1981 xj 750

howdy people, new member and first post,
first up thank you for the site, I have already got a heap of info from here, so again thanks.
right I have just inherited an old 81 xj 750. someone has had a go at makin it into a bobber, its very rough and dammed ugly lol.
but it is a great project for me and my son, and its also been his first taste of a bit of power,
needles to say we've got it running ok and he's done a fair few laps of the yard and driveway.
anyway, my questions, there are two outlets on the tank fuel tap. what is the little one for and where should it go ?? is there only one feed down to the four carbs ?
next question,, on the rubber manifolds between the carbs and heads I have ports that have been blanked off on numbers 1 3 and 4, on number two there is a vacuum hose. what should this be connected to . it has a bolt jammed in it at present.
the wiring is an absolute rats nest , so I would like to cut it all out and start again, any tips on this would be hugely appreciated.
being a kwacka and Harley kind of guy all my life, I got to say, im enjoying playing with the little Yamaha.
oh. where the hell do I find the engine capacity on the engine, (eg 749 cc) just to make sure its a 750..
ok thanks in advance guys, I look forward to reading the replys, cheers mick

 

The hose from vac port 2 goes to the small spigot on the gas tap, sounds like the vac tap has stopped working & someone has done a 'mod' on it. You can check it by connecting a tube to the small spigot & lightly sucking, if it is working gas will flow.

 

ok ok. I did wonder it it went there, but I couldn't figure out what it would achieve?

 

sounds like you have a manual petcock instead of vacuum controled one. you can go to auto parts stors for a cap to replace the hose and bolt.

what settings are shown on the petcock?
oem petcock should say prime, reserve, and on

get a manual haynes is known to have a few errors in it, you could start a new thread and ask about known errors
haynes free download
http://www.manualslib.com/manual/837861/Yamaha-Xj750.html

I would suggest reading the first post in this forum about brakes

a few links for you to consider reading
first out best supporting vendor
XJ4Ever - Supporting Vendor at the bottom of this link are some links you should look at

his alt web site
www.xj4ever.com
check out his how to links

good reading here
The Information Overload Hour


welcome you will have quite a support group here to help you.

this link will help decode vin
TECH TOPIC: Model ID and VIN's
engine size should be stamped on front of cylinders between center two cylinders.
there should be an inlet connection in rear of carbs between #3 and #4

 

750 seca wiring diagram
components list
http://members.tripod.com/dave_jack/wiringdiag/d8.jpg

http://members.tripod.com/dave_jack/wiringdiag/d11a.jpg

http://members.tripod.com/dave_jack/wiringdiag/d11b.jpg

1982 XJ750J Maxim WIRING DIAGRAM
http://members.tripod.com/dave_jack/wiringdiag/d10a.jpg
and
http://members.tripod.com/dave_jack/wiringdiag/d10b.jpg

uses same component list

 

Save yourself a lot of time and headaches: buy a used wiring harness on Ebay, or post up in the wanted section.

 

The standard petcock on these (and most similar) bikes is that the On and Res fuel positions allow fuel flow only when vacuum (from the engine intake) is applied to the small port. The idea there is that, in case of a crash, fuel gets shut off when the engine stops... and fuel is also shut off automatically when the bike is parked. This helps prevent carburetor overflow if the float needles aren't sealing well.

 

unless you plan on doing a minimal wiring harness. even then better to get the ebay harness and remove what you do not want.

 

BTW, the displacement is on the bottom front of the block.

 

Welcome aboard.

Gary H.

 

Sounds like you know what you are up against (darn PO's!).
Keep asking questions here, and tell your son how lucky he is to have an Ol' Man like you, buys a project bike and all that . . . good on ya.

 

sounds like you have a manual petcock instead of vacuum controled one (quote from XJ550H)
Original post says the tap has 2 spigots, one large one small, sounds like a vac' job to me.

 

ok, after reading the how to decode your bike information overload, my brain has now completely switched off,
tho I do know my engine and frame match, however can somebody please decode the numbers for me, they are
5NI 000740. THANK YOU , MICK

 

COUNTRY: UNITED KINGDOM

MODEL NAME: XJ750
STREET NAME: 1981 XJ750
MODEL ID CODE: 5N1
FRAME ID: 5N1
SERIAL NUMBER STARTS AT: 000101
NOTE: styling and features the same as North American XJ750 Seca model, has dual front disc brakes and an oil cooler.

 

Howdy guys. Well today i tidyed up the wiring. I set up the headlight. Tail light and stop light off the brake lever and pedal. .
However the bike is no longer charging. ???????.
Can anybody send me a simple charge diagram. I have no power coming out of the three main wires from the alt (i guess as the disappear into the engine case).
Am i just missing a wire to excite the system.
Other than that she is puring like a kitten. And i actually rode it for a few ks today.
It even sounds like it goes quite hard . Untill you look at the speedo that is. Lol.
Ok thanks in advance.
P/s i do have the minimum wiring diagram for the xj. But im not sure what or where all the components are on the bike to tackle it. And i dont see a provision for blinkers or a tacho. Cheers. Mick

 

Oh. And just incase you havnt had a chuckle today. Let me present
"black betty"...........

 

min wiring


haynes free download
http://www.manualslib.com/manual/837861/Yamaha-Xj750.html


XJ750 UK WIRING DIAGRAM
http://members.tripod.com/dave_jack/wiringdiag/d9a.jpg
and
http://members.tripod.com/dave_jack/wiringdiag/d9b.jpg


Componets Key Guide for all of the above WIRING DIAGRAM
http://members.tripod.com/dave_jack/wiringdiag/d8.jpg

check your brushes on the alternator there is a wear line on them 11mm is to short 17mm is new
from info overload
Checking alternator stators: the resistance across each pair (w1 to w2, w1 to w3, and w2 to w3) of the three white wires (w1, w2, and w3) at the alternator stator lead harness connector, on those three wires LEADING TO THE STATOR, should be as follows:

0.46 ohms +/- 10% for all XJ650, all XJ700, all XJ750, and XJ900RK, RL, N/FN, and F models.

Checking alternator rotors: the resistance across the two lead wires (usually red and green) at the alternator rotor lead harness connector, on those two wire LEADING TO THE ROTOR, should be as follows. Note that worn or damaged alternator brushes can affect these readings, as can "dirty" copper commutator strips on the rotor face (where the brushes contact the rotor):
4.0 ohms +/- 10% for all XJ650, all XJ700, all XJ750, XJ900RK, RL, N/FN, and F models, and XJ1100 models.

 

so Mad Maxish, is that electrical tape for the left grip?
I like the spear heads too, she's a beauty nice red accents

18 is alt 19 is voltage reg

 

Power from the three white wires will be AC. Are you checking for AC? Besides the brushes, a common problem with these bikes is bad connections at the plug where the alternator plugs into the rectifier. The connections oxidize, then get hot, an eventually melt the plug and wires a bit.

 

Ok. I will dig out the multi meter and have a play. Yes the red accents are nice arnt they not. Yes tape for the left hand grip. And a dodgy home made seat.
The previous owner spared no expense.
We will get to the paint and cosmetics later. For now i just want it running and charging right. As i said. She is ugly.

 

Morning guys. Another quick question if you will. The bike is running rine. Except for the not chatging issue. I dont have an air cleaner as yet as the previous owner has cut and removed the airbox and made a new battery box. I was jutt going to fit a set of aftermarket air pods to it .
Tho after reading some tech posts on here. It seems pods are not a good idea ????.
As i said the engine is running pretty well but to make that happen ive had to block off a small vent on the far left carb. Or the bike will just continue to build revs at idle.. ?????. I will attempt to attach a pic so you can see what im talking about. Cheers Mick

 

Morning guys. Ok i have posted a pic to hopefully to show you what i am on about.
Unless i block this vent/port/venturi off on the far left carb the engine continues to build revs from idle. If i have it blocked off the bike runs and idles ok. ?????
The previous owner has removed and redesigned the air box . So i was just going to run a set of pods. Will this be ok.

 

As you know BB is not a true hard tail. Keep an eye on the bolts cause if one of them break...

Gary H.

 

Yes. Good advice. Tho i will be fitting a set of short shocks i think.

 

check for a rip in the diaphram under the top cover. possible it is not seated properly.
rip or hole can be fixed with liquid electrical tape.
pods can be made to work it just makes it harder to diaignose problems. there is a few threads in the mods forum about pods and how they were made to work .
also a thread in the other bikes forum
do you have the airbox to carb boots? it has been suggested to mount the pods to them not directly to carbs, the boots work (are) as velocity stacks and are needed

 

this (short shocks) will/can screw with the drive shaft because of the angle change.

as far as " She is ugly." shes still gonna give you a wild ride.and that,s where the beauty comes in.

 

+1 on removing the top hat and checking for a torn slide diaphram. You can't run it with that port blocked or the slide won't lift (which it probably is only barely doing since the diaphram is likely torn or has pinholes).

 

Don't know how I missed it first time round but I'm lovin' the way he's 'stiffened up' the back end. Nice ..

 

This is some of why we do this. A lot of people associate "good running" with cosmetics. We can take a not so pristine looking bike and make it fire on the first hit, purr like a kitten and ride in good form based on how it was built to perform. And the best part...it'll do it over and over again. Did someone say sleeper?!

Gary H.

 

You have carb issues....

 

...and cosmetic issues, and (if he's anything like the rest of us) mental issues

 

Hey, I resemble that remark

 

Me too. I spent Wednesday afternoon and part of this morning machining a pair of clamps for a telescoping chart-stand. New clamps cost about $5.00 for a pair. Mine cost around $30 each if I count my labor, but it was a whole lot more fun than buying some.

 

Well. Put bb up on the workbench and pulled all the wiring off. Cut all the insulation off Then spent 12 hours chasing it around.
We now have blinkers (but they dont flash).
Hi and low beam. Tail and brake lights. It charges and i have a neutral light. Stand disconected and low oil switch also
New fusebox.
Struggling with starting it tho. I think the start button might be a bit hit and miss. As it winds it over sometimes. (Well once i earthed out the neutral switch it did)..
Or it could be the starter cut off relay. What does it do and do i need it. Or can i fling it ???.
The diaphragm in my far left carb was not seated correctly. Fixed that and now it idles great. .
Going to have a beer now. My head hurts.

 

This might help you:
http://www.xjbikes.com/forums/threads/how-to-test-your-starter-circuit.7828/

 

The relay prevents your bike from starting in certain conditions like going in gear from neutral but the side stand is down or the bike is in gear but the clutch handle is at rest. It can be bypassed but imo you need it. Also might want to crack open the right control, clean the contacts and visually inspect for loose or broken wires at the button.

Gary H.

 

Probably a good idea to open up left and right switch blocks / controls and clean them out, watch out for tiny spring loaded parts that can go flying ... and put them back together with some silicon grease on the contacts as preventative maintenance. My start switch contacts sure needed a clean up when i did my resto', it was good as new afterwards. Good luck with your new toy.