'80 XS1100 MS leaking float valves(?)

[gtem]

Very intriguing, I will definitely be checking what showed up from partshark as OEM Yamaha, I could have sworn they had bleed holes. Now, within the context of how these pilot screws sit in the BS34 carbs, how do they seat, and does it *matter* that they have the bleed holes or not? as I recall the pilot jets that are on these MS carbs in question *DO* have bleed holes, but I will reconfirm. I went down a similar rabbit hole with a set of I think BS32s on a GS650G a few years ago. Presumably the jets seal between the chamfer between the jet tip and the carb body. BS30/96 style pilot jets have the metering orifice down at the tip, does air from the air jet travel past and interact with those bleed holes?

[bikerphil]

There was a discussion some time back about these bleed holes in the pilot jet. After some testing, the no hole type worked exactly the same as the 6 hole type. You want to stay away from the 8 hole type, as they are for the VM type carbs and have a different type spray nozzle on the end.

[gtem]

Thank you for that info Phil! And I did confirm that the MS currently had the six bleed hole type #42.5 jets installed, and the new Yamaha #42.5s I have installed in my other set of '80 carbs on my 78E are of the no-hole variety. Very interesting. Well, I guess the search for the flooding-out culprit continues. I will try and see how the MS carbs run with my new no-hole pilot jets swapped over, just to eliminate that as a variable. Who knows maybe those old #42.5s got micro-drilled to something larger. I did also once again reconfirm that all seems well with floats and float valves.

[DEEBS11]

It's a good idea to put in a known good part to eliminate issues.

[DEEBS11]

Also check these air jets. I have seen these made in smaller size to richen carbs across the rpm range. Always better to leave stock and modify the separate circuits as needed.

[DEEBS11]

There was a discussion some time back about these bleed holes in the pilot jet. After some testing, the no hole type worked exactly the same as the 6 hole type. You want to stay away from the 8 hole type, as they are for the VM type carbs and have a different type spray nozzle on the end.

This is true Phil. I had issues with my KZ750B carbs when re-jetting. They almost look the same. They were so rich that it ran terrible.

[DEEBS11]

I have never run these so I can't comment from experience but this is the part I was referring to. If you have something like this in your carbs from the previous owner and you don't know about it, this can play havoc with trying to solve jetting problems.

https://www.ebay.com/itm/25556877974...Bk9SR56_-8bCYw

[cajun31]

Thank you for that info Phil! And I did confirm that the MS currently had the six bleed hole type #42.5 jets installed, and the new Yamaha #42.5s I have installed in my other set of '80 carbs on my 78E are of the no-hole variety. Very interesting. Well, I guess the search for the flooding-out culprit continues. I will try and see how the MS carbs run with my new no-hole pilot jets swapped over, just to eliminate that as a variable. Who knows maybe those old #42.5s got micro-drilled to something larger. I did also once again reconfirm that all seems well with floats and float valves.

If it were me, and it's not, I would leave everything stock including those pilot jets. I have three MNS and all of mine have the original type pilot jets with 6 holes and are 42.5. Of course I also run everything else OEM other then the air filters which are K&N. Make changes if you think they are necessary after you get the bike working correctly. Otherwise you might be chasing your tail.

[cajun31]

These are the correct pilot jets for the bike with those original oem carbs. Genuine Mikuni jets BS30/96 - Jets R Us

[cajun31]

These are the ones without the holes which are not oem or correct. Genuine Mikuni jet N151.067 - Jets R Us

[gtem]

These are the ones without the holes which are not oem or correct. Genuine Mikuni jet N151.067 - Jets R Us

"which are not oem or correct"

For what it's worth . ." Jet, Pilot ( 42.5) 4G0-14142-42-A0" is an OEM Yamaha part number and the jets showed up in a Yamaha made-in-japan baggie. Now it sounds like maybe in this application the bleed holes are irrelevant because the jet is fed from the bottom anyways, so the new jet is identical in function for this application, and takes fewer machining operations to produce and hence Yamaha specified it here as the OEM replacement part. But it's definitely something worth paying attention to. I'll keep the original six hole 42.5 pilots in place, I did some comparing and as far as my eye can tell they have not been opened up with a microdrill compared to the no-hole 42.5s that I've been running on my '80 carbs on the '78E

If it were me, and it's not, I would leave everything stock including those pilot jets. I have three MNS and all of mine have the original type pilot jets with 6 holes and are 42.5. Of course I also run everything else OEM other then the air filters which are K&N. Make changes if you think they are necessary after you get the bike working correctly. Otherwise you might be chasing your tail.

That's good to know that the six hole type is original to the bike, and that the no-hole type is an updated part from Yamaha.

So here's the update. Things are getting a bit complicated perhaps, as I've got two sets of '80 carbs side by side I'm messing with: the MS carbs that are the main point of this thread, and my '80 carbs that I'm running on my '78 E (https://xs11.club/forum/idle-talk-fo...s-low-rpm-funk)

MS carbs: they had 117.5 main jets in them when I opened them up, I've never heard of this as a stock setup. I installed #110 mains in their place. I've kept the original six hole #42.5 pilot jets. Float levels set at 23.5-24mm across the board. Original '80 Special style shorter/more tapered needles, spring/washer stack as assembled at the factory. It had been flooding with the idle mixture screws set to 1.5 turns out, I'm going to try them at only .75 turns out like I'd been running on my '80 carbs on my '78 E

'80 carbs on the '78E: I've installed 120 mains to go with the less sharply tapered, longer style Standard/Venturer needles that I've acquired, correct needle stack assembly. new production yamaha/mikuni no-hole #42.5 pilot jets. Float levels set at 23.5-24mm across the board. .75 turns out on the pilots

The one thing I've noticed missing is a few of the o-rings in the idle mixture screws on the MS carbs so I'm waiting on them to arrive to install before I try this setup out. But I will try my new #120 mains/less tapered needle setup on the '80 carbs on my '78E, even where I left off with that at the end of last year, it's still been too rich on the low end/roll on throttle.

Thanks again everyone for the help, this forum is an invaluable resource!

[cajun31]

You are confusing tuning issues with carb flooding issues. How those pilot jet screws are set has nothing to do with the carbs flooding. They come into play after you start the bike and play the majority of the role of how the bike idles. They are also in play as the bike revs up but to a less degree. Those o rings that you say are missing may well still be down those tunnels. They sometimes get old and brittle and flat and you really need to be sure before you throw more in there. Not doing so you will never get your idle mixture set right. Those pilot jets with no holes in them are not new replacement parts. Both are made holes and no holes for different applications. I don't want to cross threads with my opinions, but just going to larger main jets is not always the solutions unless you have made modifications to the bike from stock. What I mean for example , is if you are running pods rather than the oem airbox or an exhaust system that might require a change in jetting. Most things do not require re-jetting. There is an old thread that shows what I am talking about under the maintenance modifications section. What you are doing and suggesting will simply make the bike run rich and no one is going to want to ride behind you. Get the flooding carb situation resolved and work from there. Then you can concentrate on tuning issues. What I am saying is use the KISS method or you might be creating problems. Better to not make changes until you get the bike running properly. Just the way I approach things. I know many things run through your head trying to sort things out, but these old bikes can just be slightly out of adjustment and until you get it dialed in can be frustrating.

[bikerphil]

less sharply tapered, longer style Standard/Venturer needles that I've acquired

I find the needles in the Dynojet kit make these 80 and up carbs rip, and they are totally adjustable. Nice smooth power. I know, not genuine Mikuni, but the kit ($135) is specific for the BS34 type 3.

[gtem]

You are confusing tuning issues with carb flooding issues. How those pilot jet screws are set has nothing to do with the carbs flooding.

Jim by "flooding out' I mean running so rich that it fouls plugs out with gas within 10 seconds of starting the bike. The carbs do not overflow gas in the way a stuck float would cause. My apologies for the confusion. I've tested these things on the bench to death, I've got new OEM float needles and seats, new o-rings, floats set right and checked for any hanging-up, I will make sure to bench test them again before they go on the bike.

Those o rings that you say are missing may well still be down those tunnels. They sometimes get old and brittle and flat and you really need to be sure before you throw more in there.

Yep I very carefully inspected, like you said some are flattened, two are definitely missing.

Those pilot jets with no holes in them are not new replacement parts. Both are made holes and no holes for different applications.

Per Yamaha's own part diagram for a 1980 Special, they are. My understanding is that if the pilot jet is fed from only the bottom of the tower direct from the fuel bowl, and the air jet only interacts with the fuel below the metering orifice which on these pilot jets is at the tip of the jet, sealed by the tapered seating surface, then presence or absence of the bleed holes does not make a difference? I could be wrong, I'd love to understand this stuff better.

[cajun31]

Jim by "flooding out' I mean running so rich that it fouls plugs out with gas within 10 seconds of starting the bike. The carbs do not overflow gas in the way a stuck float would cause. My apologies for the confusion. I've tested these things on the bench to death, I've got new OEM float needles and seats, new o-rings, floats set right and checked for any hanging-up, I will make sure to bench test them again before they go on the bike.



Yep I very carefully inspected, like you said some are flattened, two are definitely missing.



Per Yamaha's own part diagram for a 1980 Special, they are. My understanding is that if the pilot jet is fed from only the bottom of the tower direct from the fuel bowl, and the air jet only interacts with the fuel below the metering orifice which on these pilot jets is at the tip of the jet, sealed by the tapered seating surface, then presence or absence of the bleed holes does not make a difference? I could be wrong, I'd love to understand this stuff better.

There was a discussion about those pilot jets several years ago now and the theory was that the holes allow more fuel on the idle circuit. If I remember correctly the discussion was concerning the venturi effect of the carbs on the idle circuit when the bike is idling. Have you checked for vacuum leaks. I mentioned those shaft seals before and they can be a source of leaks. The other place to check and is something I always do on the bike when I first start working on it, is when you have the carbs off take the intake manifolds off and check for old gaskets that are degrading. If they are present get rid of them and clean both surfaces and remount the manifolds with something similar to Yamabond if Yamabond is not available. The carbs need all the air the engine can provide going through them. I also asked before if you still had the OCTY or not. If you don't then don't turn the petcocks on unless you are immediately ready to crank the bike. These bikes are known for flooding and until you get them dialed in, as I have said before, can be frustrating. Many eliminate the OCTY and just go with manual petcocks. I like the OCTY when it is working correctly, but have to say I still immediately turn my petcocks off when I stop the bike. Even if just stopping on the road for a break the petcocks get turned off.