Hello Folks,

I have what seems like a mild vibration in my engine and I'm not sure if it is new or if it has been there since it was rebuilt.
It seems new this year, but I can't say 100%.

The engine is the original low mile 1984 302 that came in the truck.
It was rebuilt 0.020" over with new stock style pistons.
They told me all they had to do was polish the crank...it didn't need machining.
Rebuilt Explorer GT40 heads were used with Autolite 764 plugs. Stock stamped rockers.
Camshaft is a Comp Cams XE256H (Part # 31-234-3, I think)
Edelbrock intake and Holley 600 CFM 4bbl, with a set of BBK shorty headers.
Stock Duraspark distributor with Ford 8mm wires and original 1984 coil.
The original flywheel was reused, although I did have it resurfaced. Changed from a 10" clutch to an 11".
Installed a new 50oz harmonic balancer...an aftermarket one...Dorman I think.
New engine mounts...nothing fancy, just parts store specials about $15 bucks each.

My question is, I guess...how much if any vibration is normal with an XE256H cam?
The engine idles like it has a mild cam in it, which is fine.
But, there's a vibration through the rev range that seems unnatural to me. (First engine for me with anything other than a stock cam).

Should that cam be smooth above idle and/or above 2000 RPM?

What I'm trying to say is, if a little vibration is normal, then I guess everything is OK and I just don't know.

I wish there was a Ford guy close by that I could get to look at this thing.

What would be the best steps to take to diagnose a vibration?

I pulled each plug wire tonight, and there was a change in idle with each one. Not much of a change mind you, but still there was a tiny change. PLugs look OK, I think. This is with maybe 4000 miles on them since the engine was built.





Any comments good or bad? Suggestions? Things I could check on my own?

 

No an engine vibration at all rpms is not normal.It could be a problem with the balancer or flywheel weight but could also be low compression on a cylinder.

 

Thanks for the quick reply Conanski!

OK, I can do a compression test, no problem.

Is there a way to check the harmonic balancer installed or would I have to remove it and/or replace it?

Also, on the compression test. Should I pull all of the plugs at the same time? Do I need to drain the gas out of the carb? Does it matter?

 

Got the right firing order for that cam.....?

 

Pull all the plugs for a compression test. Block open choke and throttle all the way. Battery should be thoroughly charged up.

 

OK, cool. I'll check it tonight then. So with all the plugs out, the engine won't pull any gas out of the carb, right?

 

It's not a cause for concern, no.

 

Compression test turned out OK I think.

#4: 177 PSI #8: 170 PSI
#3: 170 PSI #7: 170 PSI
#2: 177 PSI #6: 170 PSI
#1: 178 PSI #5: 168 PSI

That was with the engine hot, all plugs out, throttle tied open. We never did do a baseline compression test after the engine was built, so I wasn't sure where it was going to be, but these numbers seem pretty good for a 302. The largest spread is between cylinders #1 and #5, but I assume a 10 PSI variation at this compression is OK?

Random but related question...if for any reason there was an issue with a cam lobe, would it show up in the compression tests?

I also checked EGT with an IR gun at idle, and they were mostly in the 265F range. They probably bounced a bit between 250F and 275F, with the lowest reading coming from Cylinder #5 (225F), which makes sense since it also had the lowest compression.

Anyway....back to the vibration. I also disconnected the vacuum advance and the engine runs much better without it. More power, and smoother. I'll have to play with that a bit, but for testing purposes I'll leave it disconnected for now. I did check the timing, and the engine is still sitting with a solid 12 deg advance at idle, and 32 deg fully in by 2800 RPM. The ramp up between the 12 at idle and the full 32 is not as smooth and straight as I'd like it to be, but I can work on spring adjustment later. Not really worried about that currently.

PS...I also found #4 spark plug to be not very tight. Not sure if I could have removed it with my fingers, but it was damn close. It came out with the weight of a 3/8" ratchet handle.

Lastly, what do you guys think of my spark plug pictures above? Do they look OK, or lean, or other? I really need to buy an AFR gauge, so maybe I'll move that up the priority list.

 

if disconnecting the vacuum advance caused it to run smoother its very well possible you have way too much total vacuum and the roughness is actually detonation and you just aren't hearing the ping.

 

Yes sir, too much advance. I guess I don't have the vacuum diaphragm adjusted properly...I dunno. I'll deal with that later on.

I guess the next thing is to get an AFR gauge on this thing and see what it's doing.

Those plugs look super duper clean to me...too clean even. I wonder if it's running too lean?

 

Those plugs are clean which is good but maybe a little too clean for a carbed engine. The best way to figure out what jetting needs to be changed is with a wideband O2+ gauge.

 

I'd like to know how you put an 11" clutch on a 10" flywheel.(unless it had the wrong clutch disc to start with) And if you continue to have a vibration, I would suspect that crank. Those year's cranks were bad about being weak and breaking in half. The blocks were lightweights too.

 

I have a wideband O2 gauge/sensor on order. It is at least a week away...probably longer.

In the meantime, I figured I should take care of a few other odds and ends before digging too deeply into other things. Picked up a new fuel filter. The old one felt like it was almost plugged...I could barely blow through it, and I could blow freely through the new one. In any case, no difference with a new filter.

I started wondering about vacuum leaks, and thought why not check the carburetor mounting bolts. They didn't seem to be tight at all...almost loose. Snugged them down, and went for a road test and it seemed to run a little better. I should note here that I have one of those cheap plastic 1" carb spacers installed along with the cheap gaskets that came with it. I am remembering now that this was only supposed to be temporary and my intention was to change/upgrade these parts later on. The truck sat for a year and I forgot, I guess.

I stopped by the performance shop this morning and picked up some new carb gaskets and a new Phenolic 1" spacer. I'll install them and test again asap.

To best describe what is going on with the engine now, I'll say that under light load...say cruising between 1500-2000 RPM, it sometimes seems to have a miss, or a vibration...and if I step on it, it bucks a little bit. Above 2000 RPM, it still vibrates, but runs much better. If I stop on a deserted road, and just run it hard through the gears shifting around 2000 RPM, it seems to pull nice and hard without hesitation. Cruising at 1750 RPM, it's not running as smooth as I feel it should be.

That XE256H cam is advertised as having a power band between 1000-5200 RPM, so I would like to think it would run OK between 1000-2000 RPM, but maybe it's just this cam and I'm not used to it.

 

I just bolted the 11" clutch on. The flywheel had the bolt patterns for both the 10" and 11". It is my understanding that both were available on the Bullnose trucks, but that 10" was the more common. The shop that built my engine cleaned up the flywheel and took a skim off it to smooth it out. I forget how much now...0.005 or 0.010" maybe?

I hope I don't have crank issues, but if I can't sort it out I'll consider anything. The engine has had a very easy life with the only exception being the half day it spent on the dyno getting broken in and tuned. Since then it hasn't been over 3500 RPM.

 

The advertised powerband is a suggestion. Lots of the small block cams are based on their use in a 302. In a 351, the power would be a bit lower in the rpm range. But it should be smooth at those rpms. And unless you had a bad vacuum leak, it would still smooth out at not much above idle, depending on how bad it was.