odd vacuum problem

[maddog]

i have an odd vacuum leak on Eagle Eyes that i can't figure out and i could use a little help with it. here's what is happening when i first start her up she doesn't want to idle but once she is running for a bit she idles fine but when i put her into gear she tries to die and then while going down the road if i give her half throttle i can hear a vacuum leak but not at low or full throttle. and i have checked the vacuum hose to the distributor and it is good as are the ones to the carb so to make a long story short it is driving me crazy as Eagle Eyes is my daily driver and i need her in tip top order.

[carnuck]

Sounds like the intake manifold may be loose and the torque of putting it in gear is causing a gap when you push the gas.

[BenM]

Check your emissions port and your spark port. The spark port is on the side of the carb, and the emissions port is on the base toward the font on the fender side. Those are open to the manifold at part throttle, and could cause problems. So could some of the devices they control, like the evap canister.

Check your PCV valve for noise as well. It could be sticking.

[maddog]

thanks for the responses guys i should add that at half throttle she behaves normally other than sounding like there is a very angry rattle snake under the hood. also it sounds like the hissing is coming from the passenger side of trhe car.

[BenM]

Oooh, The emissions on '80s are very different than what I'm used to. Do you have a picture of all that's over there? How much of all of that is intact?

[maddog]

not at the moment but i might be able to get one tomarrow and as far as i know it is all there. i do have one question though would the vacuum line to the brake power booster make a hissing noise at only half throttle.

[mudkicker715]

yup

[BenM]

A pic would help... There are several setups for emissions that year, and if there's an air pump that could be doing strange things. It modulates the amount of air with a couple of valves and switches.

[maddog]

it does have an air pump on it but it appears to be working correctly but i'm not 100% sure on that.

[maddog]

here is a shot of the whole engine.

here is the distributor side.

and here is the carb side without the air cleaner on.

[BenM]

I can't see any electrical switches, so it's not a CA setup! That helps.

You have one of the more complex vacuum setups, but everything looks pretty well intact. That little round plastic and metal thing hanging out on the valve cover is a vacuum modulator. It provides a modified idle vacuum.

But what I think is the first thing I'd look at is the plumbing that goes down between the manifolds between the carb and the firewall. Since the older iron intakes didn't have the coolant passage, AMC mounted the thermal vacuum switches on the block way down low in some water jacket openings.

The front one is EGR, but the back one switches your distributor vacuum signal based on engine temperature. When the engine is cold it gets manifold vacuum to help cold driveability, then when warm it switches to the vacuum modulator.

There should be manifold vacuum to the outside port, the center port goes to the distributor, and the inner port goes to the modulator.

The modulator gets the ported vacuum signal from the carb and manifold vacuum. The plastic body should be marked, and the port marked distributor goes to the switch I just talked about.

The parts going down to these switches pass very near the exhaust manifold, and so the rubber cracks and the metal tubes can rust through. I suspect that you're getting a leak somewhere in that plumbing or the switch on the block.

You ought to be able to tell if the modulator is making noise since it's way up front away from everything else, but it may be the problem as well. It keeps the distributor partially advanced at idle then transitions to ported as the throttle opens, but things act normally under full throttle.

I think you may be on the right track with the advance hose, it's just all that spaghetti in between.

I can't really find a vacuum diagram specific to this. I don't have a TSM handy. 80-82 setups can be very different.

I hope all this is helpful.

[maddog]

okay i think i know which three you are talking about as i have had to mess with them before because they occasionally pop off but they have never caused this problem before. although i'm not entirely sure i put them back on right.