Throttle Body Sync - Yamaha FJR Forum : Yamaha FJR Owners Forums
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post #1 of 14 (permalink) Old 01-14-2016, 09:14 AM Thread Starter
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Throttle Body Sync

Like many others on this forum, I grumble about buzzing in my handlebar ends when cruising at highway speeds. Having viewed TWO Brian's suggestion for a throttle body sync then reading others who say it only matters on idle, I am torn about the effectiveness of the proceedure to get rid of highway buzzing. The sync is supposed equalize the performance of each cylinder so that one does not perform differently from others and cause a vibration. One experiment I tried way to ride at a speed that creates the buzz (approx 70 MPH) then immediately close off the throttle so that the engine generates virtually no power. Now there should be virtually no power generated and so there should be no unevenness between the cylinders causing the vibration. Yet I found that the vibration remained with the power off until the engine slowed thru the RPM range where it no longer vibrates. This leads me to think that the vibration is more an inbalance characteristic of the engine rather than non-uniform combustion.
Please share your thoughts!!
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post #2 of 14 (permalink) Old 01-14-2016, 03:56 PM
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There are two "circuits" for the airflow in the throttle bodies -- the actual butterfly valves that open with throttle application are by far the biggest factor. When these are closed, only a couple tiny holes exist for air to flow through the butterflies, plus what goes through the valves that are adjusted when doing "Throttle body sync".

So these valves only really provide some adjustment from full-closed to partially open states of the butterfly valves -- the further open the butterflies are, the less effect the valves have.

The Throttle Body Sync procedure is intended to balance the bodies at idle and smooth out differences at and just above idle for cleaner transitions and so forth. The only way to truly effect the sync at mid-to-upper range throttle openings is by literally adjusting the amount of the butterfly's travel individually in each throttle body.

When you add in more of the factors of airflow through the airbox, cylinder temperature differentials (ie, inboard cylinders warmer than outboard cylinders), exhaust cooling, etc, etc, etc, the whole balance aspect starts to get wiggly. Adjusting one butterfly to open less or more than the others may in turn affect the others too.

I imagine that some powerful computers with myriad sensors across all aspect of each throttle body -- airflow, temperatures, etc -- could continue to refine the fuel and air flow rates to each cylinder to achieve a truly even balance between cylinders. However, the R&D costs, manufacturing costs, bulk, weight, etc would provide diminishing returns, so a "pretty good balance" is probably the ultimate goal in a mass-produced system like a motorcycle.

Hence we'll always likely end up with some compromises in a given design, and variations between different bikes coming off the same assembly line, all considered acceptable variations that fit within the cost/result parameters of the product.

A little more on the FJR though -- I believe there's an "Unauthorized throttle body sync" method used by some Gen 1/2 guys to tweak the angles between throttle bodies, which are either too invasive or impossible to do in the servo-driven throttle bodies of Gen3+ bikes. If one was inclined to get deep in mechanical linkages of a $15,000 motorcycle's throttle bodies to tweak out that last little bit of uneven firing to hopefully get a little bit less vibration, then I'm sure it could be done.

I think most of us just learn to accept it. For sure, the FJR is smoother than MOST inline-4 bikes out there!
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post #3 of 14 (permalink) Old 01-14-2016, 03:59 PM
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Oh, meant to also just say... doing the factory TBS is rather easy to do, so "it won't hurt to try". Maybe it will help, maybe it won't. In my case... not really. The buzz is there and I've gotten used to it. If it bothers me I'll find a slower road or speed up a little more and risk LEO interaction. If I slow down to 65 or so it's eerily smooth and I'm amazed. More reason to do the two-lane twisties instead of the slab!
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post #4 of 14 (permalink) Old 01-17-2016, 12:05 PM
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I've done carb sync on many bikes over the years with the fjr being the first injected motor that I've done, the sync does affect the balance/airflow through the entire rpm range although as stated earlier less as the rpm's go up but they still do influence the cylinder to cylinder balance. Also as stated everything else like valve adjustment, air box, cylinder head ports and even the individual cylinder to cylinder timing all have an effect on the airflow through the throttle bodies and the balance of the motor. That said the fact that the fjr across the board uniformly vibrates in. 4-4500rpm range on most bikes says that there is an inherent design issue. On my 14 the throttle bodies are within 1/2mm of variance which I believe to be about as good as it gets but still vibrates more than the other 3 fjr's that I've ridden, guess it's just that way and I'll look for some better grips to dampen the vibration in the bars a bit ( any suggestions ?). Also I'd add that because everything effects everything you need to look at everything to optimize it and things like an air box full of mouse nest (don't ask) can have a huge effect on things, like a slight hesitation off idle that no longer exists once said nest is gone. On my old Hondas I now set the ignitions up with a degree wheel because they typically use 1 pickup/points per 2 cylinders and need to be correctly phased to reduce vibration and shudder, the point is don't dismiss anything because our fjr's are actually 4 separate engines that need to be sync'd together, good luck

Last edited by passx; 01-17-2016 at 12:09 PM.
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post #5 of 14 (permalink) Old 01-17-2016, 12:10 PM
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I've done carb sync on many bikes over the years with the fjr being the first injected motor that I've done, the sync does effect the balance/airflow through the entire rpm range although as stated earlier less as the rpm' so go up but they still do influence the cylinder to cylinder balance. Also as stated everything else like valve adjustment, air box, cylinder head ports and even the individual cylinder to cylinder timing all have an effect on the airflow through the throttle bodies and the balance of the motor. That said the fact that the fjr across the board uniformly vibrates in. 4-4500rpm range on most bikes says that there is an inherent design issue. On my 14 the throttle bodies are within 1/2mm of variance which I believe to be about as good as it gets but still vibrates more than the other 3 fjr's that I've ridden, guess it's just that way and I'll look for some better grips to dampen the vibration in the bars a bit ( any suggestions ?). Also I'd add that because everything effects everything you need to look at everything to optimize it and things like an air box full of mouse nest (don't ask) can have a huge effect on things, like a slight hesitation off idle that no longer exists once said nest is gone. On my old Hondas I now set the ignitions up with a degree wheel because they typically use 1 pickup/points per 2 cylinders and need to be correctly phased to reduce vibration and shudder, the point is don't dismiss anything because our fjr's are actually 4 separate engines that need to be sync' together, good luck
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post #6 of 14 (permalink) Old 01-17-2016, 12:18 PM
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I would stick to where you are. Playing with the butterflies while the throttle bodies are on the bike, while very admirable in concept, may not gain you anything in my opinion. There are too many variables involved, I'd only recommend that if they were on a bench and you had ability to use more precise measuring methods. Just my opinion.
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post #7 of 14 (permalink) Old 01-18-2016, 09:06 AM
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Ryman, guess I'm trying to understand what you're saying, are you saying that instead of using a vacuum gauge set to balance the throttle bodies/blades that you use some other means to do that ? Over the years I've done many carb balances which takes all the individual cylinder imbalances into it and helps balance all the cylinders together to make the overall package smoother. Not being critical just trying to understand if I'm missing something or abetter method. I have tried using precision gage pins to equally set the throttle blades on the bench but have seen little success with that because it doesn't take other variances like air box differences, valve setting, port differences, cam lobe to lobe variances, etc, which all have an effect that is seen in the vacuum at the throttle blades. Could you please elaborate more ?
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post #8 of 14 (permalink) Old 01-18-2016, 09:33 AM
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The vibration issue seems to pop up regularly on FJR forums. I imagine the issue is driven in come cases by a particular bike being more buzzy than the average FJR, but I believe the problem is often more related to the rider's tolerance to the vibration. What one rider thinks is okay another rider finds to be unbearable. People have moved away from the FJR because they could not accept the vibrations or some other characteristic of the bike. After reading threads about vibrations I believe your best bet is to attack the vibrations through bar end weights, using a set of foam grips and ensuring the bike is running properly by having the valves properly adjusted and a properly done TBS. Trying to eek out some micro adjustment to the throttle bodies is not worth the effort.
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post #9 of 14 (permalink) Old 01-18-2016, 11:59 AM
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@passx , I think we're on the same page. Absolutely use a Carbtune to do the TBS. I agree there are other variances at play, even if you set the throttle blades with gage pins...... that's why I say the method while the TB's are still on the bike is not something I would do. I say just sync them the best you can with the Carbtune and it is what it is for the most part. My '07 purrs like a kitten!
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post #10 of 14 (permalink) Old 01-18-2016, 06:30 PM
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The one thing that nobody seems to take notice of in the vibration and throttle body balance discussions, is spark plugs. I've always found that old, worn, spark plugs are a significant contributer to vibration. As a general rule, when using standard plugs, you should change them every 10K Km. With Iridium plugs, I change them at 20K Km, and I always notice a reduction in vibration with fresh plugs. When I am going to carry out a throttle body balance, step one is to replace the plugs.

I know that a lot of people run their plugs 5 to 10 times longer than I do, and the bikes will run OK for that distance, but the penalty is vibration, trust me.

The reason is that, when an ignition system generates a pulse of high voltage to spark a plug, the voltage is not an instantaneous thing, it takes time to rise to the level at which it will arc across the plug gap. When the plug wears, the voltage, and therefore time, at which the arc occurs is not constant, it will change at every cycle. Thus, the ignition timing is constantly changing on all cylinders, all the time, and the result is vibration.
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