SOLVED: Very high idle when engine warm/hot

I just wanted to put this info out there since I searched all around for why my engine idle was high and never saw this as a possibility:

Symptoms:
-After ~ 20 minutes of driving, idle would begin to creep up and eventually the engine would rev @ 3000+ rpm when I came to a stop (very bad and unpleasant).
-Turning idle adjustment screw (a knurled knob in the middle of the carb rack) all the way out did not decrease idle
-I was able to drop the idle by pushing down on the sync screws (part of the throttle linkage)

Solution:
After being dumbfounded for weeks and dreading the fact I had to pay $$$ for a carb rebuild on this bike I just bought, I realized if I can drop the rpms by pushing down, the throttle must not be freely returning to position when released; that seems obvious now but it took me a while to have the 'eureka' moment. Instead of ordering a new spring and taking apart the carbs to install, I just bought a spring and some s-hooks from a hardware store and made my own little return spring attached to the throttle linkage. WORKS LIKE A DREAM!!! High idle issue has never returned! Total cost: $2.50

I think the old return spring simply loses its strength when warmed up (or there is a significant increase in internal friction of the throttle assembly) and could not force a full return of the throttle to idle position.

Hope this helps someone else with the headache!

 

This reason for a high idle has been mentioned - the throttle cable is binding or hanging up.

Those return springs are pretty beefy, even as they age. How old is your cable? How is it routed? When was it last lubricated? Do you have any handlebar modifications that could have changed or required changing it's routing/length/free length? Did you or someone else break the rack and not install the springs correctly (you have to wind them up to a degree)?

The force from your spring assembly might be overcoming a hang up in the throttle cable, but I'd say that solution is far from ideal, since now you throttle will take more force to hold open, and could cause fatigue on longer rides.

 

I'm going to try this. I don't think 1 extra spring would matter much, you're already holding 3 open with the throttle. I'll report my findings in a few days.

 

The throttle return springs are part of each individual carb.

If all 4 throttles weren't/aren't "returning" correctly, then most likely there is a bind in the "linkage" somewhere. Keep in mind that the "linkage" is simply the throttle mechanisms of the four separate carbs "acting" upon each other. If they each work fine individually; but then the aggregated "linkage" binds, there's an alignment issue between two or more of the carbs. Has to be; they each have their own strong return spring.

By adding a return spring, you dealt with the symptom, not the REAL problem-- there's a bind somewhere. Together, those springs are plenty strong enough to "return" even a slighly balky cable.

Just because you paid a shop a lot of $$$ to "rebuild" the carbs doesn't guarantee they did it right. In this case, they apparently did not.

 

I was worried about having to have the carbs rebuilt but I did not actually do it (in fact I would prob try it myself with all the great info available here).

I know for certain it is not the throttle cable binding as I can see it go slack when throttle is released.

Bigfitz, can you explain where the return springs are located on the carbs? I have looked at the exploded diagrams without luck in finding them; there is a low-quality diagram here:

http://www.boats.net/parts/search/Yamah ... parts.html

Certainly, it is theoretically possible that the springs have stretched over 30 years (and further lose tension when warm) is it not? Though you are all likely correct that it is some other binding going on...

Next post will be a pic or two of the spring location.

 

Spring

 

I should add, it has increased the force required to open throttle obviously, but it is not very bad (with the right size spring). I have gone on long rides with @ss fatigue being the controlling issue and (not wrist fatigue)...

 

Mine snap shut with authority. I also sonic cleaned the springs when I had my carbs apart for the throttle shaft seals. I used chacal's special plate and a level for reassembly.
For a couple bucks it's worth a try. Modern bikes have 2 cable push/pull system anyways.

 

Not really. They are pretty beefy, and not really under that much stress. Also, the cycling loads aren't very high or frequent so they are unlikely to suffer from fatigue. And they won't get soft at the temperatures seen in the linkage (I'm sure you can touch them, they probably don't get all that hot).

Ignore my comment on the opening force, I had the same sentiment that BigFitz did about not fixing the root problem, and was trying to give a good reason why this fix isn't right.

I was assuming cabling given your symptoms, but if you confirm that the cable is loose, then the throttle linkage is the only answer. The springs have to be wound up, oriented correctly, and secured correctly to close the linkage fully. Easiest way to verify this is to pull your carbs and take some pics of the front. Also open the linkage by hand. If it is really easy, something isn't assembled right.

And then you can check the alignment by setting the front end on plate glass and checking for gaps.

If you need to break the rack to reassemble the thorttle linkage, it'll give you a perfect opportunity to rebuild the carbs ;-) . These bikes are old, and the process of rebuilding the carbs fully can solve MANY running issues.

 

I would check your cables Durk. If your throttle snaps shut with authority, a little extra spring won't help. And if it does, then it will either have to be a very strong spring (again getting back to my fatigue comment) or there is something else wrong to begin with.

Push Pull cables probably allow for a weaker return spring (making the throttle easier to open) and act as a safety if the spring were to fail. Custom installing a push/pull system might be a neat answer to this problem, but IS NOT the same as adding an extra spring.

 

I have the GP touring bar so I had to be creative with my cable routing.
I picked up the parts last night, but I've been really sick and not able to test this out. Maybe tomorrow or Friday. At least if it works I'll know what to fix this off season (replace return springs and other stuff)

 

You can do a quick check: if you can push down on the throttle linkage (I push the sync screw) and it reduces idle speed the spring quick (temporary?) fix should do something for ya

 

Before had that problem ( i didn't know that problem can show up) I took the springs off and now I'm riding like the Harley system. I didn't want to install a cruise control, so that was the best way to go.. It took me a couple of rides to get used to the "new throttle" system...but is great in long rides...

 

Again; on the XJs, the throttle return springs are individual to each carb; together they make up the "linkage."

Take ANY carb off, hold it in your hand, and play with the throttle mechanism. It has a nice hefty return spring and will snap the butterfly shut with authority.

Bolt all four carbs to the rack, each with their throttle arm engaging with its next-door neighbor, and you now have four springs acting on four carbs all being held together tightly in a line. If they're NOT aligned properly, or somebody wasn't careful assembling them, it only takes ONE of them to keep the whole thing from closing smoothly.

The most common cause of this is a spring "ear" binding or slightly out of place in regard to the throttle "plate" of its carb, or one misaligned butterfly. The former can be caused by wear, the latter more often caused by less than careful re-assembly by somebody. Like I said, it only takes one to bind the whole mechanism.