Top Speed

Hi!! I was testing my bike last night... I was running in a high way close to my home... I try to get as fast as I could! but only get 99 miles per hour (160 km/h)... I was reading that it can do more! 127 miles (204 km/h).

The sparks are new, I wrap the headers, the coils are new! so... I don't know how to make it faster!

BTW, my bike is a 1982 Yamaha Seca XJ750

 

It's old and lost some compression you might be able to get some back with a valve clearance adjustment and some new piston rings (if you can find them).How many miles on teh engine and how much do you weigh? Dem nipponese test riders com in at under 140 lbs..

 

Also possible speedo error due to undersized rear tire, bad cable or bad gauge or dirty air filter or .....a million other reasons like a100man stated to big a load on the seat !!
Ps a heavier rider only helps with launch ..prevents wheel spin

 

I weigh 210lbs and my 650 can can do the ton fairly quick. Unless you're a experienced speed rider like some of the members your 750, properly dialed in, should scare the sh!t outta you when it hits that powerband. Idk about 127 but it should do the ton no problem.

Gary H.

 

The other thing that kills top end power is tight valves

 

Mine can hit 120 MPH on a bad day. Your actual top speed will vary depending on the weather, fuel quality, and your weight; even when the machine is in top-notch condition.

We can't assume that the compression is low without doing a compression test.
This could be a plugged needle jet orifice, a fuel quality issue, a dirty air filter, underinflated tires, or any number of small things conspiring to lower the top speed.

It is also very possible that the speedometer is not reading accurately. I would start looking there first.

 

My question is why? I've taken my bike faster than I should have, but I can't say it was smart. I would rather know it performs well at highway driving speeds - around 70 - with enough power to get out of the way if necessary. Just my two cents.

 

Going that fast on a thirty plus old bike could be not so good if a brake would delaminate or a wheel bearing were to crap out. Please consider the age of that stuff when you are running it that way.

 

Exactly.

Gary H.

 

That pops into my head sometimes even at normal highway speeds...gives me the motivation to keep up on the maintenance.

 

Agreed. I consider myself lucky to have found this model specific forum which absolutely leaves no stone unturned as far as maintenance/wrenching on these bikes. There are many things covered here that a lot of decent shops wouldn't do. These were very well built bikes for the era. A properly recommissioned and maintained xj will "get it" in good form and do it for years.

Gary H.

 

You could also replace your POD filters with a factory airbox

 

why ? Just to know it can.
I can not tell how fast my 550 maxim will go due to speedo not going up that high but I know it will exceed red line in 6th gear which should be about 110 mph.
I was testing out some tci boxes

what was your tach reading? if you redline it you would be going the max speed listed in the specs.
as long as clutch does not slip and tires are proper size

 

To each their own, but I did that kind of thing when I was younger. Sometimes I am amazed I lived through those years.

 

I did stuff like that too, but as you get older you tend to think how long it takes to heal road rash. No Thanks

 

And how nice it is to have a bike that looks and works well haha. My XJ is the first bike I can say I have had 00% interest in modifying in any manner.

 

I let the crotch rocket guys riding along on their back tire blow by me on the freeway and I just wave at them.

 

I forgot that he has pods. 99 MPH will probably be about it then.

 

If you suspect the speedo is not reading right there is a really fast way to check it.
Take your smart phone and download one of the many GPS speedo apps around. You should not need to pay for any of these.

GPS speedos work by taking GPS fixes on the earth surface in rapid succession. Their algorithms simply do a distance vs time comparison between the fixes, and then smooths out the answer. All errors are random and cancel eachother out in the smoothing. Result: when you are moving at any speed more than 10kmh, the calculated speeed will be spot on.
This is converse to the way the mechanical speedo works, which takes a reading from the front wheel in many cases, errors will be systematic and affect the speed in a constant way.

If you can´t mount your phone next to your speedo, then have it somewhere where it can receive the sat signals and run the bike up to a certain speed. These GPS speedo apps usually have a max speed function. Compare and see what the error is.

As we all know, speedos usually read a little faster than reality. Urban myth I´ve heard is that the manufacturers (cars and bikes) want to remove any chance of liability in speeding issues with drivers.
I know that my XJ900RL is always reading about 5khm over which is quite normal. But at about 80khm the speedo starts to occilate +/-10khm (making it bloody useless) and then after about 95 it settles back down and is pretty much spot in if not reading a little low.

Havent had mine beyond 140khm so far. Before I try that I want to have new brake lines at the very least.

 

Do you have a local drag strip in your area? Other than a GPS, a trip trough the traps will tell you electronically how fast you are going. You don't have to put in a full day, just a few runs would do. They also have a full onsite medical (Ambulance) available, that way if something lets go you will get taken care of right away instead of laying in a ditch or on the road until someone stops by. Plus you won't be breaking any local laws.
2 cents!