Qjet at altitude

[sierrablue]

Ok, so I started a thread on FI options for the B350 (here: http://www.ifsja.org/forums/vb/showthread.php?t=190833), but seeing as how the Qjet is one of the best carbs out there for power, efficiency, and generally being very adaptable (better metering, only downside is it's somewhat complex), how do they handle altitude changes? If it's tuned right here (~750-900 ft above sea level), how much will I run into problems with it pinging and such in the mountains? Will it be fine because of the small venturies (more precise metering, again) and the metering rods and the secondaries being vacuum (and metered) and everything? Obviously any combustion setup is gonna lose power to altitude because of the lack of oxygen, but I'm just concerned about how it'll do if I run it through the mountains.

That leads me to my next question--if I am going to run into issues (to make things easy let's assume using it daily through the mountains in Colorado), will it be better to retune it (Edelbrock tuning kit? just some different metering rods? Different jets?) for the high altitude and have it run lean down at more typical midwestern altitudes? Or is the carb just gonna be fine and I won't even have to back the timing off?

Sorry, I'm sure a billion people have asked this before but I can't find it anywhere and am just curious; I realize every motor is different and there's no way to guarantee one thing or another with it. I just want to know what other people have run into/noticed, and an AMC engine with the Qjet is fine too. Thanks guys!

[devildog80]

Carry your wrench in case you need to adjust your timing just a bit.

For just a drive through altitude for a few days, then back home, I think you will be ok.

Others will have other suggestions

[devildog80]

Do you have a spare Q-jet carcass you do not need?

Just checking as that is one of the possible changes I am thinking about doing, if the MC2150 does not play nice with my 401 here, when I get it rolling in a few months.

I have a 4 bbl OEM cast intake to put on the engine, but searching a source for a Q-jet carb I can rebuild, if the 2 bbl does not work out.

[sierrablue]

See the issue is that I might be spending weeks or months at a time in the mountains, and getting everything dialed in just right is enough of a pain as it is, let alone doing it all of the time. But on the flip side I know how to use this carb, it fires up on one crank (one long one if it's been a couple of days or more) every time, and it gets just about the same mileage that FI would, not to mention that I'd rather not drop $1100-1700 on a FI swap. And then you have to get it dialed in and everything...(nice thing there is once it's dialed in, it's set and you don't have to mess with it again).

I do have one but it has the threads stripped out of the fuel filter inlet of the body...so it'd be a good donor if you can find a body but I'm not sure the jets/rods/etc would be right for your 401 anyway (It's a B350 carb of course). If you need a good core though, around here I've seen some cores for like $50 if you're gonna get a tuning kit or something and just need something to start with...

[devildog80]

I have the 2 bbl to start with, and will see how it performs, but if I go 4 bbl the Q-jet is the way I want to go.

I grew up with them, so they are somewhat familiar, and know they are a good carb (like any other) once dialed in.

My belief is the older carbs are better then just buying one off flea bay, etc, and really do not want to buy a new one. One of the joys I have is taking the old stuff and getting it working again like it used to!

Just getting some feelers out there for future reference

Thanks

[sierrablue]

I have the 2 bbl to start with, and will see how it performs, but if I go 4 bbl the Q-jet is the way I want to go.

I grew up with them, so they are somewhat familiar, and know they are a good carb (like any other) once dialed in.

My belief is the older carbs are better then just buying one off flea bay, etc, and really do not want to buy a new one. One of the joys I have is taking the old stuff and getting it working again like it used to!

Just getting some feelers out there for future reference

Thanks

All good I think you'll find (and get frustrated by) the notorious Autolite stumble, which has been covered time and time again here and which out family has had issues with (Jeep, old Fords with the same carb). Almost always there, except when you first drive it when you try something to fix it At least that's been our experience, anyone who's finally solved it PLEASE LET EVERYONE KNOW.

Only other thing on the Qjet is that it probably has a better manifold, and the small primaries allow for better metering, so better mileage. And when you step on it, the carb won't be the limit But you know Qjets already...

Anybody with any experience with the Qjets in the rockies, I'd love all of the info I can get

[devildog80]

I have an MC2100 on my '81 CJ5 w/258, bought from another Jeeper with a '80 CJ7 w/258, and thought plug & play.

When we put it on my truck, it did have a little stumble.

We worked for hours adjusting to smooth it out, with no success until......I got some bigger jets.

As the carb was built in September 1966, I was skeptical we could get it to smooth out, as nothing we did was working.

He had .049 jets in it, and said it worked perfect on his rig, so I figured it would do the same on mine. But alas.....no!

I ordered a set of .053 jets, put them in and tweaked it a bit, and now it runs great.

From idle to WOT, when you romp on it, it goes right now with no hesitation!

Seems the only difference we had between our engines was mine has the air pump with emission rails on exhaust, and his did not.

It does run a little rich, but I only drive it through town or trails close by, so leaving it alone and enjoying the flawless performance as it is

[sierrablue]

See, our current stumbling car with the MC2100 is a bone stock '69 Cougar...it's best at cruising at 70 mph all day long and we don't want it to run rich...thanks devildog!

[devildog80]

If I put .051 jets in, it would probably take care of the rich condition, so you might see what it has now, and only bump up a couple thousands, instead of the 4 like I did.

With the .049 jets, my idle screws were out almost 4 turns.

When I went to the .053, they were back down to 1-1/2 turns, so thinking I could do the .051 and have it perfect. But it is so fun to drive as is, I am leaving it alone for now.

[sierrablue]

But it is so fun to drive as is, I am leaving it alone for now.

Boy, we all know how THAT goes around here Thanks again, will talk to people about that...

[backroadin']

Later q-jets came with altitude compensators. While it would involve another rebuild, or buying a new one, it would still be cheaper than FI. What year is your qjet from?

[sierrablue]

Qjet is off a '68 Buick, first year for the B350. I've heard/read/family has had experience with the electronic systems on the carbs ('84 XJ w/the 2.5)(it's multiport FI now)--is the compensator just one separate piece or is it a full big computer system?

[backroadin']

Sorry for the late reply...... no, the compensator is in the front corner of the carb if its got one (yours is too old I think), and I believe they came in the qjets before they went electric (84ish?) so you hopefully could find one pre-computer controlled. There's a round cavity in the body of the carb that houses the compensator thingy and if it doesnt have one, then its just empty.
You'd have to do some research on it since I dont have enough knowledge of it all. Theres a guy on the old chevy websites named Lars Grimsrund that has alot of good knowledge on qjets and there's plenty of info that he's contributed out there on the web. Search up his name with the word quadrajet and you'll find it.

[bufurd]

Your properly setup Q-jet will be fine with altitude. Getting it setup requires a stripped down body with the primary metering rods, piston, an assortment of springs and a vacuum gauge. The right spring is what you’re after, the vacuum needs to hold metering rods closed at cruising load, and begin opening at medium acceleration. NOT AT LIGHT ACCELERATION.. Only way I’ve found to do this is run a hoes into vehicle, T it so one side goes to vacuum gauge, the other to stripped carb. Under carb is a port that pulls the piston controlling primary metering rods, find it. I’d have my wife drive and I’d hold hose to carb and watch gauge and piston having her cruise at various speeds and get the medium acceleration thing down. This is all to figure out how much vacuum is needed to hold piston down, or maybe a better way to put it is at what vacuum drop is needed to began letting piston lift. I tested about a dozen springs and marked them for future use, opens at 5,7,10,12 whatever vacuum. Sounds complicated, but it’s not, actually a fun exercise that really helps understand how they work. This spring is KEY to a properly working Q-jet, performance, mileage, no bog, etc.. Of course sealing fuel wells, jetting, correct metering rods, secondary flap tension all has to be correct as well. Very forgiving carb, frankly jetting just has to be in the ballpark if spring is right.
As for altitude, changing the spring for one a bit heavier to hold it closed a bit longer would be at most the thing needed. There’s a reason it was the only carb to live into EFI era because it’s so good.

[bufurd]

Your properly setup Q-jet will be fine with altitude. Getting it setup requires a stripped down body with the primary metering rods, piston, an assortment of springs and a vacuum gauge. The right spring is what you

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