Driveability

[WW2Winger]

Mine is a 71 Z with a L28 lower end and an E88 head. My problem is low speed driveability. That is, it has a moderate "surge" when trying to maintain a speed. For example, if you try to hold 3000 RPM in third gear, you will feel a surge back and forth which makes it uncomfortable. If you accelerate, it runs very well all the way up the RPM range. Things I have done ....

New Dist. cap and rotor, new plugs and plug wires, new points, timing and dwell adjusted, new fuel filter, cleaned and adjusted, and balanced SUs, set valves cold. Plugs are whitish and clean, no soot or fouling.

Car runs very well during acceleration at all RPMs in all gears. It is just not quite right driving in normal neighborhood conditions. I have enrichened the carbs and leaned then a bit at a time to no avail..

My plans going forward are to check leak down and purchase a "color tune" to see if that shows me anything. Like I said, she runs great in all aspects except this surging when trying to maintain a speed. I have searched the forum for "surging" and not found anything that resembles this symtom.

Any ideas to try?

[justin260z]

Are they flat-top or round-top carbs? do you still have the egr valve?

[WW2Winger]

Thanks for the response. They are round top and no, there is no EGR valve. I will also add that the car starts perfectly, with some choke in cold weather. It runs perfectly under load, but just surges when at a steady cruise. Previous owner had the L28 block installed and, at the same time, had the head totally re worked to address sinking valves, etc. Thanks so much for any leads.

[Arne]

This was a fairly common complaint in the early days of emission controls - it was called "lean surge" back then. "Whitish" colored plugs would tend to support this, as plugs can burn white when lean. Proper mixture should give plugs a tan color. However, reading plug color with the "enhanced" fuel we have these days is tricky at best.

My stock '71 does not exhibit this to any noticeable degree. But I'm sure that the increased fuel demands of the L28 block could amplify this effect. However, if this were the case richening the mixture a bit should cure it.

So with the data we have here, I'm wondering about the needles. We probably have to assume that they are stock (hard to say for certain, though), but what if they were installed a touch too low in the damper piston? In that case, at idle you'd be able to adjust the mixture to be OK. And at full throttle the piston would rise to the top, pretty much taking the needle out of the picture. But at steady state cruise, the needle would be in the jet, and if it were too low the mixture would be slightly leaner than normal.

So assuming everything else is good, I'd try raising the needles in the pistons just the tiniest amount. Maybe 1/32" at most. See if that makes a difference. Don't forget to re-adjust the idle mixture after doing so. You should need to lean the idle mixture just a touch after raising the needles.

[beandip]

Just one thing I would like to add. What oil are you using in the carbs ? If you are using a thin oil like ATF or Marvel , this could also be the problem. I recommend 20w and if you cannot find that use 10-30 as a second choice. The thinner oil could cause the vacuum piston to flutter up and down slightly which in turn will do the same to the needles. With the engine completely warmed up to driving temp. If you were to go to wyed open throttle and let it drop , just blipping the engine . Do you ever get a back fire or pop out of the carbs ? If so you are for sure too lean . Gary

[WW2Winger]

Both are great suggestions. Thanks Arne and Beandip, please stay tuned (play on words) till we get this resolved. Notice I said "we" get this resolved. I think I will try these suggestions one at a time. I am using MMO for the damper, so Beandip could be right on target there. Since I don't know the history of the Distributor, could that be an issue as well? If the Distributor itself has some gear wear, under load it might not show itself but, allowed to "coast" it could be fluctuating the timing. Does this make any sense at all? I will try your suggestions first and I will be interested in your thoughts on this distributor thing. One more thing, this car has a very restrictive exhaust. It has the original manifoldm which is OK, but the rest of the pipe is very small, probably original. It will be replaced soon. Could this also present too much back pressure on this larger displacement engine at low throttle? Just a thought.

Again thanks so much. Stay with me now. I look forward to smoothing this out because this is a very nice car and I love driving it. It's a keeper as far as I'm concerned.

[Arne]

No, I don't think this is an ignition issue, so don't make any changes there yet.

I agree with Gary - 20 wt. oil is absolutely the best choice for the dampers. Much better than Marvel, which is about 5 wt. oil. If you have trouble finding it, look for 20 wt. fork oil at a motorcycle shop. Since we are pretty certain your oil is not optimum, do that first, and test it prior to moving the needles. One change at a time...

[WW2Winger]

Roger on that "one change at a time". If I get home early enough tonight, I will be changing out the damper oil as that apears to be the quickest fix and also something that should be done anyway, even if it does not address my immediate problem. Thanks again for the response and I will report back as this continues. It will be interesting to see if it is one silver bullet or a combination of several things that resolve this.

[WW2Winger]

Arne and Beandip, We (there I go again "we") are masking progress. I changed out the damper oil last night to 20 wt. fork oil and there is a noticeable difference. I had my doubts, but it is much smoother and the acceleration is still very strong. There is still a hint of the pulsing that was the original problem, but significantly reduced. I didn't have time last night, but I intend now to go back and do the whole carb setting and balancing thing again. I feel like I am very close and I appreciate your help very much. Stay with me.......

[JohnB]

Blast from the past. I have the same issue and appreciate the input you all had in 2007. Did you ever get rid of the surging? Sounds like the 20 wt. oil was a big first step and I will do that as soon as I find 20 wt. I have been using 30 wt. oil in my 240z and 260z for years. The surging has only showed its ugly head in the 260z recently.

Any update would be great.

Thanks,

JohnB