lars

For those of you who have been following along, Forum member "Jebbysan" recently procured one of the brand new Chinese-built 4MV Q-Jet knock-off carbs for evaluation: We wanted to see if these inexpensive carbs (roughly $250) are a viable alternative to a car that's missing its carb, or for someone who has a badly screwed up/damaged Q-Jet. Do these carbs run well "out-of-the'box?"

Jebby's initial thread is here:
https://www.corvetteforum.com/forums...et-review.html

We ended that thread with Jebby running the carb on his Camaro. He reported poor idle quality, and near-normal cruise driving. He then sent the carb to me.

I have just completed testing and a teardown inspection. The following issues were observed:

Very first reaction is in regard to its weight – it is unexpectedly light. The bowl and airhorn are most likely cast from aluminum rather than zinc.

Visual inspection of the throttle linkage and secondary linkage shows that the Chinese assemblers do not understand how the secondary linkage functions – it’s set up way out of whack in a way that does not allow the secondary “crack-open” feature to function. Without this correctly set up, it is possible for the secondaries to remain closed when “tipping into” the secondaries from cruise speed or from a throttle position producing a little bit of vacuum:


The gap is set up so large that the rod never touches the tang for the “crack-open” feature to operate:



The secondary airvalves are very “rough” in their operation – not smooth at all during rotation opening. This roughness will actually allow the airvalves to remain open if the choke pulloff is extended. Obviously, during operation, the pulloff will retract during throttle closure and the pulloff rod will pull the airvalves closed. But it’s not right…:



The carb’s linkage, and the airhorn casting, is set up to allow use of an idle solenoid. It uses the “short” idle solenoid tang as opposed to the tall truck tang:



Solenoid bracket retaining boss is the 1971 – 1974 style:



The vacuum nipple on the forward driver’s side is routed to ported vacuum. The fuel inlet fitting is the large 1"-20 thread size, so it has a little better reliability than the early 4MV 7/8"-20 threads:



Ported vacuum slot is in correct position on the driver’s side:



Carb is set up with the EGR ported vacuum nipple like the 1973-74 carbs:



Where’s the ported vacuum slot on the passenger side..?



Here’s a stock factory carb with the EGR ported vacuum slot in the as-designed location:



Opening the throttle to almost ¼ throttle opening reveals the EGR vacuum port – a small round hole drilled high in the throttle bore. This would eliminate EGR operation at cruise speed as designed, and only allow the EGR to receive signal under a partial power setting, which may make the EGR inoperable:



I then installed the carb on my test engine (357 ci small block Chevy with a mild “268” type street cam, Dart heads producing 9.5:1, iron Q-Jet intake, headers, 14 degree initial timing with 14 degrees vacuum advance hooked up to manifold vacuum) in the as-received configuration. The idle mixture screws, as received, were set at 2 turns. My test engine was up to temp from running another carb just prior, so the Chinese carb was installed with the choke wide open.



Carb fired right up, but had a crappy idle quality. Wideband air/fuel explains that (in case you can’t read it, it’s showing 10.5:1 A/F)



Idle speed was tweaked to produce an 850 rpm hot idle, and idle mixture screws were turned in until fully seated, closing off all idle circuit fuel flow. This got the idle mixture into the 11’s:



No fuel was dripping from the main discharge nozzles, so the carb was getting idle fuel from the transition circuit.

Pulled the carb off and flipped it upside-down to take a look at throttle blade opening at idle speed. The carb had an abnormally large throttle opening to maintain idle, exposing the entire transition slot:



Idle throttle opening is so big that the ported vacuum slot is even being partially exposed at idle:



Compare to the throttle blade setting of the successfully set up factory carb that had just come off the test engine (this carb has the passenger side vacuum nipple set up for manifold vacuum, and uses the “low” vacuum slot rather than the “high” slot intended for ported vacuum):



Chinese carb has a very short transition slot, and the entire slot was fully exposed at idle speed. Idle mixture screw can be seen fully seated in the idle circuit discharge hole:



Factory Q-Jet slot for comparison:



Teardown of the carb revealed that every “trick” possible had been utilized to get the carb to run rich. Typically, a lean-running carb that won’t richen up enough to respond to idle mixture adjustment, or which runs a little lean at cruise, can be corrected by using one or two of the little “richen-it-up” tricks to “tweak” the fuel curve a little. But this carb uses all of the “tricks” at the same time, causing a massive rich condition that cannot be corrected with basic adjustments.

The float bowl, modeled after a 1969 bowl, has been changed to eliminate the idle air bypass bleed holes:



Stock 1969+ bowl with the idle air holes:



This explains the need for the excessive throttle blade opening required to maintain idle – the idle air bypass holes allow the throttle blades to be closed down at idle, much like drilling the throttle blades on a Holley. Blocking these holes also richens up the idle and transition mixture. The throttle plate itself is set up with the air bleed passages and discharge orifices, so the system can be made operational by drilling the holes in the float bowl to allow air flow:


To richen up the main metering circuit, the airhorn is equipped with Main Air Bleeds:




These are sized at .047”. This will have little effect on idle but will certainly have an effect on richening up the main metering circuit, depending on the size of the primary jets. Carbs using these main air bleeds typically have primary jets in the 66-range:



First thing I noticed in the float bowl was that the Chinese obviously have not read the GM/Rochester Service Bulletin regarding correct installation of the float needle clip. The clip was installed backwards:



A quick check of the float level was shocking… I don’t know why this thing was not spilling fuel out of the accel pump hole:



The carb has an odd needle in it, and the float/needle/seat geometry is all messed up, as you can see from the float level being set above the rim of the bowl.

The power piston is retained by a metal retaining collar rather than a nylon collar. This collar is pressed in hard, and makes power piston removal difficult:



Power piston is close to a stock piston (Chinese on right, factory stock on left):



…but the stop pin on the Chinese piston is .040” longer than the stock pin:



To compensate, the stop pin tang (APT) in the throttle plate is depressed to allow the long-pin piston to ride at the near-correct height. This means that you cannot interchange the power piston in this carb with a stock power piston without changing the stop pin tang height:



Power piston spring is taller and stiffer than a stock spring, making it unsuitable for most mild performance applications. Chinese spring on left. Stock 7036019 spring, as used in most Chevy and Corvette carbs, on right:



The primary rods are a unique creation. 4MV rods typically have .026” diameter power tips, with M4M truck rods (“M” rods) having .036” tips. These unmarked Chinese rods have .023” power tips, and measure out on the cruise side at .030”. Chinese rod on left - OEM Rochester rod on right:



Primary jets are also unmarked, but measure out as equivalent to #71:



That makes the jet/rod combo 71/30, which is a 41-size split. A normal target range is usually around a 30-size split, making this an extreme rich condition.

Air bleeds have also been installed in the high speed circuit side of the carb. They are sized at .043”. This size high speed bleed is usually used when primary jets are in the mid-60’ish range. Combined with the MAB’s at .047, I would anticipate this carb has an extreme rich cruise mixture:



The low speed air bleed holes (seen just to the left of the high speed brass air bleed above) are .060”. “Normal” size for these bleeds in Chevy carbs of this vintage is .090”. I can often correct a lean idle/transition/light cruise condition by reducing bleed size down to around .075 - .080”, but I’ve never been able to use a bleed as small as .060” without causing a massive rich idle and transition condition.

The IFR tube restrictor orifice is the large .038” size – “normal” IFR sizing on these carbs is .035”. Further richening up idle and transition.

Pulling the float out of the bowl reveals that they have installed the M4M style (1975 – 1981) float instead of the 4MV float. Chinese float on top – standard 4MV float below:



The seat had been grossly over-torqued into the float bowl, and had to be “torch-extracted” so it could be replaced with a “real” GM/Rochester spec part:



The first round of mods took the carb in the right direction:

I first repaired and correctly adjusted all linkages. Here, the secondary actuation linkage is being bent, corrected, and properly set up to open the secondaries at the proper time, while allowing free movement and full closure of the primary throttles. If this linkage is not set up right, the secondaries may not open, and the carb may not be able to “idle-down” due to the linkage holding the primary throttles open:




Note the small air gap between the secondary actuation rod and the secondary actuation lever tang – compare to the first 2 photos above of how it was “out-of-the-box.”. This linkage is now functional. It was not before:



The spring on the secondary airvalve is not an OEM-style spring - it's much heavier. By playing with windup and lubricating the airvalve shaft, I was able to get "normal" feel and response from the airvalves at 1/2 turn windup (the "normal" 3/4 turn Chevy spec would not allow the valves to open).



The Chinese choke pulloff has a stroke that's too short. To keep the secondary airvalve rod "tight" in the airvalve lever under vacuum, the rod was bent/adjusted in a way that would not allow the secondary airvalves to fully open at WOT - the pulloff stroke is too short. I bent the rod to allow full secondary opening, but the airvalves now have some "slop" at initial opening that's not restrained by the pulloff. This could result in a secondary tip-in stumble. The pulloff should be replaced with a "real" pulloff that has a spec stroke.

Here you can see the secondary airvalves are not fully opening:



Notice how the secondary airvalve rod is preventing full travel of the airvalve shaft – the stop tang is not hitting the airhorn casting:



Secondary airvalve rod is bent to provide adequate airvalve shaft rotation:



Airvalves are now free to fully open, allowing maximum airflow at WOT:



Airvalve lever stop tang is now hitting the casting at WOT:



I installed a non-Chinese GM/Rochester type needle/seat and correct 4MV float. Set float to .300"



Float level and geometry is now correct, and fuel level control is working as it should:



I then drilled the idle air bypass holes in the bowl casting, on both sides of the float bowl:




I installed "real" #42B primary rods in the “generic” Chinese #71 jets and installed a real 7036019 power piston spring to make the power piston operate correctly:



I assembled the carb with the idle mixture screws set to 2 turns. Installed the carb on the test engine again, and fired it up. It was running much better:



Choke was operational, and the choke pulloff opened it to spec while running cold:



Cold fast idle was set to about 1230 rpm:



Cold fast idle air/fuel mixture was way too rich at 10.8:1. It should be in the mid-12’s:



Once the engine warmed up and the choke was opened, I was able to back the idle speed screw out a full turn due to the airflow provided by the new idle air bypass holes in the casting:



Idle rpm was set to a fairly smooth-running 850 rpm:



But idle mixture was in the 11's with the mixture screws at 2 turns out:



Closed the idle mixture screws completely and got the idle mixture into the 13's:



Pulled the carb back off.

The throttle blade position was now observed to be almost where it should be. Very little of the transition slot was now exposed, and the ported vacuum slot is well above the throttle blade now. This is a huge improvement:



Carb was disassembled again, and the low speed air bleeds were drilled out from .060” to .076”



I then lowered the float level to .400" and set idle mixture screws to 1 turn out.

Re-installed the carb on the test engine, and it fired right up. For the first time it was running lean: Idle mixture at 15.4:1. Hooray!!!



Backed the idle mixture screws out to 1.5 turns, and idle mixture went to 14.5:1. Right on the spec, but the 1.5 turn setting is not to my liking… Carb sounded like a "normal" carb for the first time. Elevated rpm no-load mixture was at 11.5:1, indicating that the carb still needs more air bleed in the low speed circuit:



Pulled the carb back off the test engine.

Disassembled the carb again and drilled the low-speed air bleeds out to .082":



Installed a pair of OEM #67 jets with the OEM 42B rods:



Set idle mixture screws to 3 turns, and put the carb back on the test engine. It fired right up, with smooth idle and idle mixture at 13.8:1:



Closed the mixture screws 1/2 turn to 2-1/2. Idle mixture went to a stable 14.6:1, and I was able to further back out the idle speed screw to maintain the 850 rpm idle. Elevated rpm no-load mixture is now 13.0:1. It’s running right on the money:



Carb has good throttle response, smooth idle, and runs like a "normal" carb. Throttle blades are nicely closed in the throttle bores, revealing only about .020" of the transition slots.

After 6 hours of work, repairs and tuning, several rounds of drilling, a box full of real Rochester parts and jetting components to replace faulty or out-of-spec parts, 4 times on the test engine and 3 disassembly operations, this carb now runs reasonably well at idle and no-load. It remains to be seen how it runs under load on a real vehicle and how it performs at WOT – chances are good that the secondary rods and the airvalve operation will need some work in order for it to run well. Anyone expecting to buy one of these and have a nice-running carb out of the box will be truly disappointed... No – I don’t want to do this exercise again if you buy one of these carbs.

A good, used original Q-Jet could have been rebuilt and set up in about half the amount of time with significantly less effort….


Lars

chevygod

Lars, thank you for your time and expertise spent setting up this carb to run better at idle.

Looking forward to any actual updated driving reviews.

Thank you again
Tom

Vette5311

Wow. Thanks for the in depth analysis. I guess my first impression, not being nearly the carb expert you are, would be to not buy one. Just curious, if one bought one of these and then sent it to you for proper set up. What would that cost be? Thanks again for you and Jebby for the effort to bring this information to us. Hats off.

lars

See my second-to-the-last sentence and the last sentence in my post above... Good, used, rebuildable Q-Jet cores are still readily available.

Lars

Buccaneer

Awesome complete shakedown and write-up. Very cool Lars and now we all know the rest of the story. Good Job!

theandies

Doesn't sound like your typical "shade tree" mechanic could get it running like Lars can.
Good post Lars

Torqued Off

Well.....that settles that. Good to know, and thanks Lars for doing this. I guess there is more to carbs than just Chinese folks making something that looks like a Quadrajet!!

Redvette2

Thank you for taking the time to do this most excellent evaluation. The price difference between what the original poster paid for this carb and a new US manufacturer's one or a rebuilt OEM isn't really that much when you factor in all the effort that will be needed to make it run right.

4-vettes

Thanks much for the in-depth analysis. Very interesting indeed.

Buccaneer

Maybe the Chinese will see this thread and take careful notes and make a much better carb.

lars

No, they will simply put out a hit on me and make me disappear...

I would not advise the average hobbyist mechanic to start "tuning" a carb with a box of drill bits and a bag of jetting components... Most people would be unable to get this carb running at any satisfactory level. If all these Chinese carbs are built and set up like this one, I don't foresee these guys being in the carb business for very long...

There are no US manufacturers building new QuadraJets, and the commercially rebuilt ones are pure junk... That's why the Chinese are doing it.

64drvr

Wow, great write up!

Buccaneer

Noo...That's another agency from the US that does that. But I don't think they care about carbs, they have MUCH more important things to take care of right now.

Jebbysan

Lars sent me the rough draft of this thread but has added a LOT more info.....
Thanks Lars as always for dissecting the issues this unit has......it has been nothing but interesting although we all hoped the outcome would have been a bit better.....but hey, now the community knows and that was the whole idea from the start.
Looking forward now to the 5-Gas analyzer data that will happen on its next trip......
So far we have had the “Garage Scene” by me....the dissection by Lars and soon a full analysis of its ability to meter.....a Trifecta....it has been interesting and fun so far.....
Stay tuned!

Jebby

7t9l82

My doctor says watch my carbs. Now Lars tells me the same! Thanks Lars for your detailed analysis.

70vetteinpieces

would you be concerned about the aluminum parts being affected by the poor quality gas we have in USA? I have fooled around with lawn mower carbs and noticed some of the aluminum ones the metal has pucks or badly deteriorated forcing a complete replacement.

derekderek

i assume you have done similar with an out of box autozone rebuilt? at least they didn't weld up any passages.

augiedoggy

I had this happen to a new edelbrock carb I had. The inside of the float bowls was clear jelly from sitting a few months.

Jebbysan

It is a concern if you do not take care of your equipment, period.
I have been in the vintage bike world for a long time........ALL of these carbs are aluminum......and it is NOT the fuel that deteriorates them....it is the minerals in the water that collects in the bowls, particularly calcium. There is nothing in "todays" fuels that causes long term corrosion, it evaporates before it gets a chance.......but trapped water has a way of making a vapor cloud inside a float bowl for weeks or months.....
So no.....I am not worried about poor quality gas causing issues with the material......under normal maintained circumstances. BUT what will happen here is there will be thousands of these China-Jets that were installed to drive around the block and then pulled because it doesn't work....then they sit there and get dried fuel residue inside, which is of no consequence as it doesn't work anyway.
If a person owns anything with a carb on it now......regular start-ups thwart any and all issues.....this was true 40 years ago and is true now.
I pulled a BMW R100T motorbike out of a shed in 2009....it had not been started since 1983......the insides of the carbs looked just like they would if you put todays gas in it and let it sit for 27 years......
No, fuel doesn't keep like it used to....but it doesn't do what everyone thinks it does and people have short memories.....

Jebby

OldCarBum

Thank you Jebby and Lars for both great threads and for donating your time and expense to the cause.
Greg