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Setting Fuel Level

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Here is an ingenious method from: http://datsunzgarage.com/engine/index.htm

  1. Remove the dome and main piston from each carb so you can look down the tip of both fuel nozzles.
  2. Screw each mixture nut exactly 10 turns down from fully up. Each full turn drops the nozzle tip 1 mm, so 10 turns puts the tip of the fuel nozzle 10 mm (1cm) down....which happens to be the 23mm float bowl level.
  3. Then look down the tip of each fuel nozzle and adjust each float to set the gas level at the fuel nozzle tip.

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I tried to something similar based on what Bruce suggested, but I couldn't tell where the fuel was in the nozzle. I know my eyes aren't what they used to be, but this method didn't work for me. Bruce suggested that I tune the carbs as usual and then set the floats so the fuel level is 1/16" below the nozzle. Maybe the method you show would be easier since the fuel would be at the top of the nozzle rather than down in the tiny orifice.

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I was never able to get that to work well either. Instead, I connected a clear vinyl tube to the bowl outlet, held the tube vertically next to the bowl and cranked the engine to run the fuel pump. With this method you can see the level in the tube and use the measurement in the FSM. No guesswork. Once the level was set in this manner, the fine-tuning of the mixture was a piece of cake. I don't think the settings where more than a half a turn different from carb to carb.

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Very cool. I'm going to give this a try. I'm a little worried about the stoppers, though. Mine are still intact. Anyone with experience have suggestions to remove them, if need be?

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Just kidding, stupid question. I got them off.

I also took this opportunity to take my tops off and try a different fluid. I previously had 10-40 with a little 5-20 mixed in (i think its too heavy, and causes my car to deplete its fuel bows too fast and run rich on acceleration). This time I put ATF in and my car started lean popping on throttle like crazy. I dont know how any of you folks run that stuff, it was very light. Now I've got to do it again tomorrow and just put 10-40 in it until I can find a good fluid. I guess fork oil is next on the list.

Back on topic, it was rather hard to see the fuel level from above, even with my young eyes. :P Piching the fuel line made it wiggle a bit, which helped. Its a neat method, but I think the old clear hose trick is perhaps a bit better.

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When the general consensus it that 2 1/2 turns give or take maybe a turn, is the sweet spot for tuning the typical good condition SU, I don't quite get how this 10 turns down deal is in any case the spot to shoot for. At ten turns down they ought to be lean lean lean even with the float level set just below the top of the nozzle. Seems like the air flow thru the carb would be working way too hard pulling gas up out of the nozzles being that far down. Not saying what was offered was wrong, just doesn't pencil out for me.

Also this 10 turn deal will effectively lower to fuel volume in the float bowl by nearly a 1/2" and the average "get on it" romp very conceivably will run the float bowls dry which will make for an even leaner condition as the fuel level drops from an already too low setting.....

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When the general consensus it that 2 1/2 turns give or take maybe a turn, is the sweet spot for tuning the typical good condition SU, I don't quite get how this 10 turns down deal is in any case the spot to shoot for. At ten turns down they ought to be lean lean lean even with the float level set just below the top of the nozzle. Seems like the air flow thru the carb would be working way too hard pulling gas up out of the nozzles being that far down. Not saying what was offered was wrong, just doesn't pencil out for me.

Also this 10 turn deal will effectively lower to fuel volume in the float bowl by nearly a 1/2" and the average "get on it" romp very conceivably will run the float bowls dry which will make for an even leaner condition as the fuel level drops from an already too low setting.....

You don't want to drive it at 10 turns, you just set it at 10 to see if the fuel is sitting at the tip of the nozzle. If it is, the floats should be propeely adjusted, and you return the screw to its usual positiom.

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I ran ATF in my SUs for decades and never had an issue with it. I suspect that one could tweak the mixture a bit and get ATF to work. Truthfully I don't know why it worked on mine. Perhaps because I am a bit obsessive when it comes to keeping SUs clean and tuned. I never had a reason to try anything else.

I've seen many opinions on carb oil, I'd like to hear some input from people that have used different weight oils. Mark in Portland

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I tried ATF but it was much too runny for my setup. I get 20 weight fork oil for free from a local motorcycle shop, and that's worked best of everything I tried. Other weight I've tried have mainly been multi-viscous, and I think that if you play it right, you could be one that would behave like a choke on cold days, however I was not able to find anything else that made my engine happy. Any lighter, and it lean popped like crazy. I could possibly go a bit heavier, especially in the cold, but it runs how it is, so I'm not going to fiddle and break it.

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I set up a 2000 roadster carbs at ZCON. Mr Matsuo came by and we talked for a bit. He mentioned 30wt for hot climates and 20wt for temperate climates.

The oil serves only 3 purposes:

1. To dampen the piston from engine vibrations, road vibrations, and bumps.

2. To control how fast the piston rises under abrupt accelerations (slower rising increases richness)

3. To control how fast the piston drops under abrupt decelerations (slower falling decreases richness....but it is not significant as the throttle valve is closed)

Thicker is richer under abrupt acceleration transition

Thinner is leaner under abrupt acceleration transition

Oil has no effect on stead state idle, cruise or WOT. it only affects stab-the-pedal bursts. Think of it as a mild pseudo-acceleration pump-like circuit like on a weber

Edited by Blue

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I have been running 10W30 motor oil in my SUs for several years. I started with ATF, but it ran like a cold Ford. (The engine would lean out and almost stall on sudden acceleration). The 10W30 may be a bit heavy, but it is easy to get and seems to do the job.

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The majority of posts that I had read before the refurbishment was a straight 20 weight oil. That was easily found on my shelf as "John Deere Hydrostat Oil", straight 20 weight no additives or detergents. It has worked fine in both sets of carbs that have been on this engine.

IIRC, each carb will hold about 2cc of oil. I use a 6cc medical syringe (w/o needle) to fill with no mess and correct amount.

Bonzi Lon

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The majority of posts that I had read before the refurbishment was a straight 20 weight oil. That was easily found on my shelf as "John Deere Hydrostat Oil", straight 20 weight no additives or detergents. It has worked fine in both sets of carbs that have been on this engine.

Bonzi Lon

Another straight 20 weight, no additives or detergent oil is "3 In One" _Motor Oil_. Not the stuff in the white and black cans, er, bottles found in every home in America, but an oil for larger electric motors. Comes in a blue and white container.

Chris

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ATF seems to work up in chilly Alberta, I've tried heavier oils and on cold days they seem unhappy about it. 10w30 seems to be my oil of choice for my 20-30 C ambient summer temps. ATF also seems to need to be topped up a lot more often.

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