New to the RC51 - Vibration Question - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 7 (permalink) Old 08-20-2018, 12:38 AM Thread Starter
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New to the RC51 - Vibration Question

I recently purchased an SP1 with 16,000 miles. The bike is 100% Honda with absolutely no modifications or aftermarket parts. The engine starts and idles as expected with smooth power delivery from a standing stop. What concerns me is the amount of engine vibration that occurs while the bike is being operated at normal road speeds. This is the first V-Twin that I have owned, sport bike or otherwise. Each of my previous sport bikes have been inline four cylinder liter bikes. To the best of my understanding the RC's engine is hard mounted to the frame without any rubber dampening. This may be part of what I am feeling. My concern is that when the engine is being operated under a load the vibrations intensify. The vibration is greatest, for example, when traveling at highway speeds, say 55 MPH, and ascending an incline. The vibration is such that not only can I feel it through the bike but I can feel it through my body as well. At 55 MPH with stock gearing of 16x40 the engine is running at approximate 2,800 RPMs. The vibration lessens if I downshift to a lower gear bringing the RPMs up, but it is still there. The engine appears to pull well but without having ridden another RC I have nothing to compare it to.

Can someone tell me if increased levels of vibration under load is normal for these bikes? If it is not, any suggestions as to what may be the cause?

There is one more thing that I should add. Although I am not certain, it sounds as though I am hearing a backfire from the engine when up-shifting during moderate to rapid acceleration. The noise occurs during the time when the clutch lever is first depressed and the throttle is relaxed. It does not happen every time but it does occur occasionally. Again, I do not know if this is normal or not for this machine.

Last edited by Enxss; 08-20-2018 at 03:21 AM.
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post #2 of 7 (permalink) Old 08-20-2018, 03:56 PM
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2800 RPM is rather low for these engines.
Around town in lower gears, this would usually cause shuddering and bucking, requiring an occasional de-clutching to stop the condition.
The RC51 isn't a Harley chuff-a-lump. This engine prefers 3000 revs or higher (preferably 3500 or more), which is a challenge in stop and go, or slow traffic.

The stock final ration of 16/40 is too low for the engine, with the most popular final ration typically being 15/41 or mathematically similar to place the engine in a better rev range.

Anything above 3000 and the engine should smooth out to a better degree.
Although there will always be some vibration, it will be at a lower frequency than an I-4.

There is the remote possibility the engine mounts may have loosened over time. Though doubtful.
Being an SP1, it uses adjust bolts and locking rings along with engine mount bolts to secure the engine in the frame, requiring special tools for the bolts and rings (the SP2 uses distance collars and pinch bolts instead, which makes the job much easier).
If the engine was never swapped out, then I doubt that's the case.

If the bike does not have a Power Commander fitted, then the OEM air/fuel mixture is leaned out per EPA, which, when decelerating or rolling off the throttle during shifts, can cause popping out the exhaust due to a very lean air/fuel condition from extra oxygen via the PAIR system lighting off residual fuel vapors in the exhaust.
This OEM lean condition can also cause abrupt throttle operation as well, which the SP1 is known for.
A Power Commander can smooth out this condition by richening the air/fuel ratios.
PAIR block-off plates can eliminate the extra oxygen injected into the exhaust, and this can eliminate the popping or slight back-firing it causes.
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Last edited by SubSailor; 09-04-2018 at 11:33 PM.
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post #3 of 7 (permalink) Old 08-20-2018, 09:12 PM Thread Starter
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That answers all of my questions while alleviating all of my concerns. Thank you, SubSailor.

Last edited by Enxss; 08-20-2018 at 09:32 PM.
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post #4 of 7 (permalink) Old 08-28-2018, 12:37 PM
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I actually had the exact same question. New to the RC51 myself, only had 4 cylinder bikes before. Mine has gone 43.000 miles, so i was wondering if the vibration could have been caused by high mileage, but maybe it's normal. It occurs mostly when cruising in high gear around 2500-3000rpm when throttling up, or in lower gear when accelerating past 2500-3000rpm. Never in idle or at higher rpm.

The bike performes very well, so i dont suspect anything wrong with it. I wouldnt worry about it

Dress for the slide, not the ride.
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post #5 of 7 (permalink) Old 09-04-2018, 11:45 AM
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I have -1+2 sprockets, Power Commander, and stock engine on my SP1. If I try to cruise anywhere below 3500 rpm, performance is poor compared to anything above that. Over the last eight years, I've learned my particular bike performs best between 5000 and 9000 rpm on serious rides. City commuting, I usually stay around 3500 to 5000. It just handles best for me at that range in casual traffic.
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post #6 of 7 (permalink) Old 10-06-2018, 09:44 PM Thread Starter
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Follow-up to my initial posting.

After running the bike for about a month or so I decided to perform the initial 16,000 mile valve service. An inspection prior to disassembly revealed some light oil seepage occurring from both the front and rear valve covers. But nothing more than what one would expect from a seventeen year old engine that has never been disassembled. Upon removal of the front valve cover I noticed several spots of light brown (the color of road dust) appearing on the lower surface of the rubber sealing gasket where the gasket mates to the upper surface of the head. The upper surface of the gasket had been cemented to the lower surface of the valve cover from the factory so no indications of such discoloration were present. The valve cover gasket on the rear head showed no such discoloration. An inspection of the valve lash verified that all clearances fell within specifications ... as expected. No adjustments were made.

As outlined in the factory service manual, I reassembled the engine using new valve cover gaskets on both the front and rear heads. I also installed new O-rings on both the front and rear dowel pins. In addition, new O-rings were installed on both the timing hole and crankshaft hole caps. I also decided to replace the two O-rings which reside between the top of the throttle bodies and the base of the air box, although this is not suggested in the service manual. Once reassembled the engine started and idled without issue.

Given that no valve shim adjustments were performed I expected no change in the engine's performance. The engine always stared and ran without issue. My only concern with the engine had been the low frequency vibrations that I noted in my initial posting to this thread. However, immediately upon taking the bike for a test ride I noticed a considerable change in the engine's operating characteristics. Suddenly the engine accelerated in a much smoother fashion with a more linear and predictable throttle response. In addition, the popping produced by the PAIR system when up-shifting during moderate to hard acceleration was all but gone. The engine also maintained a constant speed without continual adjustments to the throttle. Previously, the bike had a tendency to drift up in speed. The engine was also far more comfortable at lower RPMs in sixth gear. Previously, maintaining, say, 55 mph at 2,750 rpm on a relatively flat surface was a struggle. Now the engine could comfortably maintain a constant speed of 45 mph at 2,250 under similar conditions. Also, the engine was far more predictable at parking lot speeds. The rather sensitive and somewhat bursty throttle response previously experienced was gone. All of these initial conditions of course point to a fuel/air mixture that is far to lean. Such a tendency for SP1s was pointed out by SubSailor in his response to my initial posting. This is not to say that the tendencies pointed out by SubSailor had been eliminated. Rather, they were suddenly far less severe and more in line with what I would expect from the machine.

In trying to understand why this has occurred, it appears that "False Air" may have been entering the Crankcase Breather system via both the front and rear valve cover gaskets, as indicated by both the light brown areas on the front gasket and the oil seepage from both the front and rear gaskets. The Crankcase Breather system is, by design, a "closed system." Any air entering the system would be introduced to the induction system as False Air thereby causing the fuel injection system to run lean. Too lean a mixture would of course produce the erratic throttle response, upwardly drifting engine speeds and PAIR system induced popping due to unburned fuel exiting the combustion chamber. The lean mixture would also burn hotter creating hot spots within the combustion chamber leading to pre-ignition thus increasing engine vibrations under load as I expressed a concern of in my initial posting. The installation of the new gaskets and O-rings appears to have restored the crankcase breather system to its intended closed system state thereby restoring the fuel/air mixture to a richer more appropriate level.

So what should be taken away from this discussion? My thought is that in addition to the recommended mileage interval of 16,000 miles for valve clearance checks it may be appropriate to introduce a maximum time interval as well. A maximum interval between checks of, say, 10 years would not seem unreasonable. On bikes such as mine that have had low usage during their lifetimes, or owners that have decided to delay the initial valve inspection due to the low probability of the valves actually needing to be re-shimmed, this would take into account the possible deterioration of the rubber used to seal the Crankcase Breather system. My guess is that the leaks were introduced by a contraction of the rubber components over time. Note, however, that the rubber seals were neither cracked nor hardened. In fact, they still appeared quite malleable. But obviously something had changed.

I would be interested in hearing if anyone else has encountered such difficulties on their bikes. Or if anyone can suggest additional maintenance items that should be accelerated simply due to aging.

Last edited by Enxss; 11-03-2018 at 01:13 PM. Reason: Additional Comment
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post #7 of 7 (permalink) Old 11-08-2018, 03:03 PM
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Mine ('03) does the typical buck and stutter below 3500 RPM. 15-41 cogs so I run low gear in low speed (pits) riding and use the clutch a lot. Not on the street anymore but 1st gear was pretty much a given.

Duct tape can't fix stupid...but it sure helps muffle the sound
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