Bogging down at 7.5k RPM

[thumperjsa]

Can anyone tell me if the pilot jet in these carbs should be bleader type (with holes)? My next thing would be to pull them apart and look if no one can answer this.

If so, the cost from MikesXS is real reasonable at $2 each for mains and $3 each for pilot jets. I guess I can get the float bowl gaskets and intakes from partsand more since they have those and the cost is about average. I checked with Motorcyclecarbs and they were quite a bit more expensive.

[thumperjsa]

Need Carb info help

Well, the gasket stuff did not do it so it looks like I am going to break down and buy the intakes. These are probably the originals and why keep repairing them.

I answered my own question with the pilot jets as I pulled apart one of the float bowls and the pilot jet does have holes on the sides. By the way, it was 42.5. Interestingly enough, the main jet in carb 1 is 137.5. Am I missing something here? These jets should be the size of the jets of a 79 F. I looked all over the carb and other than the Mikuni Kigyo name on it, I don't see any stamped part numbers to identify the carb.

Is there supposed to be a cast marking of the part number somewhere that maybe I can't find?

From looking at the yamaha-motor website and looking at the diagram, it looks like the 79f because it has an adjuster screw (part 49) where as the 80 model does not. This is I believe the idle speed adjustment screw according to the clymer manual. Does the 80g carb have this?

Am I ok with this carb set and should I re-jet due to the change in exhaust and air filter?

Other than the part number being slightly different according to the parts list, is there a problem using this carb set?

Is it possible it is the right carb but with the over-sized jets?

I must admit, it has been a long time since I have been a wrench turner but it is coming back to me. I am just a bit confused here because I am seeing what I don't expect.

[pgg]

Thumper I'd be wary of going up too high on jet sizes, it's a fine line between performance gain and performance loss. Banging in bigger and bigger fuel jets is gonna cause bad gas milage, carboned up pistons, carboned up combustion chambers and outlet ports, and a soggy top-end!

[skids]

You said, "For now until I get the new boots, I put some Threebond on it and I will pick up some 3m Liquiod gasket as some others have recommended. Threebond dries grey and I don't like the look of it." I don't know how that stuff is with atomized gasoline, but I am cautious with things like that. If it gets disolved, it could (at least some products will do this) deposit on the intake valve stems, turn hard as a rock, and stick open your valves.

[ratbyk]

Sealing Intake Manifolds

I used black RTV silicone sealant. Seems to seal well, dries shiny black. I did it over two years ago and have not had to touch since.
Ken/Sooke

[thumperjsa]

Threebond dries pliable which is why it is good for gaskets. It does dry grey and I don't like that either. I will just break down and spend the money on the new boots so it will look good again.

The question I still have remaining though is what to do about the carbs and jets being that it looks like it is a 79f carb set on a 80g bike.

[John]

I was running a set of 78E carbs on my 80SG, and I also had an XJ 1100 engine in it at the time. Just leave the stock 79 jetting alone. All the changes were to meet EPA regulations. The 80 carbs are different internally, from the 79's. They do the same thing, just differently. I am currently using these same 78 carbs on my big bore engine, which happens to be the original engine from my bike. I am running a Jardine 4-2 pipe, and a Uni foam air filter in the stock airbox, but I have not changed any of the stock settings/jetting for the 78 carbs. Seems to run just fine, but I haven't been able to wind it up tight because I just completed the 500 mile break in on the new rings and pistons, and I took the bike out of service to replace leaking crankshaft seal. I haven't done the work yet, as I'm waiting for parts.

[thumperjsa]

Thanx John. At least this gives me a lot more information than I had or could find.

So after I get the new intakes, I'll leave the jets alone and put the carbs back on. By then, I hope to have the K&N filters.

By the way, I discovered something new on the bike last night. Apparently the PO decided the bike ran better without an air filter in the air box. I don't think a filter would matter much because there were holes in the air box for I assume where the two wingnut screws were supposed to be on the top that air would have been sucked in and bypassed the filter any way.

Doesn't make me to happy. I don't see any damage with the carbs or intakes, then again I did not lift off the heads and I hope to not plan on that. Before I run it again, I will make sure there is an air filter though.

[John]

Here is the air filter I'm using. Uni P/N NU2244. Fits all XS1100. Will not fit XJ1100. About $20. You will need to buy the filter oil, and some filter cleaner also, so be sure to order them when you order the filter. Most any bike shop will be able to get it for you. There should be four screws with wing nut heads to attach the air box lower section to the upper section. There will be a hole in the center of the lower section, but it opens up into the center of the airbox where the outside air is drawn into anyway. The hole is a drain in case the carbs should overflow because of sticking floats.

[pgg]

John, I reckon if you go to 45's and 140's on your 1178 you'll feel some extra kick from the motor.

[John]

There must be too much snow on the ground for riding, so why don't you go clean your carbs...again?

As I said earlier, I haven't had a chance to really twist it up before the breakin period was up , and now it's in pieces in my garage waiting for the new seals. One step up in jetting may not hurt, but what if I just raise the needle one notch for a little sooner kick? I don't know what it will do at WOT yet. Any ideas on that?

[pgg]

Thought you texans might need some extra kicks. I know you know the needle won't affect W.O.T. Raised needle = black plugs midrange. I'd be happy with 8 grand in 5th. 125mph. Enjoy the sunshine. Lucky Ba***rds.

[John]

Sunshine??? We have had 18 days in a row where it rained somewhere in the Dallas/ Ft. Worth area. Now it's so damned humid that you don't wont to go out. Besides that, my bike is down anyway for the crank seals. But, at least, I don't have to clean my carbs.

[TopCatGr58]

Back to Thumper's question!

Hey Thumper,

IF you truly have an 80G, then yes, your main jets should be 115 or 120, but NOT 137.5!! The way to tell if you've gotten some switched carbs is by looking at the sides where the fuel inlets are, if there is another inlet just a little above it in the same size, then that is the vent and you've got a 78-79 carb, if not an entire bank of them!? If someone messed up one of your carbs, and replaced it with the wrong year/body type then you've got a mismatched mess. You'll need to pull all your mains and check them out. The 78-79's provide fuel for the idle/pilot circuit thru the mains, that's why they are so much larger than the 80-81's.

BTW, the jet recommendation is 2 sizes for pipes, then 2 sizes for Indy type filter, but if doing both, then total both=4, then subtract 1 =3 sizes, or 7.5 larger than what you now have. You'll want to check your plugs, this goes for you, too, Big Bore John.

Mine were pearly bleach white, it ran fine, but was concerned about excessively lean burning holes in pistons, after going up sizes, got nice tan color!!!! Didn't go up on Pilot size, just backed them out a little more, runs fine!
Good Luck, HTH.
T.C.

[thumperjsa]

Thanx for the reply TC. Between carb 1 and 2 % 3 and 4 is where the fuel enters those carbs. Above them is the vent lines so I gues it is in fact a 79 carb. When I get my intakes on and put it back together, I guess the next thing is to see how it runs with the new filters on, when I finally get to them.

Looks like I won't be riding for a week or so till I get the intakes from Partsandmore.