While literally waiting for some paint to dry I got looking at the poor old carb and after fixing a broken screw in on the primary flap I got looking for some stuff I've been following on some other posts.
(can I send this carb to someone to hold onto and prevent me from looking at it until I'm ready to bolt it back on)
The back flaps move smooth but seem to "stick" and will stay open if you trip them by hand until you trip them back



This spring isn't actually touching the pin in any position. I see it's adjusted by the slotted pin on the side but it seems to be totally turned off. It doesn't seem very heavy so I'm not sure if it actually does much but if they put it there I'm figuring I should at least try to set it to do something.

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Should there be something in that hole (screw, pin, rivet ????)

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Anyone see anything hideous, disturbing or insightful in this picture that should be remedied before going back together?


Here's the manifold that it was on (being changed now which is why the carb is off) It had a totally open gasket with a shield plate so everything was basically open to everything else


Thanks
Mooser

 

That spring adjusts when the secondaries open. Lars has a good explanation on adjustment about 10 years ago on a thread here: http://forums.corvetteforum.com/c3-t...justments.html. Basically, you loosen the screw shown with an allen wrench a half turn. You then adjust the spring with the slotted screw so it is just barely touching the shaft. Give it 3/4 of a turn to tension the spring then tighten the allen-headed screw to lock it in place. Drive an adjust as needed.

Just did this and my secondaries now open and contribute how/when they should. Thanks Lars!

 

That's explains this spring perfectly and thanks for the link (I didn't find that one in my search, using the right terms would have helped, I also did not know about how the secondaries got their initial shot of fuel until reading that link.)

I've got several of the Lars papers and they really help but I think I need the "Q-Jets for dummies" version that tell you how to get the carb to the town that the ballpark is in, then work on tuning it.

The more I look at it the more I wonder how I got it to actually run in the first place.
Mooser

 

Your pic #2 shows a manifold vacuum port. You are missing the small pipe.

 

Mooser, a rebuild kit and some elbow grease would have it looking and working looking like new. All of the adjustments are ouitlined in the instructions that come with the rebuild kit, the task is a lot less daunting than it apears. Also, some one correct me if needed, but, that port where there is no nipple, is sometimes not drilled into the carb all the way if the original did not need it. Look in there and see if it's blank, just a possibility.

 

Was / is that there on all model years? Believe it or not this has actually been on the car running for the last several months

I'll have to check and see if it's plugged, I can't believe I've had a full manifold leak and still got it to idle.

Thanks Paul
Nice clean carb you've got there BTW

 

I'm desperately trying NOT to take it apart.
I have this nasty problem of find how mechanical things work waaaaay too interesting and I'll end up spending time blowing air down different passages and feeding wires through holes to see where they go and then cleaning and re-plating things and the next thing you know it'll be December and I'll have missed what's left of the season.

But these hanging springs and empty holes etc are just too hard to ignore

I figure that hole must be blocked (rightly or wrongly) surely I never got idle with a full leak there (I hope)
Mooser

 

You sound like a mechanic, a good one in the making, only thing lacking is experience and only one way to get that. That port would only leak vacuum above idle since it would be ported vacuum.

 

You need to install a 1/8" brass tube to that vacuum port and/or cap/plug it. That is a 'manifold' vacuum source, and left open your idle quality and mileage will suffer significantly. If your distributor vacuum advance can is not plumbed to a 'ported' vacuum fitting, you will benefit from switching it to a 'manifold' fitting...such as the one you need to work on.

 

Nope - that's a full manifold vacuum port, and will cause a massive leak at idle. Buy a short length of brake tubing (the pre-flared stuff with the fittings on it) from NAPA, cut the flare and fitting off and use it to make a tube for that vacuum port. You can then use that port for your distributor vacuum advance.
Lars

 

QJets are not what I would consider to be a GREAT carb, but they are a pretty good carb if done right. The small bowl can be a problem in some higher-performance applications....

Due to the very small primaries and the very large secondaries, AND the numerous adjustments you can make on a QJet, using the larger 850 CFM QJet on a SB Chev engine is not only do-able but is desirable.

NOTE: The Buick/Olds/Pontiac/Caddy QJets have the gas coming straight off the front of the carb, not at a right angle like the Chevy. You will have problems clearing your thermostat housing if you use a BOP QJet on a small block chev. HOWEVER... the BOP QJet parts are usable on a chev QJet.

The electronic QJets have minimal performance applications. Nuff said...

BUT.. After you have taken your QJet apart, consider using epoxy or JB weld to seal the unreliable lead sealed plugs on the carb, especially the ones on the bottom of the throttle body. QJets are notorious leakers, this is why some rebuild kits come with that useless sponge in them.

Check the top cover of the carb for a level surface. Nearly every QJet that I have rebuilt has had the top cover warped. This is because two of the four bolts that secure the carb to the intake pass through the top cover, and we all have seen what most people do when they think their engine is leaking vacum: they tighten these four bolts without also tightening the screws that also secure the top cover to the throttle body. This WARPS the top cover. I carefully remove the four air bleeds from the top cover and carefully file the top cover level with a quality file. File in a circular motion. Use a machinists rule to check for level.

There are MANY secondary needle sizes, MANY secondary hangers with the holes for the needles higher or lower (determines when gas will start to flow into secondary jets), and many primary needle/jet sizes out there. Most QJets have "smogger" sized secondary needles/hangers, the good ones have very small tips (more gas) and hangers with higher holes (pulls the needle out of the jet faster). Its always a good idea to collect junk QJets just to harvest these parts.

The air valve ("flapper") spring adjustment described before is critical to secondary operating right. If it is too tight, you get little to no secondary operation. Too loose will bring your secondaries in too soon and bog your engine. This is something you must adjust and then "road test." You cannot do this accurately with the engine not under load.

Your primaries are hung from a "power piston." It is adjustable when you have the top cover off, and MANY QJets have a hole in the top cover already there directly above the power piston adjusting screw. I will thread this hole and use a plastic screw to seal it but still allow access. NOTE: Your air cleaner base rests exactly where this hole is, so the screw you use to seal the hole must be level with the top cover surface.
Two things about power piston/primary needle/jet adjustment:
1. The power piston has a spring under it that works against manifold vacum. Vacum pulls the piston down and therefore the needles down into the jets. When your engine is under load, the loss of vacum allows the piston spring to pull the needles out of the jets. If you are running a cam that does NOT have "stock" vacum characteristics, the needles will always be out of the jets and your engine will always be loading up. One way to deal with this is to cut one coil from the spring, test, etc... (This is a pain in the thing you sit on) The BEST method is to get ahold of the vacum rated QJet power piston springs Edelbrock offers.
2. The power piston adjustment screw determines how high your primaries will be riding in your jets. This is another adjustment that you must road test for crisp throttle response, BUT you can get in the ball park with the engine NOT under load.

IF you are running a cam with non-stock vacum characteristics (less than 18-24 lbs) one way to improve your idle characteristics/adjustment quality is to drill a VERY small hole near both idle jet apertures in your primary butterfly valves. I would do this last, as it is a "permanent" fix.

 

 

With the car idling hit wot. The secondaries should barely open a 1/4-1/2 inch. Test drive the car and if it boggs then tighten the spring a tiny bit until it stops bogging. This is if everything else checks out of coarse :-)

 

Ok lets everybody take a deep breath and relax.....
The car has been driven with that passage open all summer. idles nicely at 600+ in gear

The hole on the side doesn't go anywhere.
Either it's blocked or it was never opened. I just washed it out and tried pulling a vacuum (hand pump) and it does nothing, hit it with compressed air and it is solid. Hole goes in about a 1/2" then stops dead.

I can't see inside, perhaps someone has blocked it off with a lead ball or something deep inside.

Did ALL q-jets have this port open or were some open and some not? I'm looking around at some images and notice that some do not have that tube and some do.

Carb s/b 1972, 454 auto. Number appears to match according to the list Lars has, but may very well be off of a dump truck.

Carb is on of the ones built by Carter

Carb may or may not have been rebuilt prior to me.

I should have it back on later today or tomorrow and I'll adjust that spring, it being off might explain a few oddities I had during the summer after some ummm experiments with the upper end of the tach.

Thanks for the insite guys
Mooser

 

Nope, just know that if I take it apart things like this start to happen
http://forums.corvetteforum.com/c3-g...r-problem.html

And I'd rather spend that time driving right now

Mooser

 

Mooser -
Not all Q-Jets used the nipple on the passenger side. If it was not used, there was no hole drilled at all into the casting. Rather, there is a slight "boss" on the end of the casting standoff that centers into the slotted hole feature in the choke pulloff bracket. If the hole is drilled, it was an open hole. I've seen people plug the holes (for whatever reason), and that appears to be what's going on with yours. When I rebuild carbs with this feature plugged, I always open them up and insert the tube so the vacuum port can be used.

Lars

 

Now there's a solid answer.
Should I be able to see where the hole comes out on the inside somewhere?

Thanks for hanging around
Mooser

 

Once I get to the workshop this evening, I can post up some photos of the vacuum path and where the hole comes out. If the wrong throttle plate gasket is used, it will block the vacuum path - the photos will show it clearly...

Lars

 

I will be very interested in what you report! A couple of years ago you advised that that port on my #7044206 Q-Jet was manifold vacuum. I had the carb rebuilt locally and gave it a try. No dice. No vacuum at all at any RPM. So like Mooser I probed that port with a wire. Mine is, unlike his, a free flow orifice until you get to the end. Then there is a blockage. I suspect the wrong "generic" plate gasket was used and that is the problem.

I wimped out and used ported vacuum (driver's front) and the car runs very well. But I did consult Cliff Ruggles and he sent me the gasket shown. So is that the correct one? I have yet to use it. It would be nice to try intake vacuum and see what it does for the car. I can get off my butt and do this job. Just darn lazy this year.

 

Paul - that looks like an airhorn gasket to me...