Racing RPM's that subside after 3 seconds

Ok - starting a new thread - directly related to this thread:
http://www.xjbikes.com/forums/index.php?threads/sticking-throttle-cable.47705/

After a week of 'normal' performance the symptoms have returned.
After the engine is warm, in fact hot would be more appropriate, the RPM's will hang high and slowly (about 3 seconds) come back down.
3 seconds is a lifetime in rush hour traffic.

See above link for what has been done so far.

A thought came to mind and would appreciate any feed back before I remove rack and have to break it.

I am quite certain that HCP2421 Buna butterfly shaft seals came with the Deluxe Rebuild Kit.
When I replaced the butterfly shaft seals I used Parker Super O-Lube (http://www.parker.com/literature/O-... MSDS/MSDS Super O-Lube 2012 - 16 Section.pdf). They describe it as "Clear Dimethyl Siloxane Polymer" with a Generic Description of "Silicone".

Here's the thought - here at work if we used the above SUPER version of Parker O-lube on Viton it would cause the viton to expand. . . . If we used Park O-Lube (sans SUPER) there was no expansion.

Buna is NOT Viton.
Could buna react the same as viton with this chemical?
Would it make sense that it took 4 months for the SUPER O-lube to permeate the rubber seals, which when upon being warmed by engine heat expand enough to cause resitance to the butterflies closing?

I know if I press down on any of the sync adjustment screws that I can close the valves as quick as I press them.

When the engine is cold it doesn't appear to have any problem.

It almost appears that if I stay below 3,500 RPM's (ABSURD!) that the butterflies are not as prone to 'hanging up'.

I am distraught - I love a crisp day for riding and our weather has been plenty crisp around here.

Any help would be greatly appreciated.

 

Does your throttle snap back? If it does then it could be the carbs need to be re synced.

 

I like the re-sync check also. like last time just to see if one carb seems to have drifted out more than the other... might give you a clue as to which carb is causing an issue.

Having to go above 3500 rpm to duplicate seems it would indicate one of the slides hanging slightly once the bike is warm.

If you pull the carbs, I would look very closely at the throttle plates to be sure they are installed correctly - if you back off the idle screw the throttle plate should just cover the small hole in the carburetor bore. It is easy to get one off slightly as the holes in the throttle plate are oversized to allow for tolerances. Proper assembly would be to hold the throttle plate against the flat portion of the throttle shaft, then verify throttle plate is completely closed, then secure the screws.



Also, if you pull them I would definitely add the purple / blue Loctite to the throttle plates. Might even have one a bit loose since Loctite was not used during the re-build. If the adjustment looks good, do one screw at a time.

 

revisit your mixture screws, theory, once your warmed up there's a cyl that doesn't like it's idle mixture. once off idle it works fine and when it comes back to idle it works for about 3 seconds(fast idle)
then it cuts out and the idle drops to *normal*. one of those IR temperature guns might help find a lazy cyl. as soon as the choke is off and you have a steady idle.
check your choke plungers, vacuum leaks and tweek the mixture screws.
defiantly do another running sync too

 

HCP2421 are fuel pipe o-rings, not throttle shaft seals..........hope you didn't use those as throttle shaft seals........if so, there's your problem and you need not theorize any further!

HCP38B are the "v"-ring throttle shaft seals, and I'm pretty sure those are made of Buna-N.

 

Right you are Len, I meant the HCP38B's (that came with the Deluxe Rebuild Kit) They have an "H" cross-section profile.

Throttle snaps back when cold.

Hanging slide is interesting, although two Sundays ago I had the rack off and everything seemed to clunk well (with hats off).
Would a hanging slide create a condition where the carbs are 'sucking' more air and causing the butterflies to not close immediately??

Checked temp at exhaust , top bend of pipe and all 4 cylinders were within 10 F.

Vacuum leak was tested multiple times last week.

Enrichment plungers are seated.

Running sync sounds logical, need to wait a couple days to borrow from a coworker.
Have a colortune spark plug and can go through air/ fuel mixture.

Carbs were broken off rack and completely rebuilt May of this year, bike ran great until about 4 weeks ago.
"Strangeness" coincided with replacing throttle cable - throttle cable is NOT the issue (read link in first post of this thread).
Keep in mind the bike starts fine, idles appropriately and runs as it should until the engine is hot.
When the butterflies are sticking, I can press down on the sync screw and reduce the 3 second 'wind down' time to as long as it takes me to push . . . To me this indicates either something is amiss with a spring or something is restricting free motion of the butterflies . . . Last week with rack off all springs visually appear fine.

Has anyone used "Clear Dimethyl Siloxane Polymer" on a rubber component and experience expansion? I admittedly was rather liberal with the silicone gr

I appreciate everyone's input - I am also thoroughly frustrated as there are a lot of other things going on in my world now and unfortunately the XJ is slipping down the priority list.
Why does shit have have to go FUBAR on multiple levels in multiple arenas at the same time????

 

P.S. this new site ROCKS!
Thank god for the 'auto save' feature when composing . . . Thought I lost my spiel, came back and it was still waiting.

 

So when you're having this sticking high RPM issue, you can manually push down on one of the carb linkages and it immediately returns the engine RPM's to normal low idle and stays there even after you stop pushing, right?

If that's the case, then you can rule out all the second hand issues like syncing, vacuum leaks, idle mixture, valve adjustment, hanging slide.... Forget all of that stuff.

You've got a mechanical binding issue somewhere. Either in the cable, or one or more of the carbs.

On my bike, I can actually see the throttle cable go a tiny bit slack at the carbs when it's fully released. I can actually feel it in the hand grip too when turning the throttle. The first few degrees don't do anything until the tiny bit of cable slack is taken up, and then after that, the cable starts to pull the carb linkages.

If you can see the same thing on your bike, then it's not the cable. Might narrow down if it's still a cable issue or if something went wrong with the carbs themselves.

 

from the Parker site:
Parker Super-O-Lube is a silicone fluid recommended for use with all rubber materials. Only a thin film of Super-O-Lube should be used with silicone rubber to prevent seal damage.
so i really doubt that's the problem.
from playing around with yics tools and vacuum sync. it seemed to me that a high idle caused by a out of sync condition that was dragged down with the clutch in gear and back brake, did not go back up. in other words a bad sync, once brought down doesn't go back up. a vacuum leak will, a stuck butterfly will. this only works if you slow it down with the clutch/brake.

 

I missed the part about pushing on the sync screw to correct, so since you already checked the slides for smoothness that should not be an issue.

And if Polock's suggestion and the running bench sync are not successful, some more thoughts:

I did find this post... not suggesting you add a spring but the symptom is very similar - except for explaining why you were good for 4 months. Bigfitz52, as usual does a nice job of explaining the relationship of the assembled rack and the throttle plates.

http://xjbikes.com/forums/index.php?threads/solved-very-high-idle-when-engine-warm-hot.38021/#post-331722

Also, from the church of clean Chacal with his usual thorough explanation:

"But BEFORE you Bench Synch:

If you have removed the butterfly valves (for whatever reason), then while reinstalling the butterflies, hold the carb up to a light source and look through it. The butterfly valve mounting holes are slotted so that the butterfly position can be fine-tuned. Assemble with the screws very lightly tightened, then hold the carb up to the light so you are looking through the throat. Adjust the butterfly valves so they make a light-proof seal all the way around it's circumference in the throat.

If you can see light, then air will leak past at idle AND the butterfly will probably bind in the throat when things get up to operating temperature. It cannot be stressed enough that the butterfly must seat fully when allowed to close gently under spring pressure without any additional force. NOTE: if in the rack, the #3 carb must have the idle stop screw fully backed off so it does not prevent closing).

Finally, tighten the screws fully and recheck to make sure they didn't move!"

We used to joke at work that bad things come in three - hope you've reached your limit and can sort things out.