Author Topic: Quick question about throttle tip in differences between my two Q-Jet vehicles  (Read 107 times)

Offline Zurbo

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Hello Cliff,

I recently had to rebuild my Q-Jet in my 72 Super Cheyenne BB with one of your kits due to the fuel sock disintegrating and junk getting past the itty bitty stock filter.
Went through the whole fuel system of course cleaning out the tank, and all new hoses including the evac system.

The truck is running better than it ever has in fact so good that I even had to juice up the HEI with a better module as it was breaking up at WOT.

Anyway, I have another Q-jet-equipped vehicle (74 Hurst/Olds) that I'll be getting a kit for as well but I was really curious about something.

I've had the Hurst/Olds for a good 36 years as I bought it when I was 14 and here I am now 50.
I did restore it back in the day so it certainly looks really good still. It is all original under the hood down to every nut and bolt other than the upgraded HEI and normal wear / tune up items that have been replaced over the years.

My big question about the quadrajet setup is that the tip-in on the Hurst/Olds has always been abysmal when compared to any other Chevrolet I have driven.
Even my old 77 Cutlass (403cid) I had ages ago was the same way.
Both had plenty of power once you opened them up WOT but it always seemed the throttle was super mushy on any Olds I drove requiring what seems like quarter to half throttle to get moving properly from a stop when compared to just cracking open the throttle a bit on a Chevrolet including my truck.

Did they tune throttle tip-in differently on Olds compared to Chevrolet?

It doesn't seem like it is only power to weight ratio differences between the Truck and the Hurst Olds
The truck is a 402 BB with TH400 and 3.08 rear end.
The Olds is a 350 SB with TH350 and 3.73 rear end.
Both are fairly even acceleration-wise at WOT.

The only thing I can think of is throttle pedal ratio differences.
Figured I check in with you to see what your thoughts are on the subject.

Offline Cliff Ruggles

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The carburetors used on each engine was specific for the engine as were the distributors.  When you switched to an HEI you may have changed the amount of timing added by the mechanical and vacuum advance.

Olds engines from that period were also low compression with dished pistons and a lot of quench area.  This hurts power, throttle response, etc.  During the rebuild I would recommend freeing up the APT system in the baseplate and adding one of my external APT screws to the rebuild kit.  This will allow for custom tuning the metering rod height without taking the carb apart.  It is a great feature and you'll be able to add some fuel to help with throttle response and power in the "normal" driving range.......Cliff

Offline Zurbo

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Cliff,

Thanks so much for the info on the APT possibilities.
I will certainly have to get one of those kits and add the APT adjuster.
The power piston spring in the truck was mangled up so that spring kit sure helped the truck too.

Both the carbs on the truck and the Olds have a little brass-looking plug swaged in that location so are both "stock".

The HEI I used was out of a 75 Olds Cutlass 350 and was left stock as far as springs and weights so no curve kit was added.
Dad had an old Sun distributor machine back in my younger years that we tested it on to make sure it was in good working order when we swapped it.
Still wish that machine was still around as it was really neat testing different units on that thing and would take the guesswork out of putting a curve kit in it.

I do eventually plan on rebuilding the motor in this H/O to wake it up.
The car doesn't have quite a 100K on it yet and still purrs like a kitten but certainly lives up to the "gutless cutlass" nickname it got before I restored it.

Planning on doing everything possible I can internally to the engine but keep it looking as stock as possible.
There were some interesting tricks they did on this Power Nation episode that looked really promising as far as getting more power out of an Olds 350.
https://www.powernationtv.com/episode/HP2012-13/an-old-olds-repurposed-for-new-power

Turning down the rod journals on the crank to accept Chevy rods really opens up a lot more possibilities it looks like.
The pistons they used don't look to be available anymore but most of the other stuff is.
They squeezed 383HP@5600 and 410Ft/lbs@4200 out of it.

Offline Cliff Ruggles

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Do NOT install one of those HEI advance curve kits....JUNK!

You can modify the vacuum advance or buy one with the amount of timing needed.

Same with the mechanical advance.  Any attempt to use "light" springs will allow too much timing in at high RPM's which can destroy your engine.

The stock curves typically aren't that bad or too far off and you can add a positive stop for the advance and set it where you want at the same time.......Cliff