Occasional top-end miss: Fuel or ignition?

[Stanley]

Is there a way to tell? I test it every time there's a clear road ahead and no cops, but so far it's clear as mud. About half the time it will pull hard with no miss up to 5000 rpm (no use going higher as it quits pulling - from my reading it seems to be inherent with the stock Y-70 head and cam, great mid-range though), but frequently there will be a miss (just feels like it's not pulling) at about 4700. Once in a while it happens about 4300.

Only seems to happen in first and second (it's an AT) but it's hard to test it in high since 4500 gets me to about 95mph (on the speedometer, running slightly smaller than stock tires).

Fuel pressure is OK, about 3.9 psi, only drops slightly on WOT. Car seem to be running great otherwise, gets from 60 to 90 very quick on the freeway, don't notice it then.

240 z, SU's with new needles and nozzles, floats set OK, Euro distributor with new Nissan points, plugs look good, new NGK wires, advance set to about 10 degrees at 1000 rpm (as high as it will go with barely perceptible ping when it's lugging - supposed to be 17 deg at 900 IIRC, but no go with higher compression Y-70 and 91 octane CA gas).

I can have a pro race mechanic mod the needles (cheap) or put in MSD (expensive), but I don't have a clue about diagnosing this.

[madkaw]

What year engine?

First thing I would do is go pertronix- just my opinion. Why deal with points.

Checked timing at 3000rpm?

Tried swapping out a dizzy?

[Stanley]

So ignition not fuel? Because intermittent problem more likely ignition?

Stock '73 240Z engine with '81 Skyline C211 head. Nissan Euro/JDM Distributor was brand new about 6 years ago. Checked timing about 3000 or whenever it stopped advancing, wrote it down somewhere. It gets to max advance faster (at lower rpm) than US emissions type. Anyway can't advance it any more without pinging. Even with a little octane booster.

Dyno run two years ago, looked like A/F ratio dropped off at top end. Had some other fuel system problems at that time, dirty gas tank, etc., now fixed, so I don't know. Maybe I'll try another dyno run.

[FastWoman]

My vote would be ignition as well. As long as your jets are clean, it's hard to starve a carbed engine for gas. Floating points are more common. I'd change over to electronic ignition as well, unless it's important to stay 100% OEM.

[madkaw]

How much mechanical advance with that dizzy?

What's your plugs look like?

Tried tweeking the floats richer?

Running vacuum advance?

[Zed Head]

I might set dwell on the points first. It's commonly overlooked, with the assumption that point gap will set the dwell, but using a dwell meter is more precise. Dwell will determine how strong the spark is at high RPM. Too short and you'll get a weak spark.

[Stanley]

Would new Nissan points in an almost new dist. float under 5000 rpm? don't know but wouldn't think so. D606-52 distributor has 12 deg mech. advance, all in by 1000 rpm, 11 deg vac advance, book (FSM) says set initial 17 deg at 700 rpm, good luck with that on CA gas.

Set dwell 37 deg with dwell meter, book says 35 to 41 deg. Plugs looked good a month ago, can put some new ones, though. Floats set per book with sight glass, any higher gas runs out. Vacuum advance is hooked up.

[Zed Head]

Bad gas? Might be worth the time to drain and refill. I posted this elsewhere, but the ethanol-blending requirements are averages over certain time periods, you can get high and low within weeks, and, also as noted by others, ethanol tends to draw water to the fuel. Today's engine systems compensate for fuel quality and blend, probably leading to a lowering/widening of quality standards for fuel. Our engines were designed for tight formulations.

Fuel age is a factor also. I know someone with a sail boat whose carbs couldn't handle 1 1/2 year old automotive fuel. This was with a new 10 hp outboard engine. Had to have the carbs serviced and get new fuel to get back to right.

[madkaw]

Wow-just 22 degrees-all in- advance at WOT! -YUK!

I would be looking at a different dizzy that doesn't 'come in' so quickly.

What size are the chambers on the Y70 head?

[beermanpete]

We had a distributor in our 73 that would float the points at about 5000 RPM due to the wrong points cam (to sharp). The problem was very stable with respect to RPM and frequency of occurrence. Your description indicates some variability as to if and when it occurs. I suspect weak spark. Look at the DC voltage at the coil, resistance of the ballast and coil, condition of the plug wires, cap, rotor, and so on. A lean fuel/air mixture is harder to light and will show up weak spark, so it could be a combination of problems.

[Stanley]

Wow-just 22 degrees-all in- advance at WOT! -YUK!

I would be looking at a different dizzy that doesn't 'come in' so quickly.

What size are the chambers on the Y70 head?

That would be plus the initial (theoretical) 17 degrees, so 37 degrees total (if you could set it to 17 initial at 650 rpm without pinging. D606-52 suppose to be the best points distributor for performance.

Can't look up the Y-70 specs now, building department needs more calcs for a shear wall, gotta work. Did take a ride, did a WOT run in first gear up to 5000 rpm, ran perfect. Stopped for a red light, about 30 seconds (plenty of time for the bowls to fill up I'd think). Made another run when the light turned green, felt weak at 4200 rpm. Typical.

Beerman, that sounds right. Coil is Crane Fireball PS-40 about 6 years old, other stuff is a few months old except cap and rotor which I just wirebrushed recently, I'll check if there are new ones in the car somewhere.

Edited April 7, 2014 by Stanley

[madkaw]

You said 12 degrees mechanical, but you meant 24? The number don't add up.

I got 22 by 10 degrees initial+ 12 mechanical= 22, how do you get your numbers.

So your saying 37 degrees total IF you ran 17 initial.

so 17 initial+ 12 mechanical= 29 total ????