Im having an odd issue with my motor. After about 3500 RPM, it develops a strange transient vibration. This also shows up on the tach. Its hard to see, but theres a high frequency vibration to the needle when this is happening. My guess is that its something along or near the valvetrain. What would be the most likely suspect? Worn dist. or cam gear? Excess endplay? Sloppy timing chain? I had the dist. apart not too long ago and I didnt see anything remarkable and the timing set is relatively new. What is the correct endplay for the dist.? .060" or so?

 

If it's showing up on the tach it's more than likely a misfire.

 

It could very well be. I was thinking spark scatter as a possibility as well, but Im still on the lookout for a misfire. Ill look over the ignition system yet again just to be sure. It all appears to be in good condition and I couldnt find anything wrong with it on the first go around. Whats odd about it is that the frequency is so high. It definatly feels and appears to be higher then 30-35 Hz (cylinder misfire), but who knows. On the tach, the needle has very small high frequency ocillations when the problem occurs. The car runs well and has very good power.

 

There could be more than one cylinder misfiring, which would be more than 30 Hz.

I don't know if you'd ever notice spark scatter without using a timing light. Distributor end-play should be more like .010"-.015" or so, IIRC. Every .010" of play is something like a degree or two of variability.

 

Ive had lean issues at idle cause lots of spark scatter in the past when the engine is unstable. The timing would dance all over the place. And as for the dist., it feels alot sloppier then .010-.020". The air fuel ratio is also tied to the engines RPMs as the computer estimates the mass of air in each cylinder from the engine speed and MAF flowrate, so if the apparent RPMs vary, then the AFR will as well.

Im going to reshim the dist., and look at the ignition again. Hopefully that does it...

 

The only problem Ive found so far is the dist. It has over 1/16" of end play. It was a rebuilt unit. In good shape, but obviously not shimmed to spec.

 

Yea, i've heard of that a fair bit, when "they" rebuild a dist they put in new electronics, but don't look too carefully at endplay.
Shim that puppy up to .012" or so with the Mr gasket kit, then give'r a shot.

 

After checking/fixing everything I could think of I checked the cam with a dial gauge out of sheer desperation. Turns out one of the intake lobes has the exact same lift as the exhaust lobes. Checked it three times. Same result each time. Neat. Talk about a curve ball. Im assuming that someone mistakenly ground the exhaust profile on the intake lobe by mistake.

One cylinder is running on a 214/214 .442/.442 cam profile while the rest are on a 204/214 .420/.442 cam profile. I guess thats a 1/8 race cam...

 

hahah, go figure. You get the weirdest problems - the offset bellhousing dowels, etc.
But then again, that cam isn't one of the ones that screams high quality now is it ?
I wouldn't think that'd really cause a whole lot of difference though. But hey, you should be able to return it - that's a pretty obvious factory defect, not like normal wear and tear would cause that.

 

Actually, I put it in there about 50,000 miles ago. Its an old edelbrock performer plus that I put in it when I was using it as a daily driver. It would be one thing if it was the wrong cam. I could live with a single pattern 214/214. But the current multipattern cam causes a strange low speed idle as well as that odd WOT problem. Check this video out. Listen and watch the interior of the car carefully as the engine reaches 5.5K. I can rev the engine under little load and its fine. But once I put the gas to the floor, it does that.

http://www.youtube.com/watch?v=ye9XOBvGFWE

One of these days Ill have to pull that POS out and replace it with something better.

 

huh, so it's been acting weird for 50,000 miles?
So I guess there isn't any warantee left on it - at least it wasn't a pricey cam.

Is the car a daily driver still? I mean, a vortec headed TBI motor, it's a shame to use a teeny cam - I can't help but think you're leaving power (fun) on the table.
Compxe262 is just crying out to be tried

 

Ive been thinking the same thing.

 

It wasnt as noticable with the old speed density setup, but when I went to mass airflow is when I really noticed it acting odd. One thing with the MAF is that the total mass of air ingested by the last cylinder is used to calculate the fuel needed for the next cylinder. So if one cylinder is different from the rest, then itll throw the fueling off.

 

I had a similar sort of problem but much worse as the powerband went totally dead past 4k at WOT but part throttle I could take it all the way up no problem. I replaced all my ignition wiring, modules, dist, coil, etc and it never went away. I finally wired it up for a MSD 6AL just for kicks and it never had a problem again. So... I'm going to say your problem is definitely ignition related.

 

what are you talking about? how would a cam witha improper grind come to ignition?

 

Must have missed the improper grind part. Oh well.
Guess I should check mine now.