Carb Issue- Hanging Idle when warm- Help! Video Attached

Seca 750 and while warming it begins to hang @ 3k
Carb Rack has been broken 3 times, and removed and reinstalled at least 8-10 times.

*New throttle shaft seals and carbs are cleaned
*Carbs have been synced
*Pistons, and bores have been polished to a mirror shine/passed the clunk test
*Mixture screws have gotten new orings and are in the correct order
*No known vacuum leaks/Starter fluid used to check

I have been working with Rick to get this figured out but no luck yet.
I am planning on removing and breaking carbs ONE more time before I give up!
I am hoping for some guesses of items to check while I am tearing into the carbs again.

My current guesses are---
*Spring is no good or not properly tensioned
*Main needle is not seated properly in the emulsion tube
*A mysterious air leak that I can not find

BUT...It could be something simple I am overlooking....

Attached is a video of the issues so please enjoy.

http://www.youtube.com/watch?v=YB3hoNGaHZo

 

Fault from your video it sure looks like your butterflys are not closing all. I would suspect the springs since you did everything else.

 

i'd like to see the throttle cable move before the linkage moves, give it some more slack
when the choke comes off can you see light between the lifting forks and the
choke rod?
you ease the throttle on and off, try twisting it way open and let it slam shut
just for testing
you checked the fat spring skinny spring thing on the sync screws, right

 

I'm working on another members carbs that are doing the same thing. The carb #3 seems to be binding for some reason. We are going to work on this problem on Saturday. I will post what we find.

Just to give back ground. The carbs were taken apart and cleaned by a carb expert here in town. The owner of the carbs had to dog the guy to put them back togeather and give them back. Believe me our member was not happy with his carbs. So he brought them over to me. I found an extra plastic spacer washer on #3 carb next to the barrel spacer. We took that out and tightened the nut that holds the linkage on. The throttle shaft is still binding for some reason and does exactly the same thing. RPMs hang untill you push the linkage back down.

 

just a thought but if you take them off again check the butterflies one of the edges could have a nic or something or could be off center. i center them when reassembling very carefully by closing them with the screws loose a few times wile progressively tightening the screws until snug.

just my 2 cents.

 

If that's happening on the video (I wasn't able to see exactly what's happening) my two cents goes to two things. First of all, carb sync. I know you did but I experienced this behavior many times and the final solution was to remove the carbs, do the bench syncing (in any ways) put them back and the high idle should dissapear. If you rotate the throttle linkage screws during sync more than a half turn after bench syncing I'm sure you have a bad sync.
Moreover, for me the sound of the engine suggest that the carbs are off sync. Not much but it should be enough to cause a high idle.

Another suspect would be the idle mixture. It's quite funny but if you put a wrong jet (smaller in size) or you changed the mixture setting then the engine can have a hanging idle. It took many tries to finally find the proper setting for my bike. If I put smaller than necessary jets, the engine always wanted to idle high like yours.

What idle jets do you have? How many turns on the mixture screw?

 

Z

The bench sync was as very good. (did it with a business card strip) I set the throttle blades and tightened with the shafts in the closed position. The idle jets are 40's and the mixture screws are at 2.5. I'm sure that #3 carb is the culprit. I am going to seperate the rack and try each carb by itself. I have my spare virgin set of carbs that have never been apart. I will compare and let you guys know.

 

Update 2/14*******


Found just what I suspected Carb #3 was binding. I would suggest pulling the rack apart and try to move each carb. See if one is binding. Pay close attention to the butterflys and how they fit in the bores. Also check the throttle shafts to make sure they fit properly.

 

your vacuum pipes dont look right on my bike the vavuum pipe runs from no1 to no 2 and not to the back those are for air to your flout bowls

 

Did you ever get this resolved? If so, what did you find?

 

Your vacuum line that opens the petcock is coming off of no.1 carb. I don't know if it makes any difference, but mine is coming off of no. 3.

 

If you're still having the hanging idle issue this is what I would suggest you try.


There are only 2 major mechanical systems at work here. The throttle cable from where it anchors to the carbs back tp the twist throttle and the throttle mechanism on the carb rack.

If you disconnect the throttle cable at the actuator on the carb rack, start the engine and exercise the throttle by hand you might be able to isolate the issue if it is mechanical in nature. I saw the slack in the cable on the video but it's a stiff piece of cable.


Might be worth a try.


Bon chance

 

AF What did you find on your carb issue? I had jbunkes carbs on my bench twice and the #3 carb was binding. I have worked on several carbs since and it seems that it is real easy to overlook binding throttle shafts. When you split the rack you have to work each carb by itself. Work it slow and just let them close with no pressure if they bind the slightest it will cause the trouble you described in your video. If you changed the throttle shaft seals you need to center the throttle blades when you re-install them. Its not hard. All you do is barely tighten the butterfly screws and close the throttle on the carb. hold it shut with slight pressure and tighten the screws. Don't forget to locktite the screws or you could suck one into the engine. I had one other problem with the throttle shafts. Sometimes the nut that holds the linkage on will put a burr on the shaft. You need to file those burrs off before you pull the shaft out of the carb body. Just use a small file and take your time. When putting the carbs back togeather you need to make sure the springs get back on in the proper position also. Hope all this helps

 

I had 1 set that when I tighten the brackets that hold the carbs together they did that I can't remember what it was that caused it specifically,,,,but I do remember something about the sync screws an their springs working against the return spring also,,,,Dan

 

mine had the same issue until i cleaned the upper fuel circut behind the mix screw for the second time. look in my gallery for a pict of the circut. you have to clean both directions. also whats your vacuum readings on each intake boot? Do they Match cold and hot?