[tom3]

Installed an Edelbrock O2 sensor. Pretty nice gizmo, seems like. I notice in adjusting the idle mixture that there is a very fine line between too rich/lean. Is there any way to adjust the metering rod depth without removing the top of the carb every time. I can see that this will take several adjustments this gets tiresome quick. Thought about trying to run the engine with the top off, or getting and old top and cutting out the area above the metering rods.
Any suggestions appreciated. Seems like there must be a way, I imagine the factory had some way to set this during the smog years before computers. tom

 

[Tom Mobley]

tom3

I'm a q-jet guy, I guess I don't understand your post. Are trying to adjust your idle mixture by raising/lowering the rods? This will not work. That adjustment affects the lean cruise. (part-throttle steady-state cruise) The small screws on either side of the front of the baseplate adjust the idle mixture.

What year is your carb? Does it have the integral choke or is the choke bi-metal spring mounted on the manifold? How are you adjusting the height of the rods? Early carbs have a screw (that's usually stuck) in the center front of the baseplate under a soft aluminum cover. It's accessible from the outside. The early carbs will have tapered rods that can be adjusted to good effect, most performance carbs had "B" series rods which have a smaller tip to give richer power mixtures.

Later carbs have a vertically oriented adjuster inside the carb, it is in a well that runs parallel to the power valve well and has a "double D" stud sticking out of the top to turn for height adjustment. This style usually has an aluminum plug in the top plate, center front under the air cleaner housing gasket. Knock it out from the bottom and don't put it back in until you're done tuning. I've seen this hole threaded for 1/8" pipe threads by tuning freaks. Put an allen head plug in and you're set. My double D tool is just a piece of thin wall steel tubing that I flattened slightly with a hammer, works fine. All you need is to turn the screw a couple turns one way or the other.

Here's the gotcha in the deal: The rods are not interchangeable between early and late carbs because of their considerably different overall length. The performance rods for the late carbs (small power tip) are the "K" series. The K rods ARE NOT TAPERED, they're stepped, it doesn't do much good to adjust them up and down. In any case, it won't affect the idle, only the part-throttle cruise. If you have the late style M or P series rods, they're tapered but have a larger power tip. I've discovered that the tip can be turned down about 0.010" (that's ten thousandths) (similar to the K rods) by chucking it in about anything that turns and using a small sharp file on it. Turn down the last 1/4" and polish it up with 400 grit. Many perf cars will really respond to this treatment. The tip of the rod is only in the jet when the manifold vacuum is low, like when there's a heavy foot on the throttle trying to mash it through the floorboard, so this doesn't usually affect the gas mileage or smog numbers. This deal is a little touchy on the size, use a dial caliper or micrometer to track the size.

For most perf engines you want about one percent CO at cruise. I don't know what O2 sensor number this translates to, but it is on the rich side. Most of these O2 sensors have some kind of indication that you're at stoichiometric A/F (14.7:1), in my experience this is too lean for a perf motor and leads to driveability problems. I *think* 1% CO is about 13.2:1. I found a table somewhere one time that converted CO% to A/F ratio but I can't find it now.

Incidentally, that 14.7 number goes by weight, not volume. A gallon of gas weighs about 7 lbs, I wonder what 105 lbs of air looks like, anyway? How many cubic feet/yards is that?

Any questions all this baloney re-post or email me.

Tom

 

[Fred Aldrich]

Tom
Excellent Q-jet info.
I've got two Q-jet problems I could use some help with.
- Extended crank time after car has set for several day. Have had several different carbs on the car, same result. Have used JB Weld and Locktite to seal main well plugs with no noticable improvement.
- Have a stripped inlet fitting I'd like to repair. Where can I get a oversize repair fitting.
Thanks

------------------
Fred Aldrich
Web Site: www.GeoCities.com/~69_chevelle




[This message has been edited by Fred Aldrich (edited 02-15-99).]

 

[tom3]

Tom; Thanks for the info and taking the time to type this out. Much appreciated. Actaully working on the cruise/part throttle mode at present. Running about 12.5 ratio all the time now. I know these carbs have to run a little rich at high vacuum steady speed to not lean out at part throttle acceleration, but I'm pretty rich all around. Running a 1970 model carb with divorced choke. I'll print this info out and get into it after while. Thanks again.

 

[Tom Mobley]

tom3,

You should be running B series rods, I'd try going one size bigger on the rods, IOW if you're running 45B, go to 46B or maybe 47B. If you can't find the rods, email me, I've got a bunch of this stuff in boxes out in the garage. I think 12.5 is prety fat, you should be able to get better than that w/o losing driveability. On the lean part throttle acceleration, sometimes going a size or two bigger on both the jets and rods fixes this. Same cruise, richer accel. If the car is running on the idle or transition circuit at cruise, you can drill the idle fuel feed with a jobber length 0.039 drill bit held in a pin vise. Allows more fuel into the idle circuit, you can re-adjust the curb idle with the screws and still get more fuel out of the transfer slots. Helps with cruise sometimes, well, lots of times. SOP for me on perf motors w/Q-Jets. You can get these carbs so dialed in you have a hard time tell it's not FI. People that ride/drive my 57 PU ask me what kind of injection I've got on it.



Fred,

I gave up on the epoxy-type repairs many years ago. NAPA and others have a little kit with two O-ringed metal plugs. There's two different diameters depending on year model. What you do is tear the carb down and file the lead rings off the plugs, this can be done on a belt sander if you're careful. Then reach down through the sec main wells from the top and drive the plugs out. My tool is a piece of welding rod about 4" long with a little crook in it to match the bend in the sec main well. Tap the plugs out with a hammer and this punch-like tool you've made. De-burr the edge of the holes and tap in the replacement plugs with a little motor oil for lube. A little foam piece goes in the recess in the baseplate, I don't think it's really necessary but I put it in anyway. Also, I have a small ball-peen hammer that I use to re-peen around the lead plugs under the primary main wells, go easy here.

If it drains off after this, try leaving it for a few days with the rubber line between the fuel pump and the frame pinched off and see if that makes any difference. I've had a couple that seemed to drain back to the tank through bad check valves in the pump. This makes absolutely no sense whatever from an engineering point of view since the needle and seat is quite a ways from the bottom of the bowl but it worked anyway. I've learned that if the physical results don't agree with my theory, believe the results and get a new theory.<g>

I'm told the Carb Shop can repair those inlet threads but I haven't tried them. They're somewhere in SoCal, usually have an ad in the back of the magazines. They're supposed to be a good source of hard to find Q-Jet parts too.

On the threads, I've never found any good solution other than the usual oversize, selftapping inlet nut, I think there's also one that a rubber part similar to a rubber expanding freeze plug. None of these deals pass visual inspection on a restored car. They don't always work, either. Good luck on the threads, if you send one to the Carb Shop, please let me know it turns out as I've got a couple of rare Q-jets with that problem. You can get the oversize fitting at NAPA or other large parts house.

Tom

[This message has been edited by tommob (edited 02-15-99).]

 

[Doug Flynn]

Keep in mind that a typical O2 sensor can not accurately measure air/fuel ratios above or below 14.7:1 (stoiciometric). I don't care if the gauge says you are at 12.5 to 1, it is not accurate other than 14.7 +/- a few tenths. Look at a chart for a regular O2 sensor and you can see this. Only wide band sensors which cost hundreds of dollars can do this. Above and below this these sensors are not even near linear. If you are tuning for cruise or idle near stoic, it is ok, but for heavy and wot you need et's, egt's, plug readings and seat of the pants to check for best a/f.

It is a tool to tell you if you're rich or lean, but can't accurately tell you that you have a 12:1 a/f ratio.

I think you've found that there is a fine line between rich and lean because that is how the output of a regular O2 sensor works.

It's useful but you need to be careful for wot tuning.

 

[68396]

I also bought a highly advertised A/F ratio indicator thinking that this would save me considerable time in tuning my wifes V-6 Buick. (I know, I know) Unable to get the Q-jet in the ballpark using the indicator as a guide I called the manufacturer. They convinced me that because the carb was originally calibrated for a V-8 I needed to use the recommended carb for the intake manifold, which was also of their manufacture. I bought the recommended carb (a Holley) and installed it with the same conditions being displayed on the ratio indicator. Leaning the idle mixture to a 14:1 ratio on the indicator produced a random lean misfire, an acceptable idle was found at 12.5:1 indicated. Part throttle cruise showed 12.5:1. A jet change one number size down, brought about lean surging at highway speeds and a varying reading running from 12.5:1 to off the scale twards the lean end of the scale. As far as I'm concerned this unit is worthless. Does anyone have experience with the units sold by K+N or CB Performance? I think my Q-jet was ok after all.



they also manufactured

 

[Junior]

Tommob
We truly are not worthy "You are the man"

 

[Sean Flanagan]

Just wondering if anyone has seen a Q from the '70 F*** Cobra Jet?

------------------
Sean Flanagan
Heavy '73

 

[Tom Mobley]

Sean

I've seen one, built it for a guy with one of those cars. I didn't see that it was all that special. After doing the carb and dist, drove the car. What an animal. Ran like a well-prepped early Camaro with a hot BB. Both hands on the wheel deal.

Tom

 

[whee]

Just a quick question, I have a '68 Q-jet that I am putting on the '71 350 in my '70 Malibu(crazy ain't it?

). I picked the carb up from a friend, but the fuel inlet nut was missing. I bought a fuel line from YO for a '71 350 4bbl. Please tell me that all the thread sizes and everything stayed the same over the years since I plan on using the fuel inlet nut from the 2bbl that is on the car now.

Also, what is the difference between a carb used with an automatic trans vs. a carb used with a manual trans?

The primararies read 46B. According the the carb numbers(7028213) this carb came off a 327 with a manual trans. I plan on sticking with my TH350. The carb is just something to start all my learning with when I finally get a garage in a few weeks.

Thank for your help.
Mike Manley

 

[Tom Mobley]

Mike

I don't think the 2bbl inlet nut is going to work for you, most 2bbls, especially earlier 2bbls, had a 5/16" fuel line, Q-jets all had a 3/8" line. I'm not sure the nut is the same on the carb end, either. Sorry, not what you wanted to hear, but stuff happens.

re: auto vs manual. most (not all) auto carbs have some kind of lever hanging down from the throttle arm for the transmission kickdown. Lots of manual carbs don't have this linkage deal, and are a real PITA to use on an automatic car unless it's a TH400 with the electric kickdown switch mounted on the gas pedal.

Looks like you've got a few things to deal with here. The TH350 really needs the kickdown cable hooked up and adjusted right else maybe the tranny is going have a short life. It would surprise me if a 68 manual carb had a kickdown lever on it.

Tom

 

[whee]

Not a big deal. I'll just have to check around at the swap meets this year for what I need.

Thanks for the information. It's always good to learn something new even if it might mean something isn't going to work easily.

------------------
Whee (or sometimes Mike)
'70 Malibu