Topic: Hard vibration at specific rpm

[ekpent]

That's just Dave and the reason why we like him so much around here. 

[dave500]

with those forks the bike thinks its going uphill!

[bryanj]

 I would say forks and choice of tyre MAY have something to do with it

[Tracksnblades1]

I came across this post and was trying to figure out this exact situation. At certain RPM's while in neutral sitting still I can feel vibration though the seat / my butt. I just rebuilt the engine and am concerned I messed something up. What do you think it would be if I can feel it while sitting still in neutral?

Things going though my head are. 1 cylinder getting more fuel from messing up needle position on one carb? My counter to my own thought on that is I did a temp reading on each exhaust and they are about 6-7 degrees of each other. None of them stand out at a different temp.

Only other thing is maybe a loose generator rotor?? I'm thinking I got that thing tightened but I can check that easy enough I guess.

Just wanted someone's thoughts on what I may have done.

Thanks in advance!

Can you duplicate the vibration by holding revs between 5900-6200 in neutral sitting still...?

I would try to eliminate engine vibrations  from chassis, wheel, chain vibrations....🤓

So are your vibrations coming from a 550 or 750…?
Tell us about your engine rebuild:
New pistons?  Or did one of the old ones need replaced?
All original rods? Or did you have to replace one?
Does the engine sound like it’s running on all four cylinders at road speeds, accelerating, and throttle transitions?
Are you sure your points and timing are spot on? 1&4 and 2&3 are the same ?

[dave500]

#$%*,the original post was #$%*ing 2019,who really gives a damn?

[Tracksnblades1]

#$%*,the original post was #$%*ing 2019,who really gives a damn?

Probably the new member RomHog. Since he’s experiencing the same problem in April of 2022.
Do you think he’ll respond or interact now..?  Asking for a friend…

[Romhog]

So are your vibrations coming from a 550 or 750…?
Tell us about your engine rebuild:
New pistons?  Or did one of the old ones need replaced?
All original rods? Or did you have to replace one?
Does the engine sound like it’s running on all four cylinders at road speeds, accelerating, and throttle transitions?
Are you sure your points and timing are spot on? 1&4 and 2&3 are the same ?

it's my 550,
yea I replaced the Pistons, didn't have to replace the rods.
I thought there same thing about maybe 1 cylinder but I checked the temps at each header and they are all about  4-5 degrees of each other. I thought it might be more fuel or less fuel going to one of them but I'm pretty confident I said all the bowl heights accurately I've done that 10-15 times on this bike so I have that down to a science. also the carburetors were completely rebuilt so I know it's not a clog issue.
you gave me a good idea to not trust my point settings and I'm actually rechecking all those right now and will admit here shortly if I messed up there.
but aside from that it seems to run good and have good acceleration it's a bit wonky when I start it up with the choke but who doesn't have imperfections with that. 😁

[Romhog]

Probably the new member RomHog. Since he’s experiencing the same problem in April of 2022.
Do you think he’ll respond or interact now..?  Asking for a friend…

new member?? heh , I've been on here since 2011. I'm mostly just an observer rarely do I have problems I can't solve on my own... jkjkj!!!
wish I could say the same thing about my phone because I wasn't seeing the posts after my question for like a day partially because I just got a new phone and for some reason it's not going to the most current page of the post thread

[Romhog]

Quick update. I checked the timing and 2-3 was off slightly from F position but after starting it up again the vibration was still there so I decided to take temp readings off the headers for each cylinder and this time to my shock... 2,3,4 were at 200 degrees when I checked and cylinder 1 was at 115 degrees.
I did also update my coils and checked the wire which looks ok. heading out there now to drop the bowl on the 1 carb  and see what's going on.

[Tracksnblades1]

Quick update. I checked the timing and 2-3 was off slightly from F position but after starting it up again the vibration was still there so I decided to take temp readings off the headers for each cylinder and this time to my shock... 2,3,4 were at 200 degrees when I checked and cylinder 1 was at 115 degrees.
I did also update my coils and checked the wire which looks ok. heading out there now to drop the bowl on the 1 carb  and see what's going on.

115* that’s cooler than an air compressor getting ready to shut off at 180 psig.
What does #1 plug look like?
Is it firing?

[Romhog]

115* that’s cooler than an air compressor getting ready to shut off at 180 psig.
What does #1 plug look like?
Is it firing?

It was black. I checked it's Om's against the others and it was getting a different value enough that I decided to go get new plugs yesterday. After putting those in and confirming 1 they are getting a spark I took it out and rode it hard. Took temps again when I got back and 2,3,4 was around  215 degrees and 1 was at 158 degrees. So better but not..

UPDATE: I checked the tappet clearances and they are fine. I did a compression check and all the cylinders are a nice 150psi COLD.  I also included a pic of No 1 Cylinder spark plug  I replaced early yesterday.  (2,3,4 looked perfect) So, again I checked the electrical and that passed and replaced that spark plug so it cant be a resistance issue.  I have new rubber boots on the carb bot on engine side and air box side so I don't think its a vacuum leak but I will triple check it. I guess I'm left with checking the top end of the carb. Do you think this could be something as simple as maybe me placing the needle jet on the wrong position for that cylinder?

[david 750f]

Before you get into the carbs swap the #1 and #4 plug leads. If the problem follows the wire it’s electrical. If it doesn’t,  it’s time to clean those carbs.  Just touch the pipes at startup, to see which are getting warm. No need to take it for a ride.

[Romhog]

Before you get into the carbs swap the #1 and #4 plug leads. If the problem follows the wire it’s electrical. If it doesn’t,  it’s time to clean those carbs.  Just touch the pipes at startup, to see which are getting warm. No need to take it for a ride.

Well, I took your advice and swapped 1 and 4 and the issue didn't follow.  so at least it's not electrical. What's interesting is that if I keep it at idle aka anything below 2k on tach. it shows a drastic difference in temp when compared to 2,3,4.

I should also mention that all 4 of my idle jet screws really don't do jack on changing the rpm's when I idle it around 1500k . Anyhow, I revved up the bike to about 5k (using the idle set screw)and took the temps. 1 started to get closer to 2,3,4. So if 2,3,4 were at about 250 degrees 1 would be at about 200-220 degrees. So it seems it balances out when the needle rises. Then for double fun, (because I'm probably going to be pulling the carbs to check the top end)  I loosened the top set nut and turned the screw for the carb sync adjustment and it seemed to get better in temps. 1 was at 275  and  2,3,4 were at about 300  But even after all that overthinking basic troubleshooting, the bike still vibrates at about 4k and up on tach.
So maybe the vibration is something else. but now that I've messed with the screw and the other issues, I will probably pull them and then do a bench sync again and then vacuum sync once back on bike. Blaaa.

[Tracksnblades1]

3 big bangs are going run hotter than 1 little bang.
You found the problem. Now you need to find the reason and cure.
A black spark plug and 3 normal ones…🤔 #1 spark plug is telling you a story..
I’m betting when spark plug #1 looks like 234, your your new vibration will go..
If #1 looked like 2,3,4, and running cooler, then I agree 2,3,4 were doing all the work
and the carb sync (throttle opening) for #1 was lagging behind..

[scottly]

I also included a pic of No 1 Cylinder spark plug  I replaced early yesterday. 

That was a brand new spark plug yesterday? You have a carburetor problem on that cylinder: too much gas/not enough air. Unlikely that a needle clip position alone would foul a plug that quickly...

[dave500]

check the plug cap resistance,a dud cap will foul a plug.

[Romhog]

That was a brand new spark plug yesterday? You have a carburetor problem on that cylinder: too much gas/not enough air. Unlikely that a needle clip position alone would foul a plug that quickly...

No. That was the old plug that was in cylinder 1. I replaced them yesterday as a part of my initial troubleshooting. I checked the new one, but its not had long enough to show any signs of that issue yet. I'm going to pull the carbs out again today, re-check needle position,  all jets for clogging, bench sync it again, put it back in and vacuum sync. See what happens. If I still feel the vibration. I guess I will need to re-check my cam (on my engine rebuild I didn't replace that because it seemed good but I didn't have a very good caliper at the time.)

[Romhog]

I don't often say this, but I give up.
 - checked tappet clearance.
Aside from the other things I mentioned above I've also
 - pulled carbs, checked all jets (which are brand new) to make sure all the numbers were matched.
- rechecked the float leveled all were perfect. Bench synced again.
- now, I've good thing is each cylinder temps are even now!
Still... Sitting parked in neutral , when revving above 4-5k I'm still getting the vibration in frame, aka in my butt when the seat on. It's enough for me  to be sick of riding it for 20 minutes. And again didn't have those issue before engine rebuild. (And initially I don't believe it did this right after the rebuild on my first few rides either)

[dave500]

only other thing that spins fast enough to vibrate is the clutch,maybe take off the cover and have a look?check the basket isnt damaged and its end float and the circlip is still in place?