Hi guys,

So my freshly rebuilt 400 (roller everything, .030 over etc.) with a blueprinted rebuilt carb is running (eyes watering) rich. The air fuel screws have no affect. The car seems to be fouling plugs and it is idling rough. It is a '68 442 by the way.

What am I doing wrong?

Thanks,

Craig

 

What is your idle speed set to?

 

The tach isn't accurate but it is probably about 750. I don't think is is getting into the main circuit if that is what you are getting at.

Craig

 

Who rebuilt the carb?

Are you saying if you turn the idle mixture screws all the way in there's absolutely no change at all?

 

A reputable guy in Los Angeles built the carb. It runs and looks beautiful. It is just too rich. I talked with the engine builder today and he suggested putting in smaller jets. i ordered a multiple size kit. It has 72's in in right now. How dramatic is on size to another? I thought I'd start with a 70

Craig

 

If you change jets, you ALSO need to change rods. Primary rod tips are all the same size, so at wide open throttle you get richer because of the larger jet. But at low or mid speed, without changing the rods, you also get much richer, and almost always, too rich. Doug Rowe talks about that in his carb book. But if you are not off idle, changing jets won't do anything except make part throttle richer or leaner, and maybe not in a good way.


Have the idle circuits been modified? Is the float set correctly? Is the needle & seat sealing and is the washer installed under the seat, and the old washer removed first?

 

Check for nozzle drip at idle. Idle Transfer slots might be over exposed.may need bypass air if you have a bigger camshaft in there now. If it is not using manifold for vacume advance try that too.

 

If the idle screws don't do anything, the idle circuits are probably plugged, etc.

 

I agree with run to rund I would start over on the carb overhaul. I suspect there is blocked passages. It is not rocket science to rebuild a qjet. Clean clean clean and make sure all passages are clear. Plugs are epoxied correctly and it should work great. Another issue not addressed is what is the status of the fuel you are using?

 

Timing off manifold vacuum may be necessary. Check your vacuum readings. I went thru a similar thing on the 403 conversion to High Compression to match the PO's cam.... "eye burning rich" is exactly right. Low vacuum, caused in part by retarded timing [if the engine is no longer stock, the factory specs are probably not optimal!], releases the power valve in the engine, effectively rendering it useless. You need to get the timing up to where the vacuum is high. Mine kept faltering at idle, the vacuum dropped to 7 which opened the Power Valve which made it richer and worse and then die. I was trying to run an all-mechanical HEI w/o the computer [not using the computer, that is]. I ended up with timing specs far from stock for any Olds engine I am used to. Like oh I forget maybe 20 BTDC static, plus 10-11 from vacuum at idle. Another 20 or so from mechanical at upper speeds- but less from the vacuum under load at speed. Manifold advance vacuum = easy starting, none of that too-early pre-fire backpedalling crap. Idle then brings on 11' more timing for good idle behavior and the OVERHEATING PROBLEM WENT AWAY. Much better seat of the pants throttle response.

Too bad that write-up was all lost in the ROP Crash of 2012 or so. I had all the distributor rework numbers and how-to, etc. Sources of parts. HEI vacuum actuator PN's.

 

X2 on this. What is your best (highest) vacuum reading at idle? How big is the cam?
Todays gas will have a negative effect too and the float level should be dropped slightly from spec to compensate. Todays gas boils at a lower temp then yesteryears gas. Look down the carb at idle when the engine is hot. Your looking for gas burbling over.

 

If you can't control your idle with the mixture screws, the idle transfer slots are exposed. Either drill holes in the primary butterfly's, or (better) enlarge the idle tubes. Then try different jets and rods.

 

Use a mirror for this. Don't put your face directly over it.

 

First off, have you checked the float setting. If it is set right the read on.
If you have a camshaft with a modern profile you most likely have a much stronger signal to the carb. A basic rebuild just won't cut it. You need someone who knows "how" to "build" a QJ. The idle circuits need modified. Different guys do it different ways.

 

Craig if you are really not familiar with taking apart quadrajets. Smitty might be right you need to take it to someone who knows what there doing.recalibrating the idle passage is a art. I have gotten good parts and advice from cliff ruggles . what is the carb number on it . sometimes you will find it looks like the carb that came with the car but it is not , calibrations will be off. Why most people jump on the holley train is because there idle is very generous therefore screw adjustment can get you close.quadrajet are set a little lean stock. When you put free flowing exhaust and a bigger camshaft it makes it too lean and most people over jet them to trying to fix it.when it really needs the idle down tubes,idle down channel enlarged .air bleeds size adjusted.some time bypass air is required to tighten up throttle angle to keep the idle transfer slots from being over exposed.if you have a early carb they sometime do not have bypass air holes in the baceplate have to modified it or drill hole in the butterfly's

 

Just look up jet size for a 68 400 442 had 72 jet 49 rod and AH rod a 400 w 30 had 75 jet 49 and AH rod. The 72 are most likely stock that you have. Give more detail on the motor. Bigger camshaft, headders could use 74 to 76 jet with the stock 68 carb .so if anything your jetting is not too rich a 350 run on 71jet in 68.
Just for kicks pull a small manifold vacume line off and see if you can ajust your idle speed then the idle mix screws .

 

Wow guys, my head is spinning. I thought it would be simple as a jet change. My engine builder sent the carb out to the carb rebuilder. i just assumed it was brought up to specs. We gave him all the data on the engine. I figured it would be plug and play. I guess a bigger cam will generate less vacuum ,thus not enough air, so it would run rich. I am just wondering if this is something I should mess with. Maybe it is time to bring it to someone who knows how to tune Q-jets. Here are the specs on the engine. It is a '68 442 convertible, automatic, .030 over 400 with forged pistons. I have a 2 1/2" H pipe with Dynomax Super Turbos exhaust system. It has a Pertronix flame thrower coil and electronic points. The cam and carb spec sheets are posted.

Thanks guys for all the input, I really appreciate it!

Craig

 

The chart look like it set up for a holley carb .so it hard to figure it out.Looking at the specs a few things stand out to me .the idle down tube (idle jet) what is it at? . what the idle down channal size ?Was the idle screw hole enlarged ?. A 8.5 power piston springis OK ,that cam is not that big at all should pull over 15 inches at idle CK rods is just like a AU rod.
I am no expert on q jets I am just telling you what I see. Your set up is warmed over 400.it looks like a good street combo to me .maybe others will chime in on this.
To tell you the truth they might just did a stock rebuild and just gave the sizes where they were at . a .044 rod is a little fatter than the stock .049 rod that looks like all was done. the accell discharge port is stock size .026. Stock size main jet and scondary rod .Even if it was stock carb with your camshaft it would not give you a fat idle.
Have you check the plugs? A really lean idle can give you a stinky idle too.

 

The carb builder will want to know how many Hg's your pulling at idle, choke off and engine warm. Try to adjust the timing and air/fuel to obtain the highest reading possible using a vac gauge. Record that as well as at what RPM that reading was obtained. A good QJet guy will take it from there. Also bring said carb guy a spark plug or two. Ask for your old parts back after the rebuild especially if you have harder to find secondary rods.
If you want some schooling pick up Cliff Ruggle and Doug Roe's Q jet books. Both go into detail how the mysterious Qjets work and how to unlock potential in fairly simple terms.