454 Engine Miss

[Pat Bush]

Hi - I have a 1970 coupe with a LS-6 crate engine that no matter what I do seems to develop an intermittent miss beginning around 2000RPM and getting worse from there. I have tried several things including a complete tune-up, adjustment of timing(currently at 9BTDC -- seems to run best there), and the dwell (currently at 30 degrees).

It has an Edelbrock performer 750 and the vacuum advance is direct manifold. While the car runs at several settings, if I drop the timing to 8 degrees, it runs fine when cold, then surges and won't hold idle when it warms up. At 9-10, it holds idle fine but develops the miss. Mind you, this is not a bucking just a "poof" that generally worsens at higher RPM's. Dwell is set at 30. I checked this engine out in several tech manuals and it says it should be around 8 degrees. I detect no pings in the 8 - 10 range.

Any help you folks can provide would be greatly appreciated!

Thanks - Pat Bush

[Dennis Rosenthal]

Re: 454 Engine Miss

Pat, are you runing a vacuum advance? if so try and disconnect it and see how your problem changes. have you checked total timing at the affected rpm's. I need a little more info for a good diagnosis. Dennis

[Pat Bush]

Re: 454 Engine Miss

David:

Yes, it has a vacuum advance and it is attatched. I of course timed the car without the advance on (vacuum hose plugged)in neutral. I set the idle rpm's this way too and adjust the carbuerator as well.

I have a beast of a time just seeing the marks on the cover and use a flashlight with the timing light to locate them - most are covered by the water pump and idler pulley. The only ones I can see when timing are 6,8,10 and 12. I have never tried running the car without the advance and have no way of reliably knowing what the advanced timing would be at idle or other higher rpm's. I have run it up in neutral after timing it and I again will get the miss starting around 2500 RPM. The only time it happens is under load. It doesn't seem to miss at idle or lower rpm's

I have worked on buckets of small blocks but this is my first big block and my first vette so pardon my stupidity on this one. The engine seems very finicky on minor changes and although the owner said the engine was stock, I suspect there is a bit of a lopey cam. Even at 800 she has a bit of a roll.

Thanks for your help -

Pat