Good vibrations (NOT!)

[Bob Booth #33372]

Hello again all,

When I get hard on the throttle in my '72, at about 3500-4000 rpm, there is a very fast-paced vibration and a perceptable loss of (flattening of) power. It feels alot like when I raced formula V and the valve float at 6000-6500 rpm would signal that I was just over the shift point (oops, amazing how they stay together). The thing is, I can't imagine valve float at such low rpm. The engine has about 3000 mi. since rebuild. I'm not sure what valve springs are in it (they were new when put in), but even with stock (light duty) ones, is this possible? Could it be the harmonic balancer or u-joints or flywheel causing vibration with natural loss of rotating momentum and thus power? I checked the u-joints and all seem ok. Could it be an ignition thing that seems like a vibration but is really a misfire?

Thanks for any advice, Bob Booth

[Bill D.]

Re: Good vibrations (NOT!)

If you are easy on the throttle, can you increase rpm through and beyond that range without vibration?

[g.r.rogers]

Re: Good vibrations (NOT!)

How long has this flattening out been going on? Since rebuild or just started. Does the car come up to its normal rev limit once you pass the vibration point? I once installed an after market hi-po timing chain on an 1970 LT-1 and installed it too far advanced and my RPM band was only 4500 max not 6500. My compression went up about 30# when this happened. A compression check wouldn't hurt. It could be lots of other things also. Good luck and I wouldn't run the car through that critical speed or vibration point to much until I looked for some externals. Pull the valve covers if your worried about the valves and turn the engine over by hand checking each valve one at a time. There could be plenty of other things but the vibration is telling you to cool it and fix it or pay later. Good luck.GRR

[Bob Booth #33372]

Re: Good vibrations (NOT!)

Hello Wild Bill & g.r.,

Y'know Wild Bill, it's funny you should mention that because I tried that just the other day to try to determine if it is speed related or rpm related and found that the vibration was still there but seemingly not as pronounced. Whether that was because I gradually worked up to it and therefore wasn't as sudden and noticable or that it actually was less is hard to tell but I didn't want to continue trying for too long for obvious reasons as g.r. so wisely advised.

I think checking each valve and doing a compession check is an excellent idea. Thanks for the advice and I will continue to look for the answer.

Regards, Bob Booth

[Bill D.]

Re: Good vibrations (NOT!)

Good advice from g.r. but before you "go deep", try this. Change fuel filter as it may be partially plugged and not allowing sufficient volume at higher rpm. Check ignition points for burnt contacts and proper dwell at high rpm. Check timing for proper advance at high rpm. Good luck! WB