SBC timing problem

[MFPvette80]

I've got a problem with a 350 in my 1980 vette. The engine is not original, but an LM-1 crate motor from GM... not that it should matter.

Here's the story: The vehicle ran well last season after I had adjusted the timing properly. During the winter I decided to clean-up the motor a little since the engine compartment needed some TLC. I took some digital photos of the distributor and then pulled it out. Then the intake manifold was removed, bead-blasted and painted. No other engine work was done, except for installation of headers.

When it came time to put the engine back together I consulted the photographs and replaced the distributor in the exact position it was in before the work began. New cap and wires followed. The motor fired-up just fine, and I thought all was well. When I put the timing light on, the mark was nowhere to be found. Actually it was over a hundred degrees off. The first thing I thought was that the distributor was off. After pulling it out and replacing it several times a tooth or two in either direction, nothing changed. The timing light is on number one of course.

I then pulled plug #1 and brought the piston to TDC on the compression stroke. The timing mark should have lined up, but again, no sign of it. The rotor was in the correct position however. No surprise there since the motor would run. I check the harmonic balancer to be sure a mark was there, and sure enough it was very visible... from under the vehicle.

Keep in mind that the balancer was never pulled, and no work was done on the cam or timing chain.

It makes no sense to me at all since the timing mark is basically a crankshaft position indicator, and that should not have moved.

The engine runs, but needs to be timed badly. It also starts breaking up around 4,000 RPM (the tach starts jumping all over).

Any advice would be appreciated. Thanks in advance people.

-Mike

PS- Please excuse the fact that I've posted this question on another site also (Chevy Talk). Just trying to tap into the expert advice I've seen offered from both groups.

 

[JohnZ]

Time for "back to basics" to chase this one down:

1. Are you sure you're at #1 TDC (compression stroke - both valves closed on #1, rocker arms parallel) and not #6 TDC? There's no way it would even run with the timing line on the balancer that far off at #1 TDC.

2. Rough check on the timing index line on the balancer - it should be about 10 degrees counter-clockwise (as viewed from the front) from a radial line drawn straight out from the keyway notch (late configuration) or exactly on the radial line from the keyway (early configuration); don't know which way the crate engine is set up. If the index line doesn't fall between these two locations, the key for the balancer may have sheared, although that's VERY rare; the outer ring of the balancer "slipping" is even more rare, contrary to popular opinion - it takes over 1,000 ft-lbs. of force between the hub and outer ring to shear the elastomeric isolator.

3. Verify TDC location for the index line on the balancer and the "0" mark on the timing tab using a piston stop - that will tell you EXACTLY how close both marks are.

4. Set it at #1 TDC (compression stroke) and verify that the rotor is pointing at the #1 wire tower on the cap, and that the wires are correctly indexed from there in firing order (1-8-4-3-6-5-7-2). What kind of distributor do you have?

5. It should fire right up in that configuration, and then you can set dwell (if you have points), timing, adjust the idle mixture screws, and idle rpm.

 

[MFPvette80]

I determined TDC on cyl #1 by "feeling" the compression through a compression tester hose. The rotor position verified that I was close. I replaced the cap and wires on the HEI distributor. With the key in the "on" position I rotated the distributor to get a spark on plug #1. The cold timing worked, but the timing index mark was no where to be found.

I tried the obvious stuff like putting the timing light sensor on other plug wires, but all that proved is that the light worked.

I can't believe that I sheared the key on the balancer. I did put a B&M shift kit in the TH350 tranny before the car was put into hibernation last year. It shifts real hard now, as it's supposed to. Could the "snap" when shifting be enough to shear the key? I'm about out of ideas. I'll see if I can get a mirror down in front of the engine and look for the key.

Time for "back to basics" to chase this one down:

1. Are you sure you're at #1 TDC (compression stroke - both valves closed on #1, rocker arms parallel) and not #6 TDC? There's no way it would even run with the timing line on the balancer that far off at #1 TDC.

2. Rough check on the timing index line on the balancer - it should be about 10 degrees counter-clockwise (as viewed from the front) from a radial line drawn straight out from the keyway notch (late configuration) or exactly on the radial line from the keyway (early configuration); don't know which way the crate engine is set up. If the index line doesn't fall between these two locations, the key for the balancer may have sheared, although that's VERY rare; the outer ring of the balancer "slipping" is even more rare, contrary to popular opinion - it takes over 1,000 ft-lbs. of force between the hub and outer ring to shear the elastomeric isolator.

3. Verify TDC location for the index line on the balancer and the "0" mark on the timing tab using a piston stop - that will tell you EXACTLY how close both marks are.

4. Set it at #1 TDC (compression stroke) and verify that the rotor is pointing at the #1 wire tower on the cap, and that the wires are correctly indexed from there in firing order (1-8-4-3-6-5-7-2). What kind of distributor do you have?

5. It should fire right up in that configuration, and then you can set dwell (if you have points), timing, adjust the idle mixture screws, and idle rpm.

 

[joe1975]

Sounds like it's 180 deg. off!

 

[MFPvette80]

I dug deeper into the problem by removing the harmonic balancer. The key was NOT sheared. The unit is a two-part set-up with the outer ring isolated by a rubber strip. The rubber insulator looked to be in good shape. The timing index mark was 180 degrees opposite from the inner hub keyway. With the balancer in place, you can draw a straight line from the keyway through the center of the crankshaft to the timing index mark. Does this sound right?

Still no obvious sign of what may be wrong. I am now officially out of ideas.

- Mike

I determined TDC on cyl #1 by "feeling" the compression through a compression tester hose. The rotor position verified that I was close. I replaced the cap and wires on the HEI distributor. With the key in the "on" position I rotated the distributor to get a spark on plug #1. The cold timing worked, but the timing index mark was no where to be found.

I tried the obvious stuff like putting the timing light sensor on other plug wires, but all that proved is that the light worked.

I can't believe that I sheared the key on the balancer. I did put a B&M shift kit in the TH350 tranny before the car was put into hibernation last year. It shifts real hard now, as it's supposed to. Could the "snap" when shifting be enough to shear the key? I'm about out of ideas. I'll see if I can get a mirror down in front of the engine and look for the key.

 

[MFPvette80]

Here's another update: I reinstalled the balancer and ran the motor with same result... no visible timing mark. Just to be sure it's not 180 off, I put the light pick-up on #6. Still no mark. This is driving me crazy.

 

[Jack]

The rubber insulator looked to be in good shape. The timing index mark was 180 degrees opposite from the inner hub keyway. With the balancer in place, you can draw a straight line from the keyway through the center of the crankshaft to the timing index mark. Does this sound right?- Mike

I dunno about an LM-1 ... but for most any typical sbc ... NO ... it is NOT anywhere near right ... the timing mark should be ONLY ABOUT TEN degrees from the keyway. Looks can be deceiving ... it's a near certainty the outer ring has slipped.

Here's another update: I reinstalled the balancer and ran the motor with same result... no visible timing mark. Just to be sure it's not 180 off, I put the light pick-up on #6. Still no mark. This is driving me crazy.Mike

No wonder ... the outer ring of damper has probably slipped ... get a NEW damper. They can and do slip for a variety of reasons ... not an everyday thing but not uncommon.
JACK:gap

 

[wishuwerehere82]

You can check the static timing by inserting a hardwood dowle into the #1 sparkplug hole(front drivers side) and "feeling" the piston as it tops out on the compression stroke, rotating the crank by hand. At that point, your timing mark should be darn close to 0 degrees. If your mark on the balancer is not there, your ring has slipped.

If it is there, then pull the distributer and align your rotor with the #1 plug tower. All should be good from there.

I sent you a P.M.

 

[bossvette]

Yes The outer rings do slip, I know from experiance mine slipped "only" 8 degrees but it was enough to give me fits set the timing with a light the car pinged BAD backed it off a little and it ran fine. When I finally pulled the engine I checked the balancer to TDC with the engine torn down to a short block it was 8 degrees advanced.
Craig sr.

 

[Jack]

More SBC timing mark info

More SBC timing mark info:
Face the FRONT of damper and draw a line through Keyway & CenterLine...

Pre-69: Mark is 2° to Left of CL.

69 thru 85: Mark is 10° to Left of CL.

86 thru about 91-92: Mark is 40° to Left of CL.

If mark is 180° from keyway, motor'd require timing tab located at bottom of motor toward passenger side ... you'd have to get under the car near the passenger front shock and point the timing light from underneath.
JACK:gap

 

[MFPvette80]

The mark is certainly about 180 from the keyway now. Since it was OK last year, I think it may have slipped. Of course the problem now is to get a balancer for an LM-1 engine that has the mark in the right place.

 

[MFPvette80]

I replaced the bad balancer that had slipped 170 degrees with a Summit OE unit. No problem now. I will make this recommendation: If you have access to your balancer, check the timing mark (10 degrees counterclockwise from the keyway) and make a mark on the inside of the belt pulley nearest the balancer. I used a triangle file to notch the pulley where the belt won't make contact with it. Since the pulleys are bolted to the center hub of the balancer and keyed directly to the crank, they will not be effected if the outer ring slips. It gives you an additional point of reference for diagnosing any future problems.

I was lucky that mine slipped a whole bunch, and it was obvious that I had a problem. What if the ring only slipped 4 degrees though? It's worth the time to check it out and give yourself a reference point.