can the vacuum advance be bad ?

[blkryno]

Hi All,

1981 XS 1100 Special- B 4RO.

Heres my question. Can the vacuum advance be weak or bad even though it seems to check-out? Can it be weak enough that it does not release or come down when the vacuum decreases? Can it be weak enough that once it is open all the way that the regular lower amount of vacuum is able to hold it open? I don't have a sensitive enough gauge to know exactly when the V.A. is coming down but the gauge I have does show that the vacuum pulling from the #2 carb body at idle is in the 2 to 5 lbs. range and at higher rpm's will run at a steady 5 lbs. Sucking on the vac line opens and closes it just fine and plugging it with my tongue will hold it open but Im not a porn star so I can't tell how much vacuum I am exerting.
I have just finished re-color tuning the carbs and using a mercury synch to set the carbs. The bike is running very well at idle and at highway speeds no backfires or glitches just a nice even growl.
However, sometimes when I come to a stop, the engine wants to stay reved at 3, 4 or 5 grand I can easily bring it down by letting out the clutch and braking a little. Then she will ldle properly with the clutch pulled in or in neutral. When it does this in garage I can remove the vacuum line from the vacuum advance and it will usually come down to nice idle. But, not always, sometimes in the garage when the vacuum is disconnected she will be running so hard that it won't come down untill I hit the kill switch. If I start it right back up it may still try to run at high R's, but if I leave it off for a 60 seconds or so it will start and idle nicely. When it does this I'am wondering if its caught in a vacuum lock of some sort. Is that possible? Well got go to the dentist now. Looking forward to any advice from you guys or gals out there. Ya'll have gotten me this far. Six weeks ago this bike had not been started in 21 yrs this last week I put on 150 miles.

Keith

Don't fall down.

[mro]

Have not got around to playing with my 81 special.
RPM problem does not sound like it's timing related.
Carbs outa synk come to mind first, but might be a slide hanging up in one carb too ?
DIAPHRAGM rubbers good?


mro

[DiverRay]

Keith,
Did you use a YCIS tool when you "synced" the carbs?? I need to ask, because you can "sync" without the tool, and it looks good, but it's NOT synced. It does sound as if you have one carb bringing up the rest, and that is a sync.

[AlanB]

Hi Keith,

Or a vac leak; usually traced to either the caps on top of the carb boots, the line to the vac advance, or the joint between the boot itself and the head.

The leak is rarely through the boot, no matter how cracked they look.

AlanB

[TopCatGr58]

Hey Ray,

You still suffereing from JET LAG? The 81 doesn't have the YICS, only the 82 XJ!

Hey Keith,

Yes, the vacuum leak is a high probability to check for, just surprising that you're able to get a good synch!?

The timing plate itself may be hanging, have you removed it, cleaned and lubed it? Also, someone reported about the actuator arm on the vacuum pot was scored or such and also hanging on the mechanism, once they filed it smooth, it moved and released properly!?

Not sure if you've read the articles that describe the role of the vacuum pot, but it's actually a combo advance/retard unit! When cruising at speed with little load, the vacuum is relatively high to the unit, and advances it quite a bit more for the fuel savings setting, but when you crack the throttle, the vacuum drops, and the unit is supposed to release and allow the timing to retard, but back to the more powerful timing point, so it's still advanced but not as advanced as when cruising without load!

You could cap the vac. adv. carb port, and with it idling, rotate the timing plate manually and see if the timing and rpms advance as high as when it hangs? Good Luck!
T.C.

[blkryno]

vacuum?

Hi All,

Thanks for the advice. I have checked for leaks around the boots and the head ( carb spray around all the possible connections ) there doesn't seem to be any problem there. Ya had me a little concerned about the YICS thought DiverRay . I'll take a close look at actuator arm and the timing plate.
I thought I read in one of the forums that this bike will run just fine with the vacuum advance disconnected. If so should the vacuum line be capped off or left open? Does it make that much difference in fuel milage?
Yea the diaphragm rubbbers are good but, I will go back and see if had a stupid moment and did not tighten down the caps over the diaphragms. I have had these carbs off and on enough that I sure could have forgotten to tighten something. I did sit and watch the vacuum pistons in the carbs and they sure seemed to be moving well and when I 've checked them on the bench there was no hang in them. Thanks Again.

Keith

Don't fall Down

[DiverRay]

Sorry, Posted just after finding out I'll have a call from France at 10:00am, FRENCH TIME! That's 1:00am my time, and I'm usually in bed about 10:00pm.
I'll try not to get work involved with the important stuff any more...

[mro]

Smething I've seen once was a guys throttle cable that seemed to work OK most of the time but now and then would bind and not release the carbs all the way back to idle. Lubed the cable over night and it has been good since.

Still don't think you can get the timing to increase the RPM that much and minor vac leaks can get up a few thousand or so and it should get worse as engine gets hotter. Can bring it down with the clutch/gear brakeing the engine but a leak should start the RPM cllimb agin as soon as theres no load.

If it turns out to be the advance unit be sure to post.
Also dosen't the 4RO TCI get it's advance info a little different than 78 to 80 XSes?


mro

[petejw]

mro
all the bike models have vac advance
the 78/79 had bob weights
for centrifigul advance the 81 model
has the advance built in the black box, not sure of the 80 model


blkryno,
if the vac advance is holding vac then its ok
unless its binding, if u do decide to disconnect it
be sure to cap off the no 2 carb.
if youre satisfied that there is no vac leaks,
and the cable isnt binding, and uve sync the carbs mayb
u could try backing off the idle adjuster, after ive mucked around
with my carbs, it has sumtimes idled really fast but would creep down to normal, after backing off the idle screw a turn or 2 it would idle normally.

[John]

The 80 also had' bob weights' in the governor assembly ( known as the centrifugal, or mechanical advance), but the gov assy has a different P/N than the one used in the 78-79. I'm not sure what the difference is, if it is in the springs or the actual weight of the weights.

[Brian_D]

Originally posted by John
...I'm not sure what the difference is, if it is in the springs or the actual weight of the weights.

according to Yamaha's fische, the 78E and 79SF share the same governor (2H7-81653-10-00), 79F lists as 2H7-81653-11-00, and the 80G/LG/SG is 3H5-81653-10-00.

Haven't gotten my hands on an 80 pickup coil assembly yet but I can tell you the ONLY difference between the 2H7-10 and 2H7-11 are the "advance" springs. Suspect that is the only difference in the 3H5-10 pickup assembly as well.

HERE is the thread where pggg posted on the differences between the 2H7-11 and 3H5-10.