Possible lean condition on #4 (260)...

[h2oskiier30]

Spark plug on #4 looks kinda lean. Is there anything OTHER than an injector that I should be on the lookout for that would cause #4 to be lean, when all the others are fine?

[patchesII]

Check the timing on all cylinders. I had one last year that had two cylinders with 6* more advance than the other cylinders. When four of them were at 25 three and six were at 31*. Turned out to be the trigger

[Jay Smith]

Good advice from Eric, I have been checking ALL the switch boxes on EVERY engine I build and I find as Erik said some switch boxes can and will fire several degrees advanced than the number 1 spark advance that many chjeck timing there and call "good". The purchase of a JSRE TDC that comes with all instructions can and will identify this problem. Also the 260 ECU's have but 3 injector drivers that fire in a series of pairs 1 & 2, 3 & 4, and 5 & 6 you could have a weak driver if this is the case keep a close eye on # 3 also if a driver is the gremlin it will also show lean..

Hope that helps.
Jay

[sschefer]

:iagree:With all. The bias circuit in the switchbox is usually the gremlin. I wish they would sell them in matched sets. This would help to ensure that the electronic components were from the same manufacturer. The average component can have a variance in values by +/- 10 pct. The more expensive ones are +/- 5 pct. When it comes to the timing accuracy you need in a performance motor this is quite a bit. You might consider CDI Race switchboxes.

I've indexed cranks and have found #5 to be more advanced on some but setting timing to 23 vs. 25 usually compensates for that. On a carb'd motor you could do like Merc did and fatten it up with jetting. I'm not sure if you can change the pulse width on an individual injector with a stock ECU. I would call Tony Brucato to see what he say's.

A bad switchbox bias circuit or anything that gives input to the bias circuit (det controller, advance module, etc.) can whack timining badly. Another reason to get rid of as much of that junk as you can in a performance motor.

[h2oskiier30]

Thanks guys. I have a few avenues to check now, anyway.

Jay, I have one of your TDC gauges...very useful tool. I'll check timing on #4 as you and Erik suggested.

Steve, I have noticed it on both a stock A6 box and a Brucato PCU.

I'll swap out the switchboxes from my other 260 and see what happens after checking the timing.