xj700 YICS

Hey guys. I have my xj700n up and running, altho she does not like to run under load at 2-4k. Carbs are sync'd, plugs are tannish white, mixture is at 2.25, float levels are good, dry at 18mm and wet right on, new plugs, oil, filter, new needles and seats, etc, etc.

this bike sat for better part of a decade, and I've never messed with the YICS and what effect it has on everything. I can't get this bike quite dialed in right. I'm beginning to think that something with the YICS needs some help.

Can someone explain to me what the YICS is and does, and what symptoms the bike would exhibit if the passages were clogged or in need of blocking during the sync process. Thanks!

 

Its a port cast in the head that is suppose to balance the mixtures and even out the intake pulses. Some folks claim that it improves performance. IMO the jury is still out on that. It should be blocked during the sync process as well as setting the idle mixtures. Just finished a XJ750 for adjustment and sync after a carb cleaning and with out the YICS tool installed I could have turned screws all day and not got it right.

 

The following thoughts courtesy of Someone Who Knows:

According to Yamaha, the function of YICS is to promote more complete combustion at low to mid-range rpm by swirling the mixture around the combustion chamber. They never made any direct power claims -- the system was promoted as increasing mileage up to 10 percent and promoting lower emissions through a cleaner burning charge. The increase in mileage was not through leaning out the mixture -- the increase in mileage and reduction in emissions was because less throttle was required to maintain cruising speed. They could use the same cam timing, jetting, etc. (for performance) and still meet EPA regulations. Yamaha stated that the greatest effect was while cruising at 50 to 60mph.

The swirling accomplishes two tasks: first, it assists mechanically in atomization of the fuel and second it speeds and distributes the flame front (and therefore combustion pressure wave) more evenly throughout the cylinder. The later "Genesis" (water-cooled "X" engines) design accomplished this even more effectively through the 5-valve design, which accomplishes the same task through a wider rpm range.

To blank or not to blank, that is the question. Ok, what is the function of synchronizing? Manufacturing tolerances and wear. Multiple carbs offer the advantage of increased power and performance over the single carb configuration for reasons beyond the scope of this discussion. But at a cost......

As each carb is dealing with a fraction of the total air and fuel flow requirement, each carb must meter things in fractions of the total. That makes for some very tiny tolerances. In effect, we have 4 separate engines -- and if one is fighting the others, then we are wasting power and fuel and are actually worse off than we would be with a single-carb setup! So it is crucial that each one be perfectly adjusted and that all 4 then be synchronized to work in perfect harmony.

So, how can we reliably adjust an individual carb based on the results of the adjustments, if it's companions are contributing to the results? Now, to be fair, if the bike has been reasonably well maintained and the cylinders are all fairly close to begin with, you can usually get away without blanking the YICS passage and still achieve a reasonable state of tune. At the opposite end of the maintenance spectrum, the YICS passages are often already plugged up with accumulated grunge and grime so the bike is effectively being tuned with the YICS system disabled and the presence of the tool is irrelevant. But we have seen some examples of carbs where one cylinder was effectively relying on it's neighbors to supply most of the mixture through the YICS system, and this most certainly affects performance, especially the top end.

So, my recommendation: for an engine with approximately equal compression across the cylinders and valve clearances in spec, make sure the YICS passages are clean and use the blanking tool to tune the carbs.

 

How would I clean out the yics passages? Any other thoughts on why the lack of power from 2-4k? Vacuum leak somewhere?

 

Go to your local gun store and buy a gun brush of the proper size and a cleaning rod

 

outstanding idea...12 or 20 guage should work

 

You'll never access the tiny transfer ports that are restricted to 2 MM.
You can easily clean the large YICS galley and see thru it.
It has been suggested that the engine will run well with the YICS system totally plugged- - therefore, your problems are elsewhere.

 

in that case, the passage is clear, I can see through it.

New problem is as follows...bike will not run right, bogs, period. I said wth, and swapped the two air jets on top under the vacuum slide...she runs like a champ, altho ungodly passout rich, even with the mixture at 0 turns.

I am seriously about done with this thing. I am going to part it if I can't get it right soon, and that is unfortunate, because she is a good looking bike.

someone give me hard core numbers for the air jets up top, pictures, I don't care....just to double check myself, the pilot jet on the bottom of the carbs IS the SMALLER jet, correct?

 

Cheat sheet at the bottom of this page sez:

http://xjbikes.com/Forums/viewtopic/t=14692.html

Years & Models: 1985-6 XJ700 air-cooled models
Carb Manufacturer: Hitachi
Carb Series: HSC33
1985 Carb Model ID: 1FG00 (N models) or 1JJ00 (NC models)
1986 Carb Model ID: 1NH00 (S models) or 1NK00 (SC models)
Main FUEL Jet Size: #107
Pilot FUEL Jet Size: #36.5
Main AIR Jet Size: #70
Pilot AIR Jet Size: #210
AIR COMPENSATOR Jet Size: not used
Starter FUEL Jet Size: #36 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #Y-20
Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 1.0mm +/- 1mm (.039" +/- .039")
Idle RPM's: 1,050 rpms

Although not part of your carbs, your valve clearances should be considered as part of your "intake system", so here are those specs, too. Note that valve clearances should be measured with the engine "cold", meaning 70-degrees Fahrenheit or less:

Valve Clearance Intake: 0.11 - 0.15mm ( = 0.004 - 0.006")
Valve Clearance Exhaust: 0.16 - 0.20mm (= 0.006 - 0.008")

 

Once again I go back to the float levels. With the bike on the center stand. Hook a clear tube of a size that will fit on the drain nipple of the carb bowl. Route the tube with one end on the nipple and point the other up along the carb body close to one of the bowl mounting screws. Sort a a J shape. Open the bowl drain screw. The fuel should fill the tube but only come up to somewhere between the base of the bowl to mount screw interface and the bottom of the carb body. Above than that and your levels are to high and you will create a rich condition. Lower than that and it could run lean at full load. The best way to do this is off the bike with the carb rack mounted to a board and leveled but you can do a fast and dirty check with them on. By the way if the tube keeps filling and overflows than your needle valve is not sealing. Could be a float hanging up or a bad valve. You can set them dry all day and it will not tell you if they are sealing or the floats are hanging up.

 

the wet float height is spot on. I had to reset them 3x to get them right. Dry they are sitting +2mm over stock 'settings'. Another forum member mentioned that the YICS could be the issue here and another mentioned he thinks it's 'fighting me like the plague'. i'll be attempting to make the yics blanking tool today and hoping to re sync...otherwise, bonfire a la maxim

 

There is no such thing as a "wet float height". There is a fuel level that can only be measured via the clear-tube method, and that requires some type of liquid---preferrably, gasoline----be used.

The proper "float height" is the float height that results in the correct fuel level being achieved, as measured by the clear-tube method.

You may be confusing methods with goals. Think military tactics. The goal is a proper (per spec) fuel level. We'll accept whatever float height it takes to achieve that goal. If that float height is the one "in the book", fine. If not, tough nuts. We're going with the "float height" that achieves the correct fuel level, thank you very much......

 

Len, I am using the wet float height and clear tube method interchangeably here. I set the heghts via a clear tube with gasoline out of a test tank. It appears to be spot on.