I recently dropped in a new GM 350 crate motor with a new 6 3/4" balancer that has the timing mark a few degrees to the left of keyway. I brought #1 piston up TDC and dropped in the new hei distributor facing #1 plug wire. The motor has fired and runs good so I know I'm close on the timing. Here's the problem, I put a timing light on it and the mark is not even close. The balancer mark is at 12:00. I did use an aftermarket tab but even the one GM supplied does not come off the top, they bolt on to the timing cover off to the right. How can it be this far off ?

Vacuum advance?

Since the engine started and ran the mark must be real close to the right place. Besides the vacuum advance, the centrifugal advance will also move the mark ahead. This usually won't be a factor under 800 rpms. Disconnect the vacuum hose at the distributer and plug the end (a golf tee works well for this) back the idle speed down as low as you can and still have the car run and then check the timing. As you adjust the timing you may have to also adjust the idle speed to get an accurate setting.
If you are in the Tucson area I would be happy to lend a hand.

I looked at a balancer I have in the shop, ( I know what cover goes with it) and what I found was the TDC mark was left of the keyway and it takes a cover that has the timing tab at about 12:30 looking at the front of the cover.(The cover also has the tube for hooking a probe on it for a scope.) I suggest you find TDC and see if you have the correct balancer matched up with the correct cover. Nobody is perfect it might be a mismatch from the factory. The people that work there are just like me. If your not screwing up once in a while, your not doing anything! If this is the case you could put a timing tape on the balancer. ( If your balancer is mismatched with the cover.) There are different covers and balancers for smallblock Chevys.

Prior to 1969 all of the SBC harmonic balancers had TDC only two degrees advanced from the keyway on the crank journal. Starting in 1969 they were machined ten degrees advanced of the keyway.

Are you sure which year this damper came off of? (the bolt on timing mark is post 1969 so I know it is late model) The earlier damper still works fine (though it is getting long in the tooth as far as the rubber elastomer goes) and will work either way you will just have to find TDC again and mark the damper and then use the tab to set timing (or file it down to a pointer only and use a degree tape).

Big Dave

Since you purchased a new balancer it should have 2 timing marks on it. One for the timing tab at 12 o clock and one for the tab that bolts to the drivers side of the timing chain cover. Why did you get the 6 3/4" and not the 8"? Are you using the short water pump?

I took the balancer off and took it back to the Chevy dealer where I purchased it ( at the same time I bought the motor) they actually called a GM hotline and they both say that it is the correct balancer for the new motor. The problem is the mark isn't a just a couple of degrees off, it's about 2 inches. The motor ( 290 hp 350 ) actually came with two timing tabs, one for an 8" and one for a 6 3/4" both bolt on between 1 and 2 oclock. I am going to look at the 8" thats on my old motor, I might try using it or remarking the new one, thanks for the tips guys.

Only one mark on the balancer and I'm using a long water pump. I used a 6 3/4" balancer because thats what the dealer sold me as a match for the motor.

Is the car stick or auto. Dis curve is early,try seting total timing. I had this befor. Eazy to fix.