Datsun Roadster Forum-311s.org

To be clear, when I say "cold start" im not referring to starting the car in cold weather. It never gets cold here in the Baja.(Well, unless you consider mid 50's (F) cold.) Im referring to after the car has set for 24 hrs. or longer. 67.5 1600 with stock S.U.s, Car is very difficult to start but only when "cold". After the first start up, it will start immediatly for the rest of the day. The symptoms seem to indicate a fuel starvation at carbs after car has been sitting for awhile. It will generally eventually start after many attemps and many (in)appropriate curse words. Occasionally, I have to resort to removing the air filter assembly and using starter fluid spray. I searched cold start and hard start in the posts, but didnt run across this specific problem. The part thats got me scratching my head is how well it starts otherwise. This problem occurs irregardless of whether I use the manual choke or not.

Check for fuel in the bowls. Just pop off one of the overflow tubes and see if you can push down on the float. I have this problem if I don't start my car for 3 weeks. Keith Williams has told me this is normal, but one day would be extreme.

Thanks Steve. I have checked in the recent past to see if the floats were in good condition and found no issues. But that was before this problem started. If it is a float problem not bringing enough fuel into the bowls, wouldnt the hard start problem be constant? And if it is an issue with the fuel leaking out of the bowl over time, wouldnt I have an obvious leak? No evidence of fuel leaking or fuel smell. Where would it be going? It will become more and more obvious that I am not an experienced mechanic as I ask more questions. But I am retired and trying to learn because 1) I enjoy the experience and 2) the nearest Datsun guru is about 800 miles away. Ive tackled alot of repairs successfully , but have zero experience with carbs.

a good fuel pump puts out a fairly significant volume of fuel. even starting from empty bowls they would fill very quickly.

how far out are you going with the mechanical choke?

-if- the choke is adjusted right -and- working right, the first few steps will richen the mix by dropping the nozzle, then the next few actually start to open the butterflies and richen the mix further. mine currently needs this to start after sitting overnight, then can pretty much immediately be dropped back down to normal.

so, first check I would do would be see if it starts up with the choke fairly far out, like 5-6 clicks

if that doesnt work, as a test try to turn the service screw in a ways (don't go crazy) - basically raising the idle speed and see if that makes any difference.

A fully cold start and cold idle is asking the engine to do hard work (running with every single part cold, poorly atomized fuel, bad mixture), so it exacerbates any other issues that won't necessarily show on a hot or even warm start.

in general recommend going through keith's list here:

https://newprotest.org/projects/510/docs/SU.pdf

make sure your hot idle mix is good

make sure the chokes are pulling the nozzles fairly evenly on both carbs and opening the butterflies (take domes off and watch them drop, they should go down the same distance at each 'notch')

make sure the idle air flow is balanced on both carbs

make sure the dog bones and chokes have play at idle

let's say we set a nice low hot idle - 650 or 750 rpm. you're asking the engine to run nicely on a tiny sip of fuel and air. since it's hot and presumably your mixtures and such are good, that's no problem.

now we're starting the same engine cold, everything is resisting it and especially the AFR is whack because everything is 130+ degrees colder. RPMs will be lower, to the point that it won't run. so the SU carb design says ok more fuel anddd I'll let you take bigger gulps of air/fuel mix too (fast idle) instead of the tiny sip.

Have a close look at the glass bowl fuel filter, assuming you have one.
If the cork gasket dries out or deteriorates, the pump will draw air into the fuel bowl, causing fuel starvation.
Coupled with the evaporation issue that we all seem to have with the SU's, life starts to get tough when we try to start the engine.

Trying a different diagnostic path here.
1. Check the condition of your plugs just before you start it (maybe partially saturated or gaps are off). Good time to also scope the valves for fouling/damage.
2. If that fails, do a quick check/adjustment of your valves. Either of the above can lead to hard cold starting.

Do a compression test completely cold ( eng not started in 6 hours)
The 1st pump 40-50 percent of total compression ?
If not... problemo
Tech lines always want this done with a cold start problem
& Do not use Starting spray as it can and will try to blow the crank backwards with it's quicker combustion ,carb spray if you must
It's, starting spray only real good use is in Arctic conditions &very humid and 30 degrees when gas will not aireate
When you get it running see if it will run fairly normal with one carb blocked off with a rag but doing it to the opposite kills it quick if so it's running on that carb only , mine was when I 1st got it going

Thanks to all for the input. I will try this useful information to see if I can find a solution.

Bwk2000 wrote: Mon Mar 21, 2022 5:10 pm Trying a different diagnostic path here.
1. Check the condition of your plugs just before you start it (maybe partially saturated or gaps are off). Good time to also scope the valves for fouling/damage.
2. If that fails, do a quick check/adjustment of your valves. Either of the above can lead to hard cold starting.

We will try those tests as well when we start working on my friend's new Datsun project with an intermittent cold start issue. Got to finish installing the soft tops and jeep accessories on the Wrangler before we can clear the garage.

I had a similar problem with a friend’s 67.5 1600 a few years ago and found that a non stock ignition was the cause. Installed was an ignition switch that had a cold start circuit in the switch that he did not upgrade to the wiring harness. So I ran a wire for the circuit and it was no longer a problem trying to start it when it was cold.