Corellian Corvette

Got a new Edelbrock #1901 Carb a few months ago, and the car has been running great. Solved a bunch of problems. I bolted the carb right on and it worked like a champ.

However, it seemes to me that the exhaust smells a little "gassy", so I thought about adjusting my fuel mixture screws since I put the carb on out of the box. I've read all the topics on this in the past, and I get the gist of it, so here goes.

Right now the idle is perfectly set at 800 RPM's in Park, drops to 600 in drive. Idle is steady and I'm pulling 16.5 - 17 Hg of vaccum in Park. If I turn the idle mixture screws in 1 full turn, the idle gets a bit rough. If I turn the mixture screws in 1/2 turn, the idle DROPS, but does not get rough. Since the idle drops, the vacuum drops, but again, it does not idle rough or seem to stumble. It's more as if turned out the idle stop screw and just dropped the idle (also note, if I turn the screw out, the idle goes up).

At this point, it seemes to me that I'm probably running a bit rich, since the way I understand it is that you want to be out 1/4 turn from rough idle.

So, what I did, to be safe, was turn each mixture screw in 1/4 turn, then adjust the idle back up to 800 rpm's where again I'm pulling 16.5-17 Hg of vacuum. I could PROBABLY go in another 1/4 turn and adjust the idle again. But it's hard to determine by ear the difference between "low" idle and "rough" idle, since these cars don't like to idle low anyway.

So, how does that sound? Am I about right? Is it better to run lean than rich? How much variance is there to know when the mixture is just right? I don't have access to an air/fuel meter, just a vacuum guage.

Thanks!

All this was done with the air filter on the carb.

gearheadz

Sounds like you're close with 17Hg vacuum. A simple way to optimize idle mixture is to use your vacuum gauge to adjust the idle mixture screws equally in 1/4 turn increments until you achieve the highest engine vacuum reading (which will also yield the highest idle speed). When you feel you have the highest vacuum reading possible, readjust idle to the desired speed. It may take you a few rounds of this to get it just right.

Again when adjusting the mixture, make sure you adjust each primary/secondary circuit screw the same number (or increment) of turns. I believe a slightly lean A/F ratio is actually better for HP but a slightly rich A/F ratio prevents detonation and is easier on the plugs and valves.

Mark

Corellian Corvette

Wow -- that means I can probably back the mixture screws OUT about 1 1/4 turns. From the point I was at, I turned them out about 1/2 turn and the idle kept increasing. Is there a point where, when turning them out, the idle will stop going up?

Corellian Corvette

OK - I tried this -- tell me if it's better.

1. Turned both mixture screws in until they stopped and backed them out 3 full turns
2. Took a Vacuum reading - Steady at 17 hg. Turned the screws in each 1/2 turn until I saw the vacuum reading drop, then backed them up 1/4 turn until it got back to 17.

Adjusted the idle back. Seems to be idling quite nice (as it was before), but I still smell gas strongly out the exhaust. Carb and base are clean. Normal?

GaryS

Mark set you right on this. I always tune the carbs for the highest vacuum. Besides, just to needle you a bit, your pre-adjustment description sounds like what most of us would want! ;) Remember, there are two axioms in the Corvette world, "if it ain't broke, don't fix it" and it corollary, "if it ain't broke, mess with it until it is".

Gary

Alwyn678

I have the sam carb and mine is adjusted properly however I still have that gassy smell as well......Maybe it is the carb itself....It tuns FANTASTIC and Gas mileage around town is around 15 which I think is pretty good

Corellian Corvette

I have to say that buying a new carb was the single best thing I did for the car. Just remove/replace and fixed a TON of nagging little issues.

So drove in today after my adjustment last night. I set the mixture for highest vacuum, then tuned down the idle speed.

Now, at a light I'm turning just over 600/625 RPM's in drive and it is AMAZINGLY smooth. It was eerie. Other than the exhaust rumble, there was no shudder, no stumbling, no feedback up the steering column. Honestly, my C5 has more shake at a stoplight than my Stingray did this morning! It's almost as smooth as my girlfirends 2001 Sentra.

Hopefully that's good -- and not the calm before the storm


[Modified by Corellian Corvette, 1:01 PM 10/5/2001]