Starter valve sync.

motofisch said:

• How does a person know if they should have their starter valves synched?
  
• Is there some type of indication from the engine or throttle response that indicates a synch is necessary?
  

I am over 90K and have never had my starter valves synched.

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To the first bullet: idle isn't as smooth with out-of-synch starter valves. And if left idling, cylinders won't equal temperature as they won't be firing equally, and you can more easily foul a plug or two.

To the second bullet: the moment you quickly and largely twist the throttle while the engine is still at idle, it could stumble before rpm increases depending on whether a 'good' firing cylinder is next to fire or a lame one (due to absence of airflow). But unless the butterfly valves are closed, the starter valve synch has negligible affect airflow or fuel flow to each cylinder. Said another way, a person can completely open or completely close the air bypass that the starter valve adjustment affects and NOT affect airflow and fuel mixture into each cylinder when the throttle is open (when the butterfly valves are open) any appreciable amount, and engine has a load on it.

I'd speculate that you'd notice a better idle and would have to turn down the idle speed adjustment once a synch was performed. But it is otherwise not a strong player in throttle response or engine behavior.

I guess it depends on whether or not the starter valves on any one bike are out much to begin with. Is the idle smooth or is there a stumble off idle to investigate?

I used a old school Morgan mercury level Carbtune to do my 1300 at 116,000 miles when I was down in the vee for an alternator R & R. I had no out of synch symptoms and the allowed adjustment was not fine enough to level them perfectly anyway. The adjustments are not screw verniers but serrated clicker-type stepped adjusters. The synch made no difference in idle or off-idle response. I did install the longer vacuum lines for the "next time" but at 162,000 miles and climbing there has been no need for a next time.

Thanks Dave & John for your excellent explanations. I'm good to go with a smooth idle and no off idle hesitation.

After seeing numerous posts related to starter valve synch, I have to ask a very basic question that someone here may be able to make a valid response to. Every automotive application that I am aware of, ie cars and light trucks, whether they have 4,6,8, or 10 cylinders, mpfi fuel injection or direct injection, only have one throttle body. So why do all motorcycles that I'm aware of have a throttle body for each cylinder? If there is just one air box and a single air filter, why not just one throttle body? A buddy of mine tried to explain it to me a month or so ago, but I still missed something in his explanation. Inquiring minds want to know - mine does anyway.

LOL, never thought about it Don. Great, now I'll be awake all night. ;-)

BakerBoy, I didn't mean to pose a question that would keep you up all night, but I really would like to know, this is not a trivial question to me, I really would like to know.

Short answer- One throttle body per cylinder allows for a more efficient combustion process and can produce more power per CC of engine which is important when you are dealing with a small engine to begin with. It also allows for better throttle response and torque control which is more important on a motorcycle as throttle control plays an important role in getting the bike into and safely through and out of a curve when riding vigorously. This is not an issue for cars.

For a longer answer- see here.

Sorry, but with all due respect for Kevin Cameron (and I do respect what he has to say in his column when I read it, most of the time anyway), I can't accept his explanation as being totally correct. First of all, "... all four fuel maps (one per cylinder) can be raised or lowered together to compensate for changes in atmospheric density or humidity..." means to me that the timing and pulse width of each cylinder's fuel injector is constantly being adjusted as necessary to maintain the correct air/fuel ratio. His response appears to be based on race bikes or bikes being tuned for track days, but I assume that a separate fuel map for each cylinder is the norm for street bikes as well, makes sense to me anyway. In the next paragraph, Kevin states: "... it has always been quite difficult to get fuel added at a single point to proportion itself equally among the four cylinders at all rpm and throttle angles." Fuel added at a single point is a reference to TBI, or throttle body injection as opposed to multi-point fuel injection or whatever you want to call modern fuel injection systems that use an injector in each intake port near the intake valve(s). TBI was used in early automotive fuel injection systems in the early 80's until ~1985, during the transition from carburetors to MPFI. This time frame was before motorcycles started to switch over from carburetors to FI. To the best of my knowledge all production fuel injected bikes use MPFI, unless there are some out there that have direct injection. The Motus prototypes used direct injection, but they decided to stay with port fuel injection on the production bikes. Aside from regulating the amount of air drawn into the cylinders, the only thing the throttle body (or bodies) has to do with the fuel injection system is to basically provide a TPS signal to the ECU as a required input for the ECU to regulate fuel to the injectors. What it all comes down to is that a single 72mm throttle body could replace the four 36mm throttle bodies on an ST1300 and provide the same volume of air to the engine. A 75mm throttle body would be even better if a little more air would be good.

Bottom line - I'm still not convinced that more than one throttle body per air box is needed.

My understanding is that it is not so much a question of the volume of air entering the engine but the control of that volume- multiple throttle bodies afford better throttle control and response which is of much less relevance on a street car than it is on a street bike. Whether that additional control is ever truly needed or even noticed by the average street rider is a whole other debate.

Don, I suspect another factor: space.

One throttle body can work (70's and some 80s 4-cylinder motorcycles were single our double carbs), but airflow distances and restrictions differ more prominently between a single throttle body and each cylinder in the tight spaces of a motorcycle, accentuating airflow and fuel flow differences to each cylinder relative to the 'large' spaces above an engine in a car.

Today, with fuel injection and modern computational fluid dynamics tools, I suspect that air and fuel flow problem can be solved more easily. But also, today, emissions standards are much more strict and so airflow and fuel flow are likely having to be much better controlled (and I think that is done best with individual throttle bodies and injectors).

... I don't know, just thinking what might be involved ...

Just out of curiosity, I looked at a Yamaha website to see how they do it on their four stroke outboard motors. Their I4 and V6 outboards use single throttle bodies, don't know about the V8. The latter has the exhaust inside the vee and the intakes on the outside, so that engine might have two throttle bodies instead of just a single.

Andrew Shadow said:

My understanding is that it is not so much a question of the volume of air entering the engine but the control of that volume- multiple throttle bodies afford better throttle control and response which is of much less relevance on a street car than it is on a street bike. Whether that additional control is ever truly needed or even noticed by the average street rider is whole other debate.

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There are other factors for certain but I think this is a major one. I recall reading something from the New Concept (NC700/750 platform) team leader about how important accurate fueling is on motorcycles compared to cars. The inertia of 2500 to 5000 lbs of vehicle masks mediocre or even poor fueling behavior that would be coarse or even unacceptable on a several hundred pound to half ton motorcycle. The NC twins have a single TB and are tuned for output at low rpms similar to a car or older slow turning twin motorcycles that commonly had single carbs.