I *finally* finished this project. As you may or may not know, I had some tuning problems with my Holley 850, mainly eye-burning at idle and poor off-idle transition. After some seat-of-the-pants tuning proved to be fruitless, I decided to get serious and throw some money at it. I bought some tunable carb parts: 2 Proform metering blocks, main body and throttle plate. To make a long story short, I have all this Proform stuff collecting dust on my workbench. It doesn’t work as good as the Holley components in my situation. If any of you want the details on that, I’ll give you the whole sordid tale via IM or email. My next purchase in the tuning quest was an O2 sensor and bung. I got a plain-jane 1-wire at O’Reilly. I had them installed at a muffler shop and used my digital multimeter to read the sensor output. There are several places on the web that map out the sensor voltage and the corresponding a/f ratio. Home-free now, right? WRONG! I think I took this 850 off my engine and disassembled it at least 100 times. Looking back, I should have gotten a quick-change kit for jetting. I finally ended up with 78s front and rear (80 is stock) and a 8.5 power valve on the primary side. The only other change I made was moving the accelerator pump cam up to the higher slot. At idle, my multimeter shows 920-940 millivolts. I could probably get it down to around 900 with another wire in the idle passage (have 2 in it now) but I think that might make it run a tad too lean on the main. At least it doesn’t burn the eyes anymore. When the main circuit kicks in, the sensor voltage drops to 890-910. This is just right for this engine as at 850-860 mV it tends to start misfiring. I don’t think it gets better gas mileage but it sure is smoother. When I fire it up cold, I have to throttle it up for about 5-10 seconds, then it will idle on its own. It’s never done that before. It hot starts easier now also. All in all, easier to live with (if a carb can be easy to live with). This will get me by until I get injected next year or the year after.

Brett

 

Thanks for sharing this... I am going to have to head down this path soon and your update will really help.

 

Brett: It would be very interesting to check your car with a wideband. I've been curious what kind of air/fuel ratios a carbed BB runs, especially with some cam in it. Mine has a rather smallish cam (Hotrod427 doesn't even consider it a cam... his only comment was "weenie"), but it still likes to run rather rich at low rpm cruise. Somewhere in the 13.8-14.0 range, no leaner. Above 2000 rpm I can go towards 14.7:1.

If you're planning on EFI you probably do want to pick up a wideband as it makes tuning so much easier. If you're handy with a soldering iron you can get the DIY tech edge 2.0. Including a display and sensor (i.e. complete) I have $250 in mine. http://www.wbo2.com

 

I puirchased the Innovate LM-1 WB kit and have used it on my 2002 C5; actually tuning the FAR via Predator programer.

I intend to also connect it to my wifes 1973 L82. ALthough the car runs fine, I am curious of how close to optimum it is.

http://www.innovatemotorsports.com

 

If I were to use a heated sensor, could I shove one into the sidepipe of my car and get decent readings? Or would I have to weld a bung into the collector?

 

Innovate has a special clip that can be used to hold it in the end of the exhaust pipe, sidepipe or wherever. The rear of the sensor has to be in "normal" fresh air...

 

You never mentioned holes in the throttle plates. It is a good way to lean out the idle mixture by keeping the transfer slots closed.
People think the jets are the only thing that affects the fuel mixture at cruising speed. The transfer slots and the idle circuit continue to dump fuel into the mixture all the time.

 

Also running restriction wires in the idle circuts helps to lean them out. Changing jets will do nothing to change your idle air/fuel ratios.

 

Norval - Thanks for reminding me. I did have to drill a 1/16" hole in EACH throttle plate.

Fevre - If you re-read my original post you will see that I have 2 wires in each of the 4 idle circuits. I didn't want to go with three because it would probably lean out the mixture when the main kicks in. However, the idle well gets it's fuel thru the main jets, so jet changes will change idle mixture. But yes, wires in the idle circuit are a BIG tuning aid. Too bad I couldn't use the Proform metering blocks with the adjustable idle feed restriction...

Brett

 

:blush: Sorry, trying to multi task at work.

 

A heated narrow band sensor will not work in the sidepipes. The heater is only supplementing on those. It gets it up to working temp faster, and keeps it working in a low heat situation such as idle. It still needs plenty of heat from the exhaust system to work, though.

A wideband sensor has a much beefier heater and will work at any distance from the engine. They even work in room temp. However, you can't shove one into the tailpipe just like that. One end of it has to be in outside air. I've seen a setup where a bung is welded into a piece of pipe and then this pipe is shoved over the tailpipe making it easy to move between vehicles. Only drawback to this is that there will be a delay in the readings.

 

Great post Brett. I think you should get some kind of inovators award for the A/F work. And really once a carb is set up right how often are you goning to need/want to adjust it. Just nice to check the mixture when suspect problems. Nice to hear how some can tune thier carb for pocket change (<$100). cardo0