Qjet on a BB Dodge

[76PW200]

Boy am I glad I found this site! I have taken the unusual route to carburetion on my big block Dodge by throwing a rebuilt Rochester Quadrajet on top and have questions on jetting. Firstly, let me just say that after reading through this site a bit, I can safely say that if you can't answer these questions than I doubt anyone can. Fantastic depth of experience here on what has become a black art these days; setting up a carb! This is not the first time I have married QJet to Dodge engine. This very same engine received a Qjet the first time I owned this truck over 29 years ago..... but that is another story. So here is some info to help with the answers to my question: The vehicle is a 1976 Dodge Power Wagon 200 (heavy). It has a 400 big block, auto trans, full time 4wd, 4.10 gearing. The engine has been rebuilt by myself, spec is mild performance upgrade with focus on torque and flat idle. The compression ratio is approx 9.2:1, stock heads, Edelbrock Performer dual plane intake, long tube headers and 2 1/2" dual exhaust. The carburetor is a 1987 Rochester QJet from a small block Dodge truck, part# 17087175, #72 main jets, #54Y rods and CG secondary rods (info on the tag that came with the carb). The carb was purchased at the Portland, Oregon swap meet a couple of years ago from an older gent that rebuilds carbs and seems to know what he is doing. I assume the carb is rebuilt to original factory specs. The cam is a flat idle, high manifold vacuum grind designed for torque and pulling power up to 5000 rpm. Running a simulation on Comp Cams website, the engine is estimated to make 360hp and 480 lb/ft torque. I live at 2900' above sea level in  Alberta, Canada where it is anywhere from -50C to +35C but this truck will not see the cold weather. It is a garage queen, brought out on nice days only. So, with what I have given for info, are there any recommendations on jetting, rod sizes, spring changes, mods or adjustments? I am going to research suitable ignition timing curves as well so assume that will be set up correctly. Thanks so much for any replies, info, suggestions! I really want this carb to work like i know it can. Many friends have scratched their heads at my choice but my previous life as a Chev owner gave me an appreciation for the QJet that I have never forgotten. Thanks!

Dwayne

[omaha]

...Nice.....I have an '80 Power Wagon with a 318, mild cam and a Q-jet. Runs real good. Those original mopar Q-jets are some good units. Not lean at all like their GM truck Q's of the mid eighties. Mine was off of a 1 ton van and I was able tu tune it for the 318 fairly easy. the jetting was right on for my application although I did mess around with the APT a bit. I'm gonna guess that you may need to open up the idle tubes just a bit maybe  and increase the size of the holes under the idle screws a bit. Maybe up one size on the main jet But, I would not try any of this until you try it out on the truck first without any mods. Then you will be able to get an idea of what it may need. I know 400's are pretty thirsty engines ( I had a 73 dodge Polara with the 400). And remember, the bore size of 4.340!  I think the performer should be a good choice in this application as it has the spread bore pattern. Make use of the manifold heat under the carb, do not block it off. Make sure you know what fuel pressure you have (I think that carb should handle 7psi with the stock needle and seat but you could open up the seat a bit and still get away with the 7 psi) anyway, good luck, and hop to hear from you again!

[76PW200]

Thanks Omaha, that sounds like good advice. I forgot to mention that I am installing an oxygen sensor and gauge to help with tuning as well. I also have an emissions tester that I can use that reads free O2, NOx and CO. What I don't know is what the ideal range is. I have heard it said that ideal is stoich air fuel ratio but does that mean 0% free O2? On the engines I tune a stoich (rich burn) engine is tuned to have 2% free o2. A lean burn is 8%. I have also heard that the best method for tuning a carb (and ignition timing) is by manifold vacuum. Maybe I am over thinking this whole process and like you said, just try the carb the way it is and see how it drives. It could surprise me. 

[Cliff Ruggles]

It's always best to put the carb in service, evaluate it in all areas, then go in and make the needed changes.  Those are excellent units, and you can fine tune part throttle A/F via the APT system without taking it apart.  Just tap the access hole in the airhorn for a 3/8" set screw.

From what I've seen with "rebuilt" carburetors, no matter where they were source at, the power piston hanger arms will be bent, and on that model they MUST be exactly even.

Most likely it will need one or two sizes larger main jets as well, just glancing at your set-up.

What I would recommend is to install the carb and do some testing, if it doesn't perform up to par in all areas, give the shop a call.  I can spec out some tuning parts for it, and some upgrades of the internal parts as well.  Most likely it was built using an over the counter kit, and will need a larger fuel inlet seat with Viton tipped needle, and upgraded accl pump assembly as well.......Cliff

[76PW200]

Thanks Cliff. I will most certainly do just that and get back to you with any questions I have. If the carb requires parts, I hope you ship to Canada.

[Cliff Ruggles]

We ship overseas and to Canada daily, and are familiar with Customs requirements, best shipping methods/least cost, avoiding brokerage fees, etc......Cliff