72 hard cold starting.........

[richard1]

I have never experienced this kind of problem before.

We are about the 50 degrees range in Vegas and It is very hard to start when the engine is cold. When hot, everything is OK. I have a 195 degree thermostat for winter, the carbs work fine, the ignition system is perfect, the engine is responsive without any hesitation when warmed up. These L6 engines are real cold blooded even in summer but in winter even more.

I read a lot of the archives about early Zs cold starting issues and one got my attention: I have always use ATF oil in these dome carbs without any problem until now.

Some posters say they use 20W in the carbs for easier start-up in cold weather.

So, I need some clues about what to use for these Hitachi carbs for cold start-up.

They have never been rebuilt and are trouble free after more than 130K as is for the car that has never been restored and a complete original.

Any suggestions?

[MikeW]

You didn't mention the choke at all but that's the primary thing to consider when starting cold. It's possible that the choke cable needs an adjustment. I'd have someone else sit in the car and move the choke lever back and forth while you observe their effect at the carbs. I doubt the damper oil matters very much but 20W is the factory recommendation anyway so it can't hurt. The thermostat has zero impact on starting as it doesn't open up until the car has been running long enough to get warm.

[richard1]

You didn't mention the choke at all but that's the primary thing to consider when starting cold. It's possible that the choke cable needs an adjustment. I'd have someone else sit in the car and move the choke lever back and forth while you observe their effect at the carbs. I doubt the damper oil matters very much but 20W is the factory recommendation anyway so it can't hurt. The thermostat has zero impact on starting as it doesn't open up until the car has been running long enough to get warm.

I never said the thermostat has any impact on the cold starting problem in cold weather but it helps these cold blooded engine to run warmer.

The choke cables have never been an issue. As many archives have reported the oil grade seems very important as I have always used ATF.

Yesterday, I cleaned up all the internal components of the dampers, pistons and removed the ATF oil and touched or adjust nothing else. i filled with 20W50 as recommended by posters on the archives.

To conclude........I can tell you that the 20W oil has completely fixed the problem in many ways: it is clear that the fuel/air mixture has something to do with the oil viscosity in the carbs.

Second, the thermostat keeps the engine at a higher temperature in climatic condition below 50 degrees for highway driving and stops......no more hesitations at pick up from a stop light in cold morning weather.

Third, all spark plugs were clean.......it was not an issue about dirty plugs.

Forth, Ignition points are gaped accordingly to the best performance settings for a 130K car and work fine as far as regular check and maintenance is provided which means every 500 miles

Fifth, As an original unrestored car....oil change is performed at every 2K with Castrol 20W50 and nothing else.

What I want to say is my classic Z is original, clean and healthy and is not a candidate to a restoration project of any kind.......furthermore I keep it "bone stock". It is not a trailer queen and I enjoy the drive in sunny Las Vegas or whatever it may rain.

I am grateful to the archives and posters of this site who keep posting to provide us essential information as owners of the first generation Zs to keep them on the road on as reliable daily transportation as bone stock enjoyment.

We are all car buffs but a first generation Z is not an easy task to be fully stock, reliable for daily driving and without add-ons as electronic ignition and some other hardware.

I am privileged to own one of these fully original early models, fully dealer optioned including A/C and A/T.

[Arne]

Richard, actually the factory specified carb damper oil has always been 20 wt. (Note - 20 wt., not 20W.) Over the decades, many shadetree mechanics have tried all sorts of other oil in an effort to "improve" on the factory setup. ATF is common, as was Marvel Mystery Oil. Both of those average about 5 wt.

These days, more and more people are realizing that Nissan knew what they were doing, and that 20 wt. oil is best.

But it's worth noting that 20 wt. (single grade) oil is NOT the same as 20W-50 (multi-grade) oil. While the 20W-50 you used may be fairly close to single grade 20 wt. during the cold weather of winter, when it warms back up in the spring it will be far too thick. I'd recommend that you change to true single grade 20 wt. at your first opportunity. True single grade oils like this are easiest to find at motorcycle shops, as fork and shock oil.

[kenz240z]

I doubt the viscosity of the damper oil has much impact on starting. The oil provides a slight delay in raising the piston which allows for a richer air/fuel mixture when accelerating.

You might want to take another look at each carby to insure the choke cable is properly adjusted and that the fuel jet is being pulled down out of the carb body about 1/4". Lowering the jet increases the area between the needle and the opening in the jet, which provides a richer mixture for cold starting. In addition to lowering the fuel jet, pulling the choke cable all the way back should also open the throttle plate slightly. Check the FSM for the specs.

While my '73 is not an unmodified, unrestored, bone stock, fully original early model, it does have round top SU's. I don't have any trouble starting the engine in cold weather. I simply pull the choke lever back, depress the clutch and turn the key. Once the engine starts, I push the choke lever 1/2 way in. When the engine is fully warmed up I push the choke lever all the way forward.

With over 130k miles, your yet to be rebuilt carbies may not be as trouble free as you think. The throttle shafts could be worn, causing air leaks. The fuel needle & seat in the float bowl might be worn, dirty or sticky. The float level may be off. The fuel jet could be stuck, causing hard starting in cold temps. The linkages may be bent out of adjustment. The piston may be worn in the suction chamber. Could be other problems with them, but hopefully you get the idea.

[MikeW]

I'm glad you got it figured out and that the fix was easy. Another thing to mention for anyone else having similar problems is common to all cars with carburetors: if the engine hasn't been run for a while it's simply harder to start as it's likely that fuel has evaporated from the float chambers. It takes a while for fuel to refill everything. Fuel injection has a clear advantage here.

[richard1]

in regards of my original post. Now, I know where to find 20wt oil and I have gone through the inspection of all major components by fellow members.

I took the opportunity to clean the pistons and chambers, nothing was bent or scratched from misalignment, the needles were fine, throttle shafts and bushings are still tight with minimal play and the engine is responsive at start-up and acceleration as it should be.

I know that one day, I will have to rebuild the carbs but for now it would be a waste of money because there is no other issues than the one mentioned in this tread.

At 130K it is now an old car that needs a close scheduled maintenance and it is what it gets.