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Discussion Starter #1
Hi,
I saw that there are a few threads that lightly discussed the vibration around highway cruising speed (somewhere in the range of 3700-4500rpm) from the engine. One of the discussions mentioned unbalanced carburetors, which I think I'm going to investigate, but I was wondering if there are other specific areas that I should have a look at. Opening up the throttle in that range from cruising increases vibration. I'd almost want to say it's gets a bit rich there, but I don't know if that's actually accurate. The bike is 1996, just a bit over 20k miles, stock carburetors, open airbox and slip on muffler. When I got it several months ago, as a part of refresh, I rebuilt the carburetors, did the valve shims, belts, all that. It idles steadily once warm, it revs cleanly all the way to the red line with no noticeable hiccups anywhere. One thing that relates to the potential carburetor issues is that when starting from cold, sometimes it stalls with half-choke, with a distinctive puff of backfire through the carburetors. It's happened maybe 3-4 times, I just have to make sure that it has enough revs and it doesn't stall, and it's not been a problem.
Appreciate your input!
 

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Peace.

Same year and model and experience moderate, yet noticeable vibrations at an average of 50 miles per hour or higher.

I threw my bike on the lift and was planning on verifying torque values on fasteners.

Keep us posted.

Peace.
 

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Discussion Starter #4
Thanks for the response.
I ruled out the chassis related issue. I had a bent front wheel, so I recently replaced it with a used, but straight one, and now, when I pull in the clutch at speed, the bike coasts smoothly (really smoothly, as a matter of fact), so the vibration is definitely from the engine, and it’s definitely rpm specific, too. I narrowed down that far, but I haven’t a clue what to look at first. I’ll start with carefully synchronizing the carburetors, I guess, just because that was mentioned before.
 

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Motor mount torque and condition of chain and sprockets is usually the first thing to check if the motor is running well. Did you replace your wires and caps ? They break down over time. Only one of the three has to be resistor type, plugs, wires or caps, to keep from interfering with the ignition. When you “rebuilt “ the carbs, did you replace the metering needles and emulsion tubes ? They are a wear item and can cause general poor running, but people usually notice it most below 4500 rpm. I use Factory Pro titanium needles and nickel plated emulsion tubes, because they should wear less over time. Also on these carbs, they are a little tricky to adjust the float level and they are sensitive to float level. Watch a YouTube video and see if you did it right.
 

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You can also watch your carb slides rise and see if they seem to be working smoothly and rising together. Clean the vacuum lines if you haven’t.
 

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Are you sure the problem is not caused by a loose flywheel?

You can do a (basic) quick check by looking at the flywheel through the inspection cover on the left side of the bike.
If the flywheel is loose, you might see it 'wobbling' slightly.
 

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Rough running

Try raising the carb needles a notch or two then adjust your low speed carb screws if you have the stock airbox. My 97' SS had this problem, I put on pod filters, lowered the needles two notches, kept the stock primary jets, went to 136 mains, and adjusted the low speed screws out one turn. I know this is counter intuitive to popular belief about jetting but I can now cruise at 35-40 MPH in high gear without all the sputtering and surging it had before.The main reason I went to pods was to access the carbs for tuning and cleaning. I don't think it affected performance any, but it is extremely LOUD.......
 

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And..... I forgot that I lowered the gearing one tooth on the front sprocket. Went from 15 to 14. Believe it or not it also increased my top speed since it will now run two hundred into redline in high gear.
 

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Discussion Starter #11
Interesting...

I'll check the flywheel, but I'd assume that if it's loose, would it rattle differently at all engine speed?

The vibration at that rev range is accompanied by a slightly laggish acceleration, but only from cruising, so I'm wondering it's the steady throttle at that particular rev range is making it either too rich, or some such things... Or it could just be how this bike is. I need to figure it out!
 

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Check generally around the bike for tight fasteners, especially stuff like the exhaust system that could shake and is relatively heavy. Make sure it’s fastened to the motorcycle the way it’s supposed to be. The gearing change is always a good idea for many reasons, and spinning a little higher is one of them. These motors seem to prefer 4500 - 6500 rpm cruising from my experience.
 

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Discussion Starter #14
I've been reading a lot of old thread, learning a lot about the carbie SS, and this is fun, there was a thread about the fuel, how it prefers lower octane fuel. The previous owner insisted that I use premium, but after reading those threads, I started using regular, and it runs much smoother. It still vibrates around the same rev range, but it's much smoother. I'll be updating plug wires and stuff like that that are 23-4 years old this winter, and keep updating little things to see if it improves further.
 
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