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View Full Version : really knowledgable Qjet people, enter please



WS6
09-15-2006, 04:32 PM
Well I had a qjet built for my 79 403 a long time ago now. Maybe a year. It never worked right from the get go and I couldn't get the builder to do anything about it. So on the shelf it went a my lesson was learned about that person. However I am ready to try and get it working correctly, if I can.

Supposedly the carb if off a 77 400 TA. I haven't checked the numbers and have no idea if that's true. Everything fits up correctly except for one vacuum line to the charcoal canister. I can do without that one. The carb would never idle no matter how much I adjusted the idle screws or throttle stop. It would die rather quickly after starting. I would have to run around the door to get to the engine to keep it running once started. The engine would die like it had a bad vacuum leak. I could keep the engine running steady at around 1200 I believe. I would spray ether all around the carb and it did not respond. So I ruled out vacuum leak. I must say the carb had the best throttle response I have ever seen. I loved that. I just wish I could make it idle.

So my question is, does anyone have any clue as to what I need to look for to try and figure out why the carb won't idle. The builder claimed it idled fine for him when bolted to a ZZ4 small block chevy.

My 403 is modified with higher compression heads(CR about 9.0:1) and a nice street cam with headers also.

formula
09-15-2006, 05:36 PM
might want to check and see if the two tubes underneath are blocked off. If not, they can drain gas straight into the engine and flood it out quickly. You should see two little symmetrical spots filled with some sort of epoxy.

WS6
09-15-2006, 06:42 PM
Yep those are fine. I knew about those from all my other issues with carbs and this engine. I love my qjets when they are working correctly. I just wish they were easier to work on. Thanks.

Skip Fix
09-16-2006, 07:13 AM
The 77s are the good 800 cfm ones BUT have a pretty lean idle circuit, often to get them to idle the throttle blades are opened enough to start pulling fuel through the main boosters. That causes a very variable and rough idle. While idling look down and see if fuel is drizzling from the primary boosters. My 78 400 with just a 228 @ 0.050 cam did this. Popped on a 74 SD carb and smooth idle.The idle down tubes and Channel restriction Orifices can be modified to help this.

Another thing some rebuilders do(including The Carb Shop) which they shouldn't is plugging the built in idle air bypass passages in the main body. This is the equivalent of drilling extra air holes in the butterflies for idle air. Many Q jets have these in the main body on either side of the fuel bowl and some carb rebuilders pop in ball bearings to plug them. Not really a good idea.

A SBC needs a totally different fuel curve than a Pontiac or Olds motor. Once you get to looking close at Q jets you will see how different the factory made them even between the same engoines from year to year. Different jet/rod combinations, power piston springs air bleeds in the top air horn etc.

To dial in one on my son's 81 TA with a warmed over 400 using a '73 750 cfm one I had to use the wide band AFR to get the jet/rods correct. Even starting with what the experts recommended.The wide band could even tel you if the idle is too fat or too lean.

Cliff Ruggles is Pontiac (and other engine) gy that has been doing Q jets for awhile and can recalibrate one. Often runs it on his own 455 Pontiac to check it. He even came out with a book recently on Q jets. Might be something to send ot to him if you can't figure it out.