1978 XS750 high speed problems

[IanDMacDonald]

I found a website "powersports360.com" that has MIKUNI™ LARGE ROUND MAIN JETS for $3.50 for a 4 pack. Everyone else is selling them for $12+. Sounds like a good deal to me, plus the inventory they have looks great!

[IanDMacDonald]

Okay guys, pulled the carbs off halfway (left the throttle cable attached) and pulled off the left-hand carb. float bowl. Suprisingly, the brass on the float and jets look good! Bowls are clean! The Main jet is a 130. Should I go 132.5, or 135 to cure my top end lean problem?

[DGXSER]

At that price, I'd order both and be set. IT is kind of a trial and error kind of thing. I'd start one size bigger and see what you get.

[IanDMacDonald]

Fellas-
Went over to a small motorcycle shop and bought (3) 132.5's and installed them. Did not notice much difference revving it in the driveway, so I hit the road. I was able to hit 75mph on the highway (might have been a straight road), instead of the usual 70mph. So, I went back to the shop and picked up some 135's. I will not have any time tomorrow to check it out, but I may Friday. I will keep you guys posted.

[IanDMacDonald]

Wtf?

Okay guys, I'm wondering if I am going at this all wrong. I got home around 9:30p after work and could not wait for the weekend, so I put the 135's in. To my astonishment, the bike turned over on the first press of the starter button (the starter is just about shot), so I saw that as a good sign. But, got the bike warmed up and took it on the highway in the rain, and still not going far over 70mph.
I have left out the fact that I have yet to dismantle the carbs and completely clean them. The float bowls looked good, so I took that as an indicator that the rest of the carbs are just as decent. Wrong? Anyways, I ordered carb. manifolds just in-case this may be an issue. My next door neighbor hinted at the fact that it may be a cam issue, and they are not opening up all the way? If that was true, wouldn't that prevent me from being able to have good performance in neutral? I really thought the answers in this thread would cure my problem. I'll take the carbs apart this weekend and give them a lookover. Any new ideas would be appreciated!

Mac

[DGXSER]

OK, maybe a few presumptions were made that should not have been.

First things first, check the valve clearances and do give the carbs a good thorough cleaning. I just finished tearing a set down that the bowls looked pretty clean, but pulling the pilot jets they looked like they were shoved in a mudhole before being installed!! And the emulsion tubes needed to be driven out and scraped the sludge off of them.

So, do not assume anything. Know for sure the carbs are good and clean. Know for sure your valve clearances are set correctly. THEN, take a run on the machine and see what you get.

[IanDMacDonald]

DGXER:

Actually, those are the plans this weekend! Valve clearance and complete carb. breakdown. Will keep you posted with regard to progress. Thank you for all of your help.

Mac

[DGXSER]

No problem Mac, hope it gets you running like you want.

[bookem]

been away for a few days so missed all of this. I would suggest if everything checks out that 135 may be way too small yet. While I still haven't got to town for new jets when I drilled mine I went to a 145 according to my drill chart. It seems that I may get even better performance when I put in 150's. the hugh diff it made when u change to airpods is quite dramatic on these bikes, bigger than any other bike I've modifed. When I get the jets I will be getting sets up to 170 to play with. Hope this helps.

[IanDMacDonald]

Wow, 150's? I may have time tonight after work to pull the carbs and give them a good cleaning. Valves I may get to depending on whats going on. Thanjs for the tip, bookem.

[IanDMacDonald]

I'm beginning to think I am overdoing this thread, but screw it, this is why it's here! So, I took a look at the carbs. after work today. I could not get the screws off the tops of the diaphragms w/o stripping them (fudged one bad), so I decided to stick my mouth on each opening and blow. I did notice something interesting though. The (piston?) on the outside carb. seems to have the greatest (compression?), whereas the middle one is second, and the other outside one is least.
Anyways, I received new carb. manifolds from Georgefix off ebay and installed them. Took it out on the highway and instantly noticed high revs., allowing me to hit 80, which seemed really fast! I bought a set of feeler gauges today, so I will check the valve clearances tomorrow.
What I will do is definitely pick up some 150's and see what they do. I'll keep you posted fellas.

Mac

[DGXSER]

On the slides, you actually want to suck on the mouth shaped opening at the top of the inlet bell. That will make the slides rise up. Then put a finger over that opening and see that the slide falls pretty slowly.

As to those screws, its pretty common to strip out those goofy jap phillips heads. I just finished upgrading a set of carbs with s/s cap head screws for the tops and the bowls. Generally, I use a dremel tool to cut a full width slot in the screw to use a standard blade screwdriver on them and they come out pretty easily.

Sounds like you may have had a vacuum leak as well. Your getting there with it!

[bookem]

good to hear the rubber "boots" helped. Mine topped out at 105 mph with the drilled jets. I hope to get 110 - 115 outta it next summer. No sense having pod filters if u don't get better than factory spec speed! Good luck

[IanDMacDonald]

Checked the valve clearances, and found them to be off within a thousandth of an inch from spec., so we're good there. Put the 145's in last night and really, not much difference than the 135's! Conclusion is carbs need to be rebuilt and cleaned. I went to Ace hardware (it's a block from my house), and bought six sets of screws for the top and bottom of the carbs. If I plan on doing any work on this bike, I do not want to play with crappy screws.

Thanks fellas!

Mac

[IanDMacDonald]

Eureka!

So, I decided to spend the day (and it turned out to be the whole day) rebuilding my carbs. I started out dremeling the screw heads, which did not work great. Granted, I did not do it properly. So, I went to Ace hardware and was talked into buying an impact screwdriver and dremel cutoff discs. Got home and my buddy took some channellocks to 'em, (with the other hand holding a Bud Lite), and out they came. Lesson learned: keep it simple.
Anyways, I pop off the diaphragm covers, and to my astonishment, no tears or rips to the diaphragms, just straight manglement. Whoever screwed with these things must have crushed the diaphragms when re-installing the covers. So, I think I found my problem with the loss of power/vacuum, at least I hope so. Took some carb cleaner to the rest of the carb, and really the carbs were already pretty clean! I was really impressed with that!
So, gotta call my shop tomorrow and see how much I will be raped on the diaphragms. If anybody has any other comments on this, feel free.

Thanks,

Mac