Building Frankenstein

[trbig]

OK... hope you guys can follow this. I have an XS motor bottom end (78 or 79). I have istalled an XJ head on this motor and I am going to use the original XS cams on this head. The valves on this head are larger than the XS heads. All this for a carb question...
I am going through the carbs, and have seen that the XS motor and head that were on it, were being run on 112.5 main jets.... which is stock for an XJ. The specs for the XS motor call for a 137.5 main. I see these comments in here about the speed of these bikes, and I am here to tell you, mine couldn't compare. It ran fine, sounded fine going down the road, but was gutt-less. I would lose speed on a hill while in 5th, at 4k on the tach... would have to downshift and wind it out. Also had to wind way up to get going from stopped. I thought it was maybe just clogged up with sit-itis. Another problem I had, was after it was warmed up, it wouldn't idle down. I would pull up to a stop, and it would idle up to about 4 grand. I don't think it was sticky slides... I pulled them out and polished them, and they slide freely.
My question is, with the XJ head on it now, do I keep the 112.5 jets, or go buy the 137.5's? I have the stock exhaust, stock air filter. Also, one of the pilot jets was really muffed up by somebody's previous explorations. I managed to finally get it out. Anybody know the cheapest place to buy one fo these or have a spare 47.5 they wanna sell or swap? I have a spare bank of carbs that apear to be off of an 80 SG/LG since the mains jets are all just 110. The pilot jets are 42.5. I tried running the bike on this set when I bought them, but it was even worse than the others, so swapped back. Thanks.

Tod

[TopCatGr58]

Hey there Tod,

Okay, IIRC, the bottom end of your engine was from a 78E, I believe you had stated that the case # was 2H7-xxxx, and therefore I would "assume" that it also had the 78E carbs? You need to tell us whether it has 2 sets of "T" fittings between each pair of carbs, one for fuel and one for bowl venting? If so, then they ARE 78-79 circa carbs, and indeed require the larger 137.5 jets.

But, if they only have 1 set of "T" fittings similar to the spare 80SG carbs you have, then the 110/112.5 jets are the correct size, not allowing for other mods, like aftermarket exhausts and Indy filters, or K&N type filter vs. totally Stock Box!

Oh yeah, the 47.5 pilots are not stock on any model IIRC? I believe the 78E had 45.0's and then were dropped to 42.5 in subsequent years! The revving idle after warmed up is a sign of too lean a condition, and sounds like the P.O. tried to fix it with the larger pilots, but with those small mains are a no no IF they are in fact the earlier model carbs!! Also, the pilot jet holes in the bowl should be capped....IF it has the extra channel hole from the pilot tower into the main jet tower!

MikesXS has pretty good prices, and you can order less than $35.00 worth, just have to pay shipping!
T.C.

[trbig]

These carbs have the original jets that my XJ book describes... including the 47.5 pilot jets. Just one set of "T's" in between each set. I read somewhere on here (Couldn't find it again) that someone else was having a rev problem, and making sure this jet in the bowl was clean was said to be a possible cause... So.. Why cap it ?? My gaskets have 2 small holes .. one over the jet on the bowl, and one on the other set of that channel leading to it.

Tod

[trbig]

TC... I wasn't trying to be a smart %#... I just didn't understand why the factory would put this jet here, and the gasket provides for it, why would you block it? I know nothing about carbs... so was just wondering what this is supposed to do.
Here's a pic of the expanded parts of the carb in my book... Note the main jet and pilot jet numbers..


Tod

[TopCatGr58]

Hey Tod,

Okay, looks like Yamaha put in 47.5 pilots in the XJ's carbs, along with 112.5 mains, so you apparently have the XJ carbs that were kept and put on your 78E engine! The holes in the gasket you are mentioning are for the CHOKE/Fuel enrichener circuit, NOT the pilot or main circuit, I was not talking about capping them!

See this picture, the brass colored jet in the middle is the MAIN jet, and the straight slotted screw in the towe next to it is the capped off Pilot jet tower, the other long brass tube at the edge of the bowl is the Fuel Enrichener circuit!


In the OLD 78-79 carbs, the Pilot jet feeds thru a separate port channel that connected to the MAIN jet tower, and so, to meter it properly, the pilot jet tower was capped off, and the mains were much larger 137.5's to allow for the extra fuel flow thru the pilots as well as the main needle!

In 80-81 Yamaha changed the carbs, and closed off that channel, and allowed the pilot to get it's fuel directly from the bowl, so it was then uncapped, and the main jet only needed to provide fuel for itself, so they were able to reduce its size considerably down to 110-120 range.

So....theoretically, both the XJ carbs and the 80SG carbs you have with only 110-112.5 mains should be plenty to provide fuel for full throttle performance to redline!

SO....have you thoroughly cleaned either bank of carbs, are the floats set right, are you using the stock box, or pods, stock pipes or aftermarket, sorry I can't remember all the details?!

I am also going to assume that you're using the XJ's TCI. Is a vacuum line connected to the vacuum sensor little black box under the tank? What does the left side timing plate assembly look like, is it the XS style with centrifugal and vacuum advance modules? I'll do a search on your previous posts to refresh my memory, we'll get it figured out!
T.C.

[trbig]

Stock airbox, filter, pipes.... everything. That's what got me to this point... I am cleaning these up. I found out that 2 of the 4 jets in the side of the bowls that that long thin brass tube goes up next to were plugged up. Everything seems clean and open now... just waiting on my 47.5 pilot jet. My lines connect to the little black octopus looking box (Diaphram?) on top of the carbs when installed. I don't know if one of them connects to a box, I will check later.
Excuse my ignorance again... but the left side timing plate appears to be unadjustable, or moveable.(No vaccum advance?) Just a metal plate with the "C" and "T" on it with 2 contact points at 3 and 9 o:clock. I will stick a pic up in a bit after I get home from work if ya need it.
I haven't adjusted my floats. I didn't know how until I read this new thread in here. The book tells me check fuel level with the bike level... run it a minute, then check with clear hose. Of course, if it's out... have to take carbs out again. I saw that people were setting them to 25mm, but not quite sure what they were setting that to. I see that the gasket mating surface was one side, but then just lower the floats until it just takes weight off the little wire that holds the needle valve to the tang? (Just as the needle seats) Then measure to the top of the float? All I have is a set of calipers for reloading for measuring.

Tod

[TopCatGr58]

Okay Tod,

Sounds like whoever did your engine swap put the XJ ignition system on the XS engine, which is okay. However, there should be a vacuum line coming off of one of the carb vacuum ports, the #2 carb has a metal port on the body of the front of the carb. I need to check the Parts Fiche to verify which hose is connected to that vacuum black box under the tank. Also, wouldn't hurt to check the TCI to make sure it has a model # starting with 10M-, to make sure the correct TCI is being used, vs. one starting with 2H7.

Yeah, trying to set the float height right side up with fuel is a P.I.T.A., but you'll want to use the Upside down technique, and set them to around 23mm, not 25. The later carbs, ie. 80 stated to use 23mm, then in 81 and 82 they said use the fuel tube method. If you have a caliper, then you want to measure from the gasket mating surface to the top of the floats with the carbs upside down, and just gravity pushing them down against the float valve pins!

You might find it helpful to take a piece of rigid plastic, or even cardboard, and cut out a squared off "U" shape that would fit over the floats, and fit the gasket lip, and make it 23mm deep, that way you can use it to measure both ends of the floats, and adjust it that way, easier than trying to fit the caliper in there!

Have you taken the vacuum slides out, and unscrewed the main fuel jet, and the gently pushed up the central NOZZLE and checked and cleaned it, has little aeration holes in it's sides that can get clogged easily!

Those jets in the bowls are only for the starter circuit, they don't do anything for the actual higher rpm running of the bike!
T.C.

[trbig]

I'm not quite sure what you mean about the central nozzle with the holes in the sides. The needle attached to the slide plunges a little bit, but the only holes on it are in the bottom of it?? I took all these out when I cleaned them... including the main jet.
I do have a little box that one of the lines connect to under where the tank would sit, but the port on the #2 carb has always been capped off since I have had it...





And feeling stupid again... but don't know what a TCI is, but here's a shot of my timing plate.



Thanks for the tips on adjusting the floats.... I will take the bowls back off and do that. I ordered an aftermarket pilot jet today from my local shop... only $2.50. Thought that was a bargain... don't know what they normally are. It's the only one he could get... couldn't get the OEM. Sorry the pics are so big... just wanted you to be able to see it.

Tod

[TopCatGr58]

Okay, Tod,

The TCI is the ignition/black box under the seat that the wires are plugged into! That plugged off port on your #2 carb IS where the vacuum line going to that little black box with a vacuum line nipple needs to be plugged into, cause that's how the black box senses the vacuum and sends those signals to the TCI so that it can compensate the advance accordingly! It does look like you have the XJ timing plate pickup assembly!

That "NOZZLE" I'm talking about is partially shown in that exploded diagram you posted of the carbs, that LONG piece that fits down inside the chamber that the vacuum slide and needle fits into!! At the bottom of that chamber is the slide Nozzle...sometimes called a NEEDLE also, and it comes out UP thru the Slide chamber, ONCE you unscrew the MAIN JET, you can gently tap the nozzle up from the bowl side, and then you'll be able to inspect it for good clean openings of the little holes in it's sides! You'll need to take the vacuum slides out again! There's a notch/pin and groove in the carb body and nozzle so it only can fit one way, make sure you align that when you put it back together!

I hope you were able to get a look at the PILOT jet that you were ordering, or that you took your old one with you, so that they could try to match it up....especially IF it had those little holes in the sides of the pilot jet. IF you get the new one, and it doesn't have any holes in it, then it will be TOO RICH...those holes allow air in to help atomize/vaporize the fuel/air mix!
T.C.

[trbig]

I did take my old jet up to him, but none of my pilot jets have those little holes. The one I ordered looked as if they might. He said it shouldn't matter as long as it fit and was a 47.5. If this new one has the holes in the side.... should I just change all of them to match?
I am sure that one of the lines that were coming off of that little octopus diaphram thing were going to the nipple for that box you were describing.
When the bike was running, if I gave it full throttle, it would crap out and wouldn't go above 4 1/2 to 5 grand rpm's.... just stay there as long as you held the throttle completely wide open. If I let off the throttle just a tiny bit, it would go ahead and shoot on past that up to red-line. I thought it may be carb related, but it did the same thing when I put this other set of carbs on. Think this may fix that? Just plug off the line that WAS going to it?

Tod

[TopCatGr58]

Hey Tod,

Prometheus will probably have a better/more knowledgable comment on whether to change ALL 4 pilots out for the ones with holes! He was doing a survey a while back on carb parts, specifically the pilot jets, and I think I remember someone else with an XJ posting that theirs didn't have the little side holes either!? The side holes allow more air to mix with it, possibly leaning it out vs. your no-holes style. The XJ used the YICS system which supposedly balanced the vacuum pulses between the carbs, and perhaps that also provided a more levelling out of the fuel air mix from the pilot circuit, so maybe that's why they can use the no-holed pilots?

The vacuum line from the Octopus assembly going to the vacuum sensor/black box is most likely the wrong connection. The port on the #2 carb is smaller and provides the proper NON PULSING vacuum signals. On the XS's, people found the vacuum hose attached to the SYNCH ports and they ran poorly, as well as made some nice clicking noises as the vacuum plunger pulsed with the overly strong surges.

So....I would think you should connect the black box sensor to the #2 metal carb port, and cap off the one from the Octopus, and see how it runs!

Perhaps some other XJ'er's reading this thread could comment on how their black box is connected, whether it's connected to the #2 carb port?
T.C.

[John]

Black box...

Is the boost sensor. It is connected to the vacuum port on the #2 carb body. The XJ doesn't use vacuum advance, the boost sensor senses vacuum and sends a signal back to the TCI, and controls the timing that way.

[trbig]

Well, I went ahead and changed them all. Was worried about trying to sync the carbs, and having one a little bit different. By the sound of it, since I have closed the YICS ports on this head, that it may have been what I needed to do anyway. Everything else on the carbs looks squeeky clean now... had em apart about 15 times now.. lol.
Right now, I am looking for something to cap off the tiny brake line that goes from my rear master cyl, to the valve that goes to the front left brake, and just leave that alone for now. When the time comes to replace lines, I will double up on the front brakes using the front M/C. Hoping to have it running this weekend... gotta work it in around the "Honey Do's" I have missed a month of 60 and 70 degree weather here.. Grrrr. Soon as I get it running, it will drop back into the 20's and 30's... anybody wanna bet? I need to find that "Murphy's Laws" guy and kick his ^%@!!

Thanks alot for all your help guys. I will try connecting the #2 carb and plug the other line... nuttin permanent yet so I can switch back if needed. Do you think this will help with the acceleration problem I was having? Timing not getting to advance enough?

Tod

[TopCatGr58]

Tod,

I think it will help, but it will be because the vacuum advance unit will NOT be getting as strong of a signal during WOT, and so the timing will be at the proper 36 degrees, instead of the maximum of 52 degrees when the vacuum advance is kicked in with full centrifugal advance which is too much!!!! See the quoted article I just posted to MainlyLinux's reluctor/pickup coil thread!

Can't help you on the capping the brake line, but I'm sure somebody will have some ideas!
T.C.

[Ken Talbot]

I'm not so sure that the vacuum nipple on the body of carb #2 gives that much less vacuum than the nipple on one of the intake manifolds. What it does do for sure is give a metered, very much evened out vacuum that is not subject to wild fluctuations.

YMMV, TMTAISTI, etc, etc....