1981 xs1100 running rough at low rpm

[leggman1]

I am finally getting my 81 out and warmed up and at the lower rpm range it runs like crap. No power but once I get it up it pulls like a beast. I pulled the left side cover to check the points and found to my pleasure that there are NO points. Cool.
When it is at idle the vacuum advance does not pull. If I take the vac line off at the intake a apply vacuum there the plate moves and the idle smoothes out.
This bike is very fast once the rpms come up but it should run better at the lower ranges.
I am guessing the carbs are out of sync.
The bike sat for many many years and I have cleaned the carbs and replaced the rubber plugs in the bowls.
As always I am open to any and all suggestions.
Thanks. Dave

[motoman]

I am finally getting my 81 out and warmed up and at the lower rpm range it runs like crap. No power but once I get it up it pulls like a beast. I pulled the left side cover to check the points and found to my pleasure that there are NO points. Cool.
When it is at idle the vacuum advance does not pull. If I take the vac line off at the intake a apply vacuum there the plate moves and the idle smoothes out.
This bike is very fast once the rpms come up but it should run better at the lower ranges.
I am guessing the carbs are out of sync.
The bike sat for many many years and I have cleaned the carbs and replaced the rubber plugs in the bowls.
As always I am open to any and all suggestions.
Thanks. Dave

For starters, that vacuum advance assembly NEEDS to floppy loose when vacuum is NOT hooked up. If it isn't, it needs to be cleaned underneath assembly and lightly lubed. Do DO that first, and an improvement of your issue WILL be noticable.

[spectra]

For starters, that vacuum advance assembly NEEDS to floppy loose when vacuum is NOT hooked up. If it isn't, it needs to be cleaned underneath assembly and lightly lubed. Do DO that first, and an improvement of your issue WILL be noticable.

Please explain this in detail.......I am new to he 1100s and kind of have the same issue. Chasing multiple things but this is something I would like to look into. Also lubed with ?

[TopCatGr58]

Bing, Bing, Bing!!

Hey Leggman,

Okay, you diagnosed your problem in your first post. You apparently got a carb kit to fix/clean your carbs. If you have an 81 and it IS truly an 81 and doesn't have a mix of parts, like other carbs from an earlier model, then the rubber plugs are NOT used !

The rubber plugs are only for the early 80 model carbs that were during the transition that Yamaha went thru when changing from the earlier model 78-79 series that used a sharing tunnel between the main and pilot jet towers, and the pilot jet tower was capped off with a threaded large head screw!

In the early 80 model, they still had the sharing tunnel, but the pilot jet tower wasn't threaded, and so they used rubber caps to seal it off so that the pilot jet(low rpm/throttle circuit) would feed from the sharing tunnel with the main jet, even though the main jet was resized down to 110 vs. the 135's of the early models.

The later 80 carbs finally eliminated the sharing tunnel, and the pilot jets feed directly from the fuel bowl, and so NO CAPS are used/needed. Putting caps on them will STARVE the pilot circuit and will give you the symptoms you have decribed of little/no low rpm throttle response. I'm actually surprised you're able to get the bike started and running with the rubber caps on.

SO...long story short, pull the carbs, take the rubber caps off, and you should then experience much better throttle low rpm response, and THEN you can do the idle/pilot circuit tuning and a vac. synch.

The reason why the bike smooths out when you apply vac. to the vac. advance is because it does increase the timing/rpms which allows it to start getting fuel from the MAIN jets with increased RPMs. During idle the vac. advance isn't supposed to be engaged when the idle speed and butterfly shaft openings are set right. The vac. adv. is really mostly for increasing timing during CRUISING speeds/rpms to help burn the leaner fuel mix to increase mileage. During hard throttles, the vac. actually drops and timing actually retards back from the maximum of 52 degrees BTDC to around 35 degrees BTDC which is the best maximum cent. adv . level for the best POWER mode.

The vac. adv. plate does need to be very loose and smooth moving. The grease can dry/harden and make it stiff and prevent it from rotating under the small amount of vac. from the engine which would prevent it from obtaining the optimum vac. adv. for cruising for best mileage.

T.C.

[motoman]

Please explain this in detail.......I am new to he 1100s and kind of have the same issue. Chasing multiple things but this is something I would like to look into. Also lubed with ?

T.C or someone here can likely show the link for removal of that assembly. Basicly, that assembly is removed after removing the large center nut. Have watched it being removed, but didn't really pay attention to actual removal sequence.

[leggman1]

I believe the PO told me the carbs were replaced, Why I don't know. The bike only has 6100 original miles on it now. I remember when I was cleaning the carbs that it does have a shared tunnel and I found remnants of the old rubber plugs in there.
I will try to clean the plate tomorrow if I can find a post telling me how. I DO know it is not loose now by any means because I had to push it with my finger to get it to move.
Thanks.

[TopCatGr58]

Hey Leggman,

Okay, it sounds like it might have the early model 80 carbs instead of the 81's. Some folks/PO's can't get bad dirty carbs clean and so they just throw another set on there. Essentially you can run any year XS11 with any year carb, you just have to jet the carbs according to the YEAR of the carbs and NOT the year of the engine.

SO....did you happen to notate what the size of the main and pilot jets were? Also, even though the carb bodies might have the sharing tunnel, the main jet needle(Emulsion tube) has to ALSO have the opening in the side/end of it to allow the fuel to be able to travel from the MAIN jet tower thru the tunnel to the pilot jet tower.

How many small round ports are on the intake bell, 3 or 4? The early models have 3, the later 80-81's have 4 because they incorporated the bowl vent into the bell instead of the "T" on the side of the body.

SO...IF it does have the sharing tunnel, the main jet needle has the proper opening so that fuel can get thru it to the pilot jet tower, the vac. adv. is a red herring for this problem. Looks more like a cleaning problem.

BTW, did you remove the PILOT JET SCREWS from the top front of the carb bodies? They often have a small brass cap over the screw tower sealing them in for EPA control. You have to drill into them to remove them to gain access to the PILOT JET SCREWS and those passages for proper cleaning. If you didn't remove those screws, then you haven't really cleaned them completely, and the pilot circuit is possibly clogged causing the poor low throttle/speed response.

Check out the official Yamaha microfiche OR the PDF manual that you should have been able to download from the links I provided during registration confirmation. They can show you how the timing plate and reluctor and shaft/bolt comes off/out. It's got a slot so it can only go on 1 way.

T.C.

[skids]

Hey Leggman,

BTW, did you remove the PILOT JET SCREWS from the top front of the carb bodies? They often have a small brass cap over the screw tower sealing them in for EPA control. You have to drill into them to remove them to gain access to the PILOT JET SCREWS and those passages for proper cleaning. If you didn't remove those screws, then you haven't really cleaned them completely, and the pilot circuit is possibly clogged causing the poor low throttle/speed response.

T.C.

Removing the cap from the pilot screw can be a real challenge. It is easy to break the "tower" cylinder when trying to remove the cap ( don't ask how I know). If you do break the tower removing the cap, the function of the pilot screw will likely be OK. Be aware that there is a washer and an oring below that you want to keep track of. Also, it would be good to record the number of turns of the pilot screw to the LIGHTLY seated position before you back it out (for each carb). The reason for removing those pilot screws is that you want to spray carb cleaner down that bore. You also want to ensure that the three tiny holes in the carb throat are free of gunk.

Spray carb cleaner through the pilot jets to ensure that those three holes are clear.

[leggman1]

I really think the problem is the timing plate. When I first got the bike II was shocked at just how clean the carbs were inside. They must have run it dry before it got put in the barn/shed many years ago. I DO have some cracking on the intake manifolds but I don't believe it goes all the way through. I will try the wd40 trick tomorrow as well.
What should I use to lubricate the plate?
Is there any tricks to getting the advance mechanism apart?
Sorry to have so many questions but a yamaha is new to me. I grew up with 750 hondas and bmws.
Thanks.
Dave
https://photos.google.com/photo/AF1Q...nVzK40kpCRp2TF
https://photos.google.com/photo/AF1Q...nDQdV3TB-6eivG

[TopCatGr58]

Well, you can work on the timing plate if you want to, I used some Spray White Lithium / silicone lubricant because grease is too thick.


The idle/pilot circuit is intricate and convoluted, and even though the carbs were DRY, that doesn't mean that it didn't evaporate and the passages could have gotten gummed/varnished from the fuel, especially ethanol.

You didn't say if you took the pilot screws out during cleaning? Poor low rpm throttle response is due to clogged/poorly flowing pilot circuit. Just saying.

T.C.

[leggman1]

makes sense. If the timing plate clean doesn't help then the carbs will be next. Thanks

[leggman1]

I looked at it again this afternoon and when I removed the vacuum hose from the advance unit it moves much easier??? That is telling me there might be a restriction either in the hose or the port where the hose connects?
Something is definitely wrong somewhere. I have owned many four cylinder Japanese bikes back in the day and they either ran great or not at all.
Like I said before when this thing starts to open up it is a beast, Scary fast, But on the low end it's a pig that cant get out of it's own way.
As much as I really don't want to get back into the carbs I think that is where this is headed.

[leggman1]

OK, I broke down this afternoon and removed the carbs. It wasn't that bad, It took longer to get the stock airbox off than to get the carbs apart.
I think I found the problem. All passages in the carbs were semi clean and I found nothing blocked EXCEPT for the shared passage in the number three carb. There is NO shared passage in the number three carb??? I have no idea what the PO did or why so I can only assume it is a different carb. I also found a wad of what looks like jb weld or something similar on the outside of the secondary jet tube inside the bowl???
The main jets are #110 and I have no clue what the secondary jets are. They are very small and my eyes are 60 years old.
So the question is should the #3 carb have a shared passage? If so can I just drill a hole of a similar size?

[leggman1]

https://photos.google.com/photo/AF1Q...XIUuBncUCQt6KL

[leggman1]

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