Got a question for anyone with the Rochester Qjet (cc). How did you all get the TPS adjustment screw cap out? I tried drilling it out and coudln't get it all because of the rounded drill bit end, it left a little bit of metal around the inside edges of the hole, thus not enabling me to get the tool inside to turn the adjustment screw...

Also, can anyone give me some idea how much .70VDC will throw me off, in terms of throttle response/WOT performance? If I dont get this plug out, I'm wondering how much I'm really losing (ie is it worth messing with), although I'd really like it to be set at .48VDC where its supposed to be at.

Thanks,
-Ben

 

4BBL,

The TPS voltage affects the shift point for TCC lockup on an automatic, the timing advance, air injection and EGR application, and fuel mixture to some extent. It would be good to get it set to the specification, but having it a little on the high side isn't the worst thing in the world.

I've had the same problem as you, in that the pressed-in plug doesn't get completely drilled out using the suggested drill size. I've stepped up the drill size a fraction and cleared out enough material to get the plug out with a pick, then adjusted easily (if you call getting the little 2mm double-"D" tool in there "easy").

One thing to remember is that the TPS voltage is set to 0.48VDC, but at the lowest idle position. Make sure you have the throttle stop screw adjusted for 500 RPM in DRIVE when doing the MC asjustment and other tests. Once all the adjustments are completed, raise teh curb idle RPM if you like. If that sets the TPS to 0.70V, so be it.

If you think those are fun, wait until you get to remove the idle mixture screw plugs.

 

Thankfully, the guy I had rebuild my carb last year had already drilled the idle mixture plugs out, so I can get to those no problem. The only plugs left in now are the TPS plug and lean stop plug (which I just set all the way down when I had the top of the carb off). I wasnt too concerned with setting the MCS travel as of yet because the computer will adjust it anyways. It's just until I get the other things squared away.

Thanks again Vader,
-Ben

 

Easiest way to get the remains of the TPS plug out is with a sheet metal screw.

You need to do something about the lean stop plug, having it botched like that is costing you far more performance than adjusting the TPS ever will. In fact, having the TPS set a little bit high tends to make the car a bit peppier, at the expense of emissions.

Set the lean stop to about 6 turns from bottomed out.

The idle mixture plugs are extremely easy to remove. Cut the throttle plate on either side of each one with a hack saw, such that there are 2 grooves at about the diameter of the plug; and use a small punch to break out the material between the grooves.

The other main adjustments are the rich stop, which also has one of the same sort of plug; and the Idle Air Bleed, which has a sheet metal cover, and is inside the air cleaner, at the highest point of the air horn, just in front of the choke tower. Both of those are similarly much more important than the TPS.

The ECM cannot adjust the duty cycle correctly when the travel of the rods (as set by the stops) is not reasonably close to its normal range. It simply doesn't have enough "authority".

 

Lean stop screw is the one holding the MCS 'solenoid' down towards the bottom of the carb right? right in the middle of the float bowl and the hinge? My thought was that having it all the way down would give it more of a range for the computer to adjust the primaries through. Needless to say, the car has gotten excellent mileage, although it still smells slightly like fumes at lights or stops, idling, but this may be because of no cat, I dont know. So am I backwards here? Does it set the lean stop lower when it is 'up' more?

 

Right, the lean stop is the screw that holds the solenoid in.

Did you epoxy over the plugs in the bottom of the fuel bowl under the primary metering wells? If not, they probably leak, like just about every other Q-jet; and that of course is a direct connection between raw fuel and manifold vacuum, which will make the car run rich under high vacuum conditions such as idle. It's impossible to adjust one of those carbs correctly with that going on.

 

Instead of my customary "thousand words", this might help (Item #65):



Or download this:

E4ME.pdf

 

Thanks fellas. I also have another quick question while you are all 'here'. Concerning the idle speed solenoid (#79), how does it operate. Is it visibly operational when the vehicle is running, if so when? I have never seen it work, ever. And yes, it is hooked up, and no I dont think the solenoid is dead. But perhaps the computer is not working it for some reason? I can find out easily enough how to 'set it', think I've read a few posts on that, I was just wondering when and how it would work. Like on startup, or when? I am thinking I should be able to test it with a 9v battery? I'll go out and try right now while I'm thinking about it.

I guess I just dont understand it's function, as there is a high idle screw beneath the choke for cold starts, and there is a throttle (idle) screw that stops the throttle that sets the idle, am I not correct? Please tell me the function of this thing, I dont get it. I was thinking it was similar to the Idle Air Control solenoid on a TBI or TPI, which the computer controls idle accordingly, but that's what the idle screw was for I thought. As far as I know, the solenoid wont even push out far enough to hit the throttle bracket anyways.

-Ben

 

Ya, I probably should have told you also about this one thing. Please dont laugh and think I'm an idiot haha. The small spring under the tiangular-like 'plate' that adjusts the primaries, in between this and the solenoid, well....I lost that last time I pulled the top off to swap the MCS. I KNOW that I shoulda replaced it first thing, before trying to tune it in, but I have no money right now to just run out and get a carb kit for just this small spring. I doubt there's any possibility of purchasing just this on spring, or finding one (besides going to a junkyard and ripping apart a carb, which I doubt would be allowed!) How much of a role does this spring play in the MCS control? Looks like it helps push the plate up, or return it to the 'up position', or from what I gather, the 'normal' position.

My theory on how the MCS works is it pulls the plate down (which would be lean). The faster, or more times it does this, the leaner the mixture. When the computer wants to richen the mixture, it widens the cycle (which would be less times per given time period) making the plate, or primaries, stay 'up' more, hence more rich of a mixture. Anyone care to elaborate, or correct my theory? Maybe its wrong in it's entirety, please tell me if so.

In all my determination and anxiety to get it tuned in, I guess I totally forgot about replacing the spring. Guess this may be why my Duty Cycly reading jumps all around from upper teens to upper thirties, unless I am once again wrong.

Anyone have any advice or anything to add?

 

.

 

Without that return spring, the MCS will contantly be in the full lean position, with the metering rods nearly filling the holes in the jets. I wouldn't purchase a carb kit just to get that spring, either. You should be able to find a compatible spring at a well-stocked hardware store (I.E., NOT Home Depot - a real hardware store).

It's always the little details that can make the big differences.

 

Incidentally, your hypothesis about the operation of the MCS is correct. The duty cycle is what controls how lean the mixture is adjusted by the ECM. The default position is full rich, as determined by the upper, or rich stop adjusting screw.

The older EGR systems operate in much the same manner, varying the available vacuum to the EGR valve by the duty cycle of the EGR solenoid. Ford uses a similar scheme on many of their EFI engines to control idle RPM, by cycling a throttle air bypass valve. GM also does this for idle speed control on some later small engines, like 3.1 and 3.3 V-6s, I-4's, etc.

As for your other question on the Idle Speed Solenoid, these are typically used to raise idle only when the air conditioning compressor is running. A few engines used them as a running throttle stop, and de-energized them to prevent dieseling on shut down. While I'm not positive, I believe your car should have used it as an A/C idle speed increase, since most of the anti-diesel setups were on mid and late '70s smogger V-8s on bigger cars.

 

Thats what I was thinking, about the spring. I actually looked around for a spring in my dad's stash of odds and ends, and nearly found one that fit, but was not quite right. I'll try looking around for one. Maybe thats why I'm getting really good mileage on the highway....and also why I'm not seeing absolute best performance I can at low-mid throttles.

That makes sense now about the idle speed control solenoid. I didnt think of that. That must be what it is, since my bird was previously equipped with AC. Guess I can just remove that now with no problems. Less junk in the way, less wiring (well, one maybe, but still!)