Qjet tuning

[br0nc0xrapt0r]

So I recently tossed on a Q-jet and I have been pleased with it's performance for what it is. A low buck what appears to be never rebuilt carb. It Idles really well and while in the powerband has lots of give and response. However from idle if I burp the throttle or from a stop light it chugs for a second then picks up and goes like a champ. I had the wife burp the throttle and I can see a puff of smoke come out when it does this. I also tested this with the accelerator pump disconnected to make sure it was not getting too big of a shot of fuel. All around it runs really well in other areas and seems to be not rich, or lean.

This is mounted on a stockish 302.

 

[broncodriver99]

I am getting ready to do the same thing.

I have been reading up on them and that bog may be a symptom of a missing or misadjusted throttle damper. They are usually on the pass side front of the carb and have a short vac tube that loops right into the carb. If they get out of adjustment or go bad they can cause the bogging.

 

[br0nc0xrapt0r]

I am getting ready to do the same thing.

I have been reading up on them and that bog may be a symptom of a missing or misadjusted throttle damper. They are usually on the pass side front of the carb and have a short vac tube that loops right into the carb. If they get out of adjustment or go bad they can cause the bogging.

Interesting, Do you know what said device does? I'd research it my self but I slipped and fell into a bunch of PBR.

 

[Rustytruck]

Try raising the idle to 1000 rpm with the throttle stop screw. Then re adjust the Idle mixture screws to bring the Idle back down to 700 rpm. The quadrajet likes a little extra flow past the primary throttle butterflys. It helps with the transition when your off idle. Some people stretch the spring a little on the plunger inside the center of the carb to help with the stumble.

 

[Explorer]

Cliff Ruggles has a book on these carbs: Rochester Quadrajet Carburetors. Susposed to be well worth the purchase. Know a guy on another forum that adapted one for a 300ci I6 with this book and it came out rather well. Lot of work though. Very good carb, but very complicated.

 

[br0nc0xrapt0r]

Well those are two things I am going to look into but I fixed it. I feel real stupid that o did not catch this but my timing somehow got off. Once I adjusted it it was back to normal.

 

[br0nc0xrapt0r]

Try raising the idle to 1000 rpm with the throttle stop screw. Then re adjust the Idle mixture screws to bring the Idle back down to 700 rpm. The quadrajet likes a little extra flow past the primary throttle butterflys. It helps with the transition when your off idle. Some people stretch the spring a little on the plunger inside the center of the carb to help with the stumble.

I just did this and I had to bring my screws all the way in and re adjust from there. Now it actually will idle properly.

 

[br0nc0xrapt0r]

I am crossing my fingers but its running pretty good now. I had to do a little back and forth with the carb and the timing but I think I got it. I may just retard the timing a bit more though since it will occasionally diesel a bit.

 

[blubuckaroo]

One of the problems with the Q is with the plugs are suppose to cap the places where the ports were drilled. Those little plugs can get loose and leak, causing the carb to run rich on both the idle and intermediate circuits. Back in the day there were kits to fix the problem but with the emergence of epoxy on the market, the problem was solved. I remove the carb body from the throttle body and fill the cavity in the bottom of the carb body with JB Weld. The other problem was stripping threads on the filter housing. There used to be a fix in the auto part stores for that too. We considered the carb junk and I've thrown a ton of Qs into the trash. Wish I had them back now. I'd use one now if someone would come up with a good dual plenum spread bore manifold for a 302.

 

[br0nc0xrapt0r]

One of the problems with the Q is with the plugs are suppose to cap the places where the ports were drilled. Those little plugs can get loose and leak, causing the carb to run rich on both the idle and intermediate circuits. Back in the day there were kits to fix the problem but with the emergence of epoxy on the market, the problem was solved. I remove the carb body from the throttle body and fill the cavity in the bottom of the carb body with JB Weld. The other problem was stripping threads on the filter housing. There used to be a fix in the auto part stores for that too. We considered the carb junk and I've thrown a ton of Qs into the trash. Wish I had them back now. I'd use one now if someone would come up with a good dual plenum spread bore manifold for a 302.

Yeah, my problem seems to manifest itself more so when the engine is hot. This leads me to believe I may have a vacuum leak that opens up when the metal expands with heat.

 

[bmc69]

Former carb rebuilder here.

The famous 'q-jet bog' is just that..famous. The easy way to cure it when all else is adjusted and working correctly is simply to remove the top and extract the 'power valve'..the mixture compensator with the two needles attached to the single piston in the center of the carb. Stretch the spring that lifts that piston and metering rod asy about 1/4" (measured..not by just eyeball) and put it back in. Betcha the bog is gone.

 

[br0nc0xrapt0r]

Power valve? Like the accelerator pump?

 

[Explorer]

One of the problems with the Q is with the plugs are suppose to cap the places where the ports were drilled. Those little plugs can get loose and leak, causing the carb to run rich on both the idle and intermediate circuits. Back in the day there were kits to fix the problem but with the emergence of epoxy on the market, the problem was solved. I remove the carb body from the throttle body and fill the cavity in the bottom of the carb body with JB Weld. The other problem was stripping threads on the filter housing. There used to be a fix in the auto part stores for that too. We considered the carb junk and I've thrown a ton of Qs into the trash. Wish I had them back now. I'd use one now if someone would come up with a good dual plenum spread bore manifold for a 302.

Check Offenhauser catalog. There was one on my son's 77. I'm thinking it was a "Port-O-Sonic?"

 

[nvrstuk]

Bruce had one of those for sale the other day...

Carburetors Unlimited
Phoenix, Arizona
(623) 388-0990

Hours Monday-Friday
8:30 AM - 6:00 PM

 

[jms5580]

I'm in the process of rebuilding a q-jet now. Picked up one of those books. Once you learn the quirks they aren't bad at all. That stumble sounds to me like the accelerator pump. Not sure if it's the same accelerator pump you mentioned though. There's a piston that gives a shot of fuel when the secondaries are opened in order to compensate for a momentary lean condition. A checkball is responsible for keeping the passageways full of fuel. If any part of the system fails, you get a lean condition when the secondaries just open.

As mentioned before, the bottom plugs are prone to leaking and must be fixed. Another common source for leaks is around the primary throttle shaft. If you have any play you likely have a vacuum leak here. They make a kit to drill this out and replace with bushings.

Why go through the hassle of finding a dual plenum spread bore manifold? I simply bought an adapter for square bore to spread bore manifolds (flip it upside down) and with a little modification you're set. Yes, I know they adapt spread bore to square bore, but none of these have separate runners into the manifold. Quadrajets do not like swirling, and the separated runners solves this.

 

[Ranchtruck]

Power valve? Like the accelerator pump?

No, there's a valve and spring in the middle of the float bowl that does the same function as a powervalve that threads into the bottom of a 2100 or a holley. It richens the mixture on the primarys when you have low engine vacuum.
Here's a link that's somewhat useful.
http://www.corvetteclub.org.uk/files/downloads/how_to_tune_a_qjet.pdf