I thought I saw a thread on diagnosing vacuum leaks but can't seem to find it now. My problem is on a 75 L48 350 Rochester Quadrajet. It won't idle under 1,000. If I put my hand over the primary on the carburetor, it revs approximately 700 rpm more. That makes it 1700! This tells me there's a vacuum leak (I believe). I've been trying to locate the source for several hours and have used water in a spray bottle and squirting it along the vacuum lines. Another suggestion I've heard and decided to try is to use propane. Neither seems to work. I feel like I'm just wasting my time. Has anyone ever encountered this problem? Any tricks of the trade would be appreciated. Thanks in advance.

 

Isolate all the vaccum sources. It is most likely your headlights or your brake booster. Go to the intake manifold and look for all the vaccum lines. One by one clamp the lines and see if the engine revs change. If it changes you know it is sucking from that source. Test the Check valve (Circle disc woth two in one side and one out on other). Probable source are the headlight vaccum relay(s) mounted between the headlight actuators or the brake booster check valve. Also look up under the dash and see if you kicked a vaccum line of a connection.

You can also go to NAPA and buy a vaccum plug kit. It has assorted caps to block of vaccum. Unplug the vaccum lines and blcok them off. especially the one on #8 intake runner.

There is a guide at corvettefaq.com from memory.

Don't give up the fun is just beginning!

 

don't forget to check the intake manifold bolts....

 

check that little "T" fitting on the back of your manifold mine is so loose you can blow on it and it lossens.....if that isnt the one that was already mentioned.

 

i also have a 75 l48 which had the same problem. i checked and checked for leaks but couldn't find one. check the throttle shaft play side to side and up and down. mine had a lot of play which was letting air in through the sides. you can have new bushings put in to fix it. but i just got a new edelbrock 750. runs a hundred times better than my old qjet.

 

I really appreciate all the comments and will try all of them, but when you say check for leaks, how are you actually physically checking for them? At the present idle of 1000 when I put it in drive it will stall 100% of the time.

Thanks

 

get a can of carburetor cleaner and spray the lines and fittings. if the idle increases, the you know its leaking .and like stingry said, plug off all of your vacuum ports and see if it runs any better. then plug in each vacuum individually to see which one is leaking.

 

Well I just spent another 2 1/2 hours trying to find that fricken vacuum leak. I pinched every line I could see and spayed water on the intake, every vacuum switch, line, around and under the carb, the linkages............theres got to be a better way. Grrrrrrrrrrrrrrr .... As soon as I put my hand partially covering the top of the front half ( primary as I call it) of the carb, it revs about 800 rpm more. I tried lowering the rpm to 900 but it kept stalling. I feel like giving up. Sniff Sniff.......and I reeeek ( how do you spell that? ) like gas. I better take a shower. I'll be having nightmares tonight about vacuum leaks. What a life.

 

Wait a sec...I am not sure that is indicative of a vacuum leak or not. You say you are partially covering the intake? Well that's what a choke does to richen the mixture on cold days to raise the RPM. Now if you totally cover the intake and it does not stall then you have a leak, if you partially cover the intake and the RPM goes up then that is exactly what should happen.

You may want to look in a different direction - timing, idle mix, etc.

 

I always thought that if you partially cover the top of the carb ( front half that is ) and the rpm increase considerably, than it's a vacuum leak. The bottom line is that the car won't idle lower than 1000 rpm in neutral. If I try to put it in drive it stalls every time.

 

The only thing that shows (hand over carb - increase RPM) is that the engine is running lean - caused by too much air or not enough fuel. Could be a vacuum leak, could be time for a carb rebuild. I'm not familiar with Quadrajets but it could be a blocked idle circuit? Maybe your throttle plates (front or rear) are stuck open and letting in too much air?
Try to totally choke the carb, if it doesn't die then I would say yeah you have a pretty big vacuum leak.

 

Hand over carb increases rpm by another 800-900. Both throttle plates are not stuck (visually checked), carb is running rich due to excessive smell of fuel on my clothes and eyes burn due to fumes. Fumes even leak into house (as my wife lovingly pointed out). Carb was rebuilt before being stored 15 years ago. If I totally choke the carb, engine stalls immediately. I appreciate the information SpyderD.....any other suggestions? I tried plugging all the lines and didn't make a difference, chris75stingray. I'm thinking maybe the baseplate gasket since I'm unable to plug or seal that entirley.

 

Anyone want to buy a vette cheap.....won't idle under 1000 rpm..... I'm starting to give up on this car. I don't seem to getting anywhere with it. I spent another 2 hours in it tonight and I'm about to give up.

Must be upset with me cause I stored it for over 15 years and now it's her way of saying paybacks are a bi... well you know the rest. Hope your all enjoying yours.......don't mind me ...just have a bad week...month....whatever.....thanks for letting me blow off some steam....good thing I erased 3/4 of this post.

 

Don't give up! I know how frustrating it can be, but if you keep trying you will eventually find the problem - and people here on the forum are great and always give great help. We're pulling for ya! And once you find the prob you will have a great feeling of satisfaction - you will tame this beast.

My .02 - did you check the mounting bolts on the carb to see of they were loose?

 

try putting on a different carb that you know runs good. i tried for weeks to get my 75 running good and it didn't run good untill i got a new carb.

 

The best way to find a vacuum leak is buy a hand vacuum pump with gauge. Lights off, wiper door closed, hook it to the small vacuum line (controller) pump about 15" and see if it holds. Repeat on headlight, wiper door, brake booster, vacuum advance lines, etc.
Just went through this and found headlight vacuum relay was bad.
Gary

 

Believe me when I say that I really appreciate all the advice and support I receive from all you members, and I know that a lot of you can relate to the problems that I have encountered. You set a goal and try very hard to achieve it but find that you're going backwards and your estimated time ends up being 4 times longer than anticipated. Either Murphy's Law or the Pandora's Box principle takes place. I'm also upset at myself because I'm the original owner and I have no one to blame but myself for neglecting my so- called baby for so long. And if I give up at this point, what does that say about me!
Now back to the matter at hand.....
Ric80, the mounting brkts are already tight as well as all the screws along the top and sides of the carb.
chris75stingray, the carburetor was rebuilt before it was stored 15 years ago but I know anything could have happened while it was stored in the garage for all that time. As for a carb that is reliable... unfortunately I don't have one at hand, my budget is already spent and I'm only halfway there. I will keep my eyes and ears open for one. (I have nothing to lose by trying.)
Garys 68 I will check what a vacuum pump costs and see if I can acquire one.

So far I have disconnected and capped both lines going to the headlights, replaced and or capped 40% of the vacuum lines coming out of the intake and sprayed water all over the carb, lines and intake on the principle that if there's a leak the water will somehow seal it up temporarily, thus identifying a leak. I've pinched every rubber line coming out of the intake.

I think that I may replace the base plate gasket under the carb since I don't know of a true test to identify if the leak is there.

All this will take place this week and thanks again for all your advice and support.

 

Man I still think you are looking in the wrong direction as far as vacuum leak, but get a vacuum guage anyways its indespensible for tuning the car. I assume you ruled out timing and distributor problems for your idle problem? Are the plugs fouled?
You know alot can happen to a carb sitting still for 15 years, all these little air or fuel passages can get plugged up. Maybe some bug crawled in there and made himself a home. Maybe some left over fuel was in the carb and turned to varnish and made everything sticky.

 

SpyderD, I'm going to check the timing and I was thinking of doing a major tune-up once I took it off the blocks so that it's easier to work with. I still need to install new brake parts ( as you can imagine what shape they were in) despite being new when it was stored. Don't know how to diagnose distributor problems so hopefully I don't have any. As for the possible bug scenario, if that's the case c'est la vie, it's dead and hopefully decomposed. I can hope can't I?

 

After fifteen years, you can bet that the carb needs rebuilt. I'd start there. Then change the plugs, wires, cap, and rotor...maybe coil. Set the timing then.

I woke up a 427 after about 15 years in a garage. You wouldn't believe what some of the gaskets and seals in the carb looked like! They shrink up like raisins!