My ‘71 won’t idle with air cleaner on.

[Bad4wd]

Took my ‘71 for its first drive around the block in 20 years. Broke in the engine on the stand without air cleaner. Everything is new on the engine except the rebuilt Quadra jet. Truck runs and idles perfect without the summit brand air cleaner base (14” base open element). Soon as I lower the base on top of the carb (no air cleaner or top) the the engine rpms start to lower and soon die in any gear, slower in park & N. What’s your thoughts.

[dmjlambert]

That is interesting. Is the base touching anything other than the carburetor? Perhaps you can post pictures and that would help.

[Bad4wd]

Here’s what I got. Soon as I put the air cleaner base down the rpm go down . This pic engine cold.

[truckster]

Check for interference on any of the linkage.

[HO455]

Can you post a photo of the carb without the air cleaner base?

[geezer#99]

I betcha your converted electric choke (from a divorced choke carb) is interfering with the lid and pushing on the choke linkage enough to close the choke blade causing your problem.
Is that a home built conversion on the choke or a kit from somewhere.

[Bad4wd]

Soon as I lay down the air cleaner base on top of the carb the rpm gos down & slowly dies. Just plain open it runs awesome no hesitation just smoke the tires and idles.

[Richard]

Air cleaner base looks awfully close to the coil. Maybe try rotating it and checking the coil wire.

[Killer Bee]

ditch that low profile base, it's a truck with ample hood clearance

[Bad4wd]

Originally Posted by HO455

Can you post a photo of the carb without the air cleaner base?

The base seems to have clearance. Not touching any linkage. The electric choke conversion from Mikes carbs work find.

[Sheepdip]

Originally Posted by Killer Bee

ditch that low profile base, it's a truck with ample hood clearance

Like he said^^^^ at least try another air cleaner and see if anything changes.

[HO455]

Does installing the filter and the top make a difference? I know it sounds odd but I have had problems in the past with engines at the dragstrip only running a base that disappear when the rest of the air cleaner is installed or when the base is removed. My theory is it somehow affects the venting of the carburetor.

[firedemon]

I am just wondering out loud here but could that deep of a base cause the fuel vent to go into a vacume and vapor lock

[Bad4wd]

Update, when I start the engine cold, fast idle works and when warm up it fall out with the gas pedal. This is where things start to act funny engine is warm up the dual electric fans kick on and blow air across the the intake and carb and the rpms go up with fans on. If I block the air from the fans the rpms go down just enough to kill the motor in gear. Truck always fires back up. Crazy

[HO455]

Yes it sounds like a problem with the air flow around the top of the carb. The base may be creating an area of low pressure above the some of the bleed orifices on the top of the carburetor. That in turn is preventing the proper mixture of fuel to the idle and possibly the main jet circuit too. Does it run okay with the filter and lid installed?
I've had engines literally die (Like the ignition was turned off) at 5000 RPM fully loaded, pulling hard just due to a bad open base plate design. 30 seconds later the same engine will start right up and run fine until another pull to 5000 RPM. Swapped a different base plate or velocity stack or no base on the carb and the problem disappeared
GM engineers are a pretty sharp group. Take a look at a factory high performance air cleaner, they didn't end up being shaped like they are by accident.
If it runs fine with the filter and top installed I wouldn't give it a second thought.

[dmjlambert]

I am thinking about vacuum leaks. Do you have a thick spacer type of gasket under the carburetor, or the thin one that matches the intake properly? Is the intake smooth where it mates with the quadrajet, or is it the type of intake with the smile shaped channel cut on top (hot slot manifold)?

With the air cleaner base off, and idling well, spray carburetor cleaner around the base of the carburetor and see if the idle changes, that will detect a vacuum leak. And also, does all this funky behavior go away when you disconnect the brake booster hose and plug the hole in the intake or back of the carburetor, disconnect the distributor vacuum hose and plug the port on the carburetor it was plugged into?

What is that gizmo on the driver side valve cover? It doesn't really look like a PCV valve to me, but then I am not all-knowledgeable.

I would tend to disconnect all optional vacuum operated accessories, including PCV, and plug all ports on the carburetor, and see if the carburetor will tune well (you'll probably need to re-adjust the idle mixture screws) and work well and have non of that funky symptoms you describe. If you are able to achieve that, add the vacuum operated accessories back on one at at time. This is all assuming you don't find a vacuum leak due to wrong gasket or something like that.

Do you have any pictures of the carburetor from various angles when it was off the engine, so we can see what all ports you have on it and perhaps can figure out which hose is supposed to go to which accessory?

[Bad4wd]

It will not idle with the air cleaner on once it’s warm up an the fans are on. If I take the air cleaner off the air from the fans make the rpms go up just enough to prevent the engine from dieing in gear and runs great. When I block the air From the fans to the carb with anything the rpms go down.

[Richard]

Originally Posted by Bad4wd

Update, when I start the engine cold, fast idle works and when warm up it fall out with the gas pedal. This is where things start to act funny engine is warm up the dual electric fans kick on and blow air across the the intake and carb and the rpms go up with fans on. If I block the air from the fans the rpms go down just enough to kill the motor in gear. Truck always fires back up. Crazy

Did you try adjusting the idle screw?

[GMC72BB]

look closely at the butterfly arm, on the inside, chances are that the base is making slight contact with that, basically to close, or the base is just slightly to small for the Quadrajet. You have an after market base, & dimensions aren't always what they should be. That base might work on an Eldebroke, but not on the Quad. I have had this very issue before, it was due to the base plate. One cheap way to check it is go to Auto Zone, & buy a spacer kit, install that, then the base on top of that ( with a gasket) see if that spaces the base plate up, there are 3 spacers in the kit, smallest one might work. As far as the running once fans come on, you are forcing air into it, its not set up to be a turbo carb, LOL. Probabaly once you fix the base plate issue, install the air filter and lid, the other will cure itself. Address 1 issue at a time,,,

[Bad4wd]

Yes, i adjusted the idle mixture screws and idle screw. If I block the air from the fans with cardboard away from the carb ( no air cleaner on)the rpm go down and kill the engine in gear and in park. I’m going to disconnect the brake booster vacuum next and plug it. No video clips allowed on this site?

[Killer Bee]

upload to youtube and share link

[hjewell2]

Originally Posted by HO455

Does installing the filter and the top make a difference? I know it sounds odd but I have had problems in the past with engines at the dragstrip only running a base that disappear when the rest of the air cleaner is installed or when the base is removed. My theory is it somehow affects the venting of the carburetor.

Bingo ,,, it causes it to pull over through the vent tube seen it many a time.

On some holley carbs I have seen it produce a heavy mist from pull over at high RPM. tube needs to have a cut at an angle most rochesters didn't have it do not know why.

[Killer Bee]

this reminds me of a funny story I posted previously of a tiny piece of cellophane sucked into the hood scoop and stalling my gto..

amazing how delicate something so simple like a carburetor can be

good luck OP!

Originally Posted by Killer Bee

so this reminds me a funny story..

the yellow 67 GTO pictured <-- that way had a 550hp 469 that we squeezed best et of 11.7 out of..

back in my young sailor days, stopped at the light on east bound 4th st. at alamitos diag in long beach..

I opened a pack of smokes and tossed the cellophane out the window, into an also easterly wind..

yah I know, I stopped that stupid crap long time ago, geez..

anyways, light turned green, I stepped on the gas...... and whomp! stalled in the intersection..

crank, crank, crank, no joy..

now, never had a shortage of volunteers to push when needed so help was there to clear me to the side..

popped the hood, and there it was, that little piece of cellophane 1" x 3" neatly covering the primaries..

so naturally I eventually found a lid to fit right in that shaker but that really happened while it was off..

hmmm, off in this picture too, dumb kid

[Bad4wd]

Originally Posted by Killer Bee

this reminds me of a funny story I posted previously of a tiny piece of cellophane sucked into the hood scoop and stalling my gto..

amazing how delicate something so simple like a carburetor can be

good luck OP!

Good times and funny stories. I Also was in the Navy. Played with the carb today, same thing nothing new. Never had a quad change rpms with your hand in front of the primaries but, I never had a 9.8:1 comp. engine and stock converter with this carb.

[HO455]

Originally Posted by Killer Bee

this reminds me of a funny story I posted previously of a tiny piece of cellophane sucked into the hood scoop and stalling my gto..

amazing how delicate something so simple like a carburetor can be

good luck OP!

Nothing better than being a sailor with a GTO! Those were good days indeed.