[Kenscougar]

I have a 390, 4V. I checked vacuum ( 20 lbs and steady ). I loosened the distributor to adjust the advance. I great a maximum idle speed, tighten down. Taking car for a test drive, I get great launch but have a loud "chatter". I loosen the distributer, set advance counterclockwise, finally got rid of chatter but performance is poor. I can't seem to get no chatter and good performance... Any ideas? I checked the vacuum line leading to the carb and it is attached to the timed spark vac source ( this is a new carb, 600 holley ).

 

[Bad69cat]

Sounds like you are on the right track - test your vac advance to be sure it is really working - just a suck on the tube you should be able to tell. Sounds like it's not advancing.....after that go through the Holley tuning steps. (On youtube for one, or download them off thier site)

 

[390XR7]

chatter= pinging?it happens when you advance the timing too much.need to step up to a different grade of fuel.you gonna be at the partscource cruise tonight Ken?

 

[Kenscougar]

Hi Terry, yes I will be at Partsource tonight.... Around 6:30

 

[1969XR7Vert]

Set the advance using a timing light and without vacuum advance (or as the shop manual says). Timing by RPM and/or ear is not the best way to set timing IMO.

 

[Kenscougar]

Can anyone let me know the "optimum" advance setting for the 390? I am now running "premium" gas. Is 10 degrees about right? Its looking more and more that my advance ( vac or mechanical ) isn't working properly. Can the chatter be caused by the carb settings too? Oddly, when I first installed the carb, all was good. I haven't touched any of the settings. The car starts easily, no smell of unspent fuel etc.

 

[Kenscougar]

Ok folks , I have the answer to timing issues. The timing, at idle, was set correctly (about 8 Degrees). The mechanical advance gave an additional 15 or so degrees at about 2500 RPM, so working fine. The Vac advance gave another 25 - 30 degrees advance at 2,500 RPM. So, way too much advance. Reason - Carb Timed advance port giving too much advance vacuum.

Solution, when changing from a stock carb or having the stock carb rebuilt, check vac advance. If too high, there is an allen key inside the vacuum advance, which is accessed through the actual distributor vacuum port hole, which needs to be turned to dampen the vac advance. Once this was done, vac advance was better "balanced" ( a total advance of about 30 - 35 degrees ), which not only got rid of the chatter, but improved performance and fuel economy. Now the distributor has way more range to play with, without chatter. Ken

 

[1969XR7Vert]

Total advance does not include vacuum so what you describe is far from optimum. 34-36 total advance means initial plus mechanical which should be around 30, not 15, twice what the advance slot says in the dist. If your engine likes 8 initial, then you need 26-28 mechanical. Vacuum should be as much as possible which doesn't result in detonation in any driving condition (accel under load, light accel, etc).

In my case, optimum was 14 degrees initial (what my cam likes), 22 mechanical for 36 total and 19 vacuum for 55 (true) total. The addition of the vacuum advance resulted in a 50% mileage increase for me and better driveability as well.

 

[Bad69cat]

50%! Wow - you could take that savings to the bank!

Sounds like you are on track now Ken - just some tweaking and I bet you will find the sweet spot.....depending on your compression ratio - you may have to battle the ping on lower octane gas by additives....

 

[Kenscougar]

Thanks for the input. Yes, there is some final tweaking to be done but at least now I get good torque, no fumes out the back, and no chatter. I do have way more room now to rotate the distributor to optimize the advance. I am running premium fuel, stock cam, 600 CFM Holley (81570). The nice thing about the setup is that this carb basically dropped on and hasn't been adjusted. Lights up on the turn of the key, not rich, not too lean.. I was fed up with my 4150 and we will see how this one reacts to changing weather etc.

Only thing I have to watch for is that vac advance coming from the carb. Did all 390's for 69 come with an adjustable vac advance ? Just curious..... Ken

Also, I read that 10 degrees of idle advance is about the best?

 

[doctordesoto]

advance is like women--each is different---we ran a 447 cu in fe motor with 42 degrees total w/no vac advance---it also has alot to do with flame travel/path in the combustion chamber---my opinion is that fe motors like advance and one needs to get rid of thec 32/34 deg chevrolet mentality--you just have to experment---i've found using delay valves in the vac advance sometimes helps--they were used in late 70's early 80's systems as a bandaid---like said earlier , vac advance is like icing on the cake---it should only work with little or no load roaring down the road

 

[Bad69cat]

10 degrees seems to get most things pretty close to just right....and I do think most of the bbf vac advance are adjustable (all of the ones I've seen anyways...), maybe there were some Ca ones that had fixed advance..? IDK Interesting that you went from the 4150 though (my favorite), but it does show that it's easy to be overcarbed - or at least over-jetted/etc. The Holley tuning vids are very good and I'd suggest anybody walk through that process to really nail it. (Having a full blown tunning kit on hand really helps too)