I know, I know.....another

[WMarshy]

... one dyno run with an Air/Fuel sensor in the pipe will tell you everything you nned to know with no conjecture...

Where do they put the sensor, just at the exit/tail of your pipe or do they have to place it in the header some where...?? If they dont use your pipe whats the sense...? Please more detail, interesting...

[Mr. Schweer]

T..good advise. I will check into local dyno shops and see what they charge per run. If I'm lean, at least with all the other info so far that will give me a direction to go.
Thanks

[T99Ford]

The sensor probe goes in the tailpipe. The dyno run would be more accurate if each individual header pipe would have an opening. But I have had good results putting the probe in the tailpipe. The can that is on my bike now is too large and not deep enough for the probe to tightly, so I have to hold the probe in by hand. It is ok, except that on the first run my bike was so lean at idle that coming off the 10K RPM run, it backfired and blew a 2 foot flame out the tailpipe, burning all the hair off my hand. It was a pretty cool special effect, especially given my flame paint job. It drew appreciative applause from the onlookers, but left my with a BBQ'd hand.
The sensor is measuring actual A/F ratio's from the exhaust gas, and then graphing it in relation to engine speed and load. The graph shows you then how the fuel mixtures are during your entire RPM range, so that you can jet accordingly to achieve a (in a perfect world) flat A/F ratio line. The probe just pushes up the tailpipe, so there is no modification to your bike.
With a 4-1, I would suspect that you are lean at idle, very rich at about 3-5K, and lean on the top end. Most likely you will be re-jetting all three ranges.
Just a word of caution: A dyno run will subject your bike to maximum power loads and RPM's. This is where all weak points will present themselves prominently. You migh be unpleasantly surprised to find more wrong with your bike than just the fuel/ power problems. But, if you are afraid of this kind of thing, you should probably just ride what you have and be content with that

[WMarshy]

... it backfired and blew a 2 foot flame out the tailpipe, burning all the hair off my hand. It was a pretty cool special effect, especially given my flame paint job. It drew appreciative applause from the onlookers, but left my with a BBQ'd hand…

Sounds like a good show to me!

“With a 4-1, I would suspect that you are lean at idle, very rich at about 3-5K, and lean on the top end. Most likely you will be re-jetting all three ranges.”

Sounds exactly like my bike, are you psychic? Every cold start I have to crack the throttle to start the bike and only 85% of the time when it’s warm, also it won’t idle on a cold engine. Off idle to 3.5K RPM isn’t exactly a “strong/smooth” power delivery with a stumble right at the 3000-3500 RPM range. Once in a great while if Im at or below that rpm going through town or something, I even hear a little “puff” out the carb (needs sync?)…After that, it pulls like a banshee and then flattens out at very top end nearing 7-7.5K RPM... How do you rejet for all three ranges, do you think a carb sync will fix some/all my problems? Sorry don’t mean to go off on such a large tangent…

[T99Ford]

Marshy--
To re-jet the idle circuit, you will be changing pilot circuit jets, and fine tuning with the idle circuit screw. The midrange adjusts with 1. needle height or 2. needle shape. WOT adjusts with the main jet. The main jet size is changed through the mid-range by the needle. The higher the needle, the earlier the main jet becomes larger (richer midrange). The lower the needle, the later the main jet becomes larger (leaner midrange). The shape of the needle also affects your midrange in that a long shallow taper will take longer to increase the main jet size, and a steep taper needle will increase the main jet size more quickly. You can go further with emulsion tubes, float heights, etc, but until you understand the other items well first, you can quickly go past the point of no return.
HOWEVER, before you do any adjusting, you first have to know where you are. The dyno is a quick easy way to do that, especially if you don't like taking your carbs off a lot. Otherwise, you may be using the "seat of the pants" method to which some members here subscribe. The latter usually takes longer, and may not achieve optimal results if you lack a large quantity of tuning experience.
Synching is easy to do, and does not cost anything, so you could start there. The problems in the midrange with a 4-1 pipe usually stem from the harmonics in the pipe under the pipes tuned performance range causing twice fueled and exhaust diluted cylinder mixtures. This is a VERY rich condition. This is why so many members here have been trying to delay the midrange needle with stiffer springs or filing down the needle washer. The midrange needle is opening the main jet too quickly and making the normally rich midrange condition of the 4-1 even worse. The lean idle and lean WOT conditions stem from the positive influences of the 4-1, where the harmonics of the pipe actually increase cylinder air supply, causing a lean condition. The problem with simply installing larger jets is that the midrange will be even more affected if the needle problem is not resolved. Diagnosing this with the seat of the pants method is very time consuming, whereas (if you don't mind paying for dyno time) dyno time is much faster and more accurate.
Performance tuning is very specific, so you want to eliminate all the variables. Try to find a book that describes motorcycle carburetion and exhaust theory and study it. The systems are much easier to mess with if you know exactly how they work first. This will save you alot of time in the long run, before you start tuning.

[TopCatGr58]

Hey James,

It's not recommended to use old used jets due to wear on the inside by the vacuum slide needle!? Many sources for new, from your local dealer, just ask for Mikuni Large Head main jets, and the size(s) you want if you want to have some variety without having to go back to the dealer. Or MikesXS.net, or BikeBandit.com , also Z1enterprises.com....see links for exact address.

You might pay a little more locally, but won't have to pay shipping and you get them immediately....YMMV!
T.C.

[Dan Hodges]

Dyno Daze

Hey Schweer--
There is a one stop shop to tell you all of the questions you need to know. I may have mentioned this earlier, but one dyno run with an Air/Fuel sensor in the pipe will tell you everything you nned to know with no conjecture. Most sport bike places can do this. You will get back a chart showing your air/fuel in relation to rpm and horsepower. It is exact and shows what your bike is doing under actual circumstances. I don't know what they would run in your area, but I have found osme sport bike places around here that will do a run for about $20. It will save you a ton of time and foolin' around, and let you tune optimally. It is hands down the most accurate and fast way to do what you are trying to do.But you have to be careful...it is also very addicting chasing after the almighty HP

A hundred dollars worth of dyno time with a sniffer is the best hundred dollars you will ever spend as there is no other way to get an XS dead nut on. On a stock 79 Eleven with a good flowing pipe you can achieve acceptable performance (for most) with one step richer on the mains, the idle screws 3 turns out with the stock pilots and pilot air jets and the needles set in the middle position or maybe raised one click with a stock setting on the floats. Elevens that were originally sold in California in 79 had # 200 pilot air jets vs # 180 for the rest of the U.S. The emulsion tubes varied also even though Yamaha list part number X-2 for all of them. These tubes were each different in that each had a different number and size of bleed holes which effects the A/F ratio particualary in the low and mid ranges. A picture is attached.

The only reason I'm bringing this up is because of the possibility of a previous owner tampering with the carburetors and because of that one must first determine exactly what he has before he can begin the tuning process other wise it' just a guessing game. I've got 14 sets of XS carburetors covering all years and models from 78 to 81 with 3 sets from California models and the damn things are all different. On my 81 Special I never did get it perfect until I sniffed it on the dyno and even then I had to use the needles from a DynoJet kit to get it right as the stock 81 needles are too short and have the wrong taper to work no matter what I did by modifying the plastic donuts to raise or lower them. Beginning in 79, Yamaha was doing some pretty weird stuff in their attempt to meet the emissions standards of the day and each year the idle and off idle lean conditions became more pronounced until they came out with the 4 valve motors with a centrally located spark plug. You can also drive yourself silly messing with the floats but if you are off a little bit off, a float adjustment will sometimes bring the A/F ratio into focus however at the end of the day only a dyno test will tell you what you have for sure. Oh yes, the timing mechanism including the vacumm advance diaphragm needs to be in proper working order with the correct ignition timing before you even think about screwing around with the carburetors. With today's gas, the spark plug porcelain tends to remain whiteish and you have to pay more attention to the shell and the first couple of threads because the porcelain doesn't color as quickly as it once did.

This is might be a somewhat long winded answer to a simple rudimentary question but Mr Schweer seems to have a voracious appetite for detail and so I hope this covers it.
Good luck and happy tuning to all.

Dan

[Mr. Schweer]

Dan you are correct about the detail aspect. LOL, I just don't wanna screw anything up, since I have NO experience building/repairing bikes...yet.

I checked the timing per the manual and it was dead-on....initial and advance...with and without vacuum. I am guessing that if timing is correct, then the ignition is good and I can move on to the carbs??

[Dan Hodges]

Dan you are correct about the detail aspect. LOL, I just don't wanna screw anything up, since I have NO experience building/repairing bikes...yet.

I checked the timing per the manual and it was dead-on....initial and advance...with and without vacuum. I am guessing that if timing is correct, then the ignition is good and I can move on to the carbs??

On an old XS, the vacuum diaphragm should be checked for proper operation and this should be done with a vacuum pump and gauge. The mechanism should fully advance @ 150mm Hg. and if it doesn't it should be replaced and yes I know, it will run without one.

If your coils, wires, plug terminals and resistors are kosher, you are good to go....to something else.

[Mr. Schweer]

On an old XS, the vacuum diaphragm should be checked for proper operation and this should be done with a vacuum pump and gauge. The mechanism should fully advance @ 150mm Hg. and if it doesn't it should be replaced and yes I know, it will run without one.

If your coils, wires, plug terminals and resistors are kosher, you are good to go....to something else.

I thought that if the timing all came in according to spec, then the diaphragm should be okay....I will check the actual diaphragm and go from there.

[Crazcnuk]

If it's working properly, then your probably ok. The check he is talking about is to test the unit if it's not working exactly as it should.

Mostly the check is to see if the diaphragm leaks internally. It's not a bad idea to check it.