Help 1980 Yamaha xs400SG Bike will not idle

[jlearning1]

Hello Everyone. I finally got my bike running. Heres my problem, my bike will not idle unless I have the choke pulled all the way out, and then it still barely idels with the choke pulled out. I have cleaned carb, checked for leaks at the carb boots, no leaks. I set my pilot screws/jets to 3 1/2 turns out when I put the carbs back together. If I ride the bike, it rides good, I just have to keep the throttle going or it will die at a stop sign. Any suggestions would be appreciated.

Thanks

 

[HoughMade]

Can you get it to idle steadily around 1200-1500 rpm using the throttle or do you have to keep revving it?

 

[jlearning1]

I have to keep reving it up. With the choke all the way out, it only idles around 800 to 1000 rpms.

Thanks for the quick respond

 

[HoughMade]

Can you make any headway with the idle air screw?

 

[jlearning1]

Where is that located? Is it the big round screw between the two carbs, or that small scew next to the throttle cable housing?

Thanks Again

 

[HoughMade]

This one (light circle, but it's there):

This is a popular picture, I guess.

 

[jlearning1]

I have turned on that screw, ( just a few turns in and out) and it made no difference. To set that screw, do I turn it all the way in (flush) and then back it out so many turns?

Does the throttle cable require an adjustment?

 

[HoughMade]

That screw is the only thing that adjusts idle speed. Just start it and screw it in until the idle is at 1200.

 

[OliverB]

As HoughMade said. Don't mess with the other lower screw as it adjusts the synchronization between the two carbs.

Have you replaced the VERY SMALL o-rings at the bottom of each idle mix screw on top of each carb? If not, do so as these are notorious for leaking air into the system and messing up the idle. The o-rings will be at the bottom of the shaft after removing the screws and can be very difficult to see.

 

[drewpy]

I've started cleaning carbs in my dreams.

here is one I've done earlier

 

[jlearning1]

Hello

Were are these idle mix screws. Are they the same as the Starter Jet Screw? I did take those screws out of the top carbs(1 On each side) and cleaned them, but I did not see any O-Rings in there.

 

[OliverB]

I have never seen the term "Starter Jet Screw" used to refer to the idle mixture screws. Don't know about that one. If your carbs are the Keihin brand, there is one screw on the top of each carb. If this screw has never been touched before, it may still be under the factory installed plug (very small hole in top of plug) placed there to keep us from messing with the idle mix adjustment. If the plugs are still present, you have to carefully drill them out to access the screws.

 

[jlearning1]

My Carbs are Mikuni BS34 I have located the idle mix screws. I will take them out tommorrow and cleaned them and check the o-ring. I did not take them out when I cleaned my carb


Thanks

 

[OliverB]

I am not familiar with the setup of the Mikuni carbs. Others here will need to chime in with some help. What I aid about the idle mix screw and o-ring may not apply to these carbs. My son recently got a '79 with these carbs but we have not yet gotten into them.

Out of curiosity, what is the month and year of manufacture as found on the Federal ID sticker?

 

[HoughMade]

This is the idle air screw. Screwing it in opens the throttle butterflies and raises the idles. Screwing it out does the opposite:

 

[HoughMade]

Sounds like you found the idle mixture screws, but for others, here they are. On my bike ('79) there is a plastic cap that pops off. Only about 1/2 turn adjustment can be done with it on. Just a note, on bikes before about 1980, there are no O rings, so don't look for them.

 

[jlearning1]

The Data Plates says 4/1980

Thanks

 

[OliverB]

HoughMade,
Thank you for the explanation. My son and I are just about ready to tackle the carbs on his '79 but haven't read up on them yet.

Interesting, didn't know this brand was used on 1980 XS400s. Maybe they were changed out sometime with an older set. The date shows the bike is a late 1980.

 

[jlearning1]

Okay My Friends

I have taken the carb back off today, and heres what I,ve completed
1. Set floats to 26mm+-27mm
2. Set pilot jets to 3 turns from tightness
3. set idle mix screws 3 turns from tightness

Hers my problem. My bike is idling at 1200-1250RPMS. I have the petcock set to the prime position. I can only set the idle using the throttle cable adjustment. I tried to use the idle air screw (screw between carbs. The Big one) but it will not adjust the idle at all. I have adjusted the throttle cable to hold the throttle open to about 1200-1250RPMS. If I back the throttle cable all the way off, and try to use the idle mix screw, it will not push the throttle up far enough to set the idle to 1200RPMS. I actually road the bike around the corner, and the bike ran good. No popping, no backfiring, no smoking. At stop signs it did idle back to 1200-1250 (with throttle cable adjusted downward). Does anyone know what my problem could be?

Thanks

Any Help

 

[jlearning1]

Any Help