Author Topic: Hesitation when taking off  (Read 471 times)

Offline Cliff Ruggles

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Re: Hesitation when taking off
« Reply #15 on: February 16, 2021, 04:23:27 AM »
You mentioned swapping out the main casting.

Did you verify that ALL of the items were the same as the first one?

Lower Idle Airbleed?

Upper Idle Airbleed (if present above the DCR's)?

DCR the same size?

Idle tubes the same size?

Once fully warmed up if you tip the choke flap in or place your hand over it does the engine speed up and smooth out?....

Offline kavesh

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Re: Hesitation when taking off
« Reply #16 on: February 16, 2021, 06:46:06 AM »
Hi Cliff, the float bowls are meant to be to identical. The only exception is that one was made by carter 29282 and the other Rochester. 7009282.

I did not verify anything I guess. I used all the internal parts from the other other float bowl. with the exception of the idle tubes. Are the idle tubes not the same on these units?

I am not sure what the DCR's are, sorry.

When I placed my hand over the carb when it was idling badly it made no difference to the idle, however now that I have swapped over the float bowl, when covering the primary side the motor want to die.

Offline kavesh

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Re: Hesitation when taking off
« Reply #17 on: February 16, 2021, 06:59:11 AM »
Hi Kenth

So the mix screws were out 2 and a half turns on both and I tried opening them further but it made no difference at all.
I have not tried to attach the vacuum gauge recently as it seems I am chasing my tail!, yes it is an automatic.

I cannot advise on rpm either as my truck does not have a tach.

In any event I have not swapped out the float bowl, I am back with the Carter built unit. Base plate and airhorn are reused. All internals were also swapped except for the idle tubes. (How does one tell the size of the idle tubes?)

I have the idle mix screws set at 1 and 3/4 turns out, and the motor is idling fine, could not go for a drive as yet. I suspect the off idle hesitation may be back but can only confirm when driving. When I bump the accelerator initially it does not respond immediately on the second attempt it increases rpm faster. What causes this issue, no blockages found in any passages.

As my airhorn gasket was looking a little damaged, I decided to use 2 gaskets, one slightly thicker one and the original thinner one that was on the carb. Will this be fine or should I go back to a single gasket.

Offline Cliff Ruggles

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Re: Hesitation when taking off
« Reply #18 on: February 17, 2021, 03:43:42 AM »
Poor response when you push the throttle on the first attempt then improved on the second could be several things.  Accl pump check ball leaking or the engine is "loading up" some at idle and needs cleared out.

I would NOT double up the gaskets or use one that wasn't in good shape.

I have .015" and .030" thicker main body to airhorn gaskets for the early units.  A poor fit between the main casting and airhorn could be part of the issues you are seeing.

I suspect the carb is lean on the idle system contributing to the off idle stumble.

Idle tubes are measured using precision drill bits.  A set with #61-80 will have the needed sizes......

Offline kavesh

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Re: Hesitation when taking off
« Reply #19 on: February 17, 2021, 04:09:25 AM »
Thanks Cliff, I will go back to the single gasket on the airhorn. Will use the thicker of the 2.

I wish I was closer to the USA to pick up a new kit from you, but right now cost of shipping to South Africa is ridiculously expensive. It is sad that my current kit from you which has not seen much miles is getting ruined due to trying to fix issues as explained on here.

Without the precision tools would you say it would be a good idea to swap in the idle tubes between the 2 float bowl casting? Perhaps this will give me the response needed. (I am no carb expert so correct me if my logic is not right).

Offline Cliff Ruggles

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Re: Hesitation when taking off
« Reply #20 on: February 17, 2021, 05:46:34 AM »
Swapping parts around isn't the answer without knowing what sizes the sizes of the other items involved. 

A Carter unit sporting the same part number may not be set up exactly the same as a Rochester.

Instead of guessing I measure everything, upper IAB's, lower IAB's, DCR's, bypass air, mixture screw hole size, transfer slot length and location, etc before making any changes.  With the correct information we can make small changes to improve fuel delivery in the areas where the engine is wanting it. 

There are ways other ways we can make educated changes.  If you suspect the carb is a tad lean at light part throttle causing the hesitation right off idle you can temporarily tip the choke flap in slightly to richen things up then lock it there for a quick test drive, for example. 

My "tip-in" test works well to tell us if the carb is lean at part throttle.  Set the fast idle around 2000rpm's then gently "tip-in" the choke flap and see if the engine speeds up?  If it speeds up more than about 200 rpms then the jet/rod relationship is pretty lean for what you are doing and going to a .001-.002" smaller primary metering rod will put some fuel in that range. 

If the problem is at a very low throttle opening transitioning from idle to the main system we can put more fuel there with a slightly larger idle tube and/or opening up the DCR's slightly.

None of these changes are irreversible, as we can also go back to the stock set-up and even the other direction if/as needed........Cliff

Offline kavesh

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Re: Hesitation when taking off
« Reply #21 on: February 18, 2021, 09:10:16 AM »
Thanks Cliff, really appreciate the advice.

I understand that you as a pro builder will make sure that the internals are what they need to be for the client.

In my novice rebuilding limited skill and tools, I am able to do the basic stuff. Understanding exactly how everything works is also not easy.

I do take your advice though and have removed the additional airhorn gasket. I have also read somewhere that raising the float level will richen the mix, which may help my off idle hesitation.
Currently I have my float set at 11mm or 0.43 inches. This level as far as I know is the factory setting.
Is there a direct relationship with float level and rich/lean at idle and off idle.

Offline Cliff Ruggles

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Re: Hesitation when taking off
« Reply #22 on: February 19, 2021, 04:40:27 AM »
Are you using an OEM float or brass?

That float setting is really low for one of those carburetors. 

I'd use the correct OEM nitrophyl float set at 1/4" (.250") and see if that helps some.....

Offline kavesh

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Re: Hesitation when taking off
« Reply #23 on: February 21, 2021, 12:50:37 AM »
I am using the OEM float.

I have raised the float level as suggested and removed the clip from the needle and seat as it was giving me some trouble.

It would appear that I am running too lean at idle. When I close the primary flap slightly and cover my palm over the primary side the idle speed picks up and the idle smooths out.

As I do not have smaller primary rods, but have 76 jets, I am going to try that today. I am assuming that the larger jets will at least confirm that I have a lean issue at idle. Yes I also realise that the 76 jets in all likelihood will be too big for my application but its will at least narrow down my problem and then I can look for suitable rods for my 71 jets.


Offline Kenth

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Re: Hesitation when taking off
« Reply #24 on: February 21, 2021, 01:09:02 AM »
Changing main jets will do nothing for idle leanness.
As we┬┤ve discussed earlier, the idle circuit is a standalone circuit and it supplies engine with A/F mixture at idle speed/low speed.
You will have to look into idle tube restrictions (idle jets), upper idle air bleeds, downchannel restrictions (DCR), lower idle airbleeds, off-idle slots, idle mixture discharge holes (at idle mixture needles) and idle bypass air for suitable sizes for what you are doing.

Offline Cliff Ruggles

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Re: Hesitation when taking off
« Reply #25 on: February 21, 2021, 06:17:48 AM »
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

Offline kavesh

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Re: Hesitation when taking off
« Reply #26 on: February 21, 2021, 09:52:59 PM »
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.