1985 Yamaha XJ 700 (Airhead) won't idle without choke

Thanks for the response... I get the idea of what the manufacturer is telling you. What I can't wrap my head around is why each carb does not have a good running idle set screw that will allow the engine to run even if the cables were not attached?

I guess the "Y" cable same thing that is on my Suzuki VT1400 intruder, does the synch. I would guess that both of those carbs have a idle screw? Or like the VT1400 only the rear carb has the idle screw and then you synch the front to the rear.

Bottom line, at idle all 4 of the cylinders should be running. You should not have been getting the intermediate firing of your cylinders that are attached to the same carb. If you have spark to all plugs and 1 and 2 are running the same off the same carb, same for 3 and 4. The coil banks are 1 &4 then 2&3, the carbs are feeding 1&2 then 3&4, still scratching my head why your bike will idle well without throttle cable action? There should be a "base setting" on the idle screws to keep the bike running like a well oiled engine without input from the throttle cable.

As long as you are happy with what the manufacturer is telling you and the bike is running the way it should be, that is great!

 

I understand your confusion.. I wish I knew more about these carbs. I’m sure they have all the bells and whistles a carb should have, but the manufacturer insisted no adjustments be made. These are Mikuni carbs but deeper reading online says that they are modified from OEM by Murray with “trade secrets”.

Thats enough info for me to be perfectly fine with sending them back for repair/tuning instead of handling it myself.

I don’t understand how 1 and four can be firing normally but 2 and 3 have issues when the left carb feeds 1 and 2 and the right carb feeds 3 and 4, like you were saying. I’d expect issues with every cylinder or just no start at all. Maybe if the carbs weren’t tuned properly to begin with or tuned for the incorrect make/model/engine, I could be experiencing these issues..

I pulled the plugs after a short run with choke and then a short run with just throttle control to keep the bike at idle and plugs 1 and 3 were perfect, 2 was wet and four was black as night.

Sounds like carb issues to me considering I’ve tested just about everything else that could be a problem but it’s kind of all up in the air until I get these carbs back and put on.

They promised a one day turn around so I’ll update with either good news or bad news on their return. Hah

 

i also dont get, how the carbs can be "sent off" to be tuned,?
surely the carbs are tuned to the engine , if one, or more ,of your cylinders is pulling more "suction" than another , then the carbs are synched to the cylinder, to make them even again , how can they do that "off the bike"

 

They have XJ test bikes and motors that the carbs are tested on before shipping out.

If all of your cylinders are in an ideal state and everything is within spec, then I don’t think there would be any issues with a tune on a test bike verifying the carbs will work for your bike. People should probably verify everything is in spec before making the purchase anyway since they aren’t cheap. I didn’t do that but I got lucky..

The owner said he keeps open communication so that anyone can call in and receive instructions to adjust carbs if they aren’t performing as they should. I called and we determined I had a bad splitter effecting the carbs slides and for safety and reassurance he wanted the carbs to be sent back for new cabling and splitter to be installed. Then they will be tested on a XJ test bike, tuned/recalibrated and sent back.

 

Every bike has different compression, valve clearances, etc that affect how carbs are tuned/synced. How do you do that on a different bike?

 

Thats where the phone number for instructions on adjustments to be made at home comes in handy.

 

I really hope this turns out well for you. Hope it is a plug and play for you when the carbs come back. Enjoy the bike and be safe out there.

 

My XS was fitted with a pair of VM34s by a PO.
It's not exactly rocket science to sync them.

 

Any update on this?

 

Carbs came back and I reinstalled with no change in performance. Home adjustments with manufacture recommendations over the phone helped a little but didn’t solve the problem. I identified a few vacuum leaks around the intake boots. No surprise the leaks are on the two cylinders running cold. It’s been a bit cold out so I haven’t been too motivated to work on it. I doused the old boots in carb cleaner while running with choke and it would rev up and fire on all four. As of right now, vacuum leaks are the new suspicion. I’ll post another update once I get the boots installed!

 

So I guess my question is, are you happy with the carb purchase?

 

Yes. I don’t regret the purchase. Granted I could have spent the same amount of money having the originals professionally rebuilt and tuned but I wanted the added performance. I’ve talked to a few people who have purchased the same carbs for the same bike and their bikes are running like a fine tuned machine with no issues. I just need to find my underlying problem.

 

**Update** Intake leaks from the carb boots was the issue after all. I do still have to use choke to start the bike if it sits long enough to cool down but after about a minute of running with choke, it will idle well. I think once the piston rings reseat then the inability to start without choke, will go away. Time will tell.

Took the bike for inspection today with no issues. I’ll post an update in the next 100 miles and see where we are at.

Here are couple photos of the bike finished and the carbs that have been much of the discussion.

Also, if anyone wants my oem carbs - you pay for shipping and you can have them. I’m fine with letting them go instead of them sitting in a box in the garage. They’ll need rebuilt.

 

Good deal. If I was keeping my airhead I would be interested in an extra set. @hogfiddles?

 

I’ll take them!!

 

@Shelby Biggs can you post a picture of the correct throttle cable routing? I'm installing the Murray's carb set on mine right now.

 

Shelby hasn’t been seen since March 28... two days after his post....

 

Well, I suppose that figures.

 

Cable routing is simple though ....

-Left hand enricher cable goes into the tunnel from the left side of the headtube

-left hand clutch cable goes into the tunnel from the left side of the headtube

-right hand throttle cable goes into the tunnel from the right side of the headtube

That way you don’t get cables being pulled tight by wrapping around the headtube as you turn the handle bars.

 

What a load of twaddle - fitting two carbs instead of four adds performance?
Sure, in the land of bullshit....