Qjet Help!

[shimniok]

I hope this is in the right forum...

Been working on getting a Qjet running right on my 86 GW with 360V8. Except for WOT, everything is good: idle quality, off-idle, cruise, throttle response, part throttle, etc.

At WOT around 3500rpm and up in 1st it starts surging; The surging (surge-stall-surge-stall) gets worse if I lock out the secondaries which suggests to me it's not a secondary issue, confirmed by the fact that adjustments on the air valves and secondary rods had no effect whatsoever. What do you think is the most likely cause:

incorrect jetting (lean or rich? The jets and rods appear to be factory and the carb is pulled from a junkyard and is for a '78 472 Caddy but am running on smallblock with mild cam, large exhaust, 4v performer intake)
incorrect float shape (the float gets thicker vertically on the end opposite the needle, rather than right next to the needle)
fuel pump (running generic 4psi electric with return line)
I've set and triple-checked the float level to factory specs for that carb.
The previous carb (AFB) never exhibited this problem.

Carb (and especially Qjet) gurus -- help! :mrgreen:

Michael

[Rogue Racer]

Probably a fuel delivery problem. The QJ suffers from a very small fuel cavity and a voratious appetite at WOT. Things to check:

1) You need a 3/8" fuel line from the pickup in the tank to the carburetor.
2) 4 psi is not enough. 6 psi works for most.
3) There have been four different sizes of needles and seats produced for the QJ over the years - .093", .110", .125" and .135" The largest one is hard to find and is not a universal cureall. In fact, depending on the particular model, it can cause problems. The .110" and .125" are easy to find in most kits and are quite adequate. The .093" should only be used when seeking maximum fuel mileage on a small displacement engine.

[shimniok]

Many thanks! That gives me some good stuff to look into tonight.

Should I be running a return line on the Qjet? I had to use one on the AFB to prevent pressure overcoming the needle/seat especially on trails.

Michael

[shimniok]

Thanks for the input, all! Not much time to do multiple test runs this am. I pinched the return line and did a WOT from stop. This time the nose off in acceleration was far more mild, no surging. As a control I ran again without the line pinched and it fell on its face severely at 3500 at which point I backed out to avoid damage. Mind you this is with the secondary lockout lever engaged, so this is primary only.

Also, if I accelerate while rolling at 2000-2500rpm in 1st, and without the line pinched, I do not get much nose off in performance above 3500. When it shifts after this, it does stumble, however. To me this behavior could support fuel supply or fuel delivery issues.

The reason I say that is that if it is running too rich or too lean, then it should perform consistently regardless of whether I start from a dead stop or start from cruise, right? But if I am somehow sucking the bowl dry that would be more pronounced if I do WOT from a dead stop.

[Rogue Racer]

Qjets did not utilize a fuel "return line" until the mid-'80s with the introduction of the computer controlled carbs. The did use a vapor control line that was routed to a vapor canister. The vapor canister would allow any fuel vapor that accumulated under pressure in the fuel bowl to condense and then return it to the fuel bowl. It is not a necessary system and can be plugged, if you are sure that the bowl is vented to the air cleaner area. There is a vent built in to the top casting around the air cleaner stud. Be sure this vent is open and the fuel vapor outlet and inlet can be plugged.

I sounds like you have the vapor vent attached to the fuel vapor pressure line to the fuel tank. If so, this can create a positive pressure differential in the fuel bowl and prevent the fuel pump from overriding that pressure, thus keeping fuel from entering the fuel bowl.

[shimniok]

The Qjet's on an 86 Grand Wagoneer which all had fuel return lines hooked up to the fuel filter, so I've left that intact.

The bowl vent is hooked up to the stock GW bowl vent line that goes to the emissions canister.

I have a friend with a Qjet on an 87 GW with 401 and he uses a return line and the stock GW mechanical pump, no problems.

In addition to physically removing the return line, I may try swapping on a spare Carter fuel pump (80gph) and see what if anything that does for me. Will try that other float I have. Also someone tipped me on the tendency for well plugs to leak... :-|