So here's the basic story. I've had an 88 GTU for about 6 years now. About 4 months after I bought it I had to get a new engine (I went ahead and opted for stage 2 porting). It has always run kinda rough, and has had a popping exhaust when cold (only when cold). Since then, I have replaced just about everything, and tuned it so that the idle (when warm) is quite nice (total cost about $2000 to do all this). Well, a little while ago I noticed that they (my mechanic that put in the engine) forgot to put in the auxiliary port valves, meaning that it's always running lean. It's actually not that big of a deal (the only drawback is a very slight lag at 3800 rpms).
Now, it was suggested to me that the popping and light shuddering when my car is cold might not be afterburn, as that would suggest I'm running rich. I was told about lean popping, or a popping exhaust as a result of running too lean.
Can anyone tell me what exactly this is? Is there any way to compensate?
It's really a pity, because other than during warmup, the car finally runs great.

 

Auxiliary port sleeves shouldn't affect your A/F mixture. If it's popping while it's could, i would adjust the idle air mixture control according to what the FSM says.

 

All the aux sleeves are going to do for you is slow down port velocity.

I know wha tyou are thinking: No Aux sleeves means more air can get into the engine and hence lean it out. The thing is that the air is being metered, so the ECU sees any extra air coming in an compensates for it. That being said the pri/sec ports should be more than capable of flowing all the air that the engine needs that low, so having no sleeves isnt going to increase the amount of air the engine sees at low rpm. Its just going to make it so that the air comes into the secodary runners at a lower velocity, hurting off the line performance.


BC

 

I see, thanks.
Speaking of the air to fuel ratio, I know the adjust screw, but everything I've read on how to adjust it properly requires a "checker light." Is there another way, or a way to build a cheap checker light?

 

search button is your friend. Im sure you could use a DMM instead.

 

uh you need to make sure you have your airpump and ACV all in good working order. The ACV contains an anti-afterburn valve which injects fresh air into the exhaust port.

removing auxilary port sleeves drastically affects your torque curve vs a completely stock setup but it doesn't result in a popping sound