Adventures with SU Carb Tuning

[keith0alan]

My thoughts,

For the leaking throttle shaft check with Z-therapy. I think they warranty those. It sounds like worn needles or nozzle. The needles are a satin finish. Look for shiny spots near the base. As they wear they get richer at idle. All the Nissan needles are rich at idle and lean mid and top end. I recommend the ADQ needles. Verify that the needle base is flush with the face of the piston. Setting it out .005"-.010" will lean out the idle. If the nozzles are worn Z-therapy rebuilds them.

[cbez]

Needles look brand new to me, no visible wear entire length is satin

Nozzles don't seem wallowed at all

It just seems odd that I can lean both carbs all the way and it still runs fine. But maybe I will just live with it if I can still get a good range.

I have a colortune on order so we'll see what that says about my mix.

[RBMann]

I had the same problem with mine where I could not get it lean enough. The needles had small shinny spot on one side. The jet measured okay but in one direction that matched the needle wear the jet was oblong.

jet.jpg

[cbez]

I did notice some decel backfire and what I think is NO smell this morning so maybe I am a little lean after all.

[cbez]

I tried Keith's suggestion of lowering the needles and it seems to have helped. Got them set at a nice adjustment that isn't full lean on the adjustment knob.

really aiming for 'lightly touch the piston plunger and RPM rises slightly' spot on both.

We'll see how it looks when my colortune gets here.

[keith0alan]

Backfire on deceleration is normal. Newer cars have bits to stop it because catalytic converters hate it a lot.

[cbez]

colortune came early. Idle was a little rich on both carbs, and fine at higher rpm. May have leaky float needles as it would idle down to blue then after a bit some yellow (rich) would come in. Neat little tool though.

[Gregs672000]

It's actually going to idle a bit smoother typically if it's a little rich... not puffing black out the tail rich, but richer than 14.7 ratio, more like 13.5. Does the color tune give an approximate of what range the different colors correspond to? You can see why I love widebands. They do require quiet a bit more installation if you go permanently mounted like I have, but there was someone on the site that had a tail pipe portable version for loan...

[RBMann]

My thoughts,

For the leaking throttle shaft check with Z-therapy. I think they warranty those. It sounds like worn needles or nozzle. The needles are a satin finish. Look for shiny spots near the base. As they wear they get richer at idle. All the Nissan needles are rich at idle and lean mid and top end. I recommend the ADQ needles. Verify that the needle base is flush with the face of the piston. Setting it out .005"-.010" will lean out the idle. If the nozzles are worn Z-therapy rebuilds them.

Hello Keith-

I was wondering if you still sell the ADQ needles for a '69-1600? If so how do I order them?

I had worn needles and jets that were replaced with stock. I now have rich idle and lean running. Lowering the needle in the piston helped but I am still not happy.

Thanks Rick Hancock

[cbez]

It's actually going to idle a bit smoother typically if it's a little rich... not puffing black out the tail rich, but richer than 14.7 ratio, more like 13.5. Does the color tune give an approximate of what range the different colors correspond to? You can see why I love widebands. They do require quiet a bit more installation if you go permanently mounted like I have, but there was someone on the site that had a tail pipe portable version for loan...

Yes they explain the colors and speeds and such pretty well.

Another point that might be of use to someone:

I was tricking myself a little with the choke as it would idle after a cold start while out 4~ notches, then get worse and want to die when I would push it further in. And I would think ok its still too cold or not rich enough. Well that was because the further choke positions also open the throttle. So there's a 'valley' of rich mix, small throttle opening you can get stuck in.

[RBMann]

I always use the choke to get it started then pull the throttle lock to hold 1500-2000rpm and release the choke.

[cbez]

No throttle lock on early cars

[cbez]

measured my latest lil road trip, 32mpg! beats the 18 I measured before...

[RBMann]

No throttle lock on early cars

Oops, forgot I have a modern car.

Because of this thread I did buy a snail style air flow tool that I am excited to try when there is some time to get back out in the shop.

[cbez]

No throttle lock on early cars

Oops, forgot I have a modern car.

Because of this thread I did buy a snail style air flow tool that I am excited to try when there is some time to get back out in the shop.

Haha early as in 66-67...I think it came with the high windshield?