'80 XS1100 MS leaking float valves(?)

[gtem]

I guess because all I own are my three MNS's setting everything up stock has always served me well and is why I always advise folks to get things back to oem and work from there as the address modifications. Glad to hear the bike is running well.

Yup, it's the most sure way to get things right. Unfortunately on the '78 I don't have that luxury, with the 4-1 exhaust and '80 carbs that it came to me with. But as I posted in my other thread, I'd say I see the light at the end of the tunnel on that other one as well.

[cajun31]

A final update on the Midnight Special: I replaced all of the o-rings on the idle mixture screws and replaced one of the screw assemblies entirely, the spring and washer were completely rusted out underneath the screw. With all of the float heights rechecked and various leakdown and pressure tests on the (new) float valves done, I put it all together and got it on the bike. It runs and rides beautifully! I made some minor tweaks to the idle mixture screws to dial them in a bit better, the bike starts up, idles, pulls hard through all the gears, etc.

The bike happens to have the "updated" "no hole" #42.5 pilots, and new OEM 110 mains installed (came to me with 117s). So it's all stock, stock exhaust, and runs like it.

Thanks all for all of the advice, wow I really got a second education on these Mikuni BS carbs between this Midnight Special and my '78 E.

I guess because all I own are my three MNS's setting everything up stock has always served me well and is why I always advise folks to get things back to oem and work from there as the address modifications. Glad to hear the bike is running well.

[gtem]

A final update on the Midnight Special: I replaced all of the o-rings on the idle mixture screws and replaced one of the screw assemblies entirely, the spring and washer were completely rusted out underneath the screw. With all of the float heights rechecked and various leakdown and pressure tests on the (new) float valves done, I put it all together and got it on the bike. It runs and rides beautifully! I made some minor tweaks to the idle mixture screws to dial them in a bit better, the bike starts up, idles, pulls hard through all the gears, etc.

The bike happens to have the "updated" "no hole" #42.5 pilots, and new OEM 110 mains installed (came to me with 117s). So it's all stock, stock exhaust, and runs like it.

Thanks all for all of the advice, wow I really got a second education on these Mikuni BS carbs between this Midnight Special and my '78 E.

[cajun31]

FWIW, I have several BS34 XS650's. They all use the rubber plug over the pilot jet. All have the six holes. My XS1100 does not employ the rubber plugs and all the pilot jets have six holes. My XS11 has a K&N in the air box and the exhaust is a four into one Jardine. My jetting is stock. It's stock not because I'm lazy and left it. It's stock because that's what I came to. It operates better than fine and has over 100,000 miles on it.

Couldn't agree more Marty. I have learned over the years to not stray too far from the way those Japanese engineers built these machines. With few exceptions mine are all stock with OEM parts. The only things I changed were, like you, the K&N air filter and I also have installed Vmax cam chain tensioners on all of mine. Nothing wrong with the original tensioner, just don't put a lot of miles on any of my bikes so thought that change would be more convenient for me in the long run not having to worry about remembering to adjust them. I always advise everyone to leave things stock until you get things running right. Causes a lot less confusion that way.

[jetmechmarty]

I think I found a logical explanation for the pilots with or without the side bleed holes. On the xs650 forum there was a discussion about whether the pilot shared with the main and had rubber plugs. If they did then the bleed holes are required because that is where the air gets in versus not plugged where the air is coming in from the top. So if that is the case then the non holed pilots should be no different in operation in the later carbs from those with holes in them. The only other things I have found related to the venturi effect of atomizing the fuel and some seem the think that those holes help with that process.

FWIW, I have several BS34 XS650's. They all use the rubber plug over the pilot jet. All have the six holes. My XS1100 does not employ the rubber plugs and all the pilot jets have six holes. My XS11 has a K&N in the air box and the exhaust is a four into one Jardine. My jetting is stock. It's stock not because I'm lazy and left it. It's stock because that's what I came to. It operates better than fine and has over 100,000 miles on it.

[cajun31]

I think I found a logical explanation for the pilots with or without the side bleed holes. On the xs650 forum there was a discussion about whether the pilot shared with the main and had rubber plugs. If they did then the bleed holes are required because that is where the air gets in versus not plugged where the air is coming in from the top. So if that is the case then the non holed pilots should be no different in operation in the later carbs from those with holes in them. The only other things I have found related to the venturi effect of atomizing the fuel and some seem the think that those holes help with that process.

[gtem]

Thanks Deebs. Yep I learned a while ago on my XS500 about those VM style pilot jets where the metering orifice is down at the thread end of the jets, totally wrong for the BS carbs! All of mine both hole and no-hole have the metering hole at the tapered end.

[DEEBS11]

Correct pilot jet specs.

[DEEBS11]

Good tip vs. bad tip (ignore the side holes).

[gtem]

All testing on the bike so far was done just with some roughly 6" fuel lines attached to the two fuel "T" and the bowls filled. So I'm excluding any sort of petcock factors, plan is to delete the OCTY all together in favor of a pair of manual petcocks. I did drain VERY fuel-diluted oil from the engine when I first got the bike (refilled with rotella 15W-40) , I'm glad I checked before running it for more than a few seconds. I've heard of gas-diluted oil actually affecting jetting before.

The MS luckily came to me with a pretty fresh set of carb boots. I haven't done any checking for vacuum leaks, as I assume that would cause problems in the form of a lean condition, not this current super rich scenario.

I'm going to look for that hole vs no hole discussion, per Phil in this thread, his conclusion from the same thread(?) was that it didn't matter, but if it does, that would be very useful info indeed!

[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.

[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.

[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.

[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.

[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!