Soup: Not enough carb connections

[soupman]

I've just replaced the stock quadrajet with one from Summit Racing (nothing fancy). The new quadrajet didn't have an electrical connector (2 prong) at the front that appears to go to some sort of "double pump system". That part of the wiring also seems to go to a coolant sensor in the thermostat housing.

Is that additional pump critical - it seems to run great without it.. although the engine does occasionally diesel after shutting it off...

 

[Greg Ducato]

That is probably the wiring for the throttle position sensor, not needed if you aren't using the computer controlled ignition etc

 

[dyeager535]

Two wire on the passenger side top front of carb? That was some sort of cold engine fuel device. I think maybe it worked only when the engine was cold, and gave it a bit more gas? TPS is three wire, drivers side front. You'd have two plugs if it was a CCC setup, and you'd then need to replace the distributor.

Just part of the complex emissions setups GM saddled us with, instead of going TBI much sooner like they should have.

 

[bandit408]

That connector in front on top is for the mixture control solenoid. The computer will crudely meter the fuel used. It is not a pump.

You can run the mechanical carb but it sounds like it needs a little tuning if it is dieseling.

 

[dyeager535]

You sure about that? Since he only mentions a two wire plug, (but not TPS), and that the two wires go to a temp switch, I highly doubt it's CCC. Just came to me, believe the name was "dual capacity pump".

CCC (and thus MCS and TPS) was quite rare on the trucks as a matter of fact.

For reference, here is a CCC carb: http://www.crankshaftcoalition.com/wiki/images/thumb/8/89/Qjet_with_CCC.jpg/300px-Qjet_with_CCC.jpg

This is a "dual capacity" setup. No TPS, thus not computer controlled: http://farm9.staticflickr.com/8521/8623575270_50dcb230b3_c.jpg

These setups are fairly "rare" (in this case, rare means worthless) because none of the US cars used them. All cars were CCC starting in 1981, so only trucks ended up with a ton of variations from no electrical components, to full blown CCC depending on year, destination, and application.

 

[paratrooper307]

You sure about that? Since he only mentions a two wire plug, (but not TPS), and that the two wires go to a temp switch, I highly doubt it's CCC. Just came to me, believe the name was "dual capacity pump".

CCC (and thus MCS and TPS) was quite rare on the trucks as a matter of fact.

For reference, here is a CCC carb: http://www.crankshaftcoalition.com/wiki/images/thumb/8/89/Qjet_with_CCC.jpg/300px-Qjet_with_CCC.jpg

This is a "dual capacity" setup. No TPS, thus not computer controlled: http://farm9.staticflickr.com/8521/8623575270_50dcb230b3_c.jpg

These setups are fairly "rare" (in this case, rare means worthless) because none of the US cars used them. All cars were CCC starting in 1981, so only trucks ended up with a ton of variations from no electrical components, to full blown CCC depending on year, destination, and application.

This is correct. I have this same setup. It seems to work pretty well, but just more unecessary stuff that cause clutter and potentially fail. The grey plug it just runs to a coolant temperature switch in the manifold, which grounds at 170 degrees, and changes the mixture from a richer cold mix to a leaner warm mix. If your new one doesn't have the plug, it doesn't require it. The CCC carbs (cars) had the blue plug, which worked with the TPS as a crude carb setup with computer control.

Also, check your timing. Dieseling can be caused by incorrect timing or too high of an idle speed as well as a tuning problem.

 

[soupman]

Thanks all, it appears from quadrajet info and from wiring diagrams like it is the dual capacity pump. There is no computer or TPS that I can tell of find. The wiring does in fact look like it wires into a thermostat outlet mounted temp sensor. neither the DCP nor the T/S are wired right now and seems to run fine ... except
1) Slight dieseling occasionally on shutdown
2) high fast idle when cold

In fairness - I didn't adjust a thing out of the box, bolted it on and drove.
I"ll check the timing, choke and fast idle adjustments...

Am glad to hear the DCP is "extra"

 

[dyeager535]

Fast idle I believe is an actual screw. I hate choke adjustment on the Q-jet, but if you adjust a screw, as long as you keep track of turns (use 1/4 turn or something else repeatable) you can easily reverse whatever happens.

 

[Blue85]

Yes, it just gives a bigger shot from the accelerator pump when the engine is cold. If your new carb isn't designed for that, just skip it. No big deal.

Any computer controlled carb will have multiple electrical connections coming off - mixture control and TPS at minimum. For trucks, CCC was only for California.

 

[Chevy911]

If you still have dieseling problems, you can use a solenoid off a truck or car with AC. they have an adjustable solenoid that bumps up the idle when the AC clutch kicks in to maintain RPM. Set the solenoid to extend with key on power, and back off the mechanical screw. use the solenoid to set idle rpm, so when the key is off the throttle plates close off as much as they can. it should limit the amount of fuel pulled in during shutdown.

 

[soupman]

interesting idea... Thanks 911