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I have a 66 Dynamic 88 with a 425. Car runs well but it has always had a slight stumble at around 1800rpm that we have never been able to find. I recently converted to an HEI distributor hoping that maybe it was the old points causing the issue but unfortunately it still remains. Plugs look fine and car idles really well and at a higher rpm on the hwy it drives no issue. I haven't driven it much since the conversion, however when timing it yesterday we noticed that the car ran better without the vacuum advance hooked up. Something we hadn't noticed with the old distributor. Hesitation disappears. I know the new one is adjustable but I am not experienced in this. Timing right now is around 7.5 degrees. Lots of strong vacuum. I am by no means a timing expert and I am not entirely sure on total timing but was wondering if anyone had any ideas. I know sometimes these are preset too advanced. Could this be my problem?
Is this an actual large cap HEI? Most HEI distributors have 21* of mechanical advance built in. So your initial of 7.5*, gives you a total advance of 28.5* which is too low. Try bumping your initial to 14-16* and leave the vacuum advance disconnected until you get the mechanical sorted.
After some trial and error I ended up regapping the plugs. I had them gapped at stock .030. I regapped at .045 and seems my issue is gone. Car sounds much better. Will road test this weekend to see how it drives under load but I am hopeful I will be good to go.
Is this an actual large cap HEI? Most HEI distributors have 21* of mechanical advance built in. So your initial of 7.5*, gives you a total advance of 28.5* which is too low. Try bumping your initial to 14-16* and leave the vacuum advance disconnected until you get the mechanical sorted.
This is an error I made, when I ran a HEI at the original initial timing spec, making it grossly retarded. I gained a ton of torque when it was timed properly.
For the original poster - I would get a dial-back timing light and set timing to 34 degrees at 2500 RPM (vacuum advance disconnected).
Beware that many aftermarket HEIs come with way too much vacuum advance, which will cause pinging.
This is an error I made, when I ran a HEI at the original initial timing spec, making it grossly retarded. I gained a ton of torque when it was timed properly.
For the original poster - I would get a dial-back timing light and set timing to 34 degrees at 2500 RPM (vacuum advance disconnected).
Beware that many aftermarket HEIs come with way too much vacuum advance, which will cause pinging.
Unless the HEi distributor has had the curve changed, setting it to 34 at 2500 will cause it to be too far advanced. The total (34) needs to be set to whatever rpm it stops advancing with vac adv disconnected. .
In case it helps, I’ve been using .045 on my Olds big blocks for years. I tried larger and got misfire even with some super aftermarket mega-spark gizmo. Discarded after a year or 2.
I’m running 16-18° degrees at idle and top out at a total of 28° at 3,000 rpm on my 455’s. That’s hard to get with a traditional HEI. You can get vacuum cans for various cars which will get you close along with swapping weights and springs. See the Lars Grimsrud article on timing for HEI part numbers in case you want to go down this road. The expensive route is Progression Ignitions Bluetooth distributor where you can build your own timing maps to get where you want to go.
If I go past 28 or 30 degrees all in, I get a knock at high (4,000+) rpm. Hopefully you can better gas than I can.
Instead of searching for a different vacuum canister, I made a limiter for the factory 24º advance canister on my 1977 403 HEI. It's set o provide 10º vacuum advance, but can be adjusted for anything between 0º and 24º.
