Occasional top-end miss: Fuel or ignition?

[Zed Head]

Walter Moore's chart would be handy here (attached it below). Yes, it would be 12 crankshaft degrees centrifugal advance. Not a bunch. 12 + 17 = 29 at WOT, factory spec. 12 +10 + 22 at WOT, current setting. Between 22 and 33 (add in the 11 vacuum) above 2000 RPM, depending on intake vacuum, current setting.

Notice how they changed things for the 260Z engines. Kind of interesting. Maybe CR-related?

http://www.classiczcars.com/forums/electrical-s30/34192-280zx-distributor-advance-curve-reference.html

The post-WOT behavior implies some sort of heat soak problem. Fuel, carbs, coil, etc.

[Mark Maras]

Point Float. Racers back in the 50s & 60s would cut a small piece of neoprene rubber & wedge it behind the movable point. This was a common way to eliminate point float from the list of things that could go wrong, or at least it did no harm & made them feel better. The neoprene usually came from a wet suit back then.

[Stanley]

Tested it again today, couldn't get it to miss when I accelerated a little slower, not quite WOT. Wonder if that means anything.

Zed, found the envelope where I wrote down the timing last time, illegible at 1000 rpm, but 26 deg at 1470 rpm. That's about where it quit advancing. That's advanced from where it was before, and it runs better. What does California have against higher octane gas? Someday I want to get some race gas and see how it runs with 17 deg initial at 700 rpm, like it's supposed to be.

[madkaw]

Lighter loading during acceleration is easier on spark and fuel.

Could mean you are a little light on fuel levels in the bowls or your coil is holding up to heavy loading

[Stanley]

Finally found my new cap and rotor and installed today. Made in Japan, thanks Courtesy. Now my whole ignition is made in Japan except the coil.

Seems to be improvement but maybe I'm imagining it. Made three first gear, (AT) runs WOT from stop, one right after another, first two pulled to 5000-5300 RPM, last one bogged at 4300 but I kept it floored and it picked up to 5000. Before it seemed when it bogged it bogged and wouldn't pick up if I let up on the gas and floored it again. So maybe it's better.

Maybe the coil is shot. Thinking about putting an MSD. They said it works with points, hope it won't burn them up. Nice Nissan points.

[Stanley]

Looks like it was fuel. I thought the floats were set correct height but when I checked them the front one was down about 5mm. Did many adjustments, finally got to about stock. Might try a mm higher. The rear float was still where I set it, but might also try raising that one a mm or so.

Took a drive, tried WOT a few time, seems OK. I'll keep checking but looks like it's fixed. I altered the mix by raising the fuel, time to reset for summer it anyway.

[madkaw]

Fatten her up and feel the power :0

[Stanley]

Proof today at the dyno at MSA. Although the fuel level in the bowls is stock or very close, it's still running out of gas over 4500 rpm. Seemed worse on the dyno than driving. Did several runs and adjusted it richer each time, which improved it, but even at 5 turns down it was still leaning out at top end. Fuel pressure should by OK, 3.6 to 3.9 psi. Fuel flow seems good too. Don't know what' up with it, sure glad I had it dyno'd, though.

[madkaw]

So what numbers did it dyno out. Don't be afraid to tweek those floats . These engines like a rich mixture I swear.

[Stanley]

They're going to email me the results tonight or Monday. They couldn't get it into the best power range, they shut it down when A/F went up too much, glad they did. I saw it on the computer, though. Didn't take much notice of the torque/HP since I was focused on the A/F ratio. It was taking a sharp turn up at top end. Sounded like it was running out of gas. I guess there's a big difference in fuel requirement between load and no load conditions.

I'll try the floats again, but starting to wonder if there's something else happening.

I retarded the timing slightly, since they heard a ping and didn't want to run it until I did. A little less peppy but I feel better about it, especially considering the lean-out issue.

[madkaw]

Don't forget you don't have a stock motor so stock specs might not work for fuel.i wouldn't be afraid to go 2mm richer

[Captain Obvious]

I guess there's a big difference in fuel requirement between load and no load conditions.

There sure is. Maybe the supply side doesn't have enough capacity to provide what the engine needs under extended high load conditions and you're sucking the bowls dry at high load?

You can check that by temporarily replacing the line between the bowl and the nozzle with a clear line and make sure you aren't pulling bubbles through. Another option would be to "T" that line with clear line to use as a level gauge and actually monitor the bowl level while the engine is running. In theory, that level should never change.

Fire extinguisher handy... :bulb: