Playing with a wideband

[madkaw]

Just curious of what you have your ignition timing set at with no advance hooked up.

[steve91tt]

At idle I am running 10°. It then advances to 30° (all in) at 2500rpm.

[steve91tt]

Bingo! You have just witnessed the inherent flaw in round top SUs.

Is that because the same part of the needle is used for idle as for 70MPH cruise?

[ozconnection]

Bingo! You have just witnessed the inherent flaw in round top SUs.

Hell, any induction setup can give the wrong mixtures to the engine if not calibrated correctly!!

That's where the time factor comes in. How anal are you to get it as close to perfect as possible? That is often the criticism of any setup, especially non factory carb setups. You can get them very close to spot on but sometimes it can take more time to do it than most people are prepared to spend and that's what gives 'them' a bad wrap.

16 to one mixtures are not too unreasonable. If the throttle is barely cracked open and the cylinders are hardly filling with each cycle of engine rotation, no real damage will occur at this point. At WOT and 16 to one, kiss your engine goodbye.

Is there surging at 75mph and 16's?? Surging represents unstable or erratic combustion ie missfire. Too much advance with this lean mixture can cause surge so you can look at that scenario as well.

Yes, good question and Madkaw just asked it as well. Is your vacuum advance hooked up?

I found that my running consistancy was all over the place with the extra 12 or so degees added with the vacuum pot on the dizzy. I now run my dizzy without vacuum advance. If the engine sat all day at 3k rpm and never climbed a hill or changed speed, I'd continue to use a vacuum advance pot.

cheers.

[steve91tt]

No vacuum advance currently hooked up. I do get a little surging if I have it leaned out to cruise in the 16's.

Good news...I changed to 20wt oil (from ATF) in the dash pots and the lean spot at tip in is gone (condition #5 in original post).

[LeonV]

Is that because the same part of the needle is used for idle as for 70MPH cruise?

Not quite. The round-top SUs have one circuit for all fuel delivery needs, the needle and jet. The needle and jet control everything; idle, transition, midrange, top end included. If you enrich the idle mixture, you then enrich all other modes. That is the inherent flaw in round-top SUs.

These issues are what the flat-tops sought to eliminate, hence the flat-tops have a dedicated idle circuit and a power valve. Download the 73/74 FSM and check it out.

OZ, I hope you understand what I'm getting at now.

Edited March 29, 2011 by LeonV

[cygnusx1]

L motors generally love to idle rich. My L28ET needs around 10.5 - 11.5 to idle buttery smooth. Fortunately, I can dial it in with the programmable EFI. When I try to make it run stoich, anywhere in the load range, it really isn't too happy. It feels good everywhere if you let it have 12's and 13's. Highway light cruise, and high revs, will be the only places it can tolerate the leaner stuff. Disclaimer: the L28ET is probably quite different than an L24, on which I have little experience.

Interesting data. I need to get one for my 240 with triples someday.

Edited March 29, 2011 by cygnusx1

[ozconnection]

Not quite. The round-top SUs have one circuit for all fuel delivery needs, the needle and jet. The needle and jet control everything; idle, transition, midrange, top end included. If you enrich the idle mixture, you then enrich all other modes. That is the inherent flaw in round-top SUs.

These issues are what the flat-tops sought to eliminate, hence the flat-tops have a dedicated idle circuit and a power valve. Download the 73/74 FSM and check it out.

OZ, I hope you understand what I'm getting at now.

Yeah, thanks for the clarification, I learnt something too LeonV, cheers mate. :classic:

[steve91tt]

Thanks for the feedback guys.

[Gary in NJ]

Steve,

Do you use a Colortune? It would be interesting to know what the Colortune indicates (my "hi-tech" tool for tuning) compared to the actual A:F ratio.

[madkaw]

I would see how much more advance you can get also. I am running 20 degrees initial and about 36 all in. I'm not sure what dizzy your using, but I would be suprised that your mechanical advance is actually adding in 20 degrees. Either way, I would think --since you are NOT using vacuum advance, then you need to bump up your timing and see what happens then. If you already tried this and your are pinging, then scratch that idea. I'm not sure what compression ratio your combo yeilded.

[steve91tt]

I'm running 10:1 compression on a stock cam so I can't go much above 30 degrees of total advance without pinging. I have a Mallory dizzy so I can change the curve fairly easily. I will try giving it a few more degrees at idle and see what's what.

Sorry never tried a colortune.