'81 Corvette, the only mods are removal of the air pump system, EGR valve and heat riser. Oh, also removed the catalytic converter and now have a true dual system with an X pipe where the cat used to be.

The problem that I'm having with the car is poor acceleration in the low end. Seems to be a minor problem when cold but a bigger issue when the engine is warmed up. Last night I did a ten mile loop, and when I pulled of the highway to get gas, a huge amount of vacuum went through the cap as I loosened it. Another two miles down the road, stopped at a light, and was barely able to pull away when it turned green. I'm thinking that the high vacuum in the tank is starving the carburetor of fuel.

Today I spent about a half day checking the vapor recovery system. My gas cap holds vacuum, and breaks it at something low, like 4 or 5 psi. I tested the vapor line (tank to carbon canister) and it's OK. The other lines, air can flow in both directions. I believe the issue is a bad check valve or valves in the carbon canister, however there is not a lot of information in the shop manual. There are five hoses that come out of the canister:
1. Purge to a ported vacuum on the carburetor, through a thermal switch.
2. Tees into the PCV hose.
3. Float bowl.
4. Air cleaner, inside the filter.
5. Fuel tank.

I plugged 1 and 2, seeing these as high levels of vacuum during operation, especially #2. I've set up other old cars this way with #4 acting as a purge, low vacuum except under hard acceleration. I drove the same loop earlier tonight and most of the problem, not all, went away.

I'm having poor acceleration in the low end. It's OK if I nurse the gas pedal like a Prius owner, but anything above 1/4 throttle and I get terrible stumbling in the off-idle to 2000 RPM range. Cruising at 50 and stomping on it is no problem.

So now I'm looking at my ignition timing. I have my initial timing set at 6 degrees BTDC. When I connect the ECM the idle speeds up, advances a lot, looks to be about 25 top 30 degrees. I have not determined what it's doing with throttle changes yet, since I found this issue as shown in the video.

This short video is taken with the four pin connector to the ECM disconnected. My apologies for not being able to control the auto focus. The distributor, plugs, wires are all new. I just replaced the ignition module last week since the one that came with the dizzy crapped out on me and left me stranded about 15 miles from home.

On the video, I get a nice steady timing strobe flash at exactly 6 degrees BTDC most of the time, but every second or so, no consistency, one errant spark advanced about 20 degrees. The amount of this "advance jump" is consistent.

Any help, as always, will be appreciated.

 

 

Just listening, I think it's your light. If it was jumping around, it would sound more erratic.Sounds very smooth.
When the smog stuff is removed on an 81 while still using the ECM, the car "thinks" that stuff is still there, and adjusts accordingly.
Advance it to 8* at idle, plug it in.
You can also unplug the carb and drive it default. It should still operate fairly well.
The carb defaults to a normal QJ, and the default timing curve is not that horrible.

There are also other things like bad fuel sock or fuel pump, your tank venting all contribute.

 

You're right it was the light. New timing light, inductive trigger is very sensitive. Has to be isolated from the other wires.

Without the ECM the dizzy does not advance at all, and the carb AF ratio between idle and WOT defaults to rich.

I'll check the tank sock next, thanks for your input.

 

If you unplug the mc solenoid the carb will run rich. Without the solenoid controlling the lean limit the spring under the paddle that pushes down on the metering rods will push the paddle up and against the lean limit and allow the rods to lift up and out of the "jets". That being said, most people don't even realize when the mc solenoid fails since these carbs were tuned very lean from the factory.

 

OK I've removed the sock and cleaned it- it wasn't very dirty, certainly not clogged. There's no gunk in the tank either. Put it all back together and it ran the same.

Next I tuned the carburetor. Set the throttle position sensor to 0.56 volts at idle.


Next I adjusted the idle mixture screws to 3-1/3 turns, then set the idle bleed valve to 30 degrees dwell.



I'm still having the same problem. Again, idle is fine, cruises OK, the problem is transition from off idle is not smooth at all Feels like it is missing one cylinder.

I double-checked the timing, set initial at 8 degrees BTDC, the computer increases it what seems to be a reasonable amount, and I get spark at all eight cylinders.

Maybe I have a dead cylinder, no compression? The car gets up to 95 mph on the highway though. Probably more, but you know how that goes. I guess I need to test that next unless anyone has a better idea.

 

Any way to change the thread title? Should read "poor throttle response".

 

For those who like video:

 

Can you drive the car with the gas cap not fully closed to eliminate a tank venting problem?

 

time to eliminate the rest of the smog stuff. carb, distributor and ecm need to join the cat and egr and smog pump. that ccc system sucked in 81. it has had 40 years to get worse. it used that time well...

 

Does the accelerator pump squirt?

 

I'll try this when it stops raining. Thanks for the idea.

 

I removed all the smog stuff including EGR. I have the 'puter working and talking to the O2 sensor, carb, ditz, and hopefully the transmission. Check engine light is doing it's thing, giving me a 12 code.

 

Absolutely.

 

Agree 100%! As I have said in the past the best things my friend did to his 81 was to ditch the carb and distributor . Installed an edelbrock intake and carb and an msd street fire distributor. World of difference and easy tuning. Later added a cam and sidepipes!

 

agree, much easier

 

I had a similar problem with my 81 and found that the temp sensor wire was broken. This sensor is located in the intake manifold by the thermostat with black and yellow wires and connected to the computer. When I repaired the broken wire the engine ran perfect. Also this is different than the engine temp gauge.

 

That is on my list to check, thanks.

 

Wiring is OK. There was a small break in the insulation and I fixed that with liquid tape.

The shop manual says that theresitance should be about 100,000 ohms when cold, 1000 or less when warm. Mine is 3300 ohms at 70F and about 200 when 160F.

Next I'll test the vacuum sender.

 

MAP sensor is 5.1 volts at 29" Hg, 0.6 at 0. The reference chart says 4.9 and 0.3; close enough.