QJ off idle lean bog.

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RW TECH
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Re: QJ off idle lean bog.

Post by RW TECH »

F-BIRD'88 wrote: Mon Nov 08, 2021 3:31 am It is critical when using a QJet that the PCV valve flow rate at idle be correct for the job. Too much idle airflow thru the PCV will make the carb run leaner at idle and low speed. This kind of stuff is critical with a QJET.
Wrong PCV valve can really mess with the calibration and drivability.
Also & looking at it from the other extreme, if you plug off the PCV or plug the opposite valve cover vent (to make crank case vacuum) you will have a LOT of transfer slot exposed to keep up with idle air demand and that will definitely cause a lean spot & stumbling right off idle.
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Re: QJ off idle lean bog.

Post by F-BIRD'88 »

True true the primary throttle blades must be in the correct sweet spot position re: transfer slot exposure at curb idle speed. If too far open or too far closed at idle , then when driving, the AFR during the critical off idle transition driving zone can be thrown out of wack.. shifting too rich or shifting too lean as throttle opening angle progresses from idle.

Many of the pretty hot rod PCV valves are way out of wack VS what the engine/carb needs. Trouble is there are no published specs re: flow rates or internal orrifice sizes of PCV valves. They are only classified by OEM applications. There are adjustable and dual mode PCV valves that can help on non stock modified engines.
Sometimes a simple fixed orrifice type is best on cammed up engines. It is a critical tuning element of the system, often overlooked.
You'd want to dial in this external stuff before you start drilling fuel calibration passages in a QJET.. Hard to restore.
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Re: QJ off idle lean bog.

Post by RW TECH »

F-BIRD'88 wrote: Mon Nov 08, 2021 1:16 pm You'd want to dial in this external stuff before you start drilling fuel calibration passages in a QJET.. Hard to restore.
Amen!!
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Re: QJ off idle lean bog.

Post by rfoll »

The main engine I am working on at the moment is a 9.6:1 350 with vortec heads. Iron exhaust manifolds with a nicely done Y pipe single 2.5" exhaust. The cam is a Comp X4 254-11. 210/218/ with 111 lsa, essentially a nice RV cam. The pcv is a new stock replacement. I opened the idle down tubes this morning from .036" to .039. It made a 50% improvement with no other adjustments. I can now leave the driveway without fear of being run over by the late for everything private school parent that come flying up the road at 65 mph in a 35 zone.
So much to do, so little time...
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Re: QJ off idle lean bog.

Post by olescarb »

I would suggest enlarging the ICR (idle channel restriction to 0.055" to enrichen the off-idle circuit. also I would try a V188 Standard motor parts pcv valve which has a lower flow rate to match an engine with a performance cam

I hope this helps Henry @ olescarb
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Re: QJ off idle lean bog.

Post by rfoll »

Thanks for the info on the pcv. It amazes me how hard it is to find specs on stock replacement parts. I have tried unsuccessfully to get specs on the opening rates for the electric choke coils. The aftermarket one on my Impala is ridiculously slow and cools completely after 5 minutes in the store.
So much to do, so little time...
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Re: QJ off idle lean bog.

Post by F-BIRD'88 »

rfoll wrote: Tue Nov 09, 2021 12:42 pm Thanks for the info on the pcv. It amazes me how hard it is to find specs on stock replacement parts. I have tried unsuccessfully to get specs on the opening rates for the electric choke coils. The aftermarket one on my Impala is ridiculously slow and cools completely after 5 minutes in the store.
Because there is no manifold heat.
It matters.
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Re: QJ off idle lean bog.

Post by novadude »

F-BIRD'88 wrote: Tue Nov 09, 2021 1:17 pm
rfoll wrote: Tue Nov 09, 2021 12:42 pm Thanks for the info on the pcv. It amazes me how hard it is to find specs on stock replacement parts. I have tried unsuccessfully to get specs on the opening rates for the electric choke coils. The aftermarket one on my Impala is ridiculously slow and cools completely after 5 minutes in the store.
Because there is no manifold heat.
It matters.
I think it's more than that. I've found that the electric coil for Q-jet is not "one size fits all". Some open slower / faster depending on OEM application. Like rfoll, I've never been able to find good info on which are "fast" and which are "slow".
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Re: QJ off idle lean bog.

Post by olescarb »

Walker products sells a electric choke element for the late style q-jets part # 102-1019 it draws 1-1/2 amps at 12 volts. if opens too fast we have used 10 watt resistors from 1 to 5 ohms inline to slow down how quick they open Henry @ olescarb
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Re: QJ off idle lean bog.

Post by Firedome8 »

novadude wrote: Tue Nov 09, 2021 1:23 pm
F-BIRD'88 wrote: Tue Nov 09, 2021 1:17 pm
rfoll wrote: Tue Nov 09, 2021 12:42 pm Thanks for the info on the pcv. It amazes me how hard it is to find specs on stock replacement parts. I have tried unsuccessfully to get specs on the opening rates for the electric choke coils. The aftermarket one on my Impala is ridiculously slow and cools completely after 5 minutes in the store.
Because there is no manifold heat.
It matters.
I think it's more than that. I've found that the electric coil for Q-jet is not "one size fits all". Some open slower / faster depending on OEM application. Like rfoll, I've never been able to find good info on which are "fast" and which are "slow".
Q Jets and others have duel element choke coils that will heat up at different rates depending on the ambient temp. I think 50* F is the point that it will slow down the rate of the choke heat.
A good test is worth a thousand opinions.
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Re: QJ off idle lean bog.

Post by rfoll »

Update. In a fit of desperation and frustration, I installed my spare wide band afr gauge that had been removed from my truck. I put it off because of the difficulty of mounting gauges in the El Camino without drilling holes in the interior panels. I mounted it on my cup holder. The carb on the car had a really horrible curve. Idle was about 14:1 and it didn't like to idle well any leaner. Transition would go full lean and the car would stall if I didn't lift or floor it. Cruise was about 17:1 or more. Wot was in the 13-14 range. With the ability to read the fuel, I decided to take the carb off the engine on the test stand. It's one of the later large venturi model from an '80 or newer vehicle. It's a night and day difference, the fuel curve is very linear. The transition is still slightly lean, but I can drive through it now. I was able to back choke settings way back. light acceleration drops back to 14 or 15 and the 60 mph cruise is around 16-17. I can now leave the driveway without the fear of being run over by some perpetually late mom taking her kids to school.
So much to do, so little time...
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