General Category > Quadrajet Carb Talk and Tips
Hesitation when taking off
Cliff Ruggles:
I've seen superficial improvement to idle fuel delivery installing HUGE jets simply because all the to the idle system goes past the jets/metering rods first and the PP is in the down position.
Even with that said Kenth is correct you are making changes to the wrong area.
If you have the engine fully heat soaked, timing is correct, and idle speed is where it should be, adjust the mixture screws for best idle quality at the leanest settings. This is done by backing them out pretty far then turning them in slowly one at a time until you notice a speed change or slight reduction in vacuum or engine RPM's. Then back them up for highest vacuum and idle speed which should be about 1/2 to 1 turn from the "lean tip-in point".
That will be a good place to be for idle fuel. Once you've established best idle quality repeat your "hand over the choke" or tip in the choke flap and see if things improve. If so you need more idle fuel to the mixture screws to correct/cure your issues, not larger jet sizes.......Cliff
Kavesh:
This is a HUGE learning experience for me.
As you quite rightly pointed out the bigger jets will do nothing for the idle, and I can confirm that, after installing the 76 jets.
I have 2 other scrap QJ and had a look at those primary rods and they are all 45B's like the ones on my motor. These carbs are the same part number 7029282 as what came from factory on the Holden 308 engine. Perhaps todays fuel has a bearing on why some of the internals are no longer suited and need other part sizes.
I will persevere with your assistance to get this carb sorted.
Kavesh:
Just an update.
A buddy of mine has lent me his spare QJ, 7026202 to test. This carb is missing the choke pulloff and auto choke assembly.
The good news is that it runs good on my engine. I took her for a short ride yesterday and she went well.
I was told that it has 76 jets and 47 primary rods. Not sure about anything else.
What I found interesting is that on face value the one difference i spotted is that on the airhorn, the upper air bleeds on mine does not have any brass inserts whereas the carb from my mate has brass inserts.
Are these different model carbs designed like this or have they been modified at some point.
Cliff Ruggles:
His carb has been modified. May have been thru one of the commercial rebuilders at some point.
Originally those had 71 main jets and 44 or 45B rods from what I can remembers, and small main airbleeds in both locations.
Your carb is a much better unit all the way around, you just need to get some more fuel to the idle/off idle and it will be fine. Those early units had a plunger/bypass type fuel inlet arrangement and are HOPELESS even with a conversion in them. They also had a weighted secondary air shaft system which gave them a bad reputation for "bogging" when you go quickly to the secondaries.
Every single plug in them typically leaks when they come in here for rebuilding, giving the entire line of Q-jets another undeserved bad reputation for leaking bottom plugs........Cliff
Kavesh:
Thanks for that information Cliff.
O am planning on sending my carb to my mate for him to try and resolve as I seem to have hit a brick wall.
He has gone through several QJ, in the last few years and seem to have a good understanding of their design and workings. He seem to have resolved whatever issues they may have had.
I am hoping that he will find the problem with mine and fix it for me.
The other noticeable difference i found is how responsive the mix screws are on his carb compared to mine. While mine does respond , it usually takes more than half a turn to notice a difference. Whereas on his carb you can hear and see a change in engine speed with 1/4 turn.
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