Hesitation when taking off

[Kavesh]

Hi

I am trying to resolve a hesitation on take off on my quadrajet 7029282 which is on my Holden 308 stock standard engine. Motor has been recently rebuilt with 30 over pistons.

I rebuilt the carb in 2018 or 2019 with Cliffs rebuild kit with a new plastic pull off.

It has always had the hesitation but due to engine issues which was only now sorted I have not tried to fix the hesitation.

The internal parts on my quadrajet are 71 jets, 45B primary rods, AN secondary rods on a B hanger. (Do the hangers make a difference). Float is set at 10mm measured from the toe end of float.
Is this combination good for this engine. I am looking for good driveability and economy.
When cruising at 100-120km/h the motor is running really well.

I would love to get rid of the hesitation on take off. I really have to feather the accelerator to be able to get away from stand still.

I would appreciate any advice on this

[Cliff Ruggles]

Light throttle hesitation is usually too lean right off idle.  I see this a lot building carbs here as nearly every one has the power piston hanger arms bent so the metering rods are not correctly located in the jets.

It can also be caused by the timing not advancing or not enough or too much of it.

Are you using vacuum advance?  If so how much and ported or manifold vacuum source?....

[Kavesh]

I am using a vacuum advance and my source is manifold

When you say lean condition, meaning that my jets and rods on the primary side may be incorrect, as in the 71 jets and 45B rods? If so where I am located its not easy to find replacement parts and right now shipping small parts from USA is just not economical.

I can also confirm that the power piston arms are not bent and the rods are indeed in the jets.

What I have noticed is that I am not getting a good squirt from the accelerator pump. My pump is good and all openings are clear. When I disconnected the accelerator pump linkage from the primary shaft, and activate the pump with my hand, I can see really strong squirts on both sides. But with the linkage back on, the squirt is rather weak. Would this have anything to do with the hesitation I am experiencing?

[Kenth]

Are you setting the idle mixture screws at idle in gear (A/T) for highest rpm/vacuum?
Can you have engine run worse with screws turned out from best setting?
If not, you may have to open idle fuel tubes and idle down channels some, ususally 2-5 thousands of an inch at both locations, start small.

[Kavesh]

Hi

Yes I have set the idle mix screws with the transmission in drive.

My understanding and I may be completely wrong here is that the mix screws does not affect off idle take off, the accelerator pump and jet and rod combination comes into play.

I would not be comfortable to make changes to the idle tubes.

This carb is the original carb from factory so in theory should not require any modifications, unless i am missing something?

[Kenth]

Today the fuels contains less energy and yesterdays calibration can not deliver proper amounts of combustible air/fuel mixture, especially from the idle/off-idle circuit.
The accelerating pump supplies extra fuel when throttle blades opens rapidly, as in WOT from standing still, and the same goes for the main fuel/air supply.
For normal take-off, the idle/off-idle/low speed circuit delivers most of the needed air/fuel mixture.
And since the todays fuels are "leaner" than fuel was when our carburetored vehicles were new this circuit needs some help.

[Kavesh]

Thanks for the replies Kenth, much appreciated and understand that factory settings may not necessarily work with current fuels today.

I will try and do a bit more research into the idle off idle system before attempting any mods.

Would you be able to confirm whether my jet/rods combo is decent for my newly rebuilt  5.0 V8 Holden 308 stock motor.

My primary jets are 71 and rods are 45B

[Kenth]

Primary jets #71 (.070"-.072") and rods 45B is not uncommon to see in units with .050" main airbleeds in air horn and float bowl.

Also,remember that the engine gets most, if not all, of the fuel / air mixture from the idle / low speed circuit at lower speeds and that it is involved in higher speeds, albeit to a lesser extent, at higher speeds, as the main circuit takes over to an increasing degree.

[Kavesh]

Thanks again.

Perhaps a silly question, but how on earth would i determine the main air bleed sizes or would they be a specific size based on carb numbers?
I would not have tools or access to tools to measure these air bleeds

[Kenth]

If you can´t find a pin gage set, some drill bits might help.

1.1811mm .0465"          #56         
1.2mm       .0472"      
1.1906mm .046875"       
1.25mm    .0492"           
1.3mm      .0512"           
1.3208mm .052"           #55   

Red lines marks the brass insert lower main air bleeds   and yellow lines marks the upper main air bleeds in air horn.

[Kavesh]

Thank you

[Kavesh]

Just a quick update, the carb no longer has the hesitation on take off, so that is good.

New problem though, after assembling and starting up, she was idling smooth and strong with the idle screw just touching the primary stop. (not sure what its called)

After about a mile of driving I stopped at a traffic light and the idle was really bad the motor was very rough and shaky. Once above idle the carb performed beautifully.

I suspect there is some dirt in the idle circuit that may have dislodged and caused a blockage.

I don't think its dirt from my tank or lines as I have a nice big fuel filter in line before the fuel pump and its clean.

[Kavesh]

I have pulled the carb apart again and there was no dirt inside. It was clean.

In any event I went through and cleaned again just in case a passage was blocked. This time I even removed the idle tubes and they were clean. I also pushed wire through all the opening.

Sadly though I am still with this terrible idle. When I start up it is fine and as soon as the motor starts to warm up, and you go around the block the poor very rough idle returns. I can confirm there are no vacuum leaks.
This problem began when I swapped out the float bowl as I was not getting a good squirt from the previous bowl which seems to have improved with the bowl in question now, the hesitation is no longer present.
I am wondering whether there may be an issue with the float bowl. I was told by a buddy that perhaps there is fuel leaking internally on the idle circuit.

Please help me diagnose this. I have opened the carb so many times now, the airhorn gasket is looking a little tired now

[Kavesh]

btw my carb number is 7029282 which is an Australian Holden 308 V8 engine carb.

[Kenth]

How far out are the idle mixture screws for best vacuum/rpm?
Can you have the engine run worse with screws further out from best lean idle?
How much vacuum in intake at idle in neutral and in gear (A/T)?
What rpms in neutral and in gear (A/T)?
All of above at normal operating engine temperature.