Hesitation Off Idle

[HuD 91gt]

Hello all,

1971 240z automatic. Stock.

If you guys have been following my threads on this vehicle, I purchased it about 6 months ago in the great US of A, and imported it into Canada. I've spent the last 6 months completely refreshing the front end, and some of the back as well as replacing some of the maintenance items for the engine. This includes points, cap, rotor, plug wires, and adjusting the carbs as per Ztherapy. After this work, the vehicle was finally legally registered yesterday and i've logged about 150km or so since.

Yesterday the car ran quite well. This morning was a different story. After sitting for a minute or two in the underground letting the car warm up, I decided to leave the CO2 filled garage and exit. This means heading up the ramp to the road. Upon adding throttle it bogged down, and even misfired. With some light throttle I made it up the hill and let it warm up completely.

Throughout the day I noticed a slight hesitation off idle. Fairly slow constant power for a few hundred RPM, and once it hits lets say 12-1500RPM it was as per normal (Or as normal as I know).

This evening the problem continued. At a stop light I even hit the throttle hard, and the car acted as as it did on the garage ramp. Sputtering and cutting out. It even died. Not a safe situation. This problem persists only at low RPM's.

Idle is sitting at around 750RPM in gear. Closer to 800 in neutral. I pulled the plugs yesterday evening and some had a light brownish tinge, and the others were cleaner indicating slightly lean. I decided to leave it be, and get a few more KM on the car before I made any more adjustments. Yesterday evening I was also fooling around with the fuse box as I was playing with the radio wiring. I don't think this had any effect, but it might right someones bell.

As suggestions on where to start? I'm thinking the lean condition may have an effect, but you would thing it would have done it yesterday too? Which way do I turn to richen? I believe it's counter clockwise, but I'm not 100%

[Captain Obvious]

It does sound lean to me too, but before you start turning knobs and stuff...

It ran fine yesterday without any of this behavior? The dead spot off idle didn't start until today?

[240260280z]

Try pulling the choke to give it more fuel when it is boggy. This will confirm CO's suggestion of being lean.

A faster idle will also help when cold (pulling choke up all the way mechanically opens the throttle valve a bit more (you can actually adjust this and calibrate it).

If the ambient temp was colder (autumn & morning being factors), that will also require more fuel. Nissan recommends adjusting jet height to compensate for ambient temperature but using the choke to give more fuel will help to get the motor up to temperature (where it is less sensitive to ambient air temp.... the cold metal of a cold intake causes fuel to condense on the metal and not make it to the combustion chamber...thus it is a lean charge... once the engine and intake are hot, the fuel condensation problem goes away....cold engines are always finicky because of this ).

Edited October 10, 2014 by Blue

[siteunseen]

Raising the needle leans it, clockwise. Richening is counter clockwise, turning the knob outward toward the fender.

[HuD 91gt]

It does sound lean to me too, but before you start turning knobs and stuff...

It ran fine yesterday without any of this behavior? The dead spot off idle didn't start until today?

Yes, Day 1. No issues. The hesitation started the next day.

I will try fiddling with the choke and will report back.

[djwarner]

Are you relying on the tach to determine idle? My tach is 200 rpm high. I also have an automatic and found that if the idle is set too low you will stall intermittently from a stop.

It's a good idea to double check your tach for accuracy.

[HuD 91gt]

Today the car was running even worse. Although it seemed inconsistent. It was also cooler today, although not much.

The brake felt lousy, so I suspect a vacuum leak is part of the problem. Perhaps even the booster itself (A bit of an erratic idle today). I also enriched the front carb half a turn and the rear a 1/4. I' pulled the plugs and took pictures. You guys figure that was enough richening to get me back in the ballpark? Is it strange that they all seem to be different colours?

I should also state, the choke was required to be on basically the entire time to keep it running (5-10 minute drive).

Edited October 11, 2014 by HuD 91gt

[HuD 91gt]

Numbered 1 through 6 from left to right. This was after running the car for 5-10 minutes with the choke on. They look the same as when I pulled them 2 days ago, but the black sewt is new.

I found a couple clamps off the balance tube which were loose, so I tightened those up. The erratic idle seems to be gone from this morning, but there is still the lack of power until 1400RPM or so when it just starts to pull hard. I really need to take it for a good drive and I won't be able to do that until at least next week.

As for the idle, yes I am going by the stock tachometer. I have no other way to measure. Suggestions?

To be continued.

Edited October 11, 2014 by HuD 91gt

[240260280z]

from the plugs, the front carb could use an additional 1/2 turn of fuel jet, the back carb could be backed off a 1/4 turn.

I'd recommend getting an SKE Syncrometer (silver one) and a timing light with adjustable advance. a good investment if you have carbs.

The timing light also shows rpm:

http://www.amazon.ca/gp/product/B000EVYGV4/ref=pd_lpo_sbs_dp_ss_3/176-2192564-9329659?pf_rd_m=A3DWYIK6Y9EEQB&pf_rd_s=lpo-top-stripe&pf_rd_r=0DB90H8HVPTQ6TKAVVP5&pf_rd_t=201&pf_rd_p=1535015942&pf_rd_i=B000BSWEHS

Weber Dellorto Mikuni SU Empi Carb Carburetor Sync Synchronizer Synchrometer NEW | eBay

If you only seem to get power at 1400 rpm, it is probably not enough advance at idle as the mechanical advance starts to come in ~1,400 rpm.

[HuD 91gt]

I have the basic sync meter (The one in the ZTherapy video) made by Holley. Is there an advantage to the one you posted. A timing light is on my xmas list to myself. I would have purchased it a while ago but the timing procedure for an automatic sounds extremely daunting. So much so i've even sourced out a manual distributor i'm supposed to pick up next week.

[djwarner]

Check with the local car parts store to see if they have tools to lend.