Adventures with SU Carb Tuning

[cbez]

Thought I would start a new thread for overall issues with my carbs. Balancing turned out to be the easy part.

Carbs are an older Z Therapy rebuild (marked 11-07). To my knowledge I'm the first one to put any miles on them.

Basically the issue I'm having now, they are tilted way rich. .087" won't idle, they need like .03. They will run well turned fully lean on the dial as well as maybe 0.75-1 turns in from lean

1) Checked float levels, they are both between 14 and 15mm which is factory spec

2) checked needles, they are M39, no shiny spots or wear. they are flush with the piston

3) nozzle is not sticking, both chokes work fine and have free play at rest

4) dog bones are balanced and adjusted with free play at idle

5) I found a vacuum leak in the throttle shaft but have temp fixed it with some grease. but is counterintuitive to my rich problem

6) pistons have 15wt motorcycle shock fluid

[cbez]

how I checked the floats



are these bearings or just seals? I'm surprised to have throttle shaft leaks cuz I thought ztherapy builds them to address that

[Bwk2000]

If the needle is fine, maybe try lowering its set position a bit (less than 1mm) so the shoulder is slightly raised (not flush) with the piston. Maybe it’s just a worn jet.

PS. Yes, just bushings in the throttle shaft.

[23yrRebuild]

Several items came to my mind...assuming yours is a R16 engine...

1) Are you using BRASS floats ?...if so, they might be leaking, and sinking, causing a rich mixture....I had a pair of NEW out of the box, brass floats installed at the same time, and had one of them sink after only 200 miles....Grrr...I replaced them with the plastic ones...

2) M39 needles aren't the best, or preferred needles, ask Keith to set you up with the ADQ's...they are a lot better, IMHO...

3) I've experienced a very tiny piece of rubber, from inserting the rubber fuel line onto the float chamber pipe, interfering with the needle & seat valve, causing it to run rich...

YMMV,
Mike

[cbez]

Hi guys, no they are plastic floats, very float-y.

I may try adjusting needle position next. I read a recommendation to seat them out a bit with the set screw somewhat loose, then sit the piston down in the carb, to let the needle set itself relative to the nozzle.

[Gregs672000]

Cbez, I forgot how thorough and knowledgeable you are, so I hope my earlier posts didn't come across as too simple.

I think you likely found the issue... the needles. That was something I did play with back in the day, though at the time is was "such n such needle is an emissions needle, this one is for a pre-emissions set up." I remember changing the needles on a set of SUs I got from a place that was importing engines. They would pull off the SU intakes/carbs and throw them away (!). Over time I learned from a Datsun/Nissan only shop owner who raced a 240z and he suggested I change the needles, which I did. A wideband A/F ratio gauge would really help here, though over time you can read plugs.

[cbez]

No such thing as too simple I'm always overlooking steps that's why I feel like I must be missing something obvious with this. Stock new needles, new nozzles, stock float levels, yet running very rich

[cbez]

Well tuned again with a vacuum gauge on each manifold side at 2000rpm

It wants to run fastest with the nozzles fully up

No idea why

When I tried lower the piston with the needles slightly loose, it set them flush



Verified fuel level is ok with clear tube



Checked nozzle, can't find any marks but looks new and measures to spec (was like 2.3xmm)



With bowl primed no fuel visible until nozzle is down this far

[Gregs672000]

So from what you posted, this is what I surmised: You adjusted the dial on the bottom while the engine was at 2000 rpm and it ran the best with the dial turned all the way tight... correct?

If so, makes perfect sense in that the higher up the nozzle the leaner the mixture. But, you may not be able to overcome an incorrect needle that is too rich for the engine. I would do a Google search for M39 needles and ADQ to see what their specs are and how they taper. Too rich a needle is kind of like running too rich a fuel jet in the carb eh? The info has got to be out there somewhere.

[cbez]

So from what you posted, this is what I surmised: You adjusted the dial on the bottom while the engine was at 2000 rpm and it ran the best with the dial turned all the way tight... correct?

If so, makes perfect sense in that the higher up the nozzle the leaner the mixture. But, you may not be able to overcome an incorrect needle that is too rich for the engine. I would do a Google search for M39 needles and ADQ to see what their specs are and how they taper. Too rich a needle is kind of like running too rich a fuel jet in the carb eh? The info has got to be out there somewhere.

yes, but m39 needle is stock, nozzle is stock as far as I can tell, float level is stock. So I can't wrap my head around why the adjustment is biased soo rich at idle and speed. Even if I push the slide down slightly manually, speed drops indicating it isn't lean yet.

[redroadster]

Mine seem to choke or run richer with the slide 1/3 the way up , what about sudden full throttle ? It revs fast , or it floods ? It idles semi normal?
You could have a fuel fowled plugs or close to it
Check for carbon tracking in the dist cap that can mimick it might watch it at night in the dark running to see arcing
Centrifugal dist weights free working ?
Possible the needle housing is worn ?

[cbez]

On a cold start it wants like 4-5 steps of choke at the current setting (about 50 degrees out).

[cbez]

It revs with a small hesitation and idles normal. Plugs are clean.

[nismou20]

Some have assumed that nozzles for 16/20 are the same but in fact not. I would verify for proper nozzles too. They do wear out too. I was trying to figure out why my 2.0 was rich at idle. Replacing them made all the difference.

[cbez]

Keith had 0.092" for the 1600 nozzles, mine match as best as I can tell with a caliper.