Hello, My 350 has been running a little warm, the exhaust valves where white, and I have a very narrow band of timing adjustment. Seems as though the timing goes from sputtering to running good to sputtering withing....like 1.5 inches of distributor movement..

The car runs 8.7's 1/8

I was just wondering. I replaced the cam sprocket and new chain. I did NOT replace the 1972 crank gear..... someone has told me that this could be a problem.

Why in the heck would a stock replacement cam sprocket be different than the one that came original.????

Different tolerances in machining??? i'm just wondering now if it would be worth digging into the engine to replace the timing set..........

Also...if I hook my vac advance up to manifold vacuum... the engine SURGES BAD... even with low timing.... I have used 3 different advance diaphrams...

I now have the advance locked out at 36 and no vac advance.. the car pulls 14.... for vacuum........ 230/230 cam.

Rob..

car runs fine...a "little" sluggish but.. thats the victor jr ....

Just wondering if its worth changing out.... and WHY the gears would be different.. thanx

 

location of the timing mark on the gears

 

But why would the timing dots be different... What ... the company makes a "replacement" part... that isn't the same ?????

 

The stock sets had some retard built into them from what I hear. Not only that but with any sort of chain/sprocket drive all the parts must be replaced together. The worn crank sprocket will cause the chain to strech and will also accelerate the wear on the cam sprocket.

 

The new stuff comes with both gears, right? So why not change them so you'll have a matched set?. If you don't have a puller, do what the old guys use to do. Locate the tooth that is directly over the keyway, the machining for the keyway creates a stress point there. Take a chisel, put it against that tooth (at the base, closest to the keyway) and whack the crap out of it with a BFH. The gear cracks and you can pull it off by hand. It takes longer to tell you about it than it does to do it.

 

You might alos be having some issues with the distributor it self.
I had a problem just like that with my truck replaced the distributor with a new not rebuilt one and the timing stoped bouncing around and I was set the correct timing.

 

Not all gears have all the teeth of the sprocket in the same relationship to the keyway; in other words, it might be impossible to avoid having the cam installed SEVERAL degrees off from where it's supposed to be, by mixing and matching parts like that.

Yes replacement ones can be different from factory ones, and from each other. Life is like that sometimes. Especially in this case.

Sluggish running could easily be the result of the cam not at the correct centerline, which would be a direct consequence of the gear mismatch.

The thing with the distributor has nothing to do with the timing chain or gears. Sounds more to me like the plug wires aren't in the right place; and as you adjust the ignition timing close to where it should be, the rotor contact is no longer pointing at the right dist cap contact. Or, the dist itself is flaky, which is less likely than an assembly error but still possible.

 

The bottom gear is not the problem, as long as you're not trying to put a double-roller chain over a link-belt gear or something idiotic like that.

On a SBC the crank keyway ALWAYS points up and to the driver's side of the block at exactly a 45* angle when the #1 piston is at TDC. The TDC mark on the crank gear will always have the "dot" pointing straight up at that point. And that's what you always line the dot on the cam gear up with anyway. Also, cam gears always have the "dot" in the same relative position vs. the cam dowel becuase they must- SBC cams always have the dowel in the same spot to index the cam so the valves open and close when the cam card says they will.

It's certainly possible that there are SLIGHT differences by playing mix-n-match with 2 different chain sets, but we're talking about maybe a few degrees of difference at the most. Not likely to be cause of a really bad running engine.

 

You're exactly right, in part; the keyway, and the dot, are ALWAYS in the same place, compared to the crank. However, the teeth aren't. They can be (and often are) shifted around the gear. So even though the keyway is ALWAYS lined up with the #1 throw, and the dot is ALWAYS straight up when the #1 & #6 pistons are at TDC, the teeth (and consequently the chain location) can vary with respect to the crank.

And yes, the dowel is ALWAYS at the same spot with respect to the cam proper; in fact, it's the thing they measure off of when grinding the cam. But again, different cam gears, just like crank gears, can have their teeth at a different place with respect to those fixed reference points. And in fact they would have to; since any set, when used as a complete set (except for a 71-up factory set) will put the cam in the same place with respect to the crank, no matter where they happened to put the teeth.

 

Many moons ago I installed a new crane cam in my big block 454 Fbird. Being the lazy cheap butz I am, I didn't bother to replace the crank gear when I installed the new top gear and chain.

Well after finding the motor didn;t run the way I thought it should, tearing the front of the motor down and degreeing the cam I found the cam was 14 degrees advanced out of spec.

Still being the cheap *** I am I ended up jumping the cam gear back one tooth and advancing the cam a lot with a offset bushing to get 'er close to the cam card specs.

Lesson learned: get the whole "matched set" when you buy a timing chain and gear set. And install the whole thing.
It makes a difference.

Doesn't guarantee that your cam will be exactly on cam card spec when you install it, but really helps.

If your motor isn't running to spec that would be a good place to look.

P.S. If the cam you have is the 287-287 230-230 /.480-.480 ground on 109 it goes in 2 degrees advance ( intake CL 107).
If you want a little more low end, after establishing the actual intalled C/L you could advance it to 102 or 103 intake C/L. this will make to power come on a little sooner.
(more low end)

 

By the way 1.5" is a lot of distributor movement. Lets move into the modern era and get a timing light.....

 

RB- Not to start an argument but every chain set I've ever seen has the "dotted" tooth of the crank gear pointing dead-straight up with #1 at TDC. The dot's always in the middle of the tooth and the tooth always points straight up. The dot's also never between 2 teeth.

Now if you have #1 at true TDC, you push on the bottom gear with the dot on top and it's obviously not pointing straight up then yes, you have a bottom gear that's clearly got some advance or retard built in it, or was manufactured with the keyway mis-indexed. But I haven't run across that with stock/stock replacement chain sets (not that I've seen everything that exists). And in this case it would have always been like that with the orginal timing chain anyway. Clearly it ran with this same orignial lower gear with the previous cam

Timing chain errors can definitely happen, like Midnight ran into, above, but I'll almost gaurantee that new top gear he got had the dot off by 1 tooth right out of the box or becuase the cam dowel hole was mix-indexed, not becuase of some bizarre interaction between the stock bottom gear and new top gear.

 

Maybe it's just a case of a brand new chain and cam gear (made by company "A") not interfacing well with a worn out crank gear (made by company "B").

 

Having just installed a new timing chain in my new motor I can tell you for a fact that the teeth can be in different spots on the gear. I had thought about cheating and not replacing the crank gear cause I didn't have a puller, but when I compared them the teeth were off.