xj 550r top speed question

ok here it goes try to stay with me, i picked up an old rusty pile of junk for free last summer and it turned out to be an 81 xj550r, the odometer read 27,xxx when i got it and i did some research and found out that they usually need to be rebuilt at 25k so i pulled it apart to discovered that it had already been rebuilt so i got it running and redgistered and started adding performance parts bit by bit and so far it has, lay down handelbars on it, the carbs have been stage three rejetted it has a mac 4-1 magaphone with removable baffle and uni pod filters, it runs smooth and pulls hard through all the lower gears all the way to 12k rpm i ride it fast and hard and it has never let me down, the problem im having is it just seems to hit a point where it just wont go any faster it is a six speed and in 5th at 8k it just wont go, i can down shift to 4th at 10k same thing the speedo is already topped out at the peg just past 85 so i don't know what speed it is but if i had to guess i would say around 110-115, but i know that the engine has the power to get past this point im just not sure how to get the power to the ground i know i can either drop a tooth in the front or go up a few in the rear to get the revs up in the gears but what i don't know is if this will help it get past this point or if i should go the other way with the sprockets and just use a lower gear but i don't want to hurt my acceleration, i know some of you are just going to think " go buy a bike ment to do that" but i love it when i pull up next to someone on one of thoes new "crotch rockets" and then the look on their face when i give them a run for their money in the lower gears up to about 80, if anybody has any ideas on how to get past this point i seem to be hitting, either with changing sproket sizes or getting more power out of the engine any info would be helpfull

 

With all the modes is it possible the motor is starving for fuel at 12K?
If you change out the rear sprocket to lower top end you will pick up acceleration, to a point.

 

yea, im just hoping that it doesnt bring the point to a lower mph, when it hits this "point" i can downshift to a lower gear and ring the bike out for all its worth and it will gain some, but when i upshift again it will just hold and not go any faster

 

110-115 mph should be fast enough for most around town applications shouldn't it?

Where do you ride that you can legally bury the speedometer?

Signed,

Aunt Bee

If you are at 110- 115 mph I should think this would be toward the top end of obtainable speed due to rolling resistance and especially wind resistance. They only have so much horsepower availabe and my guess is that wind resistance is not a linear function of the speed of the motorcycle.

 

You're gonna hate me. In STOCK configuration, BOTH my '81 and '83 550R's will pull right to redline in all 6 gears. Stock exhaust, stock airbox/filter. Right to redline; takes a BIT longer in 6th, 3rd-5th run right up the tach like now. And I weigh just over 200lbs. Honest, I'd say you've "modded away" some performance. The only "non-stock" things on both bikes that would affect top speed are lower handlebars and neither have the goofy fairing.

The 550R tops out at around 118 indicated I believe the "official" top end is about 110 or 112 which is probably about right.

 

Wouldn't Chadly89 benefit from larger main jets?
Do a high speed run, wide open, then kill the engine, no idling and read the plugs- - are they tan or white?

 

the bike is color tuned everything works the way it should, the exhaust might be too open and taking some top end away, but i was looking at the back tire size and it is a 130-90-16, and that is not the stock rim on a seca right? so im thinking that the rear sprocket may be too small and too tall geared i didn't have the time for a tooth count but i will get back to you on that as soon as i can

 

If you've got a 16" rear rim, somebody Frankensteined one from a Max or other chain-drive bike; is it "swirly" not straight-spoked? Stock rear wheel is 18" factory recommended tire size is 110/90-18; you can run a 4.00-18 too and it's fine. You may very well have too small a rear sprocket too.

back to carbs and powerband for a sec; Pod filters have one inescapable drawback to the proper operation of a CV carb: Nearly UNLIMITED airflow. The CV carburetor needs a FINITE amount of airflow in order to create the proper VELOCITIES necessary; think about it like putting your finger over the end of a garden hose to make the water squirt farther as opposed to just turning the hose on full blast. Yes, re-jetting HELPS but rejetting cannot limit airflow, and unlimited airflow will cause improper fuel delivery at certain higher RPM levels as airflow increases beyond the carbs ability to utilize it. There are a couple of long-running threads in regard to this issue, do a search on "restricting pod filters."

If you'd like a rude awakening, find someone with a well-tuned stock 550 Seca and see if he'll swap bikes for a quick ride.

 

ok well i figured out what the problem is, it has a sixteen inch rim, with a (41) tooth sprochet on the back and a 16(stock) on the front, but the rim is still a straight spoke just like the front rim, it is juct geared way too high so i think i am just goiens to go back to stock configuration for that year with a 16 front and a 46 rear but if anyone could help me out with figuring out what the rear rim is off of so i know what to order the sprocket for like i said before it is a 16 incher with straight spokes and the tire size is a 130-90-19 thanks

 

isnt the rear tire 110/90-18 ???

 

It SHOULD be on the 550 Seca. (4.00-18 is fine too.) Somebody's fitted a 16" rim running a big fat tire to his Seca; along with a power-sapping 41T rear sprocket, most likely the original sprocket on whatever bike the wheel came from.

 

i guess i shoulda read ur response alil carefuller :lol: And Chadley89 if ur going with a 45t sprocket you should either A. Find which wheel this came off of or B. buy a whole new rear tire set up soo you can go back to stock and make things a hell of alot easier in the long run :wink:

 

You mean a 130-90-16 right?

 

yea its a 16 inch sorry for the typo

 

I really want to know what this mystery wheel came off of.

 

Yeah me too. The 550 Seca was the only US-market XJ with straight-spoked mags, everything else had "swirly" wheels. But it had an 18" rear, and somebody found a matching 16" rim. OR DOES IT MATCH exactly? The RD400 had straight-spoked mags (not sure about rear wheel size) as did the twin-shock XS400 Special, although not EXACTLY the same style.

chadly does your rear wheel look like this one? I'm betting yes...

These are not exactly the same as the Seca wheels but very close:

 

thats it, that must be the kind of bike that the PO got the setup off of, does anybody think it would be a problem running it the way it is, it has been like that for over a year and i have never had a problem with it, and i ride hard, i took the rim off and looked at the "cush" assembly and the rubber pieces are like new, not cracked or rotted everything looks to be in good condition

 

Hmm. You're definitely running an XS 400 rear then.

I don't think it will "hurt" anything if the cush rubbers are holding up, except the poor bike is horribly under-geared. You'd HAVE to be riding it hard with it geared that "tall." I still recommend finding the correct rear wheel so you can get back to the stock sprocket sizes/ratios; it will be a lot easier on the motor in the long run. Your PO must have really had an allergy to high RPM cruising, but he didn't do the motor any favors.

 

yea ill definently look into that this bike has 30,000 on it so im sure the engine would appreciate it

 

Mystery solved.