high idle when hot

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beerboy

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my 73 340 duster has developed a small but aggrevating problem. car idles fine when first started. as it warms up, when you approach a stop light idle goes up. when I blip the pedal it drops to normal.possible vacume leak maybe? the guy I bought the car from said something about the brake booster having a vac leak? he said he had this happen to him and that's what it was. I just wonder what the booster would have to do with an idle problem? I have an eddy 1407 carb. brakes are fine. other owner said to try wd 40 on the throttle rod , possibly sticking a bit. did nothing. maybe idle air screws need adjusting? any ideas ? I am no pro mechanic, just another roofing contactor. cars a 4 speed
 
The brake booster gets it's vacuum directly from the manifold, if it does have a leak it could cause high idle.

You say when you blip the throttle it slows down. Is your choke/high idle sticking?
 
Blipping the throttle and having it return to normal elimates all the possibilities you mentioned except the linkage sticking. (Like Bad Sport said)

You might try a stronger throttle return spring if you don't see anything else.
One thing you might make sure of is that the high idle cam isn't causing it.
It is this type of a part you are looking for, and is part of the choke system.
It is designed to keep the idle up while the choke is on.
 

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yes there is choke linkage on carb. just not hooked up -----could it be a float adjustment thing or carb is too lean????
 
One thing at a time there beerboy.
Check that high idle link (find it and move it with the car idling high) first, then check the possible gasket issue like Old man Rick say's. (Should be easy with a flashlight)
Then you can move if needed, because jumping all around with no definite eliminated possibility's will keep ya busy for a long time with no results.
 
Yep, my first thought was the high idle cam flipping around and getting set. You need to wire the high idle cam down and /or make sure the choke plate is not floating loose. Though there is no electric choke on the 1407, the choke butterfly may still be in the carb, and if not fixed open and is loose, it may be being pulled partly closed by bumps or vibration when you close the throttle and stop and thus setting fast idle cam. You need to wire the manual choke linkage open.

Is there a throttle plate looking thingie near the top of the carb? That is the choke butterfly....It's the long flat thing in the top of this 1407 carb picture: http://www.edelbrock.com/automotive/mc/carburetors/sb/1407.jpg

It should not be easy to move but will be if the choke control is not attached to anything. With any throttle opening, the air flow SHOULD keep the choke butterfly open, but when you close the throttle, that choke opening force from the airflow is gone.
 
Throttle cable might be on the tight side.Try backing it off a little
 
there is a spring hooked to the arm of the choke plate on left side of carb, when I pull the plate down by hand it snaps back to open position pretty quick. isn,t this the same thing as wiring it open??
 
beerboy said:
there is a spring hooked to the arm of the choke plate on left side of carb, when I pull the plate down by hand it snaps back to open position pretty quick. isn,t this the same thing as wiring it open??
Click to expand...

The trick is to have the air cleaner off and start the car after sitting overnight.
Get in and push the throttle all the way to the floor (once) and let up.

How closed is the choke? (should be all the way)
Start it like you normally would and see how far open the choke plate is right after starting and while it's idling.

If it's all the way closed, push it open a bit with your finger.
Does the engine smooth out and seem closer to normal?
If it does the choke is choking too much and it needs to be backed off a bit.
 
beerboy said:
there is a spring hooked to the arm of the choke plate on left side of carb, when I pull the plate down by hand it snaps back to open position pretty quick. isn,t this the same thing as wiring it open??
Click to expand...
Sounds like it. I assume that this spring is going to fixed point not another moving point? NOW: Is the fast idle cam also locked down by this or is it free-er to move? That is what I suspect is your issue; the choke plate and fast idle cam are connected but one being fixed does not mean the other is fixed.

If that is OK (fixed so that the fast idle cam it cannot kick up and engage), then the suggestion made to check you throttle cable/linkage and spring is next on the list.
 
spring is on a fixed point not moving. I am really new to these carbs, had a stock carter on my 1970 duster 340. so you are saying even though the spring is keeping the choke plate open, which is connected to the fast idle cam , that cam should be should be fixed seperatly? did not look at that I figured it was all one unit and if one end was rigged to stay open the whole deal was. I will check that fast idle cam . It may be free to move.
 
Sometimes the linkages are connected in slots at one end or the other so they can move to some degree independently; I don't really know on this carb but just recommend that you check.
 
My guess is angle of throttle return spring. The throttle moves in rotation. If the anchored end of the spring is too high the throttle would stop in line with it.
This is one of the odd, cured it by accident, conditions I once had.
I put a thicker spacer under the carb for a totally different reason.
Funny how one thing leads to another. First I jacked the base timing up which increased engine rpm. Then I lowered the rpm with the curb idle screw. Afterwards the curb idle was not consistent. Once in a while the idle would stick just a tad high.
I did consider bending the return spring anchor but it would only bend left and right which wouldn't help. I told myself I would drill another hole in that anchor but never did.
Months later when I installed the thick spacer under the carb I was focused on correcting the link from choke actuator and never even noticed the throttle return spring angle had changed also. Anyway.. My Carter 2 brl. has worked perfectly ever since.
 
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