Vacuum leak?

[Mcromeo27]

Recently finished an engine rebuild on my 81 long bed. Bought the truck about a year ago and it is far from cherry. Believe these trucks came with an electronic carb but that's no longer the case on this truck. My problem:  truck runs fine on start up and for about the first 15 min of driving but after thy it runs terrible. Won't idle in gear without gas pedal depressed and when I go to take off from idle there is a loud and pronounced "sucking" sound. Almost a whistle. Could it be just a vacuum leak?  Idle mixture too lean perhaps?  There is a fairly significant exhaust leak at the manifold to down pipe connection that I haven't addressed yet. Could that be the cause?  Any suggestions would be great and much appreciated. Thanks.

[bd]

Welcome to the Forum, Mcromeo27.

Probably a vacuum leak.

Use some carb cleaner to spray around the base of the carburetor and along both sides of the intake manifold where it joins the cylinder heads.  Carb cleaner is very flammable, so avoid spraying directly at the distributor - and mop up any puddles immediately.  If the engine smooths out while spraying - Voila!  You found a vacuum leak. 

Sometimes the solution to a vacuum leak is as simple as snugging down the carburetor mounting bolts or replacing a bad vacuum hose.  If you can hear the leak 'sucking' it maybe a faulty carburetor base gasket or dialoged/split PCV vacuum hose.  So spray away and see if there's a vacuum leak.

After the vacuum leak is corrected you'll need to adjust the idle mixture and speed.

[Mcromeo27]

Thanks bd. vac leak is what I've been thinking too. I've already tried tracking it down with carb cleaner as you suggested, but got non of the tell-tell rpm jumps. Is it possible the leak is coming from the underside (oil gallery). I've never encountered that before, but seems plausible to me. I can't remember what the pressure conditions are in the oil gallery and I can't think of a quick way to verify if a vacuum leak is coming from that side. On initial crank up I was having a bit of a leak from around the carb from where the gasket had given a little since I'd torqued it. So that seems suspect to me , but as I stated above the carb cleaner test didn't indicate that as a continuing condition. Thanks for the previous quick reply.

[bd]

If you think it's pulling from the valley, remove the PCV valve from the valve cover, then spray carb cleaner into the valve cover through the grommet.  A valley leak that bad would probably leave evidence on the spark plugs - either fluffy coke deposits or actual oil.  If you heard it sucking air, it probably isn't a valley leak.  Does your engine have EGR?

You could also have plugged idle passages in the carburetor.  Count the number of turns to lightly seat the idle mixture screws, then remove the screws one at a time and spray carb cleaner into the holes followed by compressed air.  Repeat the process 2-3 times.  Reinstall the screws, lightly seat them, and adjust them out the same number of turns you counted initially.  The screws should be adjusted very close to the same number of turns out - typically (but, not always), 3-4 turns for a QJet.

[Mcromeo27]

Maybe I'm just running it way too lean. Only have about 2 1/2 turns on mixture screws. Possible that once the choke opens all the way I'm having problems.  Doesn't explain the "sucking" noise though. No egr. Supposed to, but not there.  Somebody put an older motor in there and it's a pre egr intake. I'll give your suggestions a try and repost any progress or lack thereof. Thanks.

[bd]

Does the idle smooth out if you mist the carb with carb spray or hoover the carb with a shop rag?  Although QJet models can vary widely between 1-1/2 and 6 turns, I think you should try setting both screws about 3-1/2 turns out and see what happens.  The sound may just be air drawing past the throttle plates.

[DnStClr]

Just recently I saw ole Scotty Kilmer on Youtube check for vacuum leaks with cigar smoke. He pulled off the vacuum hose at the intake manifold and blew cigar smoke into the hose. Found the leak when the smoke poured out an unhooked tubing ( I think it was staged) under the choke. But, anyhow, it was an easy test to do- engine off of course...  prof. mechanics use a smoke machine. Good way to enjoy a smoke anyway.

[Mcromeo27]

Well. Richened up the idle mixture last night before leaving work. Runs a lot better. Can't believe it was that simple. Hope I didn't fry anything running it that lean go a 100 miles or so. Guess I was over thinking the problem. Anybody know a good way to fine tune the mixture screws without an exhaust gas analyzer?  Go by feel, sound?  I'll check my vacuum pressure and tidy up a few other things this weekend. Thanks again.

[LTZ C20]

Use a vacuum guage to obtain the highest vacuum reading. remove all vacuum lines from carb and plug there ports. Connect vacuum guage to a MANIFOLD vacuum port. Set E brake, chock wheels, start engine and trans in D if auto, N if manual. Lightly seat mixture screws, now start turning the screws out 1/8 turn each time, alternating between each screw every time you make a turn. once you have obtained the highest possible reading (you will know because you can turn the screw out more but the guage will stop going up), readjust idle to proper spec. Now place trans in P, N is fine for manual and shut off engine. Reconnect all vac lines to there proper ports. Install any other removed items, unchock wheels, release brake. Start engine, check for vacuum leaks, drive and examine results. If it idles high or has drivability issues you may still have a vacuum leak or something else going on. Be sure to check the OTHER other ends of vacuum lines to make sure they are connect to something. 

[sphinx]

Do the smoke test to find a vac leak.  I did this on my old Ranger.  I used a pint jar with burning insense inside with a inlet tube through the lid to a small aquarium pump for fresh air to keep the insense burning.  A second tube through the lid was connected to the intake vacuum port.  The low pressure smoke will find its way out of all leaks.  I had one a sender fitting.  I would have never found it with out the smoke.