rpm 650, balance screw set! unisys flow even, ....mixture knob/disks need many turns?

[bounce]

71 240z, refurrbed carbs from ZTherapy

it seems like I'm giving these mixture knobs full turns to get any response from the motor and then I'll back it off a bit...but I was expecting 1/4 turns or using the notches to get lean/rich states....

almost lugging a bit when bumping the throttle initially...

what should I do? twirl the disk to find highest idle speed? I think I've already got it, the motor just lugs out when I go in either direction about 1.5 turns....

go watch the DVD again ? I guess I'm not recognizing?

thanks

[bounce]

I dialed the jets all the way to the top and then down 2.5 turns...did the quickNdirty piston push on each carb and had to richen a little bit on both carbs to keep the motor running with one carb disabled...

took it for a test drive and have hesitation/luggin at mid acceleration/ ... never got over 35 mph though.... but I think I'm rich...thoughts?

I do have high altitude needles....

Edited August 20, 2011 by bounce

[240260280z]

http://www.classiczcars.com/forums/showthread.php?41940-Quick-and-dirty-SU-Tuning

http://www.classiczcars.com/forums/showthread.php?43264-Vintage-SU-Carb-Adjustment

[240260280z]

Go ~ 60mph for a few minutes, turn off ignition then coast to a stop (careful turning off ignition as your steering may lock, you can turn it back on when engine is dead but not all the way to start)

After coasting to a stop, read plug #2 and #5 to see what front carb and back carb are doing.

[Walter Moore]

Make sure that the chokes are fully OFF. If they are partially engaged you can turn the mixture knobs multiple turns and have no effect.

The car WILL run semi-normally with the chokes partially engaged. But it will run rich and foul the plugs after a day or two of driving around.

[Jetaway]

First off, a couple of prefatory comments:

A) Engine RPM is much more useful for a diagnosis. I mean, if you are having hesitation problems at 35 mph in 1st (assuming a manual), I'd say, shift to 2nd!

My carbs could use a professional going over. I'm sure whatever performance improvement as a result would be 1/3rd of the reduction in time and PIA I spend tuning my carbs.

That said:

I've gotten pretty good at balancing and synchronizing the carbs. Fuel- mixture is still an iterative process. If tried every method I've come across, from turning both knobs an eighth of a turn to obtain the "smoothest and fastest idle" to the "lift to die" method and more than a few in between. My basic approach is as follows:

1) Idle at slowest RPM possible to prevent a death-spiral. On my 1972, that's 700 plus/minus a hair. This, if not apparent, is done after balance and synchro but with mixture unadjusted from previously.

2) Knobs to full lean, back off two and a half turns (as you did).

3) Tweak by 1/8ths, making it richer. Once two successive turns results in no change, go back 1/8th.

4) Lift the, uh damn, whatever its called, on one of the carbs as you did. I've been able to dial the functioning carb practically back to full lean before the engine dies, which has proven not to be all that useful on the road. Instead, and I'm sorry I can't give you anything black and white, I adjust the mixture until one foot is in the grave. I suspect this point varies by car and interpretation.

5) Drive:

a) If Smiling Jack feels as if it could kick the a*! out of cars that it realistically could not, which is a good thing, but goes through 5 gallons of gas in 60 miles, I'm running too rich. Lean'em an 1/4th of a turn and return to 5) above.

Note, out of deference to 200,000+ miles on the engine, my hard redline is 6500. Also, the gearing is on the tall side with an early 280ZX tranny and the presumably original 4-spd differential.

If SJ feels frisky, find a place that can be reasonably expected to be unmolested by Johnny L. and: run in 1st at 4500-5000 for a few seconds, floor it, shift to 2nd and keep pouring it on to 6500, shift to 3rd and keep the power on to 6500. If you can't make 6500 because of a urhh, urhh, coughing you are too lean. Roughly speaking, and possibly strictly applicable only to my car, its running 1/8 turn lean for every 500 rpms short of 6500. So, for example, if its screaming up 5500 in 3rd but then starts to cough, I'll stop and enrich by 1/4 turn. Then: return to 5) above. If it runs strong to 6500, happy motoring!

c) If you can't complete the this test, either you are way, way lean, or have a fuel supply problem or problems unrelated to to the fuel system.

I'm not a professional mechanic, hell, I don't even consider myself a highly skilled amateur, but I think this test is useful because: a) the 1st gear rev and hold and the 2nd gear run checks for fuel bowl depletion caused by a weak pump, clogged filters, incorrect float level, etc. the 1st gear run and, more demandingly, the winding out in 2nd is a check on the electrical system. My opinion is, and god knows it could be wrong, 6000 rpm is 6000 rpm to the electrical side and if its not up to snuff, it will misfire given no, light, or a heavy load. So, if everything is kosher through 2nd gear, but problems develop in third, it's _not_ because the bowls are dry and not because of bad timing / weak spark, but the heavier load caused by being in 3rd brings out a sub-optimal fuel / air mixture.

As a bit of a post-script, I recently read in an older Road and Track (early to mid 1960s) that the stochastic ideal isn't, under all conditions, exactly correct and that a gasoline engine under load performs better with a slightly richer than stochastic and the inverse, that under a light load (say, maintaing 60 mph on a level load) a slighly leaner than stochatic mixture is not damaging and improves fuel mileage.

Chris

[bounce]

I'll see what the cellphone camera can tell ya tomorrow. thanks for the help.

[bounce]

no film yet fellas, but HEY!....I didn't refill the dampner oil when I received the refurbed carbs...makes a difference already...no pop on rapid throttle movement...

balancing again this weekend.

[240260280z]

Damper oil and spring affect how slowly the piston rises and falls when the throttle valve opens and closes. It also stabilizes the piston from "fluttering up and down" caused by pulses into and out of the intake.

The conundrum:

If the piston rises too slowy, then the mixture into the motor will be richer as the venturi suction on the jet will be greater and more fuel will be sucked through the orfice between the needle and jet. As well, the higher velocity and smaller cross-sectional area should promote better air/fuel mixing. However, the higher suctions pulls the piston upward with more force.

If the piston rises too quickly then the fuel presented at the orfice will be greater as the needle will be raised higher however the venturi suction will be less thus pulling less fuel. The lower velocity across the jet and larger cross-sectional area should reduce air/fuel mixing. However, the lower suction pulls the piston with less force.

So the conundrum is the unknowns and their interactions to counter each other (venturi suction, orfice size, needle taper, air/fuel mixing, piston motion ). For example, you can over dampen the piston so that it runs richer on acceleration but correct by needle profile to compensate and at the same time, the increased Venturi effect will attempt to counter the over damping. You can also rework the bridge to promote more airflow (venturi) but disrupt air/fuel mixing from turbulence but the increased airflow will pull the piston harder and slow the airflow.

This whole mystical mechanism really needs someone with a flowbench and engine dyno to make emperical measurements to get the facts, Until then just stay as close to stock as you can and use the recommended damper oil.

Heck I was just looking at the weird bumps in the EFI manifold runners along with the offset injector points and Nissan put them there for a reason.... but why?

This gets good ~ 2:20 into it. You can actually see the fuel "fog" and the cone it makes down stream from the needle/jet point.

Edited August 31, 2011 by Blue

[Gary in NJ]

Cool videos Blue.

[ajmcforester]

Little advice about Northern Arizona many of the towns are higher than the area around them, I found it better to run a little rich so I would not lean out in drives.

[240260280z]

I had the same problem with a 510 we had shipped from Colorado to Boston. The 1 mile drop in elevation required carb adjustments before we could hit the road.