Carb issues on a 79 Yamaha XS 750 Triple

[Tourmeister]

Howdy,

:tab So Tripp, a member here and my local cable guy, has his bike sitting in my garage. He has been trying to get his bike ready for riding and is having a little trouble. We fired the bike up this evening and it started quite easily once the gas got from the tank down into the carbs. The engine came up to temperature quickly and would idle (somewhat roughly) without any choke. The first time we tried to apply any throttle, the engine burped, backfired a little and died. So we fired it up again and let it idle for another minute. I slowly increased the throttle input and the revs rose and things smoothed out. However, when I tried to release the throttle, the rpm's continued rising higher. Even with the throttle fully back to stop, the engine continued revving high. Finally we just killed the ignition. A check of the throttle shows that the cable is not snagged and is returning to the idle position with no problem. A visual check confirms that the throttle at the carb is snapping back to idle as well. So I am fairly confident that this is not a cabling issue. Nor is it an issue with the throttle sticking at the carbs. It almost seems as if the choke is not releasing even though before we started applying the throttle, the engine was idling with the choke off. I am not a carb guru and would really appreciate any guidance as to what to look for or try.

Adios,

[Tripp]

I would gladly offer to buy the beverage of choice to anyone who might be able to help :tears:

[Tx Rider]

What's the story on the bike, has it been sitting up for a while?

Has anything been changed since last time it ran well?

What kind of carbs does it use, and what kind of cabling setup?

It's hard to tell over the net what it could be, but it sounds like the carbs might want to be taken off and gone through thoroughly. Maybe a sticky slide with some gunk in the carbs?

If the controls are indeed closing all the way. It's hard to put a probability on a problem without a little history.

I'm not familiar with the carbs or control setup that bike uses. It would be much easier in person, normally you can sort out the carbs in an afternoon.

[Tripp]

I've gone through and rebuilt the carbs already. They did get a little sticky since I did that, but have since cleaned them. I'll try to attatch a schem.

http://houseofmotorcycles.bikebandit.com/partsbandit/showschematic~dept_id~641616.asp

they are Mkuni carbs

[gotdurt]

It's hard to say without tinkering with it. When you cleaned the carbs, did you check the holes in the jets to make sure they are clear? I use a thin strand of copper wire, spray carb cleaner, and compressed air to clean them.

You might also check for vacuum leaks, particularly around the intake boots. Check for cracks in the boots also.

Ignition problems (ie not advancing) can cause similar symptoms, maybe a coil or plug wire. Does it have an electronic or point ignition?

[gotdurt]

BTW- just FYI, when cleaning CV carbs with carb cleaner, make sure and remove the rubber diaphram from the top of each carb before cleaning! Carb cleaner will destroy the rubber... don't ask me how I know :headbang: . Without these diaphrams (or with a hole/tear), the slide will not operate, making the main jet ineffective.... You can count on $100 each new.

[Tx Rider]

Ok, CV carbs with a pretty standard control setup, that have been taken apart since it last ran well. That doesn't narrow the options. :-|

I would first look at the control linkage between carbs and make sure the carbs are at least at a semblance of syncronization and all of the butterflies close completely and smoothly at the same time, just a fairly small difference in one carb can make an engine race.

If the carbs are all clean inside, assuming all the carb parts such as jets etc. were all put back where they came from,(often inside carbs are jetted differently than outside carbs) needles put back at the clip setting they came from, and floats were set right, it could still be a control issue between carbs.

After that it would be slides and needles, making sure the slides operate smoothly, the vacuum diaphragms are good, the springs are all in right etc. Then also make sure the mixture screws are all at about 1-1/2 turns out. A sticky slide could both cause a burp on throttle if sticking shut, and a race if stuck open.

Sticky slides arent very apparent on a CV carb, as ya can't see em or feel em cause they aren't tied to the throttle physically. Might try tapping on the carbs when it races and see if it stops. or maybe you can get a visual look into the throat and see if a slide is stuck open at all.

After that I would be looking at possible vacuum leaks.

Did the bike sit up for a long time before you took the carbs apart?

[Tripp]

When I purchased the bike, the guy said that he had replaced the needles and plunger/diaphragm. He also said that it started but had'nt been ridden in years. So unwilling to trust his word I tore the carbs down and verified everything. The plungers were new... that's about it. I replaced needles floats gaskets and cleaned out about 20lbs of crap that had taken residence in all the jets/ports/bowls and anything else that can hold stuff. If you were to look at it, it looks new and all the spring loaded features work quite well. I did make an adjustment to the butterflies the other night that should have helped if anything. The adjustment just put the number 3 fly into "symblance" with the others. I also just cleaned everything out again while I had them off.

How would I check for Vacuum leaks?

[gotdurt]

How would I check for Vacuum leaks?
Spray some carb cleaner around the intake boots while idling, if the idle rises, then you know there's a leak there. Look carefully for cracks in the rubber, especially around the flanges, it's a common issue with old bikes. Also check any vacuum lines on the carbs, make sure none of them are open, and are connected to the right part. If there are open ends, and you can't tell where it goes, try blocking it with a screw, see if the idle changes. There's a good chance that when you put the carbs back together and on the bike that a hose(s) was(were) forgotten, etc.

[Tripp]

Look carefully for cracks in the rubber, especially around the flanges,

between the motor and the carb or the carb and the air box

[gotdurt]

Intake, between carb and head.

[HiSPL]

Here's lots of good info regarding CV carbs....

http://www.thegsresources.com/gs_carbrebuild.htm


I would definately look for a leak in the intake boots first. It's very common on these old bikes. Also, if the carb boots bolt to the head (as oppossed the spigot type) make sure that the o-ring is in good shape. Often these o-rings are dried out and split.

Did you get to the idle mixture screw when you cleaned these carbs? There's a tiny o-ring in there that is easily forgotten about when the carbs are cleaned. If these carbs were dipped with that o-ring still in there, it was probably ruined and your idle to @3000 rpm mixture can't be set properly....

These are just off the top of my head. Remember, the easiest solution is usually the right one. I would check the carb boots very closely as a first step....

Good Luck!

[Tripp]

Well I definately know what to look for now and that is a perfect place to start. I appreciate all the info. Looks like I'll be taking it out one more time. (Not one of my favorite activites but it must be done).

[HiSPL]

I don't know how the XS compares, but the GS's carbs are a beyotch to get in and out. :lol: :lol: :lol:

Good Luck :chug:

[Tripp]

Note the () :evil: :angryfir:

After the gritting and gnashing of teeth it's not that bad :roll:

[gotdurt]

Update?

[Tourmeister]

Waiting on parts.

Adios,

[Tripp]

Parts in waiting on time.

[Tripp]

Ok I figured out what was wrong but I'm too ashamed to admit it.

[Tripp]

Just Kiddin'.

Well I took the carbs off and replaced the intake boots. The boots I took off were weathered on the outside but not on the inside. They did need to be replaced but I'm not willing to yield that as the problem. I looked over the carbs and took the advice on the butterflies. They were not synched at all. It took a minute to figure out how to get them adjusted. Put everything back on, filled the tank and part of Scotts driveway. Got that fixed and she cranked right up. I let it warm up for about 4 or 5 min adjusting the idle as it warmed up. I let Dower take her for a spin seeing as how I'm not an experieced rider. When He came back I could tell right away that It had started idleing high again. I had more room to adjust the idle since synching the flaps and brought it back down. Will went out for another ride and the adjustment helped alot.

Now however it seems I got gyped on the new fork seals I just put in and have a leak worse than the one I had before. I think I will get seals from the dealer direct or a trusted OEM dealer instead of Ebay.

Lesson: Listen to your peers and never trust "New" and "Fits 'X-'Y year." from Ebay. :lol:

[Squeaky]

Good to hear that the bike is running!
Now what color do you want it to be? ;-)

[Tripp]

You Know... I like the silver Idea...but I was toying with the idea of silver background and blue flames. Like the blue that it is now. Or maybe vice versa.

What do you think???? :roll:

[Squeaky]

You Know... I like the silver Idea...but I was toying with the idea of silver background and blue flames. Like the blue that it is now. Or maybe vice versa.

What do you think???? :roll:

I like. Ideas are flowing through my brain. I'll PM you with details.

[gotdurt]

You Know... I like the silver Idea...but I was toying with the idea of silver background and blue flames. Like the blue that it is now. Or maybe vice versa.

What do you think???? :roll:
I'm partial to Flames... (http://home.earthlink.net/%7Ecfry129/mc.html)

:twisted:

[Tripp]

I think I noticed that before when I linked to your site.

Squeaky.. I like the tank on the virago and 400 twin as seen above.

Maybe that will give you some ideas.

[budzrex]

Carb Diaphrams I used to have an XS850 if it sat they would go bad about 90 dollars each if I remember

[momma_of2miracles]

check idle screw in top or bottom in center of carbs and take idle screw halfway out then start bike, then turn the screw back in until idles without holding the trottle. per hubby. thanks