High RPM issue

[TopCatGr58]

Hey Mage,

Who's bike is it?? Check for leaks at the boots yourself!

The 81's timing curves are controlled by the TCI electronically, the only timing curve that's mechanically controlled is the vacuum advance/retard portion! So...not a good idea to go messing with the timing plate. Good Luck!
T.C.

[BlkMage]

Originally posted by TopCatGr58
Hey Mage,

Who's bike is it?? Check for leaks at the boots yourself!

The 81's timing curves are controlled by the TCI electronically, the only timing curve that's mechanically controlled is the vacuum advance/retard portion! So...not a good idea to go messing with the timing plate. Good Luck!
T.C.

Who's house am I living in? Who's garage is the bike in right now? Who's tools am I using to fix it?

You're right, I'll sneak out tonight and check.

[BlkMage]

Checked a few things...

I looked for leaks, but couldn't find any. Then, I noticed that the bike was running fine (except for a couple of backfires, but I'll get to that). I then saw that my dad had left the vacuum hose to the auto-advance unhooked! Is it good to ride like this? Oh, and I figure that capping off where it would hook up to might get rid of the few backfires I'm still having.

[TopCatGr58]

The "Vacuum" advance provides for increased advance timing during low load cruising riding for better mileage, and actually retards the unit under strong load so that the timing rotates back towards the more POWER portion of the timing curve.

A recent Xsive with an XJ that had a fried TCI....the XJ uses a vacuum boost sensor to signal the TCI instead of a mechanical advance vacuum pot, well, he put a TCI from an 81 bike on it, and he reported that it runs just fine, has plenty of power and response. SO...running without the vacuum advance shouldn't necessarily cause you any problems or damage since yours has the cent. timing advance curve built into the TCI, you just may notice a slight drop in mileage!
T.C.

[BlkMage]

Cool, then it looks like I might make Tahoe after all!

One more question though, through everything I've said, might it be that the vacuum line for the advance was the problem all along and getting a new hose would get everything back to normal?

[TopCatGr58]

Anything's possible, but I don't recall whether you ever answered a question about the status of the timing plate and the vacuum unit, it's ability to rotate without hanging up. Also, any test on the vacuum unit....applying vacuum to, and then sealing the hose and seeing IF the vacuum held, or if the plunger slowly returned back to it's unadvanced position?

However, a leak would not necessarily allow the vacuum unit to pull to the advanced position, but it possibly could throw off the synch allowing a hanging idle!
T.C.

[BlkMage]

Vacuum advance is fine, I checked when we had it out to rewire. Anyways, did a bit of a test run, it still backfires every once in a while, aka less than it used to, with that hose off, but I still have the revving problem once it warms up. Maybe it is something with the carbs...maybe I'll still need a bit o' help Friday afternoon...

Oh, and the plate frame with LED bars works great, if requested I'll do a full write-up on it.

[BlkMage]

Bit of an update.

Trying something new here...

After looking for an air leak, I realized today that I actually MADE one when I left the vac advance hose off. I pulled a hose off the parts bike, linked the 3 and 4 carb boots, and pulled the caps off. One of those caps went on the 2 carb and it still runs ok. Full test run is tonight, then I'll buy a new cap and pull the hose off 3 and 4 if this works.

[BlkMage]

No change, still the same revving problem, but I had an idea. Could this all be caused by a faulty TCI?

[ratbyk]

No, it is caused by the fact that there is a vacuum leak or the throttle butterflys are not closing properly or the carbs are not synced and one or two butterflys are not closing the same amount as the other pair.

[BlkMage]

Originally posted by ratbyk
No, it is caused by the fact that there is a vacuum leak or the throttle butterflys are not closing properly or the carbs are not synced and one or two butterflys are not closing the same amount as the other pair.

But that's the thing, those were already checked. Butterflies were checked multiple times and synced before they went on the bike and I've checked the entire vacuum system and all the rubber between the carbs and cylinders, it's all good. This is getting bad...

[Ken Talbot]

When the carbs were rebuilt, is there any chance they were dunked in carb cleaner?

[ratbyk]

Lift the rear of the tank enough to allow you to get your hand into where the throttle cable connects to the throttle shaft. With the engine running put a little pressure on it forcing the butterflys closed. If the high idle goes away you know that they are not closing far enough to allow a normal idle.
Now figure out why.
Weak return spring.
maladjusted throttle cable.
Carbs not synced.
Throttle shaft hanging up on something.
Something is preventing the butterflys from closing. If they were closing the engine would slow down until it stalled out.
If you still have trouble getting the engine to slow down then you have a vacuum leak. If the engine is not getting air from someplace it won't run. Find the source, either past the throttle plates or a vacuum leak.

[pggg]

unscrew each idle mixture screw a couple of turns, then start the motor, see what happens then..

[AlanB]

Hi Blkmage,

I said it before, and others have said it too; if it is revving up, there is air coming in somewhere...... you just need to find it.

It does not matter what the TCi unit does, or when the spark arrives; the revs can't go up without air!!

AlanB