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Old 12-26-2008, 09:46 AM
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Modifying Electronic Controlled Quadrajet?

I want to replace my 600 Holley 4150 with my ECM controlled Quadrajet, but don't know if I can bypass the ECM controlled plunger assembly in the fuel bowl. The quadrajet is # 17085226 for an '86 K5 Blazer 5.0L. The engine is a '95 LT1 converted to conventional intake/carb/distributor using the 242 cam.
It runs good with the Holley, but appears to run out of carburator at higher rpm, so I assume I need some of that 800 cfm the quadrajet has available.
This quadrajet's ECM control isn't totally electronic in that there appears to be a conventional power valve with related primary rods. The electronic part is a solenoid in the fuel bowl with some type of button valve at the bottom. I don't know if this assembly controls vacuum or extra fuel to the primaries, so I really don't know what bypassing this assembly will do/cause. I did run this carb and engine together on a test stand with no connection to this ecm assembly, and with no load on the engine, it responded very well to idle and acceleration... Any experience out there regarding installation/running this combination with a load on the engine--ie. installed in my truck?
Any help or enlightenment is appreciated.

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Old 12-26-2008, 11:19 AM
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That Q Jet needs the ECM to function properly and there is very little you can do to modify it.
Stick with the Holley.
Try your library for this book.
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Old 12-26-2008, 01:05 PM
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Thanks for the advice

I've already read two books on the general subject, but neither have been any real help. I'll visit Amazon for your recommendation. I found Cliff Ruggles' "Rochester Quadrajet Carburetors" a good general source. It did a good job of outlining mods to generic quadrajets, but not what I was looking for. I too like my Holley, but need more flow and don't want to buy another carb...
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Old 12-26-2008, 01:48 PM
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The part you are describing is the TPS.
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Old 12-26-2008, 02:45 PM
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This is about the most you can do but the carb needs the ECM to control it, itīs all about emissions.
This might come in handy, I did a lot of hanger/needle swapping with my Q Jet, but never found the sweet spot.
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Old 12-26-2008, 05:54 PM
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Quote:
Originally Posted by malc
That Q Jet needs the ECM to function properly and there is very little you can do to modify it.
You're half right. Some computer control is required to get the CCC Qjet to operate properly, but the computer only controls the primary side mixture ratio. The secondaries are fully mechanical and respond to the same metering rod changes as any other Qjet. The other thing is that the CCC Qjets used on the 307 Olds and 305 Chevy have the secondary air valve opening limited to 70 deg. You need to file the stop to get a full 90 deg opening.

The reason why you need the ECU is that the primary side metering rods are spring loaded open (fully rich) and the mixture control solenoid pulls them down when energizes to the lean position. The duty cycle of the M/C solenoid is what determines the mixture ratio on the primary side. Without the ECU, the primary side stays full rich. If you want to run without the ECU, get a non-computer Qjet.

Note that the CCC Qjet from the Olds 307 has been modified to run just fine on a 403 and even a 455 Olds. The biggest problem with the stock system is the fixed timing advance curve. You need to burn a new PROM to change the curve. The CCC system has gotten a bad rap. Usually people have problems because they don't take the time to understand how it works or to properly set the system up. The factory Chassis Service Manual has the adjustment process in excruciating detail. It must be followed exactly.
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Old 12-26-2008, 09:03 PM
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Thanks everyone for the help. Per Malc's recommendation, I ordered Doug Roe's Quadrajet book.
It's a good carburator (except for the ecm stuff) The primaries are the big ones, 1 7/32", so in theory it should flow 800 when I take joe_padavano's advice on the secondary air valve modification. I never knew nor looked at how far those air valves opened--the throttle blades fully open, but the air valves are in-fact limited!
Regarding the TPS: This carb appears different from those described in the books. This quadrajet's ECM control isn't totally electronic in that there appears to be a conventional power valve with related primary rods. The electronic part is a solenoid in the fuel bowl with some type of button valve at the bottom. Anyone venture to guess what would happen if I pulled the button valve out of the bottom from under the solenoid and filled in the resulting hole?
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Old 12-27-2008, 07:34 AM
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Quote:
Originally Posted by Turbo-CBX
Regarding the TPS: This carb appears different from those described in the books. This quadrajet's ECM control isn't totally electronic in that there appears to be a conventional power valve with related primary rods. The electronic part is a solenoid in the fuel bowl with some type of button valve at the bottom. Anyone venture to guess what would happen if I pulled the button valve out of the bottom from under the solenoid and filled in the resulting hole?
I don't think you have a CCC Qjet. The CCC Qjet will have a large mixture control solenoid in the middle of the float bowl. It completely takes the place of the normal power valve and metering rod hanger on the primary side. If your carb has a normal power valve and rods, it is NOT computer controlled - at least the primary mixture ratio is not computer controlled.

In the mid 1980s there were some weird Qjet designs, primarily used on trucks, that had a dual volume accelerator pump. A small solenoid valve was opened when the engine was cold to increase the size of the pump shot to avoid a cold stumble. It sounds like this may be what you have. In addition, there were some with variable volume float bowls, again with a solenoid valve that controlled the float bowl volume. Posting the carb number and some photos would help a lot. Also, what did this come off of?
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Old 12-27-2008, 10:36 AM
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Thanks Joe--I'm learning more with every post!

The carburator is # 17085226 for an '86 K5 Blazer 5.0L. Everything looks quadrajet conventional except for that fuel bowl solenoid and related button valve. I'll try to figure out how to post a couple of photos--I can rebuild a carb., but I'll have to learn how to post those photos.
Thanks again.
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Old 12-27-2008, 11:17 AM
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Quadrajet photos posted

I learned something new--I was able to post three good photos of the subject carburator. I haven't yet learned how to attach the photos to this bulletin board, but they are located in the photo gallery for Turbo-CBX.
Does a review of these photos confirm this is not a CCC Quadrajet? Does it appear this carb can be easily modified to eliminate/bypass the ECM input/control?
Thanks Again.
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Old 12-27-2008, 11:47 AM
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Yours looks to be computer controlled, compare it to my old carb.


The blue connector on top is for the "dancing needles" activated
by the M/C in the float bowl.The other, white is the TPS.
When you get the book go to page 59, itīll explain it all.
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Old 12-27-2008, 12:52 PM
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Thanks Malc. Only major difference is your carb is clean
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Old 12-27-2008, 02:34 PM
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That carb is R.I.P.
Bought me a BG Speed Demon
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Old 12-27-2008, 04:09 PM
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Quote:
Originally Posted by malc
Yours looks to be computer controlled, compare it to my old carb.
No, it is not. I just happen to have the 1986 Chevy Truck Shop Manual in front of me. 17085226 is NOT a CCC Qjet. As I suspected, it has the dual volume accelerator pump. The valve for the pump uses the same two pin connector on the top as does the CCC Qjet, but it does not have the three pin TPS connector. 17085226 does have a conventional power valve and primary metering rods.

The only CCC Qjets used on Chevy trucks in 1986 were on California models and had different carb numbers.
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Old 12-27-2008, 04:38 PM
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The valve for the pump uses the same two pin connector on the top as does the CCC Qjet

The two pin connector on the top, blue, is for the M/C solenoid which controls primary A/F metering under direction of the ECM.
So what controls those needles on his non CCC carburetor ?
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