# Quadra Jet Bog Blues #7040250



## Roqetman (Aug 31, 2015)

I‘ve got a freshly rebuilt ‘70 model Q-Jet on my ‘65. It was running great until I recently had the throttle shafts re-bushed due to a vacuum leak. Now it’s got the dreaded Qjet bog at WOT from a standing start or even a rolling start. Once the rpm’s get up it does great. I tried numerous times adjusting the secondary air doors by tightening the spring tension and again by relieving the spring tension but still snap closed with the engine off but to no avail. After a week of this, the trial and error method is getting pretty old. Does anyone know how to tell me a how to find the sweet spot, not too loose, not to tight but just Right? You know, like Goldie Locks. Thanks, Roqetman


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## Jerry H. (Mar 19, 2020)

Check out the PY forum and contact Cliff R(uggles). He is the quadrajet guru. Not many folks can correctly rebuild a quadrajet.


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## AbodyNtn (May 19, 2011)

Jerry H. said:


> Check out the PY forum and contact Cliff R(uggles). He is the quadrajet guru. Not many folks can correctly rebuild a quadrajet.


Contact info for Cliff? thank you.....


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## Roqetman (Aug 31, 2015)

Ok, so to close the loop on my original post... I did get the tuning spec’s for this model qjet from Cliff’s and re-adjusted the spring tension on the secondary air doors and va-la... the bog is gone. I backed the tension all the way off and then tightened 1/2 turn CW, that’s all it took. I had previously talked to a local qjet expert and watched several youtube videos, they all had we convinced the spring tension needed to be tighter... Not So! The spec’s call for 1/2 turn CW and that all it took to get rid of the low end bog. If anyone else is having issues with the dreaded qjet bog, get the specs and follow them. Thank you Cliff Ruggles for the specs and Thank you Jerry H for the recommendation.


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## BearGFR (Aug 25, 2008)

That's the problem with all the "trick of the month club" recommendations for tuning a QJet. They are NOT one size fits all. An off idle bog can have multiple causes, air flap spring tension being just one of them. Another is the choke pull off diaphragm on the front. It controls how quickly the air valves are allowed to open via an orifice that regulates how fast the manifold vacuum inside it can bleed off. One of the 'back yard' tricks is to hog that orifice out so that the air valves can nearly instantaneously 'snap open' --- which is pretty much guaranteed to cause a hellacious bog because it dumps a ton of air into the secondaries before the fuel flow can 'catch up'. Opening 'too slow' can cause a bog too, similar to having the spring tension too tight. It takes time and tuning to discern what your particular engine "wants" in terms of spring tension and opening rate. There's another thing that can cause a bog too, and this one bit me on my '69. I spent a lot of time tuning on my QJet, working with both spring tension and making my own custom orifices to tune the vacuum bleed/pull off rate, but none of what I did in those areas completely solved my problem. I learned about this through a conversation with Cliff Ruggles. If you have a car that makes some power and tends to leave hard, what can happen is that at launch, the fuel sloshes to the back of the bowl, up through the holes where the secondary metering rods pass through the upper air horn, and over into the secondaries causing a momentary super-rich condition and accompanying bog. The fix is to make baffles to stop that "slosh-over" from happening. Apparently, the 455SD Qjets came with these and there's a recess machined out of the upper air horn for them to fit into, but for some reason mine didn't have one, perhaps because it was a service replacement carb I bought a long time ago from H.O. Racing and had been saving. I made one myself out of a thin piece of sheet aluminum, fitted it so that it fit tight around the rods without dragging on them or hindering their movement, and that cured my bog problem. You can also make a temporary fix out of black electrical tape and pushing the rods through it. That works but you'll have to replace/redo it periodically.

Cheers,
Bear


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## Roqetman (Aug 31, 2015)

Does anyone have a good resource or link for Q-jet specs for a ‘69 400 with ram air III heads. I’d like to know the size of the needle And seat, primary jets and primary rods, and secondary rods? Thanks!


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## PontiacJim (Dec 29, 2012)

Roqetman said:


> Does anyone have a good resource or link for Q-jet specs for a ‘69 400 with ram air III heads. I’d like to know the size of the needle And seat, primary jets and primary rods, and secondary rods? Thanks!


What you have is a 1970 310 HP Oldsmobile carb. Specs are Primary Jets - .070, Primary Metering rods don't get messed with as they are all the same, Secondary Metering Rod - AT which has the .067" rod end.

I recall the Pontiac Q-jets use a .073 Primary Jet. As a comparison, the 1969 W30 Olds option uses .075 Primary Jets, AU Secondary Metering Rod which have the .053" rod end.

The 1969 W30 specs is probably more in tune with what you want. The AU rods will make the secondaries richer. Each .001" of jet size up will provide 1 1/2% change in full throttle fuel mixture.

If you go on any of the carb parts suppliers, they offer stock needle and seats. I think the stock size is .120". You might want to invest in a fuel gauge to monitor fuel pressure. You can mount one inside your car with what is called an "Isolator." https://www.autometer.com/media/manual/2650-1930.pdf

You may want to try the carb without a fuel filter just to make sure the fuel filter is not the bottleneck.


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