SU Cold start question

[Ryoskatekov]

Hey, I have a 83 280zx that I've converted to SU carbs about half an year ago. (don't even try to lure me into EFI again.) winter came and became apparent that it produces shitload of whiteish greyish smoke on cold start with full choke.

 

 It never did it when it was EFI so it shouldn't be oil. Maybe full-choke is too rich?  if so, is there any way to adjust the choke so iIt won't smoke like hell and make my neighbours mad until the car warms up?  (yes, it goes away after it's wormed up.)  I'm rebuilding the carbs for leaks so that may-be the problem but. looking for answers.  (please don't comeback with BS answers like "go buy Ztherapy dvd" If you don't know it, don't reply) 



 

[datzenmike]

Colder weather shows the steamy exhaust just like your breath in the winter. Carbs with chokes will always stink more than an EFI set up. Try not pulling the choke out all the way or start with full and reduce it slightly once running.

[ggzilla]

^^ this

 

Push the choke knob in more as soon as it starts.

 

Greyish-white smoke is sunburned fuel

Bluish-white smoke is oil

White smoke is water vapor

 

You may have a combination of different types.

[dr.feltersnatch]

What oil are you running in the domes? Have you checked it lately?

[Ryoskatekov]

I'm running 20w50 since I have 38mm on the L28.  figured 10w30 was too thin but I don't know

 

Its not cold enough for white water vapor so I assume its the "greyish" unburnt fuel. wasn't burning oil when it was EFI which is about half year ago and oil level stayed the same.

 

and another one of my question is "how much smoke is normal" ? 

[ggzilla]

Normal is zero greyish smoke. A little white smoke is normal.

 

For 240Z carbs, Datsun recommends 10W-30 or 20W. I have no idea what is recommended by S.U. In any case it will have no effect on idling, but only on throttle transitions.

[dr.feltersnatch]

Reason I asked is I run atf and I lose a tiny bit. I also get some steam at start up and always assumed it was the atf leaking and burning up.

[dr.feltersnatch]

I only run 3cc of atf, so not much. How much oil are you putting in?

[datzenmike]

ATF has no clearly defined viscosity. I checked for use in my strut dampers. It can range from 8-12W depending on maker. The thicker the damper oil the slower the slide will raise and the richer it will run as you accelerate when the throttle is opened suddenly.  It's sort of like an accelerator pump.

[dr.feltersnatch]

I agree it is way thin and I am sure thats contributing to it slowly leaking. I have tried 30w 3in1 oil etc and by my seat of the pants dyno (purely scientific) it runs the best with atf. I have also heard atf will burn more white than blue like motor oil. I have not dumped atf in the cyl to be sure.

[Ryoskatekov]

well every once in a while the oil gets dirty when I check it. some black stuff mixed with the oil but it doesn't seem to be burning too much.  
I used 20W50 because I thought slower needle movement meant leaner? lower the needle, leaner right?  I'm using .100 nozzle and SM needles. 

I appreciate all the info but What can I do to adjust the choke other than the half choke and releasing it early? (which I will keep in mind) 
Will adjustment of butterfly rod thing help at all? 

 

[datzenmike]

The damper oil is just that. It dampens sudden movement just like a shock absorber. The throttle plate gets suddenly opened and full vacuum is applied to the venturi and needle and seat. If the slide were to suddenly lift the vacuum would be lost and it would not suck in fuel and go lean. This is why the SU is called a constant velocity carb. It tries to maintain a constant air flow through it by changing the size of the venturi to meed demand. You don't want the slide to lift faster than the motor can use it or it will bog.

[ggzilla]

20W-50 is thinner than 30W

[ggzilla]

What can I do to adjust the choke other than the half choke and releasing it early? (which I will keep in mind)

That is how a manual choke works - you have to manually adjust it as it warms up. The only adjustment is for making sure the butterfly closes all the way when choke knob is pulled out and opens all the way when pushed in.

[Just Joel]

well every once in a while the oil gets dirty when I check it. some black stuff mixed with the oil but it doesn't seem to be burning too much.

I used 20W50 because I thought slower needle movement meant leaner? lower the needle, leaner right? I'm using .100 nozzle and SM needles.

I appreciate all the info but What can I do to adjust the choke other than the half choke and releasing it early? (which I will keep in mind)

Will adjustment of butterfly rod thing help at all?

 

Exact opposite... Under heavy acceleration, lower dome position will richen the mixture by restricting mass air flow and increasing the air speed over the nozzle drawing more fuel into the motor.

[Ryoskatekov]

All right thanks everyone. I just finished final touches on the carb today so I'll fire the car up tommorow and see.