I recently rebuild my engine on my 1977 vette, changes included new Edelbrock performer intake, mild preformance Comp cam, flat head pistons (compression now about 8.8:1). Now the car runs pretty good on the road, but it idles real rough and normally I have the RPM's up at idle to keep the car from stalling. Sometimes the car is hard to start. I set the choke back to original settings after fussing with that, and when I adjust the low and high throttle screws, basically I get no change even if I bottom them out. The vacuum is ok, but at idle I do get a little shake in the needle at idle, which can mean carb issues. At a high RPM, the needle is steady. I know there can be issues with a little less vacuum because of the cam change and some removed hoses (which are plugged on the carb). I am looking for some possible anwers to my issues. The carb before the engine rebuild did have some issues. You would have to pump the accelerator pedel several times to start and the idle had to be set higher, but the issues have gotten much worse now.

 

Rebuild carb, many have done it & not difficult.

The accel. pump seal may need replacing.

more see
http://forums.corvetteforum.com/c3-t...ump-issue.html

Idle mixture screws- there is limited adj. even w/ the limiter caps removed & on some setups even set all cc will not be too rich.

 

One symptom I forgot to list is theat the exhaust smells very rich at idle.

 

Do a search for links on using a vacuum gauge for diagnostics.
Double check all wires are routed to correct plugs.

 

What hoses?

I suspect a vacuum leak - easy enough to check for with a little carb spray.

 

vacuum is used to keep the power piston closed at idle. If I find a pic, I'll post it but it sounds like this is your rich idle concern if all other settings are good like float level and all... a spring change is required...

Another problem area is the bottom wells for the fuel bowl...which can cause starting problems.

 

If you did the mods to the engine as you describe and kept the '77 carb in its stock configuration, you are now running far too lean. Youo need to reconfigure the carb as outlined in my paper under the "Quickie Performance Setup" section after you set up your timing correctly for 36 degrees total.

The other concern is that you appear to be turning screws without knowing what they are or what they should do: "when I adjust the low and high throttle screws, basically I get no change even if I bottom them out." There is no such thing as a "high and low" throttle screw on a Q-Jet. Chances are, you're turning the idle mixture screws. The fact that there is no response on the mixture screws indicates that you are running so lean that the carb is running on the main metering circuit at idle, thereby causing the poor idle and rich condition (yes, believe it or not, but running too lean will make it run rich...).

Pull the carb off and set it up correctly - you're way too lean for your engine setup.

 

Idle fuel is metered and restricted by the oriface in the Idle Fuel Restrictor Tubes. Power piston position has very little effect on idle quality if the carb is idling on the idle circuit. If the carb is set up so lean that it won't idle on the idle circuit, the raised power piston will cause an extreme rich condition through the main metering circuit if the carb is idling on the main circuit.

The 77 carbs tend to run very lean, both at idle and at WOT. You may need to enlarge the idle tube restrictions along with a significant jetting change. As a last resort, you can block the idle air bypass holes to force engine to pull more fuel through the idle and transition circuits.

Lars

 

You have already changed a lot of things, so stock is obviously not what your trying to stay with. Get rid of the Q-Jet! I have had them and they are a huge pain in the A$$!!.Don't even get me started on their impossible idle circuit. I've rebuilt them, run them and eventually threw them out. I know they can be made to run well, and some guys think they're great(god knows why), but save yourself the hassle, get a holley, and be done with it. Q-Jets are not, and were never intended to be performance carbs anyway.

As a possible answer to your question, make sure the two lead plugs in the carb body aren't loose and leaking(they always do). I forget what exactly what they cover because I haven't worked on one for years,(main seats I think) you'll need to epoxy them if they are. Also check for play in the base plate butterfly shafts. They all seem to wear, and will produce a vacuum leak thats all but impossible to diagnose. They make a bushing kit to repair this.
Once you have $150.00 and 80 hrs into it, and still can't get it to run right for more than a month, you will officially be a member of the Quadrajunk club.

As stated on the forum before: Give it the water test
Throw it in a lake....if it floats, take it home.
If it sinks...it was evil and needed to die.

 

check the vacuum and be sure your vacuum advance can is correct for the vacuum you now pull at idle with the new cam.
See Lars' paper on GM vacuum advance control units. You probably need a B26.

My 327/300 has a performace cam installed by p/o during rebuild. It pulled 13-14 inches Hg at idle. It had a B1 vac can on the distributor. This put the vac advance can in it's midrange and it was wandering the advance and thus the idle. Runs much better with the B26.
Thanks Lars!
oh and btw be sure your total timing is at 36 degrees with vac can plugged.
thanks for that, also, Lars!

 

This guy doesn't have a clue what he's talking about - none of his comments (including those about the well plugs and shaft bushings) are correct or valid and he should be ignored.

Lars

 

Really Lars?

The base plate throttle shafts aren't often worn and causing a vacuum leak?

Those cast-in plugs don't often leak fuel?

A Holley isn't easier to work with and often times more reliable?

Yep...I have no clue

 

Quite simply, you don't know what you're talking about.

The well plugs rarely leak. I have seen 2 leaky well plugs in over 700 Q-Jet rebuilds. How many Q-Jets have you rebuilt..? I've been in this business for 35 years, including teaching Q-Jet carbs at the GM Training Center. Probably before you were born.

The throttle shafts on a Q-Jet are intentionally designed loose from the factory and do not need to be re-bushed unless there is so much wear that the throttle blades are allowed to **** and jam in the bores and produce an inconsistent return-to-idle. This happens rarely, and only on the primary driver's side when it does, just as it does on any other carbs, including Holleys. The length of the throttle shaft bore in the throttle plate causes enough of a pressure drop for the designed vacuum leak that the leakage through the shaft bore is irrelevant to engine operation.

A Holley contains over twice as many parts as a Q-Jet, and was not used extensively by the OEM due to high cost and high warranty claims on the Holley-equipped vehicles. The Q-Jet was designed and used for maintenance-free operation, and is superior to the Holley for reliability, emissions, fuel economy, and low-end torque (not my opinion - this is factual information gathered by GM, which I had access to as an instructor).

Your comments are not technically accurate and are not based on facts.

Lars

 

I'm not getting into a pissing match here Lars.

The guy asked what could be causing the problem.

leaky plugs COULD cause a rich condition.
Bad throttle shafts COULD cause a vacuum issue. It's just something to check.
Without having the carb in front of us it's really impossible to accurately diagnose the problem. Hell....it could partially be a weak ignition for that matter.

I've been around for a bit more than 35 years, and I personally have rebuilt quite a few Q-Jets(not 700), and I don't like them.
That's my opinion. I think they are difficult for the average garage mechanic to get set up properly.
The guy wants his car to run well. In my opinion he would be better served getting something new thats easier for the home mechanic to deal with. Not everyone wants to spend the time to become a master mechanic on every part of their car. Sometimes it easier and more cost and time effective to buy a new part. Again....my opinion.
I have one sitting in my garage right now (7045626....1975 chevy auto trans I believe) that you can have for the cost of shipping.
Pm me if interested.

 

Lars, you are right I am new to quadrajets. I was working with the idle screws on the bottom front of the carb. I set both screws to bottom and then 2 1/2 to three turns out, hooked up a vacuum gauge and no changes occurred as I slowly turned the screws in or out. I misspoke when I called them the throttle screws. And to clarify when you spoke about 36 advance did you mean 36 degrees advance BTDC at higher RPM's with the vacuum advance connected up? If so that is what the engine is running at. 8 degrees at 800 or so RPM with out the vacuum advance at the specs to set the car at initially and that is where it is. I will pull the carb on Wednesday and try the quickie set up and see how it goes. I have looked for a vacuum leak, but I can not find any. But the vacuum gauge is still at about 15-17 inches at idle, which I feel is too low. I will keep looking for reasons why the vacuum is too low. Would a bad PCV valve cause vacuum issues?

 

There is another way to check for a vacuum leak. Back off the throttle adjust screw all the way (to close the primary throttle plate). If the engine does not idle down, you have a vacuum leak, because air must be getting in somewhere. Don't ask me how I know... My problem turned out to be the wrong carb base gasket. Just another way to test...

 

Set timing with vaccum can disconnected and line plugged. Most small blocks like 36 degrees of centifugal (mechanical) advance all in by 2800 rpm or so. Add another 14 degrees of advance from the vacuum can.

Read the timing sticky at the top of the page for more info.

Why set timing at idle? Do you drive with the engine at idle? Standard timing is for emissions, not performance.

 

Gun Laker,

Heed Lars' advice and I recommend that you read the sticky on timing that SteveG75 mentioned. It will help you understand the relationship between timing and carb tuning. BTW, I can vouch for Lars' skill and knowledge as I participated in one of his tuning sessions a couple of years back. Anyone that has participated in one of these events can attest to his abilities in this area. Hell, Lars has forgotten more then most people know. One other thing, Q-Jets are not difficult to work on. Good luck

 

I fell for the Holley hype...out of the box performance upgrade from stock my butt. With the q-jet at WOT it sounds like cannons under the hood, the Holley at WOT was not the same. Now that Holley 4175 sits on a shelf in my garage, and the q-jet is back. Nothing feeds a motor like the q-jet.

 

Ok, today I went out and worked with the timing. The vacuum line to vacuum advance was removed and plugged. My initial setting of
8 degrees was already set (factory specs). I double checked the idle setting with and without a rubber band on the weights, no change. At about 2800 RPM (any higher RPM's did not change the reading) the timing was 14 degrees more or at about 22. I turned the distributor until the reading was at 32 degrees (8 initial + 24). The can added on about 14 degrees. The engine seems to run ok, but now the initial setting is at 16-18 degrees at the slowest idle I can get. Ok now if the difference between idle and high RPM should be about 24 degrees difference, what do I need to do to correct the issue. It seems to me I may need lighter springs? I have a stock HEI distributor that has never been modified as far as I know. The manifold vacuum at idle at the higher initial setting of 18 was better. Maybe I am confused so I am looking for thoughts.