I agree with RapTech on the carb adjustments. Only thing Rap, the slides do fall on smaller jumps than a big triple. For example, at the sand drags in Glamis, CA, towards the end of a big weekend, if you hit the bumps at speed, you will experience the problem. You will also experience the same problem at Comp Hill after a big weekend. I have seen it, felt it and worked on it with a fairly large number of Raptors. The problem is most pronounced when you let off the throttle even slightly just before a bump. The bike will go into a momentary dead spot then clear up.
We minimized the problem with an altered riding style, two loops out of the slide springs and by insuring that the bike is not over jetted. Over jetting seemed to be the biggest problem since the condition also reduces vac. signal strength.

 

RAPTECH,
I have only one problem with your statement and that is.
The left carb feeds 1.25 valves and the right feeds 1.75,Right?
As if you were sitting on the bike.
Otherwise very informative.

 

Actually, the right carb feeds two valves via one oval port that splits in the last 2" into the right two inlet valves. The left carb feeds through one small round port to one valve. There is a 1/2" balance transfer from the left port to the center valve area. The effect is still the same on the carbs. Perhaps one slide may drop or both. In either case, the effect is the same.

On the design of the head, the purpose of the two sizes of intake ports is to keep air speed up through a wide range of engine rpm. That is how broad torque is made in the motor.

The swirl of the air in the cylinder is mostly a result of air speed, valve angle, valve seat radius, seat radius, valve pocket design and chamber design.

 

Something like that guys. You're all on the right track understanding the unique design/technology Yamaha uses. It's good to hear that some of you guys have figured out a way to work around the CV carb characteristics. Keep jamming. Whatever I can do to help, just ask, I'll be glad to shed some light (I hope).

 

Raptech, if a leaner mixture raises engine rpm, then why does the rpm not rev the highest with the air/fuel screws .5 turns out when compared too 2 or 3 turns out? An air leak and tuning the idle circuit are totally different.
Is a 1/2 turn to a turn out from standard what your manual calls for? Is this EXACTLY what that particular engine calls for? Maybe....maybe not. It's just an easy to use guide.

 

Good question Gary. As you lean out the mixture, the idle will raise but only to a certain point. When the mixture gets too lean, it will go into a lean misfire where fuel modecules are too far apart for the flame to travel.

The air leak theroy is more so on the 2 stroke engines where there is a lot of residual fuel in the crankcase and when sucking air, it'll rev crazy.

On 4 stroke engines, you can tell if the mixture is too lean by applying the choke (enrichener circuit) when the engine is hot. If the idle rpm goes up past 3000 rpm - too lean. If the idle rpm rises slighty but the engine seem to be loading (rough idle) it's OK or too rich.

On my Raptor, I first noticed that idle mixture was too lean when I was riding at slow speeds at around 1/8 throttle. The engine rpm would not stay steady and was surging like it wanted to fall on it's face. If it backfire through intake at that condition, it probably will blow the carbs off.

I turned the mixture screws out an additional 1/2 turn and she liked it. The 3.5~4.0 turns out on mixture screws from lightly bottomed position is just a ball park figure. I can't say that all machines will be OK at that setting but close. Each carbs are flow benched at Mikuni for proper setting before they are delivered to Yamaha for production assembly. Due to the manufacturing tolerances, one carb may be set right at 2.5 turns where other may be at 3.5 turns.

This is why it's important not to disturb the factory setting. On an untouched carbs, count the number of turns in and write it down somewhere. This way you can always go back to the factory setting when in doubt. When adjusting from factory setting, turn out about 1/2 turn on both and try it. So on.

One correction: the CV carb stands for Constant Volocity (sp) meaning constant air speed through the carb venturi. The diaphram slide moves up and down consistently keep the air speed consistent for optimum fuel/air ratio.

 

I changed the clamps and split a boot. The automotive type clamps I used were to wide and the left boot split after about 10 hours of riding. If you change to this type of clamp, use one that is narrow enough to fit in the boot groove and do not over tighten. New boots are about $35.
FMFracing.com has a chart that may help you on jetting and air mixture.

 

<< The air leak theroy is more so on the 2 stroke engines where there is a lot of residual fuel in the crankcase and when sucking air, it'll >>



Do a search on my nickname. You will find that my Raptor once blew a carb half off and the machine hung wide open. An air intake leak can clearly a problem on 2 and 4 strokes.

Adjust your Raptor the way I mentioned and see how your machine responds. It will NOT be lean, when done correctly, your machine will be dialed in for your current set-up and evevation. Later.

 

Ah, the 35 dollar lesson. You were lucky, Yamaha has had the boots on back order for months.

There is no reason to use a wider hose clamp especially a clamp designed and intended for radiator hoses. On the 660, there is so much movement of the boot as a result of intake suction, you can't use a wide flat clamp. The stock clamp, with the limiting barrel filed slightly smaller is all you need.

The process for installing the carbs is simple. Use carb cleaner to remove all oil from the carb and boot interior. Press the carbs in all the way and when you think they are all the way in press harder. Next, tighten the stock clamps with filed barrel stop making sure that the clamps are in their groove but nearer the carburetor than the engine. That is the best you can do. (Obviously, proper jetting and air adjustments are a must.)

 

Raptor720,
Do you put back on those dual small white locking straps ( 2 on each carb) that are next to the clamps or are these not needed now after you file down limiting barrel? Chet