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Old 10-12-2018, 12:13 AM   #1
Mike_The_Grad
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1972 quadrajet 4MV. :(

Heres the skinny. O.E.M. 1972 quadrajet 4MV 7042910 on my 72c10 with 350. I've been experiencing irregularities with my trucks performance as of late. Engine info: 350 with .040 over bore, compcams XE250H RV cam, 882 heads, roller tip rockers, double roller timing set. And about 20k miles kn the rebuild. Engine started acting funny a couple of months ago. Sounded different drove different. Almost like it was lean and had a vacuum leak with to much timing. I KNOW RIGHT!

So j finally got around to pulling the top of the carb of tonight. Other than the typical stuff u might expect from a 40 something year old carb. (Its been rebuilt 3 times, twice by the same shop, and once by me.) I was depressed to find the Venturi booster walls of the primaries with hairline cracks running vertically. Also the secondary pull over wells full of fuel and the side vacuum chambers full of fuel as well. Which by the way, might explain why one of my hei vacuum canisters was full of fuel a couple of years ago. ARRGGHHH!! It's all beginning to make sense! Dammit. Oh, sorry, you're still here? Cool. Thanks. I'm pretty sure that cracked venturi walls is not the most common issue that many Qjet owners face. But I can only imagine what kind of issues this causes. Let's try and name them off: lean mixture, loss of vacuum, flooding, idle instabilty, and weird noises. Honestly sometimes my truck sounds like a boiling kettle on the stove or like one of those bird whistles that you put water in and blew threw. Yeah, exactly like those whistles. I swear it is the weirdest sound and I was afraid to ask someone for fear of THE LOOK. The look of " Hey buddy, are you ok? You been reading on the internet to much." You know the look. Anyways. Maybe this all goes back to that one time when I first had my carb rebuilt and I dropped it upside down on the concrete? Who knows. Point is, it is what it is. And I'm not happy. But all is not lost. My really good friend had offered me his Brand New, seriously a week old, 1406 edelbrock with his brand new Performer EPS intake. Yeah, that's the type of friend he is. Well that and the fact he needs to rebuild his motor. Which we found out 2 days ago. So he gets a 383 stroker with aluminum heads and a scar rotating assembly, and hes gonna give me his 041 350HP heads, new intake and carb setup with 1" spcaer. The world works in mysterious ways. And you might ask what I have done for him? I rewired his 71 c10 swb, helped install all of his suspension and interior parts, and help him diagnose his own engine issues. All without asking for a single thing, just only to let me help him and maybe a beer or two. The reason why I did this for my friend. Because when I blew the head gasket on my truck about 4 years ago, I had NO ONE to help me. NO ONE. Sure i had a buddy with a hoist, and a buddy that could help drink my beer. But I did not have some one to show me how to reassemble my engine, how to set valve lash, how to torque engine fasteners, how to rebuild a carburetor, how to bench bleed a master cylinder, how to recurve mu distributor. I did it all myself. I loved it, but it was hard. And I wish I did have someone with some knowledge to learn from and bounce ideas off of. So, I wanted to be that guy to my friend. And I dont regret one bit. But I'm rambling now and I have to get this thing back together so I can go to work in the morning. As my friend always tells me, "Its called hotrodding, Mike."
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Old 10-12-2018, 01:33 PM   #2
demian5
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Re: 1972 quadrajet 4MV. :(

Yes you rambled quite a bit...

Bad choke? Adjusted too high?
Ive seen what look like cracks where you mention. How about some pictures?

I ran a small block Q-jet on a 468 with a decent size crower power beast cam, big oval corvette heads, roller rockers, hooker long tubes and about 12 degrees of initial timing. Ran like a raped ape and passed everything bu a gas station (which is why i no longer have it)...

All I changed was the main jet metering rods.

The 4-digit number on the side of the carb will get you a new kit.

But post some pictures.
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Old 10-12-2018, 02:37 PM   #3
68Gold/white
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Re: 1972 quadrajet 4MV. :(

Not all Quadrajets are the same. SMI is a company in California that can fix your carb. Stands for Sean Murphy Induction. www,smicarburetor.com He can help you. He's in Huntington Beach 714-843-9169.


If you don't like what they have to offer. www.Cliffshighperformance.com can selll any AND ALL parts, kits you need to do it yourself. Not sure if SMI sells kits, etc...
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Old 10-12-2018, 04:33 PM   #4
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Re: 1972 quadrajet 4MV. :(

http://www.guaranteedcarburetors.com
I just ordered one from these guys. Cheapest one I could find that sounded like they knew what they were doing. Anyone heard of them?
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Old 10-12-2018, 09:07 PM   #5
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Re: 1972 quadrajet 4MV. :(

I had an E-brock 1406 on a SBC, and my experience tells me that you will be putting primary throttle shaft bushings in it at about 30k miles. Once that is done, it'll be probably good to go, but I also had a ton of boil over from the heat here in California. And the insulator that was supposed to keep the carb cool leaked vacuum so badly that I ended up sealing it with shellac. I'll never buy another E-Brock. YMMV.
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Old 10-16-2018, 01:28 AM   #6
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Re: 1972 quadrajet 4MV. :(

Steeveedee, thanks for your opinion and experience with the 1406. Its brand new out of the box and I'm the only one to adjust anything on it. Well I spent the weekend swapping intakes and carbs. And it literally took me all weekend. I had to reconfigure my throttle linkage, throttle return springs, carb mounting studs, carb insulator gasket, brake booster vacuum line, wire in the electric choke, reconfigure the fuel supply line from pump to carb. After all that, it runs. Not well, but it runs. It's got a huge hesitation from idle, runs hotter than normal just going down the road, and gets worse mileage than my qjet did. But I have been tuning it. I moved the accelerator pump rod to the inside hole, this helped out quite a bit. I also installed the chrome fuel line that edelbrock sells that drops the fuel inlet down and to the front of the carb. I set my timing at 12 BTDC. And as I was driving through town I would pull over every 1/2 mile or so into a vacant parking lot and would trim the i.m.s. a bit and adjust the idle speed. I actually got it to run quite a bit better. But no adjustment can change the fact that it's just running to lean for my engine. So I'm picking up a Calibration Kit tomorow to hopefully make it run right.

Trust me, I'm not happy about the change. I'm happy that my friend came through in a huge way, but I knew that it would come at a price because I was changing from my stock setup to aftermarket and it was a PITA. But I'm trying to remain hopeful that I'll have this thing dialed in by the end of the week.
I'll post pictures of the qjet internals but they dont compare to seeing it with your own two eyes. How i know for certain that the main body casting is indeed cracked is because as i had the airhorn top off, I noticed the fuel evaporating from the float bowl and the surrounding chambers in the main body so I started cleaning the venturi bores with qtips soaked with Berryman's Carb Cleaner because there was brownish residue in them. Well as I did this I noticed they stayed wet with something. Berrymans evaporates pretty quickly. So what i did was wipe the backside of the venturi booster walls with just straight gasoline that i soaked up out of the float bowl with a qtip. And sure enough it soaked through to the interior of the venturi bores. So I kinda took a step away and came back to look at it, as I started looking for hairline cracks I could see them on both primary booster bores. That's when everything clicked for me as to why I had a bunch of unexplainable symptoms that would come and go. Which was almost always related to when the truck was warmed up and driven for a while. Every morning I get in it and drive it, it drives great. But after about 20 minutes or so and warmed up it would change. when ever I parked it and would try to start it after more than about 10-15 minutes it would take a few seconds to start. Also the float had a leak. So as many of you might suggest a fuel boiling issue or a vapor lock issue. I've only experienced that one time in the 14 years I've owned the truck. And it was indeed both of those things together.
Back a few months ago when it was 110 out and I hit 2 of the longest lasting stoplights I swear I've ever encountered in my life back to back in 2 blocks, I knew things weren't gonna be right when I went to start it 5 minutes later. It was so hot under the hood I couldn't grab the wingnut off the aircleaner for almost 5 minutes. I could hear the fuel boiling in the carb. But I got it running after clearing the accelerator pump and airing out the carb bores. So the symptoms I would experience daily for the last 4 years are different than this.

I've had a exhaust gas analyzer connected and tuned on the spot only to be told it needed to be left with the shop for a day. Which I never got around to. Plus, the carb shop that did the exhaust gas test also did both of the first 2 rebuilds never seen the carb on my truck. I took it to them for a bench rebuild because my truck wasnt running or even completely together then. The next time I did the rebuild myself. So it's most likely self inflicted. Which I guess it's better to be mad at myself rather than some shop or someone else. It's a reality check to me and a reminder that I need to recognize my limits and know when to go to the professionals and let them do what they do every day for a living.
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Old Yesterday, 01:21 AM   #7
Mike_The_Grad
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Re: 1972 quadrajet 4MV. :(

In case anyone is still interested. I have been able to get better results with the 1406 carb setup after I purchased Calibration Kit #1487.
I'm thinking I may have the wrong PCV valve as I can hear it actually sucking air into the base of the carb as I'm driving down the road. It is the 90 type with a plastic barbed connection. I changed the primary metering rods and went to the next strongest set of step-up springs(Orange Vs. The Yellow factory ones.) It is definitely better. But it is still running lean in the "cruise" mode. It took longer for the temperature gauge to rise above normal operating temp. But it did rise. I also moved the accelerator pump linkage back to the middle hole in the accelerator pump arm. I'm hoping to resolve this lean condition with another rod change instead of a jet change. So as to avoid removing the airhorn. Will check back with results. Also, if anyone knows of anyone with a usable quadrajet main body casting#7042910 I'm interested. Thanks.
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Old Yesterday, 08:30 AM   #8
Steeveedee
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Re: 1972 quadrajet 4MV. :(

Good progress! I had a 1406 on my 305 Chevy that I put in a Buick Skylark. Edelbrock cam (smallest one) and Performer intake with brand X headers. I went up 4 numbers on the jets, and that cleaned up the off-idle stumble that I had. Keep at it, you'll get it working fine!
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Old Yesterday, 01:35 PM   #9
Mike_The_Grad
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Re: 1972 quadrajet 4MV. :(

Steeveedee, thanks. I was kind of doubting this carb but it appears that it has potential. I'm gonna swap in another set of rods today after work. Which set of step up springs worked the best for you? With the comp cam XE250H I have I'm getting about 19.5"-20" Hg. At idle. I swapped in the orange step up springs but am considering going to the pink ones(next strongest set) and see how it likes it.
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Old Yesterday, 07:46 PM   #10
Steeveedee
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Re: 1972 quadrajet 4MV. :(

I did the setup in 2000; I only remember the jet part. I may not have even messed with the springs. Not much help.
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Old Today, 08:05 AM   #11
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Re: 1972 quadrajet 4MV. :(

Quote:
Originally Posted by External View Post
http://www.guaranteedcarburetors.com
I just ordered one from these guys. Cheapest one I could find that sounded like they knew what they were doing. Anyone heard of them?
I bought a rebuilt Qjet from them a few months back. Bolted it on and worked perfectly. Hope you have similar results.

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