su adjustment

[Bruce Palmer]

"Now that I have a good base line"...... meaning now that they are running fine I'm going to start fiddling 'til they don't.

Actual case study - Customer installed his ZTherapy carbs and was so taken with how well they ran he thought "gee this much improvement was so easy I'll bet there is more improvement to be had". Had a friend work on them with as "to be expected results", sent them to a second "expert, and then a third....

Bottom line, they were ultimately so screwed up he sent them back to Steve to un-ring the bell so to speak.

Good luck. We are only a phone call away.....

[d240zx2]

Yeah, Bruce, you damned ol' engineers don't know nuthin'. We gotta re-do everything until you have to do it again. .

That case study was an excellent example of what my poor attempt at humor was trying to illustrate.

[racer88]

Yes I know it sounds kind of crazy, but for me the fun in old cars is the constent fooling with them in an attempt to make them perform better. I would be pretty arrogant to think that my first attempt at tuning SU carbs would be the best it could ever be. I know there are many experts on this site that have been down the same road (that's how they became experts). In any form of motorsport, the moment people stop trying to improve, they start to go backwards. I still have my "baseline" to return to, if I screw it up to badly.

: )

Bob

Edited April 6, 2010 by racer88
Spelling error

[whamo]

I have a friend with a small machine shop to change the profile of the needles

That sounds like a really bad idea to me...... :stupid:

[Weasel73240Z]

Bruce is dead on with this one, and I'm another example. I got my new ZTherapy carbs, bolted them on, and they ran really good. But I couldn't leave well enough alone. I adjusted them and adjusted them, until I had them so screwed up I almost had to send them back to Steve to reset. And I'm not a newbie, I've been around cars my whole life, but not SU's. I finally went back and very carefully set them to baseline, and they're running great.

Today I passed a truck on the way to work, and they sung right up to 6,000 rpm's without a hint of hesitation. :laugh:

If you're gonna tweak, make small tweaks, that's what I learned. These carbs really respond to very small adjustments in the A/F mixture if you have you floats heights right. It sounds to me like your floats are definitely off.

[racer88]

Ok I get the hint. I will leave well enough alone. It's that I am bored. The car is at a friends body shop awaiting prep and paint. I have already built and installed tubular control arms front and rear, shortened the struts, installed coil overs, Zx ign, maxi fuses, gm alt. rebuilt the engine. lightened the flywheel ect. Maybe I need a hobby or a mistress .

I only mention all the modifications so that you do not think I am a total newbie wandering around in the dark. I spent 20yrs building late model stock cars for myself and customers around the local tracks.

Having said that I do appreciate all the advice and opnions. As soon a I figure out how to do it I will post some pics. of my soon to be bright yellow 260 zed

Thanks

Bob

[Bruce Palmer]

By Zx ign I asume you mean distributor. If so did you shorten the advance curve any?

[Arne]

I've got over 20 years in with SU-equipped cars, both Japanese and British. And I agree that most all carb "issues" I've dealt with over the years was from un-needed fiddling. Either from people certain they could be adjusted better than they are, or people who start tinkering with the carbs anytime the car hiccups. FWIW, once set they don't change themselves, so there's no reason to suspect them when things don't work normally. Problems are more often ignition or fuel delivery TO the carbs, not the carbs themselves.

[beandip]

Arne this is so true! The thing is , these little carbs have all these neat little screws and adjustments just waiting to be

screwed with. The same with the timing. I find it funny that these cars have been around and raced and tuned and re-tuned for what, 40 years ! I think that EVERYTHING under the sun has been tried before.

I had a guy tell me he was trying brake fluid in the carbs insted of oil to see if it would improve performance... So maybe everything hasn't been tried. :stupid::finger:

Gary

[racer88]

Hi Bruce

Yes I did use the Zx distributor. I have not checked or changed the curve in the dist. The advance mechanism has the numbers 9.5 on it. I assume that means 9.5 degrees in the dist. that would equal 19 degress crankshaft. I have left the vacum advance unhooked and set the initial advance at 16 degrees. I have searched many post about the vacum advance debate. I like the way it idles with lots of initial timing, but at this time I am still up in the air about it. Any thoughts on that matter would be great. I have had the dist. totally apart to clean it and the advance mechanism is in perfect working order.

P.S. The e12-80 module was non functioning so I am using an MSD GM style module with a built in rev limiter.

Bob

[Weasel73240Z]

By Zx ign I asume you mean distributor. If so did you shorten the advance curve any?

I'm using a ZX dizzy with an MSB Blaster 2, but I didn't do anything to the advance curve. Mine is an 8.5, so I get 17 advance at the crank. Is "shortening the curve" just a matter of changing the dizzy weights, and what would the benefit be to shortening the curve? Thanks as usual for the help.

[racer88]

Shortening the curve requires you to either weld up or use JB weld in the slots in the advance mechanism to limit the amount of movement. Allowing you to run more initial advance for a smoother idle and hopefuly faster off idle response. if you use JB weld make sure you use it in the at rest portion of the slot so that if it falls out or wears away your engine will not go to far advanced. It works great on small block chevs, I am just not sure what the datsun engines like in this regard.

Bob