Air-fuel mixture screws - missing?

[Remi]

First time motorcycle owner - decided to go for a vintage I could work on, to learn about the machinery behind the bike!

Got a 1980 XS400 Special II

Currently struggling with idling the bike, it will idle with the choke fully open but as soon as I try closing it it slows rpms and dies. When I tried to test ride it with the choke open, it backfired quite a bit (understandable?).

Changed oil, spark plugs, and battery thus far, everything helped but nothing fixed it.

Looking for info, I read something about fuel mixture screws, decided to try to unscrew them a bit, considering the engine might be getting too much air... go look for them and I can't find them?

I think I'm looking in the right place, there's just no screws there. I need confirmation if I am looking in the right place, if someone would please help me!

Thanks a lot!! Ride safe

 

[Revvy Kevvy]

Oh, the wonderful mukonis, the air fuel mix is here as I recall, the Hayned manual tells you where to start but its 1 and a 1/4 out just from memory.

 

[Durney]

The idle mix screws should be turned out 2.5 to 3 turns out then fine tune while the engine idles.

 

[Revvy Kevvy]

Manual says 1 and 1/4

 

[CaptChrome]

Manual says 1 and 1/4

That might be for the early bikes, especially the non-North American bikes which started to get different jetting to meet emissions regulations. For the later ones like the 1980 mentioned above, I would follow the 2.5 to 3 turns out advice. An unmolested 1980 carb will have anti-tamper plugs in the holes for the screws that must be removed before they can be operated. Checking the factory setting immediately after removing the screws would be good to do also.

On my 1981 Special II, I started with 3 turns out for the first start and then tuned the carbs from there. I would do the same again on any 1980-1982 SOHC bike.

 

[ElementalWarrior]

Here have been my learnings. I have basically the same bike, same year.
- There is a fuel pickup built into the float bowl side wall. If the bike has sat for more than like 6 weeks, it will undoubtedly be plugged. The only thing I could get to clear this was sitting there with the smallest guitar wire, and pliars, and continuously poking until I threaded the needle.
- The carbs work on a fair amount of suction/vacuum, if you have the stock intake boot and box filters, make sure you also have the drain tub connected between the rubber intake boot and the crankcase.
- My mix screw is at 1.5 turns, per the manual.
- I ordered and put in the plug for the carb pickup tube inside the carburetor https://www.mikesxs.net/carb-rubber-passage-plug-5.html. It came standard on my bike.
- If you are low on fuel, the stock petcock doesn't have enough gravity pressure when in the on position (IE float bowl may be empty, even if you have a clear fuel tube that appears to have fuel), try reserve, or put more fuel in.

All of the above will affect fuel mixture/idle.

It takes a minute or two for the bike to warm up, you should have quick/responsive throttle while the choke(its a fuel enrichener, not a choke, it just adds more fuel) is pulled out. If you don't, wait longer.

From what I've read, the people running pod filters need the fix mixture screw at like 3 turns. If you run the stock setup, ~1.5 works for me.