High Idle Issue

Hey guys. Name is Colin. This is my first post here and I was hoping maybe someone could help me out.

I bought a 1981 XJ750 Seca in winter. I finally got it out after a long Minnesota winter. After I rode it for awhile and got it warmed up it wouldn't idle below 3000RPM. I replaced all the seals and cleaned the jets in the carbs but it still idled high. I took it to the local motorsports place so they could check my work and make sure the carbs were synced properly. They could not get the bike to run below 3000 rpm with out eventually dying.

Has anyone had the same problem with this bike? The only thing I can think that might still be causing the issue is throttle cable. When I bought the bike they had clubman handle bars on that were considerably shorter than the standard. The previous owner did not change the throttle cables an I have since put the standard bars back on. I'm wondering if the maybe the cable is bent inside the line and not returning to fully closed?

Any ideas?

 

Disconnect the throttle cable at the carbs and see if it idles ok. If not try adjusting with the idle adjustment screw with the throttle still disconnected.

 

Have you cleaned the carbs + checked valve shims? Don't worry about your idle until things are in the proper operating condition first.

Also, have you visually inspected your rear brakes and front brake hoses + pads?

 

I have a problem close to this. my bike will not idle at 1 speed, if i set the idle one day it would be at 1100 and be fine, the next cupple hours it would be ideling at 4000, than in a cupple more hours it would idle at like 400 then die.
When do you set your idle? bike warmed up and choke closed? than turn the big screw on the back of the carbs, how do you get your hand in there, it likes to burn me.

 

Jchalo99 - using a long screwdriver you can turn the idle knob by using the notches in it.

MNStig - if this is the first time you are really digging into the bike since you bought I'd be willing to bet that the previous owner (PO) cranked up the idle to compensate for poor fuel delivery from the pilot circuit (dirty carbs). Are your pilot screw plug caps off? If so, you can bet the PO most likely richened the mix as well. When you attempt to lower the idle it is most likely getting too rich and bogs out. Of course, this is best guess as only you can see/hear the machine. There is no sense in doing ANYTHING further until you first (1) pull the rack (carbs) and clean the carbs. You don't necessarily have to rebuild them, but then again, you don't really have to use both lungs either (Chacal can set you up with a full rebuild kit for the whole rack - it is relatively cheap), (2) pull the valve cover and properly check you clearances - if any are out of spec pull the shims and use Fitz' chart to deduce which one you need to get to replace the existing shim(s), (3) wet set you fuel levels (after carb cleaning), (4) bench synch your carbs.

Before reinstalling the rack do a thorough inspection of your intake boots. Slight cracking on the outside is quite normal for OE boots - however if there is any cracking on the inside or any of the outside cracks are deeper than "on the surface" you need to replace the boot(s). You can RTV your boots to "fix" them but I would only ever suggest this if you are only trying to troubleshoot (IOW - if RTV'ing the boot fixed your problem then you now have justification to buy a replacement set). Now is a good time to ensure the vacuum nipple on each boot is covered by a rubber plug (save one for the petcock vacuum line if still equipped with OE petcock) and that those plugs haven't petrified - if they have (or if you are anal like many of us), bring one to your local automotive store and tell them you need 3 of them (4 if you are using a manual petcock).

Re-install the carb rack. Connect the fuel line and turn the petcock to "PRI" and wait and watch. If you see fuel start to leak anywhere (especially from the throttle body) turn the petcock back to "ON", drain your bowls, and pull the carbs back off and recheck/wet-set your fuel levels. This should not happen if you properly wet-set your fuel levels on the bench in the first place.

Assuming no fuel leaks, connect the airbox and tighten all clamps. Connect the throttle and "choke" cable.

With the petcock still in the "PRI" position, fire it up (with thumb on the kill switch in case the RPMs are way too high). Use the idle adjustment knob to bring the RPM's down to about 1,100 --> 1,300 RPM. If you get this far then (a) you have found and fixed the source of your issue and (b) now need to dynamic synch your carbs (I am a huge proponent of the loop manometer method) and properly set the lean/rich mixtures (Colortune is the best way, although a good ear will get you very close).

Good luck!! These bikes are worth the effort - take your time, DO NOT take shortcuts and be willing to spend the money to fix/replace things properly. Or, you could always break out 15 grand and go buy a new one - your choice

 

Yes, you DO have to rebuild them... unless you want to continually have carb problems. There are microscopic passages in these carbs. Trust me.... I "cleaned" my carbs 8 times and was still having problems, until i finally broke down and listened to the Wizards on here. Did it the RIGHT way and it SOLVED the PROBLEM

***EDIT***
http://www.xjbikes.com/Forums/viewtopic/t=8918.html - how to clean the enrichment circuit well
http://www.xjbikes.com/Forums/viewtopic/t=14692.html - in the church of clean
http://www.xj4ever.com/mikuni%20carb%20cleaning.pdf - mikuni carb cleaning (basic idea applies to hitachi's as well)

 

Did you miss the sarcasm? ;-)

Rebuilding carbs on a newly bought {used} bike is a no-brainer - ESPECIALLY if it is exhibiting issues. Be sure to replace the throttle shaft seals as well (these do NOT come in a typical rebuild kit).

 

These will also need replacing. A very common problem for high idle.