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-   -   Thinking of an air-fuel meter to tune carb - any experience? (http://www.clubcobra.com/forums/all-cobra-talk/127932-thinking-air-fuel-meter-tune-carb-any-experience.html)

Ace23 03-27-2015 12:14 AM
Quote:
Originally Posted by lippy (Post 1306661)
A follow up on this topic. Wideband meters can be great, but they also can drive you pretty far off course if you don't use them in context. After a ton of fiddling with main jets, air bleeds, restrictors, etc, I still couldn't get this carb dialed in. As it turns out, AFR readings for engines with big cams with a lot overlap are useless and inaccurate at low load/RPM because a lot of unburnt fuel and air pass through the engine, and the Wideband meter reads lean. Because I used the meter to adjust the idle screws, my idle mixture was rich and this was affecting my tune at all conditions. Once I learned this and used the wideband only for WOT and cruise under load, and tuned idle using idle speed and vacuum, the carb tune fell into place and I think I'm very close now. To be fair, Brent and one of the tech guys at Holley warned me that the best way to tune is to read plugs, and not to use a wideband. Brent also added that this is one of the reasons he specs racing plugs, as they are easier to read with unleaded gas.
Lippy,intersting thread. Did you continue with the wideband tuning and if so did you get the results you were looking for?

zrayr 03-29-2015 08:15 PM
They are a great tool, just don't get hung up on getting to a certain number. Youu can easily find where your car runs good , and then use the gauges as a baseline to troubleshoot when you are having issues

http://i151.photobucket.com/albums/s...ps1be60c9c.jpg

I have a gauge for each bank of Weber's. If I could, I've have eight A/F gauges, an altimeter, and who knows what else. You can't have too many gauges.

http://i151.photobucket.com/albums/s...ps3a8884f3.jpg
http://i151.photobucket.com/albums/s...ps6adadae7.jpg

http://i151.photobucket.com/albums/s...ps3af166c9.jpg

Z

lippy 03-29-2015 10:01 PM
One lingering problem I had was that my plugs were fouling with fuel. It was a tough nut to crack but I finally did. Turns out that with such a big cam, you need to run almost full advance at idle. I purchased a 10 degree MSD bushing from 4secondsflat.com, set my full advance at 36, and my initial at 26, and it solved the problem. At that point I wasn't using the wideband anymore to chase this problem. I left the wideband on just for WOT and tuning about 3k RPM, and I think I got it pretty close. Plugs look good now, even when I cut them apart, and I took the wideband off.

A98Coupe 11-12-2015 05:57 AM
Quote:
Originally Posted by lippy (Post 1343905)
One lingering problem I had was that my plugs were fouling with fuel. It was a tough nut to crack but I finally did. Turns out that with such a big cam, you need to run almost full advance at idle. I purchased a 10 degree MSD bushing from 4secondsflat.com, set my full advance at 36, and my initial at 26, and it solved the problem. At that point I wasn't using the wideband anymore to chase this problem. I left the wideband on just for WOT and tuning about 3k RPM, and I think I got it pretty close. Plugs look good now, even when I cut them apart, and I took the wideband off.
Lippy,
I just picked up theis thread after doing some researching to solve similar problems to yours. I have just installed a new 468 shelby stroker with dual quads and a fairly large cam/overlap. Runs fine under load, but surges like hell below 2,500rpm, part throttle cruise and runs-on after ignition off. I am considering to get a wideband to help me tune the carb, but your point about increasing the initial timing is interesting. I'm at around 15degrees initial and 36 Total.

Did you solve the run-on problem by increasing the initial advance or something else?

Thanks
Stuart

lippy 11-12-2015 10:08 AM
Got it in the ballpark with the wideband. But you can get wrapped up in your shorts with too much data, so I took it off when I got close. Drove the car for awhile, cut some of the plugs with a hacksaw for a good read, and the only change since then is leaning out all four jets by one number. Car runs and idles well, and the plugs are clean.

Run on is a different matter. Since my car needs to idle at 1200 to oil the solid rollers, it's prone to run on simply because of the idle speed. I think Chas suggested I put the car in second, put the parking and regular brake on, and let out the clutch slowly to drag down the idle speed. At that point I cut the ignition and let the clutch out fully. Sounds scary but it isn't. Very easy and solves the problem with a racy engine.

lippy 11-12-2015 10:14 AM
Quote:
Originally Posted by A98Coupe (Post 1371147)
Lippy,
I just picked up theis thread after doing some researching to solve similar problems to yours. I have just installed a new 468 shelby stroker with dual quads and a fairly large cam/overlap. Runs fine under load, but surges like hell below 2,500rpm, part throttle cruise and runs-on after ignition off. I am considering to get a wideband to help me tune the carb, but your point about increasing the initial timing is interesting. I'm at around 15degrees initial and 36 Total.

Did you solve the run-on problem by increasing the initial advance or something else?

Thanks
Stuart
If your plugs are wet try the bigger bushing and bump your full advance a bit. Pm me if you'd like to chat.

A98Coupe 11-12-2015 03:09 PM
Hi Lippy,

Just sent you a PM

Stuart


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