'85 XJ750 - Little Power at high speed

Just bought a bike here in France (I'm a NZer though), a beautifully chromed '85 XJ750 (41Y) with very cool twin roadlights although I'm not sure how legal they are....

When I bought it, I did a test run around city streets and it did alright.
The problem is at speed over, say, 80kmph (50mph).
I get up to around 4500rpm and feel like I should hit a nice powerband, but its just not there. It goes up slowly and it doesn't jump or gasp for fuel but doesnt take off either.

I took the plugs out and it there is a light carbon layer (carbon fouled according to The Sparkplug Chart) so my guess, or more to the point what the chart told me, is that there is too rich a mixture.

It just feels like there is less power in this bike than my old 400 (albeit a Kawi Sports GPZ).

I really really dont want to take the carbs apart. I know that if I used the "Caketin Method" I would kick them and spread carb parts across France.

I am not adverse to using feeler gauges on the tappets and a vacuum gauge combined with the YICS tool, just that these things are expensive when you combine it all.

So to sum up the question.. Has anyone fixed this issue?
Cheers,
Dan

 

I would look into the carbs. I just bought a 750 and had some of the same problems. It turned out to be the slide diaphragm unseated itself somehow. As for the feeler guage, shim tool and sync tools needed you can probley get all those for about $40 usd. You can make your own sync guage and yics tool for little or nothing. There are threads on the site to show you how to do it.
What I do when I work on carbs is to put the parts into labled zip-lock bags (1,2,3,4). Also make sure you have a service manual or the detailed teardown procedure you used to make sure you put them back together the right way. They are not that complicated to work on.

 

Does it speed up when you turn those lights off?

Seriously, I think I'd look for a restriction at the air intake or air box first.
Then work my way to checking voltage while running at that speed. Low voltage kind of confuses TCI's.
When I get a bike in with customer installed electrics,I usually disconnect them at they're source and try them to make sure they're not the root of the problem.
You could also have a problem at the fuse block. i.e. loose fuses or even just a failing battery.
Let us know what you find,Dan

 

heres a little test, when it seems to go flat at 4.5k does quickly backing off the throttle just a bit seem to make it want to pickup
just for a second ?
that would indicate a lean condition, if it sat for a while the carbs might have some goo in there
try some carb cleaner in the gas for a few tanks and see what happens
nice looking bike