Idle drops with brake on?

I have an 1986 Xj700 with the usual carb problems and 7800 miles. Have not done the required sync/cleaning/ fine tuning of the carbs yet. Will do that at the end of this season. My issue for the experts is...when my idle creeps up to around 1500-2000, which it does after running at higher speeds for a while, if I apply the front brake the idle will drop. It can drop as much as 200RPM, but mostly its a 50-100rpm drop. In some cases the idle will ,then settle down to a somewhat normal range of 1050-1150. For the life of me i can't noodle this one through. :roll:

 

Does this happen when in gear?

 

It is in gear but with the clutch disengaged.

 

So..th bikes in gear, th clutch is disengaged and you're sitting there engine idling and you release th front brake and th engine revs a bit higher.Seems to me to be electric, th power drop to th brake light....battery, alternator, connections,fuses..??

 

I think he said that the RPM dropped.
Wouldn't that be natural though, because even though the clutch is 100% engaged, it may not be enough to fully disengage the friction plates inside. I'd think an RPM drop in gear is to be expected because of resistance at the rear wheel/drive.

 

That seems to be the common thread, but I didn't think that a load on the electrical system should effect the idle.

 

This happens when stopped at a light with the clutch pulled in and the idle has creaped up. Brake on=idle drops...release brake=idle creaps up. Not sure if its front brake only or not. I'll check that out today if it happens.

 

I think...before you send a lot of time on hunting and guessing,
take a couple of hours on the weekend and eliminate one
possibility for sure. Dirty carbs.
My first thought was in line with Erman. but an easy way to see
if it's elec is have someone watch your lights as you apply the brakes.
If you have a voltage drop that effects the idle, it will also dimminish
voltage throughout the system ( that is it's likely too ).
A vary dirty air filter will do the same, and seem like it may be
elec related when it's just not getting enough air when the motor
is under low idle strain.
The batt is in charge at idle below about 2k, so if the carbs are not supply-
ing the proper air/fuel mix, the extra stress of the elec system will
cause all kinds of strange symtems.

 

I know that I have to do the carbs, just hate to dive into that when the riding season is finally here.
Being new to this I wasn't sure if the brake light coming on would really effect the idle. I don't want to go through some huge learning curve trying to troubleshoot this, but I have not seen this mention on the site in the year i've been here.
I'll check the voltage this weekend and see what happens.

 

You might find it well worth a day or two delay in starting
the season ( in fact I know you will) after the carbs are cleaned,
sync'ed, pilots corrected, valves adj, etc.
One thing you may have over looked during your searches was
the reaction of members after having done all the above.
The usual statment is some variation of "it's a dfferent bike".

Mine sure was. It had no guts when I bought her-garaged but
neglected- The po had her 5yrs and rarely rode and did no
maint.
After finding this site and listening to the advice of the memebers,
I dove in (with little experience) and HOLY CRAP!
It WAS a different bike!

If your bike is not running to it's peek of performance you
will not be dissapointed doing a proper tune-up.