1981 xj750 cafe racer help!

[T-Thunder13]

i bought an unfinished cafe racer project a few months ago. re assembled the newly painted bike got it running (and it looks sexy as hell) so took it to henderson yamaha to have it tuned up for the new pod filters on it.
thats when it all went wrong after waiting for weeks to have it looked at the technician said this model was very dependant on the airbox preassure so the pods wouldnt work, so i put the old air box back on and they took it for a test ride, gave it death, where upon it blew all its oil out the breather pipe.
the bike has a flat spot from 5-7rpm and they recon that the engine is probably tired and needs re boring citing that air was getting past the pistons and pushing to much pressure into the crank case, forcing the oil out the breather. buger.
does this sound about right?
they also broke my new wing mirror and said nothing and i didnt see it until i rolled it out of the van at home.:angry:
so, if i need to and or after i get it rebored id like to find an older mechanic or just one that knows a bit about these older bikes. any suggestions?
i found the staff at henderson yamaha couldnt have been more friendly , but totaly useless in my case.

[CookMySock]

Don't panic. It's not expensive to do rebore and pistons. (edit: oops theres four of them)

But yeah, it pays to do your own engineering on things like that, or else you end up buying a gold-plated lemon.

Steve

[hmmmnz]

they sound like a bunch of tits, you can get any bike running with pods, generally not any better than the airbox, the fact he didnt even try is pretty slack,
i would get a compression tester, and find out the compression of each cylinder, it may be that just 1 cylinder is down or the rings are worn,
rebuilding a old bike generally isnt too bad cost wise if you can do most of yourself, obviously if you need rebores new pistons rings ect then the price starts mounting, and if you going to to that then you may just want to look for a complete donor engine, or go the big bore route :D

[blackdog]

pics pls.........

[FJRider]

I have a "very tired" 84 XJ750 ... no issues with oil filling the air-box. It did a Chatto-creek 1000 miler and used 8 litres of oil.(and a top up at the end) With a CLEAN air filter at the finish.

look elsewhere for the problem maybe ...

[FROSTY]

XJ750/900s do not like being overfilled with oil. Sit it on its wheels and make sure you can still see the bubbles at the top of the oil sight glass. otherwise yea she will blow oil out of the crankcase breather.Also I guess it goes without saying that if the bikes had a lie down then the oil will run out of that breather.
I've had XJ900's that had been for a little liedown and some of the oil ran into the-up the breather tube airbox then somehow the oil would keep sucking up the breather pipe. The solution was to wash the airbox out completely especially the area just behind the ait intakes and esprecially the bit of foam at the end of the breather tube
Incidently if it did fall over likely the air filter got clogged with oil.
As for the flat spot --sorry dude thats XJ's for ya--Have a look at the factory torque curve. You can reduce it a bit with jets and needle changes.
Ps--your bike isn't black and looks like an XJ650 is it?

[Kickaha]

http://xjbikes.com/ try there if you aren't already a member

A compression test and a leak down test would be a good starting point

[jellywrestler]

technician .



theres your mistake !!

[T-Thunder13]

[ Ps--your bike isn't black and looks like an XJ650 is it?[/QUOTE]
yeah thats the one, howd you know?
well i cant see the buble in the viewing glass so ill take a bit of the oil out. im going to take it to bits and just make sure everythings where it should be and if that doesnt work ill probably go down the re bore path.!
thanks for your comments!

[FROSTY]

[ Ps--your bike isn't black and looks like an XJ650 is it?
yeah thats the one, howd you know?
well i cant see the buble in the viewing glass so ill take a bit of the oil out. im going to take it to bits and just make sure everythings where it should be and if that doesnt work ill probably go down the re bore path.!
thanks for your comments![/QUOTE]
before you do. Come see me. I might be able to save ya a truck load of time n effort.
I saw the bike sitting outside Henderson Yamaha when I was getting wof's on a few bikes I've sold.
I just thought of something else. whip ya left engine sidecover off and make sure everything is dry in there. And make sure the contacts on the rotor are all shiney.

[T-Thunder13]

before you do. Come see me. I might be able to save ya a truck load of time n effort.
I saw the bike sitting outside Henderson Yamaha when I was getting wof's on a few bikes I've sold.
I just thought of something else. whip ya left engine sidecover off and make sure everything is dry in there. And make sure the contacts on the rotor are all shiney.
yeah for sure where abouts are you? what will making sure the rotor points are shiney achieve ?( hope that isnt a naieve question)ill do it strait away though.

[CookMySock]

We had a near-brand-new GT250R that rattled and pinged real loud, like it was going to shit itself baaaaadly. Was too much oil in it. :shit:

Steve

[FROSTY]

Millars Car Centre

[T-Thunder13]

cheers frosty il drop in for a yarn next time im cruisen past.
today i took the oil out of it and there was about 2 1/2 litres too much in there!! and after i did it went much better no oil through the breather and in neutral i can rev through the flat spot but it still goes weak when its in gear on the road, when it hits around 5gs it backfires a bit too.
so a bit of progress there. with the recomended oil amount in it , the oil doesnt show up in the viewing glass at all. it seems to want about an extra 500mls to appear. should i be checking the oil level when the bike is running or when its off? it doesnt say in the manual.
cheers

[hayd3n]

should i be checking the oil level when the bike is running or when its off? it doesnt say in the manual.
cheers
if im correct check the engine oil level when the bike is level and cold

[CookMySock]

Good stuff! What sort of bike shop would not pick up on that? I guessed it just reading your posts.. without even checking anything.

For my next guess, I'd say it has CV carbs on it, and the pod filters are making backfire. Put the original airbox on it and it will work perfectly.

Steve

[T-Thunder13]

For my next guess, I'd say it has CV carbs on it, and the pod filters are making backfire. Put the original airbox on it and it will work perfectly.

Steve


yeah the monkeys at the work shop said theses carbs were very dependant on the air restrictions given by the old air box so they put the old air box back on it but it seemed to make no difference at all, except it doesnt look as good heh.

[CookMySock]

Did you put the standard air filter element in it? Just removing the element from my airbox makes it fart and bang real bad.

Steve