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Turbo engine hesitation off idle
Posted by: dryenko ()
Date: June 17, 2017 06:05AM

Read all the FAQ's but looking for further clarification regarding a large hesitation off idle on test bench / stand.
Rebuilt engine specs, which are not completely stock :
164 CI with +.030 Forged TRW pistons.
GM turbo cam and lifter.
Rebuilt 164 CI Turbo open chamber heads.
Stock correct turbo distributor with excellent cap , rotor and Seth's wires.
1964 style rebuilt 3840830 turbo [ chamfered outlet flange ] spins freely
Rebuilt 0-1769 carb with correct jet and needle per FAQ's
Float level set to stock specs.
Gross jet ball type Turbo specific needle and seat for fuel level control.
Timing set to 24 degrees, static.

Engine starts almost instantly, runs nicely on fast idle and then drops to a <1000 rpm very smooth idle when warmed up.
Turning idle mixture screw in from good mixture / idle position [ 1 1/4 turns out ] kills the engine.
Engine speeds up fine with no black smoke once rpms are above 2000 after hesitation.
Exhaust pipe from turbo is a nice medium gray color.
I did this engine combo many years ago on a 1965 Corsa Turbo to have a engine with a little quicker low rpm acceleration and it worked fine for years.

Ideas ?

Also looking for info for bending a stock 102 HP throttle rod to properly mate up with the Turbo cross link arm. IE , where to bend and how much from stock.
This will save a bunch of experimentation.
FAQ's were not specific in this regard.

Bob C aka Dryenko
Dobson, NC 27017

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Re: Turbo engine hesitation off idle
Posted by: jjohnsonjo ()
Date: June 17, 2017 06:57AM

Most likely is a vacuum leak between the throttle body and main carb. Also a bad diaphragm in the accel pump, or the square block that retains it. It must be flat, if not sand it down. Mine needs 3 psi of pressure or it will do that. Also try opening the mixture jet another 1/4 turn, see if it gets better.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa

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Re: Turbo engine hesitation off idle
Posted by: jjohnsonjo ()
Date: June 17, 2017 06:57AM

Most likely is a vacuum leak between the throttle body and main carb. Also a bad diaphragm in the accel pump, or the square block that retains it. It must be flat, if not sand it down. Mine needs 3 psi of pressure or it will do that. Also try opening the mixture jet another 1/4 turn, see if it gets better.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa

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Re: Turbo engine hesitation off idle
Posted by: dryenko ()
Date: June 17, 2017 07:43AM

Thank You for your quick response :
Opened the idle mixture 1/4 and 1/2 turn with no noticeable response improvement.
The carb body to the throttle body is sealed tight.
All new parts [ thick gasket spacer ]
No air noise there when carb is restricted at choke opening, or change in engine speed when WD 40 or starting fluid is prayed in the area.
Did you mean 3 psi fuel pressure, or vacuum under the accelerator pump diaphragm?
Is there a easy way to check the vacuum amount under the diaphragm?
I am suspecting accelerator fuel shot is the culprit.
Going to fill the carb with fuel and check for that being present.
I also wonder if the bigger venturii carb might be contributing to the issue ??
Maybe Steve Goodman could chime in.
Thanks again,
BC

Bob C aka Dryenko
Dobson, NC 27017

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Re: Turbo engine hesitation off idle
Posted by: jjohnsonjo ()
Date: June 17, 2017 08:23AM

I don't know that you can check the actual vacuum under the pump. I check mine with a hand vacuum off the car at the port the feeds it. It should hold a vacuum. Also its hard to check the accelerator pump when not running because it needs a vacuum to work. Some models have a needle or ball in the circuit. It must be there.

The 3 psi is inlet fuel pressure. With my float lowered and the ball valve the carb fuel level will go too low at 2 psi.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa

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Re: Turbo engine hesitation off idle
Posted by: Nickshu ()
Date: June 17, 2017 08:27AM

I wonder if the fact that you are running a large bore LM YH carb with a small bore early turbocharger could be contributing?

If you want to hear from Steve specifically just send him a PM.

Nick
_______________________________________________________________________
1964 Monza Spyder Convertible #435 - Rotisserie restored - Check out my restoration thread here: [corvaircenter.com]
Recreational Mechanic/CORSA Member/US Navy Veteran
Other cars: 2003 Porsche 996 Carrera 2; Boring daily drivers; 2005 Lotus Elise supercharged (SOLD); 1982 Porsche 911SC (SOLD)
Northern Colorado, USA



Edited 1 time(s). Last edit at 06/17/2017 08:28AM by Nickshu.

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Re: Turbo engine hesitation off idle
Posted by: steve c goodman ()
Date: June 17, 2017 08:36AM

Throttle shaft wear in body

Linkage 'motion' when throttle plate opens but due to wear or excess motion there is no movement of accel pump diaphragm

Both above may contribute to 'lean' off idle


Uneven diaphragm cover over accel pump

Is accel pump check needle movable

EDIT to add: metering rod at incorrect height

best wishes, Steve
Rear Engine Spec. Inc. Golden, Colo.

1962 spyder 3.0L turbo 1965 Crown V8
1967 monza 110/4 1968 monza 110/4
1971 amante gt 110/4
CORSA/RMC/PPCC



Edited 1 time(s). Last edit at 06/17/2017 08:38AM by steve c goodman.

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Re: Turbo engine hesitation off idle
Posted by: Nickshu ()
Date: June 17, 2017 08:43AM

[www.throttleshaftrepair.com]

The 5/16 variety.

Nick
_______________________________________________________________________
1964 Monza Spyder Convertible #435 - Rotisserie restored - Check out my restoration thread here: [corvaircenter.com]
Recreational Mechanic/CORSA Member/US Navy Veteran
Other cars: 2003 Porsche 996 Carrera 2; Boring daily drivers; 2005 Lotus Elise supercharged (SOLD); 1982 Porsche 911SC (SOLD)
Northern Colorado, USA



Edited 1 time(s). Last edit at 06/17/2017 08:47AM by Nickshu.

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Re: Turbo engine hesitation off idle
Posted by: dryenko ()
Date: June 17, 2017 08:53AM

Thanks, Steve for the quick reply..
Throttle shaft is tight and linkage has no motion.
The engine has the hesitation with manual throttle actuation at the carb.
Will recheck the pump cover today.
The check needle, [ little brass one down the hole in the carb body ? ], is free, not stuck.
It falls out when carb is inverted.
The metering rod is as installed from the factory, never been touched except to lightly Scotch Brite the needle to clean it.
I will run the engine after checking the pump cover and see if I see a pump shot when the throttle is activated.
What is the method to confirm accelerator pump shot passage is clear ?
Carb cleaner spray with nozzle ??
Thanks again.
BC

Bob C aka Dryenko
Dobson, NC 27017

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Re: Turbo engine hesitation off idle
Posted by: jjohnsonjo ()
Date: June 17, 2017 08:58AM

I have also had an issue where the top of the accel/metering rod was rubbing in the cavity of the float cover. That took a while to find. Look in the cavity and look for signs of rubbing.

The rod that goes from the throttle shaft to the metering rod control arm may be improperly bent and not started the pump soon enough.

Also, check you boost retard for a vacuum leak.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa

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Re: Turbo engine hesitation off idle
Posted by: 63turbo ()
Date: June 17, 2017 09:52AM

I hope this doesn't sound like a stupid question, as it certainly was not intended to be.... are you doing this test with the air cleaner on? The reason
I asked is that an otherwise well dialed in YH will hesitate and pop without the air cleaner on and wont do it when it is installed.

------------------------------------

Kevin Nash
Friday Harbor Washington
63 Spyder, Daily driver, EFI read about my project here: [corvaircenter.com]
first test start on EFI here:[www.youtube.com]
first official EFI boost test here:[www.youtube.com]
engine less 62 Spyder
Canadian 64 Monza Parts car



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