Should I rejet to fix my carb issues?

[dgr8one85]

Hello all,

This is my first post on these forums.

I purchased my 82' for $100, the previous owner was not as patient as I am in dealing with these seemingly complicated carbs.  When I purchased the bike, I knew it was going to be a project bike with carb issues.  The body is in decent shape, and all other mechanicals seem to be in decent working order, and seemed like a decent barn bike find.

I immediately got a charge on the battery and was able to get it to fire up with a lot of choke, and throttle.  That was good enough for me at the time, and parked it for a period in time where I could devote some time to it.  About a year passes.

Fast track to this past week.  My buddy bets me $100 that this bike will never be reliable enough to take a 90 mile trip to Louisville and back.  Bold bet, but I'm confident.  And lights a fire in my ass to work on a bike thats been sitting in my garage for almost a year.

 I knew the tank had a "bit of" rust in it.  This was seemingly the demise of the bike, and had to be the first issue resolved.  Had the tank acid flushed and redlined by a local shop.  Next, fuel petcock rebuild kit.  Complete teardown of the carb, being extremely careful with the gaskets.  Looking at all the blocked, corroded, jets, I have no idea how this thing started at all.  The rust in the tank was the sole culprit, and blocked every passage and venturi pressent in each of the carbs.  I proceeded with soaking everything in diesel in my ultra sonic cleaner, and removed, and blew out every passage I could see with carb cleaner.  I used an entire can of carb cleaner on this carb, and am going to purchase another. 

I bench synced the carbs, because at full close, one of them must have been adjusted to accommodate the clogged jets, and had about 2 mm opened. Reassembled.  I Cant read the jets due to corrosion, so I have no idea if the previous owner even placed the correct jets between the mains and air jets.  Set the Idle mixture screws to 3 turns from soft bottom on front and rear. 

The bike fires up!  I am so happy at this point I take a video to scare my buddy, and let him know he's about to owe me $100 bucks. 

...then the problems.  She's getting more and more difficult to start.  I have a brand new battery, and starter selenoid.  At full choke, she turns and turns.  And more time sitting pissing my neigbors off, she eventually sputters, i apply some throttle, and by some random sequence of throttle input, choke adjustment, rain dance,  i'm able to get her to idle.  I set idle to a little over 1000 rpm, but she's bumpy.  When I rev her up, she screams to 3000 rpm and holds there till I can't take it, and spin the idle screw down, and she settles back down and dies.  I tried to sync her, and thought i accomplished a sync.  But the rev issue did not go away. 

I will admit, i did not check the float level.  I'm not too sure how to do this, and cant find pictures.  From what i've read, this is a must along with every single little detail with this carb. 

Also, should i bother with the oversized jets that i just purchased from Sigma 6 jet kit?  Anyone ever use this?  It came with very vague and generic instructions.  From what i've read, it sounds like stock jets are fine.  If so, where can I source some?

Anyways, any help is appreciated.  I have big plans for this bike....and a bet to win.

Thanks!

[fret not]

The issue of not idling down after revving the engine may be related to the carbs, or may be related to air leaks in the carb manifold boots.  They can become cracked over the years and leak air, which leans the incoming mixture.  At least check this area.

[Prophet Of Doom]

Starting varies from bike to bike.  On mine, petcock on PRI, 1/4 choke only and if you touch the throttle before it fires, it won't start for hours.

Can't comment on 6 sigma and don't recall anyone posting about them.  Do they have numbers on them?? Trouble is that there is so much variation in "spec" for these bikes that it's hard to know what their starting point was.  You can bet $5 they didn't buy an XZ550 and run it up on the dyno to get the right sizing.


I wrote a post ages back listing all the published specs for various models.  Do a search.  I like the 120/120 pilot air over the narrower US spec, but generally stock jets are fine with stock exhaust and airbox.  JetsRus.com for jets - or your local bike shop, or just about anywhere really.  They aren't rare, but advised to replace - if you can't even read what they are they probably aren't metering all that well.

High hanging revs mean you almost certainly have an air leak.  Cracked manifolds as Fet nut says, or the YICS is another highly likely cause. Buy some rubber caps and eliminate the YICS entirely till you have it running nicely.  Liquid electrical tape to repair the manifolds apparently, or replace them.


Float level is important. Put some clear hose on the carb overflows, then crack open the drain screws.  Measure between the fuel level and the seam between carb halves.  Procedure is in both Haynes and the Yamaha service manuals.


If you are up to it, think about doing your valve shims or getting them done - especially if your bike has many km on the clock.

[dgr8one85]

Thanks for the tips,

She currently has 17k miles.  The main jets from that kit were 130 front and 134 rear.  I put them in and she was actually capable of moving under her own power around my neighborhood.  This was quite a milestone, being this bike hasn't moved under its own power in almost 10 years.  She shifted through the gears nicely, stopped when I wanted her to.  But I still have the RPM hangup around 3000. 

From what I've sourced throughout the tips and tricks on this post, I have plasti-dip sealed the carb boots (very generously I might add), along with fuel resistant RTV silicone when attached to the intakes.  I have also already capped off the YICS.  As far as I can tell, I can't possibly see a vacuum leak coming from these area.  The only thing I haven't done is check the float levels.  I have sourced a Haynes manual, and will be setting levels per the suggested spec.  I have also already ordered stock main jet sizes. 

I forgot what post I read from, but I heard someone else having the same issue I'm having, and it ended up being a float issue, and somehow vacuum being pulled from the bottom of an empty float bowl.  That seems like a tremendous amount of air passing through very small passages to maintain a 3000 rpm hangup, but I guess anything is possible. 

[fret not]

Maybe it is time to test with carb cleaner while the engine is running.  Spray a bit around possible leak locations and listen for the RPM to increase as the carb cleaner is sucked in and added to the air/fuel mixture.  Spray around the butterfly shaft, carb boots, etc.

[dgr8one85]

crap, I didnt even think about the butterfly shaft seals.  I attempted to take the butterflies off to attempt a complete tear down of the carbs, but halted when I was approaching an uncomfortable amount of force, and the screws not breaking loose.  Anyone have any tips or tricks?  i contemplated a torch...but i assume there are rubber/polyurethane seals on these shafts, and i'd hate to destroy possible intact seals. 

[jefferson]

The best thing I have done in a long time is get a set of the Vessel screwdrivers that are made to fit the phillips screw heads that the Japanese use as they are different from ours. I always wondered why the heads rounded off so easily and it was because of the difference between the two types of phillips heads. I can't believe how well they lock into the screws and if you don't use a the vessel screwdriver you will most likely round off the head and cause problems. They are a must have if you do much work on these or any other Japanese bikes.

[Prophet Of Doom]

It's hard to believe how much better using JIS (Japanese Industry Standard) screwdrivers is till you actually try them.  If you don't want to splash out on JIS drivers, knocking the rounded tip off a Phillips driver will give a somewhat bit better fit.  Vessels aren't the only brand, but they have nice rubbery handles, magnetic tips and seem to be of good quality so I'll stick with that brand.  They also do an impact driver version which look pretty slick.

JIS drivers won't always get the butterfly screws out though as they are heavily peened with a cross-hatch tool.  You are best to file the backside off first, and then use the JIS driver.  Drilling out makes things worse, as then you will have nothing to grip.

[dgr8one85]

is vacuum leak from the butterfly seals that common of an issue?  I will definitely run some carb cleaner around it while idling to see if they are the culprit, but i'm going to make sure my floats are set correctly first.  I'm also not too convinced my run through of the carbs through the ultrasonic cleaner and diesel broke all the passages completely clean and free.  I've been reading up on people using lemon juice and boiling.  I'm going to try lemon juice and my ultrasonic cleaner next.  The bottom of each of the bowl still looks gummy and corroded.  I'm going to assume my passages still looked the same.  I did have to drill out one of the emulsion tubes and pull it out with a screw.  Do i need to close this hole back up?  it looks like the gasket seals it off and leaves nowhere for fuel to go otherwise?