initial back up?
-
Yeah I guess I could easily put in the stock advance center piece and weights, just to try if this would solve the problem. That should put my curve down around 10-12 degrees again.
I prefer it up around 15-18 degress though.
I just dont get why I get a noticable hesitation, just because I back the timing down from 20 to 15 initial.
-
The accl pump will only effect off idle or transition if you are getting into the throttle quickly, or going to heavy/full throttle.
A hesitation caused by reducing the timing tells us in most cases that the mixture is pretty lean and requires a lot of timing to burn it effectively.
You can at this point add timing back in or fuel, or a little of both to correct the issue.
I'd start by raising the APT screw, to richen it up some in that range.....Cliff
-
re-checked timing today. It turned out that was a bit more retarded, than I thought.
It was at about 5 BTDC initial.
Got it up to around 15, like it should. That helped a bit.
Then I re-checked pirmary calibration and it WAS a bit lean... ( did both the tip-in and put in a wideband too )
I was at about 16:1 at cruise.
backed the APT out a bout 1 full turn. Tip-in was better and the wideband now said 14.5-14.8:1
Ideal. Car feels a bit stronger and has much less of a stumble / hesitation.
The hesitation is noticable on of-idle take off.
The stumble is not really there, once the car is moving.
I have a feeling that the 214/224@050 cam ( 112 lsa ) is struggling a bit with the 2.41 rear end and the stock converter?
Do you think .031 discharge could be causing this? I know .031 is propably a couple of sizes larger than needed, but do you think it will cause a stumble?
-
found an air horn with .026 discharge holes and installed it. This seemed to help some. The off-idle hsesitation is still there. I tried different settings for the APT... turning it down for a leaner jetting didn't really hange the hesitation. Raising the APT for a richer jetting didn't either.
I backed up the APT adjustment with a wideband ( mostly because I wanted to see if I could log the data and maybe spot a lean or rich situation when accelarating ). at 17:1 car runs fine but hesitates, at 14.8-15:1 the car runs a bit stronger, bit still hesitates. At 14-14.2:1 the car runs fine, but still hesitates.
What the heck?
What do you think about a 214/224@050 cam against a 2.41 gearing? potential trouble?
-
If the hesitation is right OFF idle, can you monitor what the A/F is doing at that moment? Might be a longshot, but maybe you can tell if it's a lean or rich problem.
Maybe the secondary air flaps are coming in too quick???
-
Open up the DCR a few thou.
-
I haven't been able to log my runs, but it seems that the AFR takes a quick dive from 15:1 to around 12:1, when touching the accelerator. The secondaries are adjusted real well. Loosened them to the point of bogging and backed up a little. They are at about 3/4 of a turn. (3800 lbs car with 2.41 rear )
The engine performs excellent in every area, except for this off-idle hesitation.
I don't get it... Discharge is at .025-.026... Not exactly huge...
Why open up the DCR?
I am completely open to trying anything, but why?
:-)
-
Thought you had a lean hesitation, but I doesn't look to be.
Is the accel. pump giving a shot as soon as you touch the throttle?
-
It feels exactly like a lean stumble, but it appears it aint :-) Yes, the pump shoots simultaneous with the throttle moving. Nothing wrong anywere it seems.
-
Have you tried taking the vacuum advance out of the equation? Just unplugging it just to see what happens.
-
Yup, I ran ported before and it makes no difference if it I shooed up to full or ported
-
I've ran into this problem a couple of times with "low" compression engines and bigger than stock cams. You get a very momentary hesitation from going rich right off idle.
Haven't seen this with higher compression. You may simply need to limit fuel to the mixture screws and transfer slots slightly.
I'd also hook up a vacuum gauge and see where the vacuum is at the same moment you get the "stumble". The power piston may be coming up if the spring under it is too strong.....Cliff
-
Hmm I haven't given the pp spring any thought. I'll try and mess with it.
I'll report back.
-
Decided to grab the bull by the horns today and get this problem fixed.
It turned out that I did indeed need a softer PP spring.
drove around with a vacum gauge hooked up and it read 15-16 hg doing 50. ( on my cheap vacuum gauge anyways )
I figured my stock spring might be too stiff, causing the PP to be in the up position most of the time.
Anyways, I did some measuring and found a green spring to seem to push the PP up right below 14-15 hg. I installed it an tweaked the APT. Bye-bye off-idle hesitation.
Oh yeah, the car also got back it's ability to do a burnout :-)
It most certainly has been an educational day. Funny how you can read up on these things over and over again, but the troubleshooting dont seem to sink in as well, untill you actually deal with this IRL and fix it.
-
Good news!
-
dang... decided to try a stiffer spring, since I felt the engine was not as crisp as it could be...
At the same time, I went from 70 to 71 jets, because I felt the 70's might have been on the smaller side...
Problem came RIGHT back.
Obviously I need to put back in the 70 jets and find the right spring again, but something puzzles me, that I wanted to run by you guys...
As of now ( with the 71 jets and 44 full tapered rods ) I get a nice tip-in result, with the APT raised about 3 turns. ( 50-80 rpm increase ). At that point the off-idle hesitation is quite noticeable. The car runs strong and you hardly notice when the pp-spring raises the pp and richen things up.
If I lower the APT ( for a higer rpm increase ), the off-idle response becomes less noticeable, but the car feels like it looses some torque and you can clearly feel the point where the PP jumps up ( torque comes back in )
I am starring myself blind on this situation...
What do you think of this?
-
70 jets installed, 150 rpm increase at tip-in: hesitation is gone again. :-)
Phew, that was educational :-)
-
although... at very light throttle, the engine tends to surge a bit, like it lacks power. Once I go into deliberate part throttle ( PP comes up ) the engine comes to life.
I mean, I can literally feel the engine needing fuel at light throttle, but if I raise the APT to bring in a bit more fuel, I get the hesitation RIGHT back...
What the heck does this tell me?
Idle system aint that rich, I think... 15hg at 800 rpm... Idle quality seems alright?
I have been testing a couple other Qjets, the last couple of days, and they all tended to have this light hesitation right off idle, Even though they have had slightly different calibration than the Qjet that causes this problem.
Could this be a simple case of too much cam and not enough converter and gearing?
I have 214/224@050 cam with stock converter and 2.41 gears.
-
Could try opening the DCR a few thou.
-
So your are pointing towards a regula lean spot.
I can up the DCR a bit, no problem.
I just don see how this can be a lean spot, when rain sing the apt makes it worse and having .030 discharge holes didn't change a thing...
I'm not arguing, I'm just speaking my mind, you know :-)
I guess I gotta try going a bit in each direction, on the idle system