likely dumb question re air screw and gas cap vent

so first off i dont know if i have an air screw or a fuel screw on my 78 kz650 sr. That said, is there a recommended good place to start with them, or a good how to on how to determine if I should mess with them in the first place? Also i punted the factory tank because i hate the look of it and put on a tank from a 77 650c. I am wondering how do I check the gas cap vent? blow pressurized air into the petcock? 

thanks in advance. 

You can take the gas cap apart and make sure it's clean.  

Take a look at your carbs.  Air screws are on the outside of each one.  Fuel screws if you have them are directly underneath each carb outlet. 

It sounds like you need a service manual.  

Rough rule of thumb on idle mixture screws found on slide carbs:
If the screw is on the airbox side of the slide, it's an air screw, likely with a relatively large, blunt taper.  Think worn tip of a pencil lead.
If the screw is on the intake port side of the slide, it's a fuel screw, likely with a tiny, sharp tip at the end of a long, gradual taper.  Think freshly sharpened pencil with a sharp, delicate tip.

If there are exceptions we don't know about them.

Good Ridin'
slmjim & Z1BEBE

To the best of my knowledge the 1978 KZ650 SR is also known as a D model. There were 2 versions for that model and year, the D1 and the D1A. The D1 came stock with vm24ss carbs that have only an air screw. While the D1A came with vm22ss carbs that have only a pilot screw. So depending upon which model you have the stock carbs will have one or the other type of screws. As indicated air screws are on the air side of the carb, near the intake bell. Pilot screws are located on the bottom of the carb/float bowl on the engine side of the carb. I have attached a pic of a carb that has both types of screws (note the carbs in the pic are vm26ss carbs and not the 24s or 22s that your bike came stock with) But they do show the locations of the 2 different type of screws. 

As to if you need to make any adjustments to which ever type of screw your carbs have is dependent upon how it runs currently.

there are 4 screws all accessible from the outside so yes sorry air screws. 

As to if you need to make any adjustments to which ever type of screw your carbs have is dependent upon how it runs currently. [attachment=170227]vm26 apart.jpg


It ran and started poorly but so far I have found and corrected (and not tried running yet) 
1. valves out of adjustment - adjusted and put in appropriate shims, re checked all in spec now 
2. float heights off - measured and ensured all were bang on
3. bowl fuel levels - one was good, two were low one was too high - all adjusted 
4. main jet oring on one carb was destroyed and jet was sitting high (not tightened down all the way. Rectified  and cleaned o ring debris
5. checked and cleaned out needle and pilot jets

so it may run great now who knows but I want to do what I can to prevent going through all this stuff again. I was wondering is there a starting spot for these air screws located on the outside of all four carbs? like two turns out to start and then adjust as needed etc? 

thank you!!!! 
 

Here is a list of several models/years of stock carb settings from FSM's, the bottom list covers a D1 model.

this is a great resource thank you. I am also not running stock jets, as I have pods and a 4 into 1. So I should have started with that perhaps as I am guessing that also throws the air screws out a bit from the factory settings. I think I will make markings on them all and turn the screws in and figure out where they are all currently set to as a start, use the charts and also see how she runs. 

thanks so much! 

OK, then you might want to read more on the air corrector jet that Zed has offered as a solution to overcoming the lean issue that happens when the slide opens up more of the carb throat, when running pods.  As I understand it he indicates using all stock settings re jets other than increasing the main and installing the air corrector jet on the port that provides air into the needle jet, jet needle, main fuel jet. But please search for posts on the subject as I do not mean to speak for him, as he does it much better than I do.

Keep in mind that the air screw has most effect when at idle and just off idle.

I took my gas cap apart, the paper seal disc was mis aligined, mine has a curve notched our of the cap and the disc has a hole that lines up wit it. I assume its made that way to stop it from allowing water in or gas splashing out. Vents now. 

after all the valve adjusting and carb work, she flashed right up, and for the first time i turn the choke off and it idles well, no more carb backfire, ni more exhaust backfire, no more fuel pouring out of the carbs or via overflow through the bowl drain. Yiu guys are awesome. Thanks so much. 

Perhaps better explain HOW to determine which model 1978 KZ650 you have. That's by the VIN, correct? Is there a decorder on-line?

Though not a decoder here is a table I made in the past with frame and engine numbers for the (K)Z650 models. This is info I could find and make sense of, but was never intended to be 100% correct, nor have I ever claimed it so. It was more to see what I could learn about it.