Regulator / Gauge - Do I Need It?

[Mark Maras]

 More thoughts on oil wt. If you're using thinner oil than 20 wt., you may want to try 20 wt. or even thicker oil in the SU's. The higher viscosity oil will prevent the piston and jet from rising too fast, preventing a low RPM stumble. That seems to be the opinion of most clubs. Personally, I always used Dexron ATF and never had a low RPM stumble but the higher temps in Panama could add to the problem too. Anyway, It sounds like a fun, cheap and easy, but valuable, afternoon project. Try 3 or 4 different weight oils in the SU's. Take it for a drive on the same route and let your butt dyno tell you what she likes. Oh, and be sure to share the results.

[JSM]

1 hour ago, Mark Maras said:

 More thoughts on oil wt. If you're using thinner oil than 20 wt., you may want to try 20 wt. or even thicker oil in the SU's. The higher viscosity oil will prevent the piston and jet from rising too fast, preventing a low RPM stumble. That seems to be the opinion of most clubs. Personally, I always used Dexron ATF and never had a low RPM stumble but the higher temps in Panama could add to the problem too. Anyway, It sounds like a fun, cheap and easy, but valuable, afternoon project. Try 3 or 4 different weight oils in the SU's. Take it for a drive on the same route and let your butt dyno tell you what she likes. Oh, and be sure to share the results.

I'm gonna have to try this. I have a low rpm stumble. 

Edited November 20, 2017 by JSM

[JSM]

So the stumble is caused by the fact the piston moves up to fast flooding the engine right? 

Mine is only a slight stunmble usually when in second and start to accelerate.  Mine is not like jalexquijano. 

[Mark Maras]

1 hour ago, JSM said:

So the stumble is caused by the fact the piston moves up to fast flooding the engine right? 

 

 You would think so because the needle is moving up allowing more fuel but in fact the piston rising too rapidly causes a lean condition. Piston goes up increasing the venturi size, at the same time air speed past the fuel supply slows down and doesn't pick up as much fuel because the engine is at low RPM and isn't pulling as much air. At least that's the way I understand it. There are undoubtedly better ways to explain it. How do your plugs look and what wt. oil have you been using?

[jalexquijano]

8 hours ago, Mark Maras said:

 You would think so because the needle is moving up allowing more fuel but in fact the piston rising too rapidly causes a lean condition. Piston goes up increasing the venturi size, at the same time air speed past the fuel supply slows down and doesn't pick up as much fuel because the engine is at low RPM and isn't pulling as much air. At least that's the way I understand it. There are undoubtedly better ways to explain it. How do your plugs look and what wt. oil have you been using?

Picture of my plugs at 2.65 turns clockwise on both idle mixture knobs

[Mark Maras]

Those look lean for 2.65 turns. Richen them up a bit. Perhaps 2.75, drive it and check the plugs again. Also try the thicker oil. Perhaps that will help with the low RPM stumble. That said, I suspect that your floats may be set a little low. I know you don't want to hear that but we've all struggled with getting the floats right. Have you pulled the domes and pistons to see if the fuel is about 1/16" below the top of the nozzle when the mixture screws are at 2 1/2 turns? That's where the fuel should be when the floats are set correctly.

[jalexquijano]

I was recommended Castrol ATF oil. I also have 3 in 1 motor oil sre 20 engineered for 1/4 hp motors or larger. Which would you recommend? As i said my problem is at idle when launching from 1st gear. It needs quicker response.

[Mark Maras]

If the 3 in 1 is thicker than the ATF, try that. Personally, I'd throw some 30 wt into them just to see what difference it makes, then I'd try 20 wt.  I suspect one of the reasons that there isn't one wt. oil that works for everyone is wear (increased clearances) in the carbs. In the many posts I've read on the subject of SU oil, The opinions run from A to Zed. Racers that run no oil to a gentleman that used 80w-90w gear oil and swore it was best. Don't be afraid to experiment.

[zKars]

We need to come up with a guide line for flow rate, not pressure. Pressure just needs to be in a range between 0 and whatever your needle valves wil hold without letting fuel through. New ones might hold 5 psi. 2 or 3 psi is usually fine for all but the most beaten up needles and seats. 

What matters is being able to supply enough fuel volume coming in, to refill the volume leaving the bowl and going into the engine. And to do it across the entire engine operating range. 

Had a lovely roadster with a cheap electrical pump that drove the owner carzy with what acted like fuel starvation, but because that lovely pressure gauge said 3 psi, he wouldn't believe it had anything to do with fuel supply. He even changed the distributor. Until I took the line off the carb and watched fuel barely dribble into a yogurt container... Put my finger over the end of the tubing and it stopped the pump dead. New pump, car ran like a champ.

And it's a system. If the lines are plugged, or the fuel filter is dirty, or the tank has crud that gets intermittently sucked in, a great pump (at either end, electrical or mechical) is not going to be able to do its job. 

Edited November 20, 2017 by zKars

[Stanley]

FSM says use 10W-30, and don't use 30 wt. I'm currently using 30 weight but don't know enough to make a recommendation. Springs can also make a difference. I went to APT's red springs which got rid of a lean-out at about 4500 rpm.

According to the "Just SU's" book stiffer springs improve performance at the expense of gas mileage, but very stiff springs reduce performance and increase mileage. It also states that SU's can equal side-draft Weber's if expertly tuned. Book learning can help but experience plus knowledge of theory plus experimentation is better. Lacking that I guess trial and error, with a stopwatch or dragstrip to check performance, is a good way to test modifications.

I've got this badass math program that will do 3D (and 4D animated) graphs which I hope to use to illustrate some of this stuff. If I can just figure out how to use my badass math program, that is. Anybody up to speed on Maple 16?

Page EF-3 of '72 FSM says (mechanical) fuel pump should deliver 1600 cc (3 3/8 quarts) in one minute or less. That's a lot.

The '73 fuel system mod. bulletin says  3.4 to 4.2 psi and 3 pints/minute at 1000 rpm. 3 pints is way less the 3 quarts so dunno. 1600 cc = 1.69 US quart. Seems to apply to both mech. & electric pumps.

Edited November 20, 2017 by Stanley

[w3wilkes]

1 hour ago, Stanley said:

Page EF-3 of '72 FSM says (mechanical) fuel pump should deliver 1600 cc (3 3/8 quarts) in one minute or less. That's a lot.

MY FSM with no designation other  than "Model L20A, L24 Series Engine" that came with the car from the PO on page EF-4 says "1600 cc (3.71 US pts)". So Nissan must have introduced a mistake sometime after my manual and then corrected it in the 73 manual.

Edited November 20, 2017 by w3wilkes

[Stanley]

Note that the pressure and flow in the FSM  are tested by attaching gauge to the fuel pump outlet (return line out of the loop). So in a system with a return line, pressure and flow should be somewhat less. I'm not using a return line, getting 3.9 to 4.1 psi. Never had an overflow (with the floats at stock setting). It varies depending on whether the float bowl valves are open or closed. My gauge only goes from 0 to 5 psi so it's easy to see the variations. The gauge goes nuts when the car starts running out of gas.

Edited November 20, 2017 by Stanley