'70 SS 396 motor tuning - Chevelle Tech
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post #1 of 9 (permalink) Old Jun 5th, 20, 11:08 AM Thread Starter
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'70 SS 396 motor tuning

Hello guys, it has been a while since I have been on here. I am looking for some knowledge from the engine tuners on here to assist me. My BB has 10.5 compression and I am running a Lunati VooDoo 60203 hydraulic flat tappet cam with a reworked Q-jet and a re-curved distributor. So here is the problem: when I back off the gas it tends to buck, I have to give it gas to stop the bucking or push in the clutch. Might this be caused by the carburator circuits out of adjustment? If anyone is running this cam and compression, where would you set your total timing? I run whatever the premium gas stations sell as premium gas 91 octane and I am pinging on acceleration, I am going to try octane booster, but my timing probably needs to be adjusted. Sometimes my hot motor does not start till it cools down, I may need a new stronger starter, but the timing could be creating this condition. Any thoughts? Thanks,
Roland

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post #2 of 9 (permalink) Old Jun 5th, 20, 12:02 PM
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Re: '70 SS 396 motor tuning

I was working with a 427" SBC that bucked and surged and you could not take off from a stop without that happening no matter how you slipped the clutch.
Unknown internals.
I messed with it for 3 days trying every tune and jet combo and timing curve. It still bucked.
I noticed a hint of sweet smell from the exhaust and mentioned it to the customer.

He said he got the car at an auction and it had race fuel in it and he had just been using 91 octane pump gas.

Off to the race fuel shop i went and got 5 gallons of 112 octane.
Drained the fuel cell and poured in the good stuff.
No more bucking and surging. It ran as smooth as a new car after that.

Fix your Too much timing issue and see if that fixes it.

Depending on a lot of things your compression and cam choice will not run like another of the same compression and cam.
Surface finish, chamber shape, port shape, the list goes on and on.

If you are running 200+ degree coolant temps try to get them down 180--195 and see if that helps.

I built an 11:1 396" with 23[email protected] .050 cam 108LSA it has 210 psi cranking pressure and no bad manners at all.
I ported the heads roughed up the runners and all sharp edges in combustion space have been smoothed.

It runs 91 octane with 22 initial and 38 total timing 180-185 coolant temps. 4200 lb truck.

That does not mean your build will like that timing though.

If you feel it is carburetor related then add some jet to it or twist the idle mix screws out some more.
If you back off the throttle all the way you should be in the idle circuit.

Easy enough to try.. I would turn them out 1/2 turn each and retest then 1 more turn each.

Usually 3 turns out from fully seated is a good spot to start from.
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post #3 of 9 (permalink) Old Jun 5th, 20, 12:41 PM
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Re: '70 SS 396 motor tuning

Do you have a vacuum advance distributor ? What RPM does it start bucking at ?

Bill Koustenis
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post #4 of 9 (permalink) Old Jun 5th, 20, 1:31 PM Thread Starter
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Re: '70 SS 396 motor tuning

the bucking really happens when I come off the gas.

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post #5 of 9 (permalink) Old Jun 5th, 20, 2:26 PM
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Re: '70 SS 396 motor tuning

Bill may be onto something with too much vac advance.
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post #6 of 9 (permalink) Old Jun 5th, 20, 2:40 PM
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Re: '70 SS 396 motor tuning

Sounds just like BillK is onto, the dreaded "no vacuum advance Top Fuel/Funny Car street timing disaster".

Please advise
What:
Initial timing?
vacuum advance added timing degrees?
Vacuum advance is running on full manifold vacuum, or PORTED vacuum?
Total timing at IDLE?
Mechanical advance degrees?

All timing readings should be done with a marked balancer and a timing light with the "DIAL BACK" FEATURE LEFT OFF.

If you are unaware on the vacuum advance degrees stop you will need, send an email for a FREE, NO ADS vacuum advance modifications package on how to get the vacuum advance woring correctly, with a stand alone degrees stop plate, one screw, one flat washer.

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post #7 of 9 (permalink) Old Jun 7th, 20, 10:17 AM
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Re: '70 SS 396 motor tuning

try and disconnect vacuum advance and plug line coming off carb. and take car for a test drive. This is a simple test to see if conditions repeat. If drive ability improves you need to either replace vacuum advance unit or you can also leave it disconnected. You might want to give timing settings of initial and total for the guys here, maybe something is not quite right.
Too much initial timing will cause hard starting when hot, as will weak starter, battery and/or cables.
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post #8 of 9 (permalink) Old Jun 10th, 20, 8:20 PM Thread Starter
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Re: '70 SS 396 motor tuning

I usually just go to 36*-38* total timing on my light. i have not tried initial 10* btc. I can try that. I am not sure about the conversion of static timing to total timing. i usually just set mt timing light to the desired * for total timing. I can go back to setting as just 10* btdc. Advice is requested.

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post #9 of 9 (permalink) Old Jun 11th, 20, 1:33 PM
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Re: '70 SS 396 motor tuning

Another disaster, using a "dial-back" feature on an ANALOG ignition system.....DON'T DO IT, LEAVE THE DIAL-BACK OFF, MARK THE BALANCER. DIAL BACK IS ONLY FOR COMPUTERIZED TIMING SYSTEMS, LIKE EFI.

Now, TOTAL timing is the combination of both INITIAL timing and MECHANICAL ADVANCE DEGREES. Example, 10 degrees INITIL, 26 degrees mechanical is 36 TOTAL degrees. Both settings are essential to getting the curves correct. Simply setting the timing to 36 degrees tells you, and we, NOTHING about the INITIAL.

An example of that is on a Pontiac engine, later emissions, last of the analog ignitions. Total was 36 degrees, BUT, the distributor had 40 mechanical degrees from the factory, so, what was the initial if set with just the 36 degrees total? Well, 40 minus 36 is 4 degrees, and since this is not within the 36 degrees, initial timing was a whopping MINUS 4 degrees AFTER TDC (retarded). YES< a Pontiac isn't a Chevy, but tmng is timing, and this is only an example of what bad can occur when we don't know all the particulars, for either engine.

So, what is the initial, what is the total, we already know, 36, and what does the vacuum advane add when plugged into full manifold vacuum??? This will tell us what you have for the other parameters we need to know to help.
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