This may be a long story...

Hopefully someone can point me in the right direction. 1976 L 48 completely stock setup w/ turbo 350 trans At this point only thing bubbafied was the radio so I'm guessing no bubba issue here. When I would accelerate WOT I would get a major bog as I approached the 2nd gear shift. If I decelerated a bit it would recover from this bog. As I continued to accelerate it would then surge. Checked all vacuum connections. A friend had a timing light so we checked the timing. Was advanced to approx 12 degrees. Reset to fact specs. Now the problem occurs at lower speed on acceleration. I am thinking at this point it may be the distributor or carb. I read some information by Lars who stated most carburetor problems are usually ignition problems. Therefore I thought I toss this problem out there for you savvy wolves before doing the carb rebuild thing. Thanks in advance.

 

Waiting for Lars, but it sounds to me like a carb issue.

 

What carb is currently on the car? Sounds like it's fuel related. I recently had what sounds like the same experience on a different vehicle with a Q-Jet. What I found was that the fuel pump had gone bad and was only putting out about 2-3psi, so when the secondaries went to open, it didn't have enough fuel and went lean. If you have a Q-jet, make sure that the plastic "cam" on the secondary needles isn't broken and not letting the needles pull up when the secondary flaps open.
If it's a holley, check the fuel psi, and rear float level.

 

PS-A surge is also a sign of a lean condition.

 

Has new fuel pump as I was trying to eliminate the easy stuff (easy hell..ever do your fuel pump lately?) Putting enough miles on her so tank fuel is ok. 1 to 2 tanks a month. Lean condition sounds like an angle. The Q-jet has an adjustment on passenger side for lean/rich. How much adjustment do you do to it? ANd I'll check the plastic cam. Not familiar with it but will look.

 

if it still has the original cat converter, that would be bubba.
(they clog),
if not,
fuel supply problem

 

Sounds like a fuel filter issue to me. The Q-jet filter is pretty poor in the sense that is starves the engine at higher rpms.

You have issues at lower speeds now though....so maybe some dirt got in the lines when you changed the pump?

How old is your filter?

Just throwing out some thoughts, (since this is an easy thing to check....)

 

Does your vehicle still downshift with authority when you depress the gas pedal?

 

Change the filter and go from there.

 

Not really

 

Ha! u dont even know what a cat is
better take it in for a cat scan

 

Before you do anything else take a flashlight and look in the tank at the sock filter over the pickup. Over time they tend to clog and especially if you get a bad batch of gas. This causes a restriction in the fuel system which will become progressively worse. I got a bad batch of gas in my 74 and the sock clogged so bad I couldn't take the engine over 1500 rpm without it stalling. If the sock is clogged or if you see a lot of junk in the tank you can change it by removing the gas door and its assembly and then the tank neck. You will need a new gasket when you reinstall the neck so have that ahead of time. The sock is removed by carefully twisting it off and installing a new one over the pickup pipe. Junk in the tank can be vacuumed out by making a vacuum device. It works on the same principle as a brake bleeder and hand held pump. Use a hand held pump attached to a plastic bottle by a vinyl line with another vinyl line attached to a piece of aluminum or steel tubing. You can do this by drilling the correct side holes in the bottle cap and inserting the tubing through them. When you pump the bottle you can insert the tubing into the tank and vacuum the debris out of the bottom. I removed about a half gallon of sludge this way out of my tank. Once the tank is clean you can reinstall the gas neck and gas door. Good luck.

 

I asked because on my 76 with th-400 there is a downshift switch which lays next to the gas pedal rod up under the dash. As you depress the pedal it advances a short rod on the switch. At a certain point of travel it sends an electrical impulse to a downshift solenoid in the transmission causing the accelleration demanded. part# GM 1242101 switch interupter commuateur. Price through dealer was $27.00 labor was pita (need to take out seat to access).

 

Check your timing. 90% of carb problems are timing related. I am convinced that this is probably an ignition issue since you changed the timing and the problem changed.

Basically, the factory timing settings are for emissions quality at idle, not performance. Most small blocks like 36 degrees of mechanical advance, all in by 2500-2800 rpm. The factory has a lazy curve that does not peak till 4500 rpm. Also, vacuum advance should have about 16 degrees of avance for a total of 52 degrees under light load.

Now, by setting the car back to factory specs, you will not improve performance.

For example, I run about 16 degrees BTDC at idle. But I don't drive at idle so that doesn't worry me. I time my engine for total mechanical advance (36 degrees at 2800 rpm).

Now as for the carb. You can try changing the fuel filter but thyat is as far as I would go. There is no lean/rich adjustment for the secondarys on a Q-Jet other than changing the rods. The only thing that people do is tighten up the secondary air valve spring. That just prevents air flow into the secondaries, reducing performance.

Here is a good paper on timing:
http://www.corvettefaq.com/c3/HowToSetTiming2.doc

Info on Q-Jet secondaries;
http://forums.corvetteforum.com/show...051&highlight=

 

Rayluka -
A few comments on your issue:

First, there is no mixture adjustment on the passenger side on a Q-Jet. The "Rich-Lean" is a choke adjustment that sets the duration of the choke application. It has nothing to do with carb mixture. You don't have a choke problem.

Next, you need to set your timing correctly. Check out my papers referenced by SteveG75. Setting your timing to 12 degrees initial means nothing - you need to set up for 36 total and verify that your advance is working correctly. If you want the latest copies of the papers, drop me an e-mail request for the Timing Papers:
V8FastCars@msn.com

If you still have a problem, you have one of the following issues:

1. Restriction in suction side of the fuel system. Check the "sock" in the tank and make sure it's not plugged. Then, make sure none of your rubber hose connections between the tank and the frame lines and between the frame line and the fuel pump are kinked. This is very common.
   
2. Bad fuel pump. "T" in a fuel pressure guage with a long fuel line into your fuel line to the carb and tape the pressure gauge to the outside of your windshield so you can see it while driving. If pressure drops below 3 psi when the problemn occurs, you have a fuel pump issue.
   
3. Plugged fuel filter. Replace your in-carb fuel filter with a new NAPA part number 23052. Do this first before you do anything else.
   
4. Multiple fuel filters installed. A lot of Bubbas install a fuel filter in the suction side of the fuel system (back by the tank or just before the pump). This will cause the problem you describe. Make sure you have no in-line fuel filter installed in addition to the stock in-carb filter.
   
5. Low float level. Float level on your carb should be .420". Many people lower the level to try to get better economy. A low float will cause your problem.
   
6. Loose secondary airvalve spring windup. If Bubba has loosened the secondary airvalve spring windup, the secondary airvalve will open prematurely when you go into the secondaries and the car will fall on its face from the resulting lean condition. Make sure the spring is set at 3/4 turn windup.
   
7. Ruptured or disconnected choke pulloff. Bubba, not understanding the Q-Jet secondary airvalve operation, will often remove the secondary airvalve rod. This will cause the airvalve to open prematurely, and will produce a bog going into the secondaries. Also, if the choke pulloff of leaking or inoperative it will cause the same problem. Make sure the pulloff is working and that it is connected to the secondary airvalve lever.
   
8. Lean or rich secondary rods. Verify the secondary rods are correct for you carb number.

Any one of these issues, or any of these issues in combination, will cause your problem. Have fun with the troubleshooting.
Lars

 

Huge PITA and ran like a mule until I changed it......lug lug sputter sputter.

New switch and install ,,,,ran great!
Just a thought.

 

OP has a TH350. No kickdown switch like a TH400.