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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
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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
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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 10-18-2018, 01:21 AM   #7
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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 10-18-2018, 08:30 AM   #8
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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 10-18-2018, 01:35 PM   #9
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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 10-18-2018, 07:46 PM   #10
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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 10-19-2018, 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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Old 10-21-2018, 09:37 AM   #12
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Re: 1972 quadrajet 4MV. :(

The 1406 isn't a bad carb at all once it's calibrated and they're stupid simple to work on. I didn't see you mention it, but since the carb is new, did you set the float levels on it? Off float levels and/or too high fuel pressure make Edelbrocks flood. For your step up springs, they should be half of what your vacuum level is, so the strongest springs in your case. The manual for the carb actually goes over tuning very well. Here's a good little tuning video from Jegs as well: https://youtu.be/wR_AfQjyT-A
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Old 10-21-2018, 09:58 AM   #13
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Re: 1972 quadrajet 4MV. :(

The problem with throttle shaft bushing wear is exacerbated by too much throttle return spring tension. All of the return springs I have found these days at too strong. If you find a stock spring and check the tension you will find they are not very strong. If you add a second spring you have doubled the wear potential on the throttle shaft. I have modified the springs on my 1406 to get positive return at low tension. Notice the spaces between some of the coils on both springs.
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Old 10-21-2018, 01:37 PM   #14
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Re: 1972 quadrajet 4MV. :(

68 P.O.S.
No, I didnt set the float level. I haven't noticed any telltale signs of flooding yet. Is setting the float level necessary right out of the box? If it is then I'll do it. I installed a brand new mechanical fuel pump at the same time as the carb and intake swap. I plumbed in a new chrome drop down fuel line with banjo bolt connection and a barbed inlet with a new liquid filled 0-15 psi. Fuel gauge. At idle, after driving for about an hour, the needle is bouncing around at about 6-6.5 psi. I do have a new plastic inline filter right before the chrome fuel line. I swapped in a richer set of primary rods and the pink step up springs which is the next strongest set, but I'll try the strongest set of springs with this new set of rods and see how it goes. And its true how easy it is to make these kinds of changes to the carb. That is one area I wish the qjet had. I haven't touched anything else on the carb and I'm already liking the results. it's got nice throttle response, and goes a little higher into the RPM range. Something my quadrajet didnt do. Which tells me my qjet was having issues for quite some time. I know the qjet pretty well, and I actually miss it on my truck being that it's the original carb. My truck will have another qjet in it someday.

HO455
I agree with ya on the throttle return springs. Those stainless steel ones you see on the shelf everywhere are a lot stiffer than the stock one. But I broke my stock one. So I went just the big one out of the pair of those stainless springs. And I modified it also. I do recall a number of years ago when I was trying different springs out on my qjet, I had a really stiff spring on there. I liked the pedal feel, but I had to put way to much effort into it. I'm glad I changed it out for a softer spring. I had never heard of throttle shaft wear at that time. I did find a neat little diagram online that shows ideal throttle returns and why some of the ways I've tried in the past were really bad for the carb.
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Old 10-21-2018, 01:43 PM   #15
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Re: 1972 quadrajet 4MV. :(

Just updating you guys on my qjet purchase from guaranteedcarbuetor. Com,
It blots right on, only have to adjust the idle screw. I'm impressed. When you get ready to return the Qjet to your truck, I'd suggest giving these guys a call.
Oh, and it's a purdy silver coated too!
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Old 10-21-2018, 01:59 PM   #16
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Re: 1972 quadrajet 4MV. :(

Quote:
Originally Posted by Mike_The_Grad View Post
68 P.O.S.

HO455
I had never heard of throttle shaft wear at that time. I did find a neat little diagram online that shows ideal throttle returns and why some of the ways I've tried in the past were really bad for the carb.
Do you have a link or copy of the diagram?
Thanks.
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Old 10-21-2018, 09:43 PM   #17
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Re: 1972 quadrajet 4MV. :(

Here is the chart.
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Old 10-22-2018, 08:35 AM   #18
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Re: 1972 quadrajet 4MV. :(

Thank you. I am now done hijacking this thread. Apologies to all.
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Old 10-22-2018, 12:47 PM   #19
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Re: 1972 quadrajet 4MV. :(

Glad you’re making progress, you’ll get there. Yes, setting the float level is necessary. It’s very common for the floats to be off right out of the box due to shipping and who knows what else. Which fuel pump did you end up going with? Sounds like your new setup is pretty good. Keep us up to date on your progress.
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Old 10-23-2018, 10:06 PM   #20
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Re: 1972 quadrajet 4MV. :(

External,
That is a nice looking qjet you got there. Reminds me of how mine looked getting it back from the shop that did my overhaul.

HO455,
I dont think you've hijacked the thread at all. I appreciate your contributions to the thread. And I'm glad rsgt knew exactly which diagram I was referring to and posted it. Thanks. Because I didnt have it saved anywhere.

68 P.O.S.
Well then I'll make sure to adjust the floats. Thanks for the heads up. I remember when I replaced the float in my qjet but I didnt know about float level and all that goes along with it. Sure enough it wasnt performing properly. So I mustered up the courage to tackle the issue myself.I was able to correct the float level and realized how important these kinds of things really are to how a carburetor functions and performs. One thing I haven't really heard about regarding these edelbrock carbs is if the brass floats are subject to the same issues as the fuel tank sending unit floats are for our trucks. I bring this up because I just replaced the sending unit in my '72. And I had to replace the brass float twice within 2 weeks and ended up going the plastic float route after realizing that is the only way it's really going to be fixed once and for all.
I installed an Airtex fuel pump because that's what my parts guy stocks.
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Old 10-24-2018, 02:50 PM   #21
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Re: 1972 quadrajet 4MV. :(

I really haven't heard of many issues with the Edelbrock floats and I haven't had any myself. Although I'm sure it happens like everything else, there isn't a rampant problem going around with them.

Hopefully the new fuel pump doesn't give you any flooding problems.
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Old 10-28-2018, 12:31 PM   #22
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Re: 1972 quadrajet 4MV. :(

Well that airtex fuel pump was junk. My fuel pressure gauge at the carb inlet was reading only about 2-3 psi even if I revved the engine. So it was either a bad gauge or a bad pump. My parts guy said he hasn't had one of those specific gauges come back to him, ever. So he offered me a different brand pump. I swapped the airtex flr a spectra premium fuel pump. For some reason this spectra pump was a PITA to install. The gaskets were junk. So I cut my own out of some Fel-Pro karropak gasket material my brother bought some time ago. And sealed it with some of "The Right Stuff" one-minute gasket maker. What I do like about the spectra pump is that the inverted flare fitting sealed great the first time. The airtex pump leaked and leaked until I finally just went all in and torqued the you-know-what outta it.

The spectra pumpbos putting out about 7 p.s.i. according to the gauge. I also had to swap out the clear plastic online fuel filter for a steel one because the filter element separated from the base and was free floating around inside, and not providing any filtration. I think I might be getting some flooding now with the newer pump. I'll keep an eye on it and if it is indeed flooding then I'll put a regulator on it. I'm still running lean because my temp gauge is riding higher than normal. Do I think I have to go ahead and take the top of the carb off and replace the primary jets. I've already gone through the range of metering rods in the calibration kit and it has gotten a lot better. But it's not quite there yet.
It makes sense to me that a jet change is necessary because regardless of the rods ability to meter fuel the jet is a fixed orifice and will only allow a specific amount of fuel to flow. I'll be tackling that today along with checking float height and drop.
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Old 10-31-2018, 12:38 AM   #23
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Re: 1972 quadrajet 4MV. :(

I went ahead and removed the airhorn on the 1406. I wished they would have mentioned that the fuel inlet would need to be removed in order to fully remove the airhorn from the carburetor. Would it have made a difference to me actually removing the airhorn? No. But I wasnt sure if banjo fitting washers were reusable. And I didnt have a spare set to use in case they weren't. I say this because I have the chrome drop down fuel line with inline fuel pressure gauge.

Anyways, the float height checked out on both floats. But the float drop on the passenger side float was out of spec at 1-1/8". So I bent the tab that adjusts float drop to somewhere around 15/16"-1".

I also swapped the stock primary jets from 0.098 to the largest set of jets in the calibration kit, 0.101. These actually are the only set larger than stock in kit. I buttoned everything back up and put the stock metering rods along with the strongest step-up springs back into the carb to get me at around the #18 setting according to the edelbrock tuning chart for the 1406 carb. I definitely noticed a change. Mostly in the idle and off idle characteristics. The engine seems quieter and a little "lazy" in the idle to off idle performance. And seems to be running lean in the "power" mode. Which is not good from what the tuning guide suggests. Running lean in the power mode has the potential for detonation and pinging. But I will say that the jet change seemed to cure my engine temp rising issue during "cruise" mode. Now it appears that I need to riches up the "power" mode and I should have this carb dialed in for my setup.

I'm sure I also need to go back and readjust my idle mixture screws to compensate for the jet change. One thing at a time and eventually it'll all get worked out.

And I think I am experiencing flooding now with the new spectra pump and the primary jet change. So, I guess an adjustable pressure regulator is neccessary. It seems like edelbrock knows all this stuff and could easily factor into its design all of these "necessities", but I also think they dont do this purely for the fact that they could profit more by piecing all of the parts out. This is purely my own opinion and has no bearing on anything other than that.

That being said. I'm still happy with the 1406 carb and performer EPS manifold sitting on my engine. Next thing to tackle is if and when I get my buddies "041" casting sbc heads and have them gone through at the machine shop so I can replace the Reman "882" heads I currently have. Then I'll be looking at some real improvement.

Oh yeah, I threw a pic of the LS1 my buddy is putting in his 71 swb factory a/c cab. It's got a stage 3 MRE custom grind cam, LS3 heads, Holley sniper EFI intake with 92mm throttle body. Should be making somewhere around 520 HP. This is gonna be fun to install and drive. I know very little about LS engines, but me and him are gonna tackle this one head on.
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