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Old 04-05-2013, 06:30 AM
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Newbie seeking tune-up tips for fresh 383

Mornin' all,

Have done my initial break in on this baby, but now I need to dial it in a bit. I have a slightly lopey cam and it's throwin me off a bit. I seem to have forgotten most of my early years of hot rodding skills from high school those 40 plus years ago, and now with a little more time and money available I figure I would give it another shot. Da Queen and her fresh, tight 383 will mainly be on the street...with a few "grudge night" runs when the challenge arises

Did some searches here on the site and found what looks to be some sage advice, but there is a statement I'm a little fuzzy on. It reads:

The stock GM HEI distributor has 20 degrees of built in mechanical advance.

Your 28 degrees BTDC at 1000 RPM more than likely has got the initial and some mechanical advance together because of the RPM.

With the engine fully warmed up and the choke fully open, dis-connect and plug the vacuum advance hose from the distributor. Bring the engine RPM up to 2500-3000 RPM. Assuming that the centrifugal advance weights and springs have reached the maximum mechanical advance at the above RPM range (this can be adjusted by lighter or heavier combination of springs), set the timing at 32-38 degrees. Do not exceed 40 degrees. What you are looking for is the maximum amount of timing with no pinging. At this point you can check the idle timing and just leave it at whatever shows up. Remember that because the idle RPM is above 800 that some mechanical advance will be added to the actual initial timing. Re-connect the vacuum advance hose. If connected to the full vacuum source the idle will go up as well as the timing. Leave the timing as is and lower the idle RPM to whatever you like. Even with your combo you should be able to get the idle in the 650-750 RPM range if the carb is adjusted correctly.

Now, regarding that pinging...am I to assume this means no pinging at the 2500 to 3000 RPM...or when I bring it back down to idle? I was unable to find a sweet spot for an idle anywhere near the 650 range, but I attribute that to the fact that I am running a Turbo 350...and this cam.

Any help / advice / tips are heartily welcomed...

Da Queen and I thank you

Chris

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Old 04-05-2013, 06:42 AM
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I can help if you can be more specific than "slightly lopey cam". Also need to know your convertors stall speed.
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Old 04-05-2013, 07:20 AM
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you will likely find that a maximum of 36 total mechanical timing is all you can use.The stock H.E.I. needs serious upgrading. Depending on camshaft,somewhere around 20 at idle is usually needed.This higher initial than stock sometimes requires a limiter of mechanical advance.
Like B.C. stated,a little more information would help.
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Old 04-05-2013, 08:31 AM
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re: Help with 383 tuning

Thanks guys...I don't have the cam specs here with me at work, and the convertor is a stock one, replaced when I had all the guts/seals refreshed in the trans. Sorry I can't be more specific. I will post cam specs tonite when I get home.

During the initial break in I found if I set the timing at @ 18 BTDC initial, then with about 1500 RPM I got a pretty smoothe response (34 total), but when I tried to lower the revs it would stay smoothe til about 1000, then drop to about 750 with a sweet sounding lope...but drop it into drive and it would crap out...then really hard to start and had to retard timing about 5 to get it to start.

Thanks for the input...and I will try and be a bit more prepared next time

Chris
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Old 04-05-2013, 08:34 AM
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Vinnie...I have the parts to put the stop on the mechanical advance...this is an MSD Street Fire btw...the quote I pulled up refers to a stock HEI unit and the Street Fire is similar in design (no control box)

Where would you think I should start with limiting the mechanical advance?

Thanks again,

Chris
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Old 04-05-2013, 08:44 AM
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I use 14 advance in my distributor,some people use as little as 10 or less.Every engine will be slightly different.An idle speed of 900-950 is normal with a moderate cam. I think you are close and just need to tweek the carb/ignition.
Will your car start ok with the idle at 1,000?Do you have any run on after shutting engine off?
What are you using for fuel management?If its a carb,give details,IE; jet sizes and power valve sizes,all when you get cam specs.
Welcome to the forum,,,
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Old 04-05-2013, 09:41 PM
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Cam specs

Alright then...here's what the card says:

Comp Cams Part# 12-422-8 Grind # CS XR270HR-10
Gross Valve Lift Int-.495 Exh - .502
Duration @ .006 Int - 270 Exh - 276
Valve Timing @ .006 Int Open - 29 BTDC Close - 61 ABDC
Exh Open - 72 BBDC Close - 24 ATDC

Duration @ .050 Int. - 218 Exh - 224
Lobe Lift Int - .3300 Exh - .3350
Lobe Separation - 110.0

Hope this helps

Thanks again,

Chris
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