Factory Five Racing Forum banner
1 - 14 of 14 Posts

·
Charter Member
Joined
·
1,576 Posts
Discussion Starter · #1 ·
UPDATE: I took off the belt as per Hind Site's recommendation - to rule out accessory vibration - and the vibration went away. Put it back on, and you guessed it, it was back. I put my hand on top of the alternator and felt a strong vibration. Thanks to all of you who replied for your ideas - very helpful. Surprising that little guy could cause such a vibration.

Have a great night,

Geoff


Hi all. I have a vibration that is rpm related at 2100 rpm, any gear, any speed. Tried it in the garage tonight and it is present with the clutch in or out at 2100 rpm. Runs smooth from idle to 5500 rpm other than this vibration.

Here is what I have:

rebuilt 306

new damper - stock replacement from FMS.

new billet flywheel - came scratched so I had it ground, then balanced the whole rotating assembly with the same machinist.

new FMS cobra clutch

New FMS dowels that are in place. I even took pictures during assembly to make sure. I rechecked the pictures tonight and the dowels were still in after pressure plate was tightened down.

Any other ideas?

Thanks,

Geoff

[ May 02, 2007, 11:30 PM: Message edited by: Geoff H. ]
 

·
Registered
Joined
·
3,427 Posts
Since your engine was ballanced and your flywheel clutch assembly is ok, I'm gonna guess it to be ignition / fuel related. Sometimes the only way to find these issues is to use an engine analyser / scope. It may be as simple as a bad plug wire or a faulty injector.
 

·
FFCobra Fanatic
Joined
·
636 Posts
I would agree with Michael it is most likely one of you cylinders is not getting proper spark or possibly a sticking valve.
MadDog
 

·
FFCobra Fanatic
Joined
·
12,975 Posts
Goeff, have you driven the car or is this only being tested in garage with no real load on engine? Since it's not speed dependant but rpm brings it on thinking may have a tune problem that may show up better if driving at that rpm under load. How is the idle. How does engine respond with a quick jab of the throttle. Hesitation or instand rpm increase?
 

·
Charter Member
Joined
·
1,576 Posts
Discussion Starter · #5 ·
Thanks for the help. I have driven the car 150 miles or so.

I had not considered that it might be tuning related. It idles fine - a bit of a lumpy idle with the F cam - at about 900 rpm, but no misses I can hear or feel touching the engine. There is no hesitation on blipping the throttle - instant rpm.

I have only really got on it full two or three times up to about 5500 rpm. It pulls strong with no hesitation or missing feeling. You can feel a momentary vibration at around 2100 as it pulls past it, but it comes and goes so quick it is hard to detect unless you are looking for it.

Thanks again for your help,

Geoff

Thanks
 

·
FFCobra Fanatic
Joined
·
12,975 Posts
I see where you had it balanced. Did you have the same balancer on engine now used when assembly was balanced? Has anything been changed on rotating assembly after the initial balancing was done? Being able to make it happen at same rpm each time under load or not does sound like it could be a balance problem but caused by something small that even a missing water pump pulley bolt could do. No wobbling pulleys when watching?
 

·
Charter Member
Joined
·
1,576 Posts
Discussion Starter · #7 ·
I had the same balancer included with the rotating balance. Nothing has changed, I also had the pistons weighted, rods trued, arp bolts installed, then the rods were balanced. I assembled the engine myself and all the new specs after machining were dead on, including all the endplays.

I have looked at the pulleys, but I think you may have something there. I will take a closer look at the bolts. I do not see any wobble, but maybe I would not see it. I am wondering about my alternator as it is old and there is play in the shaft - maybe at rpm it is enough to bring on a vibration.

Thanks once again, it is great to have people to bounce this off.

Geoff
 

·
FFCobra Fanatic
Joined
·
12,975 Posts
Bad alternator may be enough to cause this. In the case of a possible bad bearing and or end play in the altenator it could be sorta rattling around till say the 2100 rpm then it takes a set where all the free play is taken up so vibration stops. Worth checking by removeing the belt. Taht would eliminate all accesorys as a cause.
 

·
Registered
Joined
·
589 Posts
I had the same issue...when installed the engine was stock, 36,000 miles & never opened, with a new damper, flywheel, clutch, bolts and dowels etc.

I checked many of the items suggested here without success.

A local garage run by a local drag racer and his crew diagnosed the issue as a "harmonic vibration" in the FFR side pipes and offered to run an exhaust bypass between the pipes in front of the engine to solve it.

I declined and somewhere around 12-15,000 miles later the vibration stopped. Not sure why unless, if they were correct the pipes burned out some inside and that changed the harmonic characterics of the air flow.

I'm certainly not an engineer so I don't really know why it stopped but the car now has 22,000 trouble free miles on it and no vibration at 2100 RPMs.

Good Luck!

Bob Jones/Raleigh, NC
 

·
Charter Member
Joined
·
1,576 Posts
Discussion Starter · #10 ·
Wow Bob! Thanks for the info. It is good to hear I am not crazy! Maybe these guys nailed it for you. Once I have checked my alternator, I have run out of ideas and am at a loss as to what else to check. I just want to make sure I will not harm anything by continuing to driver it. It does not seem like it the way it runs.

Thanks again,

Geoff
 

·
Charter Member
Joined
·
1,576 Posts
Discussion Starter · #11 ·
Found it!

I took off the belt as per Hind Site's recommendation - to rule out accessory vibration - and the vibration went away. Put it back on, and you guessed it, it was back. I put my hand on top of the alternator and felt a strong vibration. Thanks to all of you who replied for your ideas - very helpful. Surprising that little guy could cause such a vibration.

Have a great night,

Geoff
 

·
Charter Member
Joined
·
1,117 Posts
Good call HS! You da man


Eric
 

·
New
Joined
·
183 Posts
Is your distributor set up for the right firing order? In your modified small block,many aftermarket cams are ground with a different firing order than the stock engine. Correcting involves swapping two spark plug wires on the distributor.
Good luck.
 
1 - 14 of 14 Posts
Top