covert689

History - The engine was completely rebuilt and included all new and/or rebuilt parts including the distributor, points, condenser, cap and rotor and new high performance ignition coil. While trying to set the timing the engine would change RPM plus or minus 50 and the timing mark on the harmonic balance was erratic and jumped or bounced about 10 degrees. At idle the engine sounded great and would reeve up nicely, under load the engine hesitated and stumbled, all indications of bad timing. All the standard troubleshooting steps were completed including, vacuum tests, vacuum lines being pinched and plugged etc., timing advance, retarded, timed by ear, timed by light. Nothing seemed to fix the erratic timing mark problem or fix the performance..

On a hunch, I replaced the high performance coil and put the old coil and coil condenser back in place. POW. the problem was resolved no more timing mark bounce, no more RPM change and after the timing was set …. the tires will smoke. .

Posted just in case anybody else experiences this problem. There are several posts on the internet about this issue and several videos are on you tube but not post a cure that I could find.

Hope this helps.

Vintage Chief

Pretty sure an Oldsmobile 5.7L 350 CID is considered a small block - JS

covert689

Yep I guess it is. Maybe someday he/she will grow up to be a big block. I will make sure she gets her veggies.

coppercutlass

Another think to keep in mind is your advance curve. You could have been advancing it to the point the RPM raise was kicking in your mechanical making timing eratic. The RPM should raise when you advance the initial naturally.

I have a pretty stout 355 with a solid lifter cam. Its likes to idle between 850 to 900. I use my old distributor from my last 355 which was low 12 sec mill. It liked to idle about 800 to 850. my advance on the mechanical end would come in right off idle. So when I put it in my new engine I found all kinds of idle quality issues bouncing timing similar to what you posted. I put one medium spring in to stiffin the advance up and viola problem solved. All it takes it 50 to 100 rpm sometimes for the weights to start flopping around specially with a cam with more duration it will bounce around a lot.

oldcutlass

Moved your thread to the sbo section. I wouldn't say you found the cure, but you did narrow it down to a few parts. I had this happen with a replacement distributor, turned out to be grease in the points.

covert689

thanks

olds 307 and 403

Cutlassefi helped the SBO become a big boy, see half an inch does matter.

Hammerdrop

I suspect that Chinesium HEI module.