What Size Jets?

[XJ Rob]

My dad recently picked up an 81 Special with only 14,000 miles. After installing 4 new plugs the bike fired right up. It runs a bit rough but I'm going over tomorrow to check the valves and take a look inside the carbs. The bike has an after market 4-1 exhaust on it. The PO put carb kits in it but I don't know what size jets or exactly what parts were replaced. All he gave my dad was the receipt for the kits. It doesn't state what jet size or contents. Just "carb kit" I am wondering what size jets SHOULD be in the carbs with this exhaust.

I am also going to try and adjust the carbs after I investigate. I was wondering if the vacuum advance has to be disconnected while adjusting mixture or syncing.

One last question.......for now. The Vacuum advance wasn't hooked up when he brought it home. The PO had a vacuum hose running from the #2 carb to the #2 manifold sync nipple. So the octi wasn't hooked up and neither was the vac advance. Does it matter which nipple the vac advance gets hooked up to?
Thanks

[WMarshy]

It does matter, IIRC the vac advanced should be hooked to the #2 carb holder. Stock jetting information can be found here and almost all other information can be found regarding tuning. GL and Welcome.

P.S. Most will tell you that no rejetting is required if your running a 4-1 with a stock air box. I have a 4-1 and stock air box but am running a K&N. Im going to try a larger pilot jet as It runs lean off idle to ~3800 rpm. Then the primary jets kick in and you hit warp speed.

[randy]

NO.

The vac advance should be hooked up to the nipple on the carb, not the carb holder (manifold boot). If the Octy is gone, cap the nipple on the carb holder.

[dbeardslee]

Just "carb kit" I am wondering what size jets SHOULD be in the carbs with this exhaust.

Usually the carb kits contain jets for a factory setup. The 4/1 is going to make things a little lean. A trip to Yamaha's fiche will tell you what the stock jets were, and a trip to the jetting recommendations tech tip will tell you what they should be.

[DGXSER]

Stock jetting should be 110s across the board for main jets, 42.5 pilot jets, and 185 air jets. Sometimes, you will find 120 main jets in the middle two carbs, not wrong either.

Often if your running stock air box with the 4-1 stock jetting will still work. Open the carbs up and clean up what is there. Replacing the jets is not an requirement. Typically, replacing the float seat and needle valve is done and I would recommend it. If it were me, I'd see what is there, try it stock if it is jetted stock now and see what you get. If it is lean, then rejet up per the recommendations.

[XJ Rob]

Update

Thanks for the info on the Vac advance and links to the jet sizes.
I went by my dad's today to check things out. I was able to check the valve clearance and he is in need of some shims so I didn't make it to pulling the carbs off. I used the "Zip Tie" method to pull the shims out,so when I pulled the new plugs that he installed I noticed they were BLACK!! So obviously he's running rich. I checked the mixture screws just for S&Gs and they were all out 2 turns. It makes me wonder even more if the PO didn't put in bigger jets, thinking he could get more power. The PO was very young and didn't seem very knowledgeable. There is also a beat up K&N air filter in the air box. I told him to order a new "Stock" filter so when we do get to the carb adjustments we'll be ready. I was also thinking the floats could be set too high. There is no gas leaking out from anywhere so that is a good thing.

I'm in the process of cleaning up the calipers and rebuilding the M/Cs while we wait for the shims. (still have to PM randy) We don't even know if it has the 2nd gear syndrome yet. Or any other shifting problems. I'm not worried though. I know where the dremel fix thread is and I'm not afraid to get on my back and get my hands dirty.One thing at a time though...Well maybe two things.

It's been a couple years since I have ridden with my dad so I'm pretty excited to get this bike on the road.

p.s. Does anyone have experience with the 1300 Ventures?? 1988 model Another one of his/our projects. Runs like Sh!t!!
Thanks Ride Safe

[WMarshy]

Rob,
My advice is to get a new K&N, they are worth the money and at this point, it wont add any more tuning work than what you already have en front of you! For valve shims, look for at the 'Valvle Shim Library' sticky at the top of the discussion forum.

P.S. If you didnt know already, when you tear down to clean the carbs take the tops of your off and check the diaphrams for holes and make sure all the needles are at the same clip position... I take that back, you dont have the adjustable clips, just make sure they have a washer under them they are all the same thickness.

[zie_kraut]

easy jetting guidelines

I have researched and worked on Mikunis for a little while now, and an easy way to guestimate what main jets you'll need after modifying air flow is:
Intake- high-flow aftermarket air filters in factory housings +5 from factory jets
- high-flow cone filters +10 from factory jets
Exhaust- 4 to 1 exhausts and other performance parts +5
- open headers +10
So, if you had a K&N air filter with a 4 to 1 exhaust you would go 10 over factory jetting. It's not exact, but it is pretty close to what you would get crunching the numbers (+/-5). It's also a hell of a lot cheaper than buying kits of asst. jets or buying a set at a time trying to figure it out.

[XJ Rob]

Stock jetting should be 110s across the board for main jets, 42.5 pilot jets, and 185 air jets. Sometimes, you will find 120 main jets in the middle two carbs, not wrong either.

Often if your running stock air box with the 4-1 stock jetting will still work. Open the carbs up and clean up what is there. Replacing the jets is not an requirement. Typically, replacing the float seat and needle valve is done and I would recommend it. If it were me, I'd see what is there, try it stock if it is jetted stock now and see what you get. If it is lean, then rejet up per the recommendations.

I opened up the carbs today after trying to sync them. They synced pretty easy. but it would die every time I tried to rev it. Idels fine but you can only get a tiny rev out out it, any more throttle and it stalls.

What I found in the carb was the PO had put new main jets 125 and 42.5 pilots. air jets are 185. I checked the needles and they were on the center slot. The bike is running super rich. The plugs were very very black. I'm not sure if just lowering the needles one notch will be enough to lean it out at all. I'm just looking for some other advice where or how to correct the problem. I haven't checked fuel level yet but I dont want to put them together, if I have to put in smaller jets.
The bike is an 81 Special. K&N filter. stock airbox with 4-1 exhaust. Thanks

[TopCatGr58]

And the light bulb goes ON!!!

Hey Rob,

You said that the carbs vacuum slide needles were in the MIDDLE NOTCH!?

The 81 carbs had NON adjustable needles...so this makes me think that you may have the wrong or earlier model carbs on the bike, and IF they are jetted for the 81 model, they are ALL WRONG!!!!!

You'll need to verify what model/year the carbs are. 78-79 carbs have only 3 small vent holes in the intake bell along with the large arced vent for the slide, the 80-81 carbs have 4 small ports.

The 78-79 have two sets of "T" fittings between the carbs....lower for FUEL, and upper for Bowl Vents. The 80-81's only have the lower T's, the 4 inlet port is the bowl vent!!

The 78-79's have large pilot/idle jet screws on the top front of the carb body with the spring visible on the outside of the carb. The 80-81's have small screws and springs that fit down inside a small tunnel!

My bet is that you have the early model carbs. They used and need 137.5 or larger MAIN JETS because they share the supply with the pilot jets thru a sharing TUNNEL between the pilot and main jet towers! The Pilot JET Tower should be capped OFF, because it's getting fed thru the MAIN JET tunnel...but if the pilot jet tower is not plugged/capped, it's getting too much fuel directly from the bowl.

Also, some kits have the wrong type pilot jet..the VM style vs. the BS style, and they can also cause an overly rich pilot circuit problem. With the bike stalling when you try to give it throttle points to it being too lean for the mains, but may still be too rich for the pilots due to the above reasons!
T.C.

[XJ Rob]

Thanks TC I'll get some pics and post them for an accurate Identification.

[XJ Rob]

Got Pics HELP!!!

So my dad took some pics of the carbs. I'm hoping they help identify these carbs so we can get them straightened out.

It looks like the pilot tower is open. The screw inside is a 42.5. The Main is 125.


Unless I'm looking in the wrong place, These would be 81 carbs/4 ports and the arced part at the top.And only 2 T's for fuel inlet.


He didn't take any pics of the slides or needles.But they DO HAVE 5 slots for the adjustable clip.

So I take it we need to get some carb kits and some plugs for the pilot towers. I know there was a thread about the difference between the two different pilot jets. Which ones should be in there VM or BS??

If anyone has a set of used/good jets they are willing to sell as long as they are the correct ones. Send me a PM.
Thanks for all the help.

[bikerphil]

Jets are cheap, compare your pilot jets to the chart on the page link below...

http://www.mikesxs.net/products-39.html#products

Look down the tower after removing the MAIN jet and look for a cross drilled hole just below the threads. If there is a hole, you need the rubber plugs over the pilots, no hole, no plug needed. Your carbs are the later BS34-III type which are supposed to have non-adjustable needles. Someone probably put a Dynojet kit in. Beware that Dynojet numbers do not coincide with Mikuni jet numbers.

[petejw]

youve probably seen this thread
but it might help you out

http://www.xs11.com/forum/showthread.php?t=28559

tho the later carbs xs and xj are similar, the jetting seems
to be different, especially the large pilot jet (47.5/50) and the smaller
air jet (180).

[DGXSER]

Bowl drains

The drains on the float bowls indicate they are a mixed bag. The float bowl drains on the 80 models had just a plug in the bottom as your outer no 4 carb shows, the 81 model carbs had the side screw bottom outlet drains as shown on your two inner carb bowls.

All four look to be the version III of the 80-81 model years, but there were some "bastardized" sets with all the newer looks, but still had the crossover in the tower and required the pilot jet to be plugged and jetted like the 78-79 carbs.