xj700 dif on xj750

Any one know the rear end ratio of a 86 xj700? And if it would be possible to modify it and install it in a 82 xj750 Not looking for more speed just lower RPM's.

 

ive been trying to find this outmyself and it seems that no one has any answers. So far im thinking the easiest way is to just do a manual spin count on your bike and the donor bike...

 

My bike is no problem. 2.909 By looking at a different post. Just not sure what the 86 xj 700's are. I am sure I could make it work. Part numbers aren't off too much looking at the Yamaha Parts manual online. Maybe a slightly larger rear tire?

 

you guys have the tallest ratios already. Put an 18" wheel on with tallest tire possible....

 

Hate to waste a perfectly good tire. May have to wait untill next year for a new one. Thanks for the reply though. I put a little more air in the rear tire and it seems to have helped a bit. The windshield on the front has seemed to work wonders for the milage a well. Was getting 34mpg. Havent filled up since but I will tomorrow. I will let you know how it goes. Thanks again.

 

Lower RPM's won't necessarilary give you better mileage since it depends on the load on the engine and throttle opening.
You should be getting about 45 MPG. If it's less your carbs are too rich or else you ride at full throttle all the time like Dean. I would check and adjust the mixture but if the tires are nearly flat or the brakes are dragging that will bring it down too.

 

How about living at 6000 feet in elevation. The previous owner put a mac 4 into 1 exhaust and a k&N filter on it. The idle mixture screws at at about 4 turns to get it to Idle. I dont know what the jet numbers are supposed to be at stock but it appears to run pretty good. I think the transition between idle and open throttle could be better though. It has a hesitation and then roars to life. I cleaned the carbs this spring, balanced the carbs and colortuned them as well. Seems to run well just doesn't get the mileage I would expect. The windshield did increase the mileage by 3 so I am up to 37 now. Any suggestions. Or do you know the jet numbers in stock carbs. Think I am going to take mine off again. and get the numbers so I can compair. Any clue on what jets I should have in it with the above changes to it. including the altitude?

 

IIRC, the 750's have 120's in them stock, but I could be wrong on that. Just going off of memory when we had my friend's carbs torn down from his '82 750 Max.

 

Ah ha! Now we get to the heart of the matter.
Living at 6000 feet in elevation is going to cause the mixture to be too rich because of the thinner air. There is no altitude compensation in carbs like with fuel injection. To drive at that altitude you would need to decrease the pilot jet a size, as indicated by the idle mixture screws out 4 turns, to compensate.
However, if you have installed a mac 4 into 1 exhaust and a k&N filter that would usually mean you need to increase the jets a size at sea level so it may have balanced out for you somewhat at the higher altitude. The main jet won't matter unless you drive at full throttle a lot but I would decrease the pilot jets a size at least to help fuel economy. After you do that and adjust it to run more efficiently at 6000 ft. it will be too lean at sea level however.

 

I doubt I will ever have the opertunity to ride it at sea level so it really wont matter. I will see what jet sizes I have and jet it down one size and see if that helps. Colortune says it is right on though. Nice blue color. Thats the baffeling thing. I will play with the carbs and find out.

 

The colortune is a good start but only works at idle so you can't go riding with it and check the mixure on the highway! I have a Heath EGA that runs on 12 volts that I can strap on the tank to check the mixture at highway speeds but it might not be a good investment if you only need to tune one bike. You could rent some time on a dyno but it's a lot more economical to just change the pilot jets first and see if that does the trick.

 

Just got my carbs appart and back together again. To recap. I have a xj750j with the hitachi carbs. Mods include mac 4 into 1 exhaust and a k&N filter in the stock air box. Here is the clincher. I live in Colorado Springs with an average altitude of around 5500 feet above sea level.

The problem is I have a hesitation off Idle when starting out. Just falls flat on its face until you give it more throttle. The carbs are squeeky clean. I am wondering if the jetting could be off. Previous owner did the mods and I am not to sure of the jetting. I am looking for an average of what you guys might think of the jetting in my carbs. The Idle jet is a 40. the mains are a 110. and the metering rods are a y-13. Any help is appreciated and needed. The Idle mixture screws are turned out 4 times to get it to Idle. Runs like a raped ape at around 2500rpm and farther. Sorry for the long post just trying to get all the info to you guys. Thanks

 

Tweak the Pilot Screws in until the idle stumbles. Bring it out until you reach the spot where further improvement in idle ends. Back in until you find the best idle across the board. You might find that you are really opening them up.

 

Tried that. it likes to Idle at 4 turns. I am just trying to figure out a base line on the jets. I dont know what came stock on the bike and would like to know which direction to go on the jets.

 

Idle mixture screws let in more mixture when you open them so all the way out to 4 turns means the pilot jet is too small. Go up a size.

 

Be careful taking them out ... oil the top of the carb ... good!

 

Are we talking about the same thing here. Are we talking about the pilot fuel jet or the pilot air jet. Or maybe the idle mixture screw. All come out easily. I have had these out befor when I cleand them. Please advise. I ordered the fuel pilot jets today. going from a 40 to a 41. Let me know if I am going about this the right way please.

 

I got down and dirty with the carbs last winter, Changed out the pilots to a 41. Ran really bad with tose in there. Reinstalled the 40'sI read some more recent posts and decided to check out the fuel level in the carbs. Guess I should have checked them earlier. Found all low. by 4 to 5 mm's. Adjusted the level on the bench Using a old 90 weight bottle hung from a coat hanger off of a shelf above the carbs. Fuel line running to the carbs. Benching these is a great Idea espacially if you dont like installing and removing the carbs all that much. After removing the bowls 4 times they are perfect. Reinstalled them on the bike and found a leak. Stupid me had left out a gasket. Runs great now. No hesitation at all. Mixture seems a bit rich. plugs are brown not tan. But that will be a different time. slightly rich is acceptable but lean is BAD. Thanks for all the help with this ongoing problem. Will update the mileage when I fuel up.
I still would like to go with a slightly tall tire. Andy suggestions. Installing a Seca rim, 18 inch, and tire would only increase diameter by 1/4 inch with stock tire on it. That is from the dunlop site, specs. Any suggestions?

 

Getting back on topic, changing the final drive will not do what you want. All the shaftie XJs have the same rear end ratio, with the 900 Seca getting its' longer legs via a taller secondary reduction gearset inside the cases.

 

What was it doing ... when you say "Running bad" with the 41's in there?

Not getting out of the hole good? That 6,000 Feet's got to be making a big difference.

I'm right smack-dab at Sea Level, here in Massachusetts -- near Boston. I want to bump-up my '83 Maxs' Jetting to compensate for my Bigness!

I can tell she'd be very, very-happy with some hotter Mains.

 

It may have just been just a coincidence but it started gouling the plugs. I had also put some seafoam in it at the same time. I have noticed that if you put in too much it tends to foul plugs though. So I am not sure which was doing it. Maybe I will try them in there durring the summer. At this altitude what I really need is just some more air in there. It is runnig well right now and hate to mess with it again. Maybe if I have to pull the carbs again I will put them in and retune.
Sounds like some tranny modifying may be in order when I have to split the case in the future to get taller gears. I will keep an eye on the mileage and let you guys know what it is doing and what I do. Thanks for the help.