I have an 81 and did away with the computer carb and dist. I was told to just get an older qjet. So I found a qjet from a company that I bought. Not knowing what I was in for I bought it. After putting it on the car it just lost it performance. So I started checking into the code and found that it was off a 75 caddy. Now what?? Do I need to hire someone and put all new jets in this? Are all qjets the same except for the jets? Does corvette have a special qjet? Lost in carburator land.. Thanks Randy

 

There's all kinds of things that can be adjusted all different kinds of ways. Which is why I never messed with mine.

You can either get another QJet that's been set up for your engine, or you can hire someone to make all the correct adjustments.

 

Send it to Lars and have it reworked before you use it.

 

Suggest you get a copy of Lars Q-Jet tuning paper over at www.corvettefaq.com . He has done a pretty good job of researching the stock jetting in many different Q-Jets.

The Q-Jet is a sophisicated piece but not too hard to work on if you take your time. A good reference is the Doug Roe book, Rochester Carburetors (check Barnes and Noble).

 

I'm not sure if carb tuning is the solution to your problems, but I will tell you that changing the jets and rods on a qjet is not that big of a deal. Lars' paper has step by step detailed instructions for taking the top off the carb and changing jets (adjusting float and other things as well). I also found the doug roe book very helpful because it has pics and also step by step procedures. Go for it!

 

What exactly did you buy? From whom?

I would've suggested one of the QJets that Jet Performance sells through Summit Racing...

 

A caddy carb should be OK. There isn't anything special about the different carbs except for the vacuum ports and choke mechanisms. All a carb knows is the vacuum coming from the intake manifold and supplies the correct amount of fuel for proper combustion. As long as the vacuum is in the right range then any carb will be fine. If you put too small of a carb on an engine then the vacuum will start sucking fuel through the main jets even at idle and won't provide sufficient flow for higher RPMs. Putting a carb that is too big on an engine will find it hard to get the idle to work right due to a lack of sufficient vacuum to get the idle system to function properly...again the idle will be hard to get working, though in this case high RPMs will not be limited.

Thus, the real trick to a carb is in getting an idle system that works at the desired RPM and vacuum. The 75 Caddy had a 472 BB and idled at around 700RPM. Thus a 350 which I assume you have may not want to idle nicely below say 900RPM. You can try to adjust the idle jet and idle speed to get it dialed in better.

Or, when in doubt send it to Lars...bless his soul.

 

It's a LOT more complex than that. Different jet and needle combinations allow differing amounts of fuel to flow for the same exact amount of vacuum. Too big a jet and too much fuel flows. Too big a needle and not enough fuel at idle flows., and potentially too much (because the needles taper to a point that can be much smaller than the maximum size of a large needle) Yes, venturi size matters (okay, guys, sometimes size does matter), but a QJet body is, so far as I know, the same regardless of which application it was originally designed for.

So a QJet that's been jetted for a Caddy is very likely NOT jetted correctly for a Corvette. Especially if the Caddy had a big block and the Corvette a small block.

 

Why did you get rid of your electric qjet? The electronics have little to no impact on anything past part throttle operations and 0 on WOT.

 

Yes...there is more to a carb than what is visible. There are several vacuum and fuel passages running through the carb...one of the major problems with some carbs is when one of them gets clogged...even partially. And yes, the size is actually what the first selection is for a carb...its CFM or Cubic Feet per Minute rating. And, yes different jets are a good idea to have available to customize the carb for the specific application. Keep in mind though that carbs basically have just two systems...with idle jets and main jets from which metering and Air/Fuel mixture will be determined. And, it's nearly impossible to adjust a carb so that it provides perfect A/F mix throughout the entire range of RPMs. Thus, the reason for customizing to the application using jetting, for two purposes..one is to have a nice mix during the desired idle, and another is for a nice mix during the desired cruising speed. The transition between idle system and main system must then be sacraficed...ergo an accelerator pump is added and the transition allowed to become lean by design...the accelerator pump has some adjustment as well with lever arm and piston length.

It's really cool to see a girl getting into a nice tech discussion. You make some good points. Keep in mind that there are two systems...at idle the main system needs to be shut off, thus a carb must be designed for an engine with the right CFM at the required idle speed so that only the idle system is supplying the juice. Also keep in mind that if the caddy idle was like 550RPM while the vette required something like 750RPM then the carb would probably be fine given that the ratio of cubic inches to carb idle settings would equate to the same amount of air flow through the carb. Luckily, the idle system can be adjusted externally with a screw to assist in idle mixure tweaking, but the transition point can not be adjusted even if all the jets are changed...the transition point is fixed via venturi port size and angle, and the only way to change it is modifying the physical characteristics of the carb venturi. The most common and easiest for a rich carb is to drill a hole in butterflies away from the venturi port openings to allow more air in without disturbing the idle system port that sits right on the edge of the butterlies.

So, at the end of the discussion I would have to agree with you that a BB caddy carb is not likely to be the best match for a sb vette, but I disagree in that I believe it can work fine with only a change in the idle speed of the vette and an idle mixture adjustment. Of course if you are trying to tweak as much horsepower as you can then yes jetting would be an issue too. But, for a daily driver I think a caddy carb would work until a carb with better specs is secured.

 

Thanks for all the great advise Randy

 

I've counted to 10 enough times to count to a thousand or so, and I still can't resist the urge to respond.

Chronically mis-jetted carbs are BAD for engines and BAD for performance and BAD for fuel economy and BAD for the environment. Dumping unburnt gas down your exhaust manifold is a great way to do all kinds of bad things, including fry your cats. Getting an engine to idle doesn't mean that any other parameters are going to be right.

Combustion is a more precise process than "put air and fuel together in a confined space, ignite, extract energy." That's what jetting does -- it makes sure the correct stoichiometric ratio of fuel and air (the "14.7 to 1" ratio biz) is at least close. Yes, an unmodified carb off a reasonable sized V8 might run under some circumstances if you adjust stuff you really shouldn't have to adjust. But why not just put the right damned jets and needles in there in the first place and be done with? I've never jetted a QJet (I've jetted a lot of Mikunis and Keihins, tho ...), but I'm sure there are books and people who can answer questions. Learning to read spark plugs is plenty easy and a very effective way of answering questions about F/A mix under a variety of operating conditions. Come on, this is easy stuff -- don't be lazy!

 

Oh...don't bother counting...just let me have it between the eyes, I always ask for it, and deserve it since I rarely follow any popular advice, and come up with some off-the wall stuff which I can only back up with math & science stuff...for example I claim that removing the thermostat will help cool an engine since higher flow rates cool the engine better. Keep in mind though that I have a really tough forehead so not much gets through my thick skull, unless I want it to.

Yes...yes...But, my point was that the main jets are there to match the size of the venturi and the other specs of the internal system in the carb. And once the jets are matched correctly for the carb, then it doesn't matter if you hook even a vacuum cleaner to the carb as long as it can apply the right amount of vacuum and draw the right CFM through the carb the A/F mix will remain the same...there's really no magic going on or anything...there's just a vacuum being applied to the carb. The only major criteria in swapping a carb that has already had it's jets tweaked is where the idle transition point is and that the carb can handle the CFM required for the full range of RPMs of the engine.

OK...OK...if you want to get an extra horse or two then fine tuning with jets for a specific engine can be done in the event that the compression ratio is very different, typical air pressure is different (for example high altitude), etc. But, the improvement is minimal compared to the horses gained from timing, exhaust, etc. And, if you try to get that small of a gain by jetting a carb that already has the right jets for that carb under standard atmospheric conditions then you will be re-jetting at the change of the seasons right?

OK...standing by with the face shield down.

er...uh...sorry 81vettelover1, but did you intend to open a thread for a thorough discussion of jetting? If not I'll just shut my fuel port.

 

Have you opened this carb up? If so, what size primary jets does it have? Also what size primary rods and secondary rods are in there?

You say the car laid down. Is it going rich or lean? If you can tell us the calibration in the carb and if it is going rich or lean we can make some recommendations as to which way you should go.

Be careful when buying parts for this carb. 1974 and older carbs use a primary rod that is a different length than 1975 and newer carbs.

http://www.jegs.com/cgi-bin/ncommerc...51&prmenbr=361

The one you want is 350-1992.

BigBlockk

Later.....

 

My My getting pretty deep in carburator land. Its not a matter of being lazy Tallgirl, its a matter of letting all the horses lose at that giving second. If I wanted to ride a pony I would be driving a Mustang. SORRY Ford people. The main problem that started was I couldnt find the exact code for the carb. So with that said not sure what jets to buy. So to make it real easy I found a place called Carburator exchange in Arizonia that is building a carb to fit my needs. Yea lazy but easy. Bigblock you asked if I had the top off and checked for jet sizes, and rich or lean. If I had to guess I would say VERY rich at idle and very lean mid to higher RPM. Thats with the timing set at 12/36. Ready to duck in PA.

 

Wasn't saying you are lazy, Randy. I was talking to the guy who insists that Caddy carb is just fine. He seems like a smart guy -- just a wee bit on the lazy side.

Oh, and glad to hear you've found a place to build your carb. Getting one that's closer to start with should make your life easier.

 

Rods play an important part, regardless of jets. As Lars stated, I believe, you can mix/match jets and rods to suit your purpose, especially as not all jets/rods are available.

Also check Doug Roe's book on Q-Jets.

 

81vettelover1: Great news! Carburator Exchange sounds like a great place...do they have a web site you can post for us? If you no longer need the caddy carb then send it to Lars so he can use it as one of his loaner carbs for when one of us members send our carb to him for work...a vette that lacks ***** on the road is better than a vette which lacks the road itself, so I don't think anyone would mind using a caddy carb for a short period of time.

Uhm...thanks for the smart thing. As for lazy...I have rebuit my tranny, rear end (though goofed that up and had to get another one), brakes, did some fiberglass work to match up new bumpers, painted them with the help of VetteDan, installed them, did many small details including rebuilding my carb, and am now in the middle of completely rebuilding my engine along with painting every thing including some of the smaller parts to get that OEM look. I also spend some time looking stuff up in my manuals for forum members and offer my advice and stuff. Just because I think someone should go ahead and use a caddy carb doesn't mean that I'm lazy. Perhaps it means that I've worked on caddies (which I have) and know that they pretty much use the same Rochester carbs that go on our Vettes except for some small details like idle-main transition point, choke controls, and emission ports.

 

The Caddies are about 75 CFM larger than all other Quads.

 

According to Doug Roe, the Q-Jet guru, Q-Jets came in only 2 sizes - 750 cfm and 800 cfm. Happened to be looking at his book tonight, that's how I remembered the numbers.

Seems to me, though, I remember a 795 cfm Q-Jet, unless that was what he called the 800 cfm.