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Quick and dirty SU Tuning

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Guess I should chime in since I'm having the same sort of fun with carbs. Hope it will add some useful information but maybe not.

I'd been running SM's with the mix nuts set 3.5 turns down to keep from going lean under load at low rpm's. Had to clean the fouled plugs every couple weeks. Got tired of that so two weeks ago tried moving the needles up in the pistons from stock position as far as they'd go, about 1/8 inch or one station. Set the mix the usual way and ended up with the mix nuts about 1.75 turns down. It idled great but wouldn't run well and didn't want to start, finally got it started and home. Checked the plugs, they were fouled even though I'd cleaned them before the "tune-up" an hour earlier. Had a couple construction jobs to go to, one 40 miles away, so rented a car, didn't have time to mess with the Datsun for a couple weeks.

So yesterday cleaned the plugs again. Decided to lower the fuel level since I'd previously set it a couple mm above stock height (before installing the SM's). Pulled the air cleaners and discovered the probable reason it wouldn't start before - I had installed one of the velocity stacks upside-down, covering the vent holes. Today I hooked up the sightglasses. Got lucky and nailed the float setting where I wanted it first try. Set the mix by combination of methods including white paper towel at tailpipe, ended up with both carbs set 1.5 turns down. Started easily and sounded good at idle but drove around the parking lot a few times and it ran crappy. Richened it up a flat and made it to the store and back, not impressed with the lean popping though.

I'll rich it up some more tomorrow and try again but I think I'm about done with the SM's. Think I'll put the N-27's in the drill and polish some off the upper-mid and top end zones instead. Or maybe polish off the first few stations of the SM's. Probably should have done that before.

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'72 three-screw roundtops swapped on my '73 car by the P.O.

The sightglass is a pair of clear plastic tubes with metric fittings that screw into the drain plugs on the sides of the float bowls. I bend a piece of wire around the tube and hook it over a fuel hose or something so it doesn't flop down and let fuel run out. Fuel in the tubes rises to the height of fuel in the bowls. Handy for setting the floats. One of them has a slight leak so I remove them when the float adjustment is done.

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Thaks Stanley, do have a picture of this setup?

Well today was not a good day, I wanted to hook up my choke cables. I started the car and 2 minutes later the rear carb started to overflow AGAIN. I don't know what else to do. The floats are adjusted to specs, and carbs were cleaned. Is it possible that the floats should be adjusted differently. The spec I used was 14mm with the float cover inverted.

Iam going to try to set the carb to the 70/71 S.U specs.

S.U adj bowls..jpg

 

Edited by dat260

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The newer needle valves cannot be set upside down. I learned working on that the hard way. Hold them the correct way as installed and blow through the inlet pushing up on the float until your breathe is stopped. Or set up a jar thats about the same circumference as the float bowls and make a mark on the glass. I'm using my phone now and can't provide pics, sorry. I will in the morning though.

Google "setting floats 240z classiczcars.com" or "float bowl lids 240z classiczcars.com".

Edited by siteunseen
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Suggest running hoses from the vents on the lids to some containers so gas will go there if it overflows.  Make sure it's not overflowing before the test drive. I run the vent hoses to a couple of empty plastic Shiracha bottles lodged under the master cylinder. I leave them there so I'll know if they ever overflow.

The rubber tip of the needle valve needs to come to a sharp point or it might not seal. If it's rounded get a new one. While it's out maybe give a blast of carb cleaner out through the banjo bolt, in case there's gunk in there. Also If the float isn't sitting just right in the bowl, especially if the fuel level is set too high, or if the float is tilted up, it can stick against the side of the bowl causing overflow.

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Thanks guys, I will tackle this issue my next day off. I found out my carbs are from 70/71 series. The float specs are different for those years.

 

Stanley, my needle valve does not have a rubber tip and the tip is rounded. I also cleaned them last time they were out, needles and banjo fitting, they were dirty. The problem Iam facing now is that Iam running out of bowl gaskets. Everytime I open the lid the gaskets warps. It took two weeks to get them from ztherapy. Can I install the lids without gaskets?

I appreciate your input

 

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You can make paper gaskets.  Scan the bottom of you bowl lid or use the template below. 

Scale to correct size using regular paper and a printer. When you have the scaling right, put thicker paper or gasket paper in the printer then print to it and cut out with scissors or Xacto knife.

 

carb gasket.gif

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When the gaskets are removed, put them in a plastic bag or wrap them in foil and put them in a book to keep them flat.

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17 hours ago, dat260 said:

Thaks Stanley, do have a picture of this setup?

Well today was not a good day, I wanted to hook up my choke cables. I started the car and 2 minutes later the rear carb started to overflow AGAIN. I don't know what else to do. The floats are adjusted to specs, and carbs were cleaned. Is it possible that the floats should be adjusted differently. The spec I used was 14mm with the float cover inverted.

Iam going to try to set the carb to the 70/71 S.U specs.

S.U adj bowls..jpg

 

Here's what I did.  Hope something here may help. :)

 

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12 hours ago, dat260 said:

my needle valve does not have a rubber tip and the tip is rounded.

I don't understand that, maybe someone else does. I meant the end that goes into the banjo, not the part that contacts the float. There was a different type that was used for a while, I haven't seen one though.

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 It's possible he has Gros Jet needle valves if there's a steel ball bearing in one end of the needle. The standard needles have a rounded end that contacts the float tab and a cone shaped end that is the fuel sealing surface. The cone shaped end is either rubber? or brass.

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17 hours ago, 240260280 said:

You can make paper gaskets.  Scan the bottom of you bowl lid or use the template below. 

Scale to correct size using regular paper and a printer. When you have the scaling right, put thicker paper or gasket paper in the printer then print to it and cut out with scissors or Xacto knife.

 

carb gasket.gif

Thanks 240/260/280

 

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10 hours ago, Mark Maras said:

 It's possible he has Gros Jet needle valves if there's a steel ball bearing in one end of the needle. The standard needles have a rounded end that contacts the float tab and a cone shaped end that is the fuel sealing surface. The cone shaped end is either rubber? or brass.

That's correct Mark

 

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I am having a heck of time trying to adjust the jet adjustment/ fuel mixture knobs to get the browish tanned color spark plug. Is there any better tool than the Gunson Colortune i bought on ebay? Im at 4 turns at each of the carbs on my 72 240z!

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On 6/20/2017 at 10:16 PM, dat260 said:

Thanks guys, I will tackle this issue my next day off. I found out my carbs are from 70/71 series. The float specs are different for those years.

 

Stanley, my needle valve does not have a rubber tip and the tip is rounded. I also cleaned them last time they were out, needles and banjo fitting, they were dirty. The problem Iam facing now is that Iam running out of bowl gaskets. Everytime I open the lid the gaskets warps. It took two weeks to get them from ztherapy. Can I install the lids without gaskets?

I appreciate your input

 

 Which direction are the Gros Jet needle valves installed? The rounded end (not the ball) is the contact point to the float tab (tang).They were supposed to be a better design (better mouse trap) but they too leaked at times undoubtedly from foreign matter in the valve. Could some more crud be in them? I've not installed the lids without gaskets before. I don't see why it couldn't be done temporarily. The float will be a little lower (the thickness of the gasket) but I doubt that will affect anything. I doubt that most floats are set exact.

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1 hour ago, jalexquijano said:

I am having a heck of time trying to adjust the jet adjustment/ fuel mixture knobs to get the browish tanned color spark plug. Is there any better tool than the Gunson Colortune i bought on ebay? Im at 4 turns at each of the carbs on my 72 240z!

 What color are the plugs at 4 turns down?

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I have always thought the color of plugs should be checked after driving the car, not just reving up the engine. So I have always driven on the freeway near a rest stop and coasting in from speed with the engine off. Then pull the plugs to examine. Must be the backyard mechanic method ??????


Don's iPhone using Classic Zcar Club mobile

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Checking plugs after revving is useless! Check them after being driven like suggested

Adding turns is just gonna put more fuel down the exhaust for more popping. Get a wideband sensor!!!!

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They will probably both work for what you need. Jegs sells the AEM too

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Modded my SM's today. Measured a new set of N27's and compared to C.O.'s numbers on this thread. One of his numbers looked like an outlier so I used mine. Also one of mine looked like an outlier so I used his. There's a better way using statistics and curve matching software, but I wanted to get it done today. Only planned to modify the first two stations anyway, to equal the N-27's so they might work better with the Hitachi nozzles.

Pulled an SM and put it in my drill, used the edge of a piece of used 320 grit with a credit card as a backer. Checked it at station 1 with caliper set at 0.0950, and station 2 at 0.0940, (not that I trust the numbers in ten  thousandths place) took about 5 or 10 minutes each to get from 0.0990 to 0.0950. Used some 1000 grit for a few seconds to shine them up. Unintentionally took station 3 down a little too on both needles. Only 1/4 inch from 1 to 3. It runs, will test and tune tomorrow.

Edited by Stanley
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