Wow, congrats. That old plumbing is looking nicer than mine, haha. So, now that Leno's Law has passed in CA, do you still plan to smog her? If I remember correctly, you're in CA, right? You need either classic car insurance -or- keep the miles below 1k annually. The law for our year goes into effect on January 1st.

Suggestion for the future tuning of the fuel system. Install a FP gauge between the filter and fuel rail. This helps a lot determining fuel issues in the future and monitoring the fuel supply. IMHO

1 hour ago, AndysPlit said:

Wow, congrats. That old plumbing is looking nicer than mine, haha. So, now that Leno's Law has passed in CA, do you still plan to smog her? If I remember correctly, you're in CA, right? You need either classic car insurance -or- keep the miles below 1k annually. The law for our year goes into effect on January 1st.

thank ya👍🏼

Yeah, still planning on smogging just to see the numbers so I can know if it’s running right and if all my various fixes worked.

But after that, yep, I’ll be getting hagerty insurance and she’ll be smog exempt. Can’t believe that actually passed🤯 I’ll take it. Yes, starting jan 1, 1981 and older is exempt as long as one of the conditions you mentioned are met.

14 hours ago, kcpope said:

thank ya👍🏼

Yeah, still planning on smogging just to see the numbers so I can know if it’s running right and if all my various fixes worked.

But after that, yep, I’ll be getting hagerty insurance and she’ll be smog exempt. Can’t believe that actually passed🤯 I’ll take it. Yes, starting jan 1, 1981 and older is exempt as long as one of the conditions you mentioned are met.

Sounds like a good plan. I passed smog earlier this year, not only feeling good about getting the test over with, but getting the car properly dialed in. Many seem to overlook the value in the latter part. I'll be changing my policy in the next few months, as well.

I have been very pleased with Hagerty! Good coverage and cheap. I have 3 cars with them and should have a couple more in a few years. If I could have nothing but classic cars, I would

Haven’t been able to find a good answer for this question…

Does anyone know roughly how many turns out the AFM air bypass screw should be?

Mine appears to have been monkeyed with by the PO and I want to return it to the “factory” setting before going to the smog station.

It’s my understanding that this screw was set at the factory using a CO meter in the tailpipe to measure emissions. So there isn’t an exact “factory” setting because it varied vehicle to vehicle. I obviously don’t have a exhaust gas analyzer, so I’m just looking for a ballpark setting.

Any insights on this would be greatly appreciated.

That’s the screw adj. not knowing a better phrase is where you can increase/decrease the rpm at idle speed.

This is a very helpful guide if you haven't stumbled upon it yet. It doesn't have a lot of information about the bypass screw, but it seems most of the work you want to perform is inside the AFM as opposed to the outside screw.

"It is used to close a passage under the primary AFM passage. This passage is primarily used to vary the air to fuel ratio at idle. The screw controls how much "extra air" bypasses the AFM. Closing passage is accomplished by turning the screw CW. btw When the intake "pops" aka front fire usually due to running lean;. the energy passes through this passage. If it is closed off, the energy will damage the flap."

https://www.atlanticz.ca/zclub/techtips/afm/index.html

Edited October 6Oct 6 by AndysPlit

@AndysPlit Thanks for the info👍 Yes, this is the exact article I had open while I was inspecting/cleaning my AFM. Mine looked completely factory inside, so I simply cleaned it, and sealed it back up.

Yeah, no mention of the outside bypass screw "setting"...

Maybe someone else will chime in with some insights on this screw.

Yarb, the idle RPM control screw is on the throttle body.

I don't think that the "btw" was relevant on the atlanticz page. Intake backfires will damage the AFM vane even if the passage is open. That's why Nissan put a spring-loaded relief valve in the later years of AFM.

The screw controls a passage that lets air past the AFM vane so that it doesn't tell the ECU to add more fuel. I used to tune mine for emissions by backing it out until the engine starts to stumble at idle, then running it back in a half turn. In theory, once it is correctly set it never needs adjustment. That's why Nissan sealed it shut before it left the factory, in later years. But, of course, we can't help messing with things, and they left it open with only a rubber plug stopping us.

Stole this picture from an eBay listing, showing the backfire valve, the round thing. Your 78 probably has one. Doesn't really matter, just interesting.

Here's the extract from the 1976 FSM that kind of summarizes things.

@Zed Head Yep, all correct info. You mentioned your "tuning" method and I developed my own rudimentary plan similar to yours, if no one chimes in with a ball park setting.

Here's my plan:

1. Back the screw out entirely, counting turns. Then set it at the midpoint. This will be "baseline".

2. Refine baseline setting with lean-drop method. Found an adjacently relevant BMW K-bike article describing this method. Attached here. Fully warm the engine, set the idle speed screw to spec, and turn the bypass screw for the highest smooth idle. Then turn it counterclockwise (leaner) until rpm just starts to fall, about 20–50 rpm, and stop there.

   See attachment explaining the process. Sound reasonable enough?
   

Adjust_CO_Mixture-K75K100.Exhaust.pdf

Edited October 6Oct 6 by kcpope

8 hours ago, kcpope said:

1. Back the screw out entirely, counting turns. Then set it at the midpoint. This will be "baseline".

2. Refine baseline setting with lean-drop method. Found an adjacently relevant BMW K-bike article describing this method. Attached here. Fully warm the engine, set the idle speed screw to spec, and turn the bypass screw for the highest smooth idle. Then turn it counterclockwise (leaner) until rpm just starts to fall, about 20–50 rpm, and stop there.

   See attachment explaining the process. Sound reasonable enough?

It's been a while since I've messed with one of those idle mixture screws and don't remember how many turns it takes in order to take it all the way out, but I believe there is an O-ring seal around the head. (Either there's an O-ring on it, or maybe I made one and put an O-ring on it? Don't remember.)

But, in any event... I wouldn't want that screw to be run out more than maybe five or six turns. If it takes more than that, I think you might want to look into possibilities in other areas for running rich.

Without and AFR gauge I think your plan is sound assuming you aren't having to run that screw out more than five or six.

Edited October 7Oct 7 by Captain Obvious
So many words