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Author Topic:   Jetting an alcohol carb
BrianW
Dirt Freak

Total posts: 385
posted December 16, 2004 08:28 AM  
OK - I've mostly delt with gas carbs where you can read the plugs or use a cheap O2 sensor and meter to get it close to ideal, but haven't played with the jetting on my Alcohol carb - I know I run rich at idle and that should be the mixture screws - I've adjusted those leaner and now it doesn't burn the eyes at all when idling in the staging line. I'm wondering how I can tell if the part throttle/WOT mixture is rich or lean (OK it's definately not lean from what I can tell...) I've looked at the plugs and they all are clean and even colored...

Would taking it to a Dyno with a Wideband O2 sensor help?

-Brian

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted December 16, 2004 08:44 AM  
Taking it to a dyno would be the best way. Then you would have a baseline that you knew was correct that you could always go back to. We put one on the dyno once and ended up going up from a 161 we had been running to a 177 jet, to get to an ideal mix, but we would have never known that without the dyno.

The problem with reading alky plugs is it takes a long time for them to show any color, and at the same time any excessive idling or part throttle use will hide the full throttle reading coloring just like on gas. I was told to look at the ground strap and if there was a color change near the bend in the strap that was about right. Closer to the center and it was rich, closer to the threads and it was lean. At the same time still look for pepper on the porcelain for signs of detonation.

dluna
Dirt Freak

Total posts: 334
posted December 16, 2004 11:39 AM  
I always heard that you look at the coloration on the strap for timing, and you look at the base of the porcelain for fuel mixture. If the greenish tint (coloration) is right at the bend of the strap, you are running the correct timing. If the tint is more towards the center, you could run more timing if you’d like, and more towards the threads (pass the bend), you are running too much timing! Then you look for a “cardboardish” tint around the base of the porcelain to know that you are just a little rich (better safe than sorry). My engine builder hates it when I bring him the plugs and they don’t look like they have even been ran.

Does anyone know if this is correct? Maybe we should start a “spark plug reading” thread

BrianW
Dirt Freak

Total posts: 385
posted December 16, 2004 01:24 PM  
quote:
Originally posted by dirtbuster:
Taking it to a dyno would be the best way. Then you would have a baseline that you knew was correct that you could always go back to. We put one on the dyno once and ended up going up from a 161 we had been running to a 177 jet, to get to an ideal mix, but we would have never known that without the dyno.

OK - this is my dumb question - did you use a wideband O2 sensor to tell if it's rich or lean or just by reading the plugs or just looking for max HP???


dirtbuster
Dirt Forum Champ
Total posts: 2007
posted December 16, 2004 01:57 PM  
The dyno measured the amount of air used and the amount of fuel used in the run and then calculated the ratio from that.

OKC9
Dirt Full Roller

Total posts: 52
posted December 16, 2004 02:44 PM  
We full car dyno'd last week with our new engine to get the carb right. We ended up going from a 154 to a 161 and gained really big hp numbers. There was an exhaust gas sensor installed in the even side header. I think you can get in the ball park by reading the plugs, but the dyno takes the subjective element out. It's a bit pricey, but worth it to us.

ctfarm
Dirt Freak

Total posts: 206
posted December 16, 2004 04:17 PM  
quote:
Originally posted by dluna:
I always heard that you look at the coloration on the strap for timing, and you look at the base of the porcelain for fuel mixture. If the greenish tint (coloration) is right at the bend of the strap, you are running the correct timing. If the tint is more towards the center, you could run more timing if you’d like, and more towards the threads (pass the bend), you are running too much timing! Then you look for a “cardboardish” tint around the base of the porcelain to know that you are just a little rich (better safe than sorry). My engine builder hates it when I bring him the plugs and they don’t look like they have even been ran.

Does anyone know if this is correct? Maybe we should start a “spark plug reading” thread



You are correct,been doing it this way for 20 plus years.It's allways worked for me.

juliaferrell
Dirt Freak

Total posts: 370
posted December 16, 2004 08:56 PM  
LM-1 exhaust gas analyzer or any wide band 02 analyzer will read the oxygen content in the exhaust. The millivolt reading of 6 to 8 hundred is optimal for any fuel.

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