1981 Low end doggishness

[AzusaMan]

Hi all, I have a 1981 Vette automatic. Runs pretty good in general, but lacks punch at the low end. I don't have another 81 to compare against, but IMO doesn't have the oomph a muscle car should have . I other words, it doesn't burn rubber - but will chirp going into second....

Back story : I had a GM 350 replacement motor installed a couple years ago (the 290 HP variety). Should have a lot more oomph than the stock motor. After the installation, the mechanic said he "disconnected the computer". He also removed the plugs covering the mix screws and adjusted those by hand.

Honestly I don't know what he did with the ECM, it does pass Calif smog, and I assume it would not have passed if the ECM was not connected. There is one green plug that is disconnected on the passenger side of the carb, I have never been able to figure out where that should be plugged...

I would like to get your opinions on how to proceed to unleashing some decent low end....

Thanks in advance
Steve

 

[Peer81]

Sound to me like the ECM is still working. The green connector is there to connect a dwell meter to, to see if the carb needs adjustment or not.

I think you first need to know what you have right now. Run it on a dyno and see what you have. There are a number of things that could cause this problem so first try to locate where the problem is.

Greetings Peter

 

[spikebot 81]

There are alot of people here that know way more than me but I'll try to give my 2 cents worth. Keep in mind that with independent rear suspension these cars are made for rear wheel traction. If your rear end is still stock it has a 2:87 rear gear. Swaping to a 3:55 gear will launch your car to the next county. The exhaust is very restrictive. Stock cat converter and the 2 into one pipe choke the engine. Because you live in a smog nazi state not sure what mod for the exhaust you can install but at the least install a high flow cat converter.

As far as the engine, the ecm controls spark and air fuel ratio. If the system is unpluged or has sensor failures the ecm goes into limp home mode and runs very rich. Stock timing for 81 is 6 deg btdc. Before I removed the ecm system on my car I Advanced the timing to about 12 to 16 deg. That helped a bunch for power across the board. To do this just get the engine to operating temp, shut the engine off and unplug the 4 pin connector at the distributor. Set the timing and shut the engine off and plug the connector back in.

There could be alot of other issues to keep your engine from performing like it should. For me the stock ecm, carb, distributor and exhaust where all holding the engine back. I still have the 2:87rear gear but after the mods to the timing, exhaust and carb the engine has alot more grunt .

 

[AzusaMan]

Response to suggestions

Great answers, thanks.

1) I did have a high-flow cat installed when I first got the car (to pass smog; forgot to mention that), That did make it seem better, but still no grunt. Probably still restrictive by its 2-1 pipes design.
2) The dwell meter/carb adustment is intriguing. I'll look for more info on this - if someone knows a good thread on these, pls repost
3) The timing seems like its worth a try.
4) The gearing - I've seen this suggested before - if someone's done it, please share.


Thanks all

 

[AzusaMan]

Dwell and air mix

Regarding the air mix - I checked my smog test chart and it read .5% Oxygen at 25 MPH. Does that mean anything to anyone? Does it sound too rich? (I wouldn't think so...)

Also, doesn't the ECM control the air mix (or does it kick in at higher RPMs)

Regarding the dwell meter: Can someone make a suggestion for a suitable unit? I read somewhere that we should use the 6 cylinder setting (by the way, why?), but the first unit (see below) I saw only had an 8 cyl readout. I didn't find any other units that looked promising, perhaps I'm off-target

Amazon.com: Actron CP7605 Dwell/Tachometer/Voltmeter Analyzer: Automotive


Sorry for all the simple questions and thanks.......

 

[Wajulia]

The ECM controls the fuel mixture in the primary side of the carb only. WOT performance is mostly unaffected by the ECM. With the ECM disconnected or not working properly, the car will run VERY rich (10MPG kind of rich) and the timing will stay at 6deg all the time (Slugishness!). If you want to disconnect the ECM, the carb and distributor need to be replaced to a pre 1981 models. If it is an automatic, the ECM controls the converter lockup and that will need to be addressed.

My question is -- why did he remove it? The 1981 corvette is no Hot Rod, but it is not due to the ECM, it is more due to the heads, exhaust and cat. If the ECM is in the car and operational, connect it back up and make it work, a lot cheaper than changing the carb and Dist.

Almost all of the E4ME Carbs (computer controlled Qjets) have their idle adjusters drilled out, and almost none of them needed to. It is a matter of every other Qjet idle being adjusted that way, the E4ME idle is actually adjusted by the idle air screw on the top of the carb and measured by the Dwell meter connector discussed earlier. A lot of mechanics that know Qjet carbs are unaware of how to adjust this unique beast. So yours, as so many other, thought he was bringing it back to something he understood (Idle adjustment screws with no computer). But this is the wrong approach for the 81.

How did he disconnect the ECM? If he disconnected the carb connections (there are two, one on the top of the carb on the passenger side and one on the front of the carb on the drivers side ) plug them back in and see what happens. If he Disconnected the ECM itself (in the battery compartment, two big connectors) plug them back in.

After you plug them in we can read codes from the computer to determine how the car is operating.

Feel free to contact me if you need any help.

 

[AzusaMan]

More on my ECM

Wajulia, thanks for your help. I checked with the mechanic who installed my engine and he confirmed that he hooked up the ECM and adjusted the air valve etc (we had kind of concluded this earlier in the thread)..... He did however drill out the caps on the air mix screws and tweaked them - this might have been necessary for the new engine.....

Thanks everyone who contributed to this thread (and my education), it is appreciated.

Will follow up when I make some progress on this....

Steve

 

[Wajulia]

OK, The green connector is indeed a way to measure the carb during idle -- no other time. And it should not be connected to anything normally. If you are not getting a check engine light during idle, and the idle performance is acceptable, there is no reason to measure anything at this connector. It will NOT effect the off idle performance. Have you ever gotten any check engine lights? does the check engine light come on when you start the car?

 

[AzusaMan]

Thanks Wajulia, the check engine light comes on normally when I turn the key and turns off when I start the car, so I guess the computer is happy enough with the air/fuel ratio and the emissions.

On the other hand, I would say the idle is a little bit rougher than I would like, and it seems like the air/fuel ratio is slightly off. If this was a standard non-computer carb, I would adjust the idle mixture screws. But I see that on this carb, the procedure is to
1)adjust the idle air bleed valve
2)set idle mix
3)readjust idle air bleed valve

So I would like to do that, unfortunately I have two issues:
1) I don't have a dwell meter or the idle air bleed tool;
2) I can't get at the idle mix needles with the carb on the car

Thanks for your help, and thanks in advance for your future comments and advice

 

[AzusaMan]

A little more searching and I found there is a tool K-D 2776 for adjusting the idle mix. So will order one of these along with the air bleed valve tool Thextron 362 and a dwell meter. This should get my idle right, I would hope.

Do I need any other tools?

Thanks