Author Topic: Basic best practice tuning.  (Read 2605 times)

Offline Pav8427

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Basic best practice tuning.
« on: March 11, 2018, 11:25:53 AM »
So far I have had decent results dialing in my carbs. For some of you with way more experience, this may seem basic, but I am wondering if there is a best practice procedure for initial tune. Because the APT setting has an effect on idle mixture tune and vis versa, would it be best to start with the tip in procedure then move to idle mixture then repeat for best tune? Or would it be better to do an initial idle mixture then tip in then back to idle mixture. Maybe once more to'make sure'. I have also equalized idle mixture screws when done, but recently read on and older post, that that applies to single open holed base gasket. Would that make any noticable difference whether it is a single plane vs. dual plane manifold? When adjusting idle mixture on a dual plane, would it be best to adjust rt-lft, lft-rt, rt-lft or
rt-lft, rt-lft, rt-lft. Or does it even matter? Would this 'best practice' be the same for both a newly rebuilt unit and the re-tune of a running unit? If retuning would you have better results resetting APT and idle mixture screw to a base setting as if it was a newly rebuilt unit?

Offline Cliff Ruggles

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Re: Basic best practice tuning.
« Reply #1 on: March 11, 2018, 12:52:57 PM »
The APT system tunes part throttle fuel, not idle fuel.  If you see any noticeable change in idle fuel delivery messing with the APT then most likely the idle system is not adequate for the engine combo right to start with.

When tuning, I start by setting the idle speed an mixture screws with the engine fully heat soaked.

Then do some heavy part throttle testing to determine the best jet size.

Then dial in the APT for best part throttle efficiency.

Last tune the secondaries for smooth transition and best power.  I prefer to do this at the track with ET and MPH as it's a little difficult to measure that sort of thing by the "seat of your pants"......Cliff

Offline Pav8427

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Re: Basic best practice tuning.
« Reply #2 on: March 11, 2018, 02:43:53 PM »
Went back and read the book(cant count the times I have)and may have found where I thought that. Page 31 states "At the transfer point both idle and primary main system work together to supply necessary air/fuel ratio". By that statement,would the idle settings have an effect on the APT at higher vacuum when the primary rods are 'seated'? Would there be any situations where APT setting(pri rods seated) would have an effect on mixture screw settings? For example: set idle for best vacuum, tip in, tweak APT for best throttle response while driving, retune idle and find mixture screws at different settings than previous.

Offline Cliff Ruggles

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Re: Basic best practice tuning.
« Reply #3 on: March 12, 2018, 05:00:14 AM »
All of the idle fuel must flow past the main jets.  Most idle tubes are only .030-036", some even smaller.  You'd have to be using a jet/rod combination that has less metering area with the rods fully seated to provide a restriction to the idle fuel

The idle system however does feed the main system.  As the throttle angle increases and the engine sees a pressure differential change above and below the throttle plates fuel will be introduced from the transfer slots in addition to the holes under the mixture screws.

So yes, idle fuel has an impact on main fuel delivery to the primary side of the carburetors for "normal" driving.

The big difference for a Q-jet vs many other square flange designs is that it gets fuel much sooner (lower throttle angle) from the boosters due to the small primary bores and triple venture areas created by the rings around the boosters.

The design of the boosters and venture areas also greatly increase efficiency and atomization and one of the reasons Q-jets (and Thermoquads) are excellent street carburetors and rival the very best fuel injection set-ups for efficiency and fuel economy.

I've actually built scores of Q-jets to replace both factory and aftermarket Throttle Body style fuel injection systems and not once to date has any of those folks complained about increase fuel consumption or not as user friendly anyplace.  Matter of fact most absolutely LOVE the switch back to a carb.

Case in point.  A very good friend of mine owns a Chevy 1 ton dually crew cab with a 454, TH400 and 4.10 gears.  It came from the factory with a throttle body FI system.

Despite his best efforts and everything with the factory ECM working correctly he got HORRIBLE mileage and not overly impressive for power output.  We selected a 1985 dedicated 454 Motorhome Q-jet and he installed a factory cast iron spread bore truck intake.  We also built him an HEI distributor with a custom advance curve and vacuum advance.

He had to purchase a fuel pressure regulator and set up a return system for the high pressure factory fuel pump, but overall the swap was relatively easy.  He couldn't believe the improvement that the carburetor and HEI had over the TB and ECM set-up.  Fuel economy went up, as did throttle response and power output clear across the load/speed range. 

The only possible negative was that he had to "pat" the throttle once in the morning to set the choke, otherwise he said the new set-up was better to the FI in every other area.......Cliff