Problem with starting and idle

[BPK2]

I am having a problem with my 1976 ‘02. Last week when I was driving, it suddenly would not idle although I could keep it running by holding the accelerator pedal down. I suspected a vacuum leak and replaced a very old brake booster hose with a new one. The check valve seems to stick a little but otherwise was functioning (preventing backflow). When I tried to start it, it would not idle at all but later maintained a higher idle (around 1500-2000) when I adjusted the idle screw on the carburetor (Weber 38). The in-dash vacuum gauge fluctuated slowly between 10 to 15 inches.

 

I worked on it some this weekend and here is what it is doing now. I can get it to start by holding the accelerator pedal way down but it won’t go above 1000 rpm, even with the pedal completely down, until it warms up some. Interestingly, the vacuum gauge shows it is drawing 0 inches. After it warms up a bit, it can be reved higher and vacuum gauge will read steady around 15 inches. When it is fully warm, I can get it to maintain an idle at 1500 rpm and the vacuum will hold steady at 18 inches and occasionally at 21 inches. Normally, it draws 20 inches at idle. I drove it down a residential street for about a mile and it seemed to have normal power when moving.

 

Any ideas what might be causing this?

[halboyles]

It sounds like an issue with your choke mechanism.  Take the air filter top off and check to see if the choke butterflies are closed when cold and open up when warm.  When cold you should be able to push down on the choke butterflies with only slight resistance and they should spring back closed when released.  You don't say what kind of choke you have.  If it is electric make sure the connection to the coil is good.

 

['76mintgrün'02]

does your carb have the electric idle cut off solenoid?

If so, is it getting 12V+ with the key on?

If so, does it make a clicking noise when you give/take power to/from it?

If not, it may be sticking.

 

It is also possible that there is a blockage in the idle circuit.

That jet is easily accessed while the carb is on the car.

I would probably pull it out and look at it and blow through that passage.

 

Are you running points?

If so, do you have a dwell meter?

If so, what does it say?

 

The change in your vacuum 'map' indicates that you may have a leak.

Are the nuts that hold the carb on tight?

If you have a vacuum advance distributor, you can check the pod by sucking on the line.

A failed pod is a vacuum leak.

 

Just a few things I would look at....  Tom

 

[BPK2]

Thanks for the suggestions.

 

The carb does not have an electric idle cut off solenoid and it has an electronic ignition system, electric choke, and mechanical advance on the distributor. I checked for vacuum leaks and the carb nuts but did not find anything amiss. I will check the choke and the jets.  

[zinz]

plugged idle jets ?

['76mintgrün'02]

I recently had this problem in my old Subaru.  

 

Everything was fine and then suddenly it would only run if I revved it up over 1200 rpm, or so.  

 

Taking the top off of that Hitachi carb is a bit 'involved', with all of the emissions stuff still intact; so I removed the idle mixture screw and blew through the opening with compressed air.  I also cut a piece of tubing at an angle, to slip in over the primary side air jet and blew through there as well.  Long story short, that fixed the problem and it idles happily again.

 

 

[Hans]

If  teh problem came on suddenly, I would support clogged jet diagnoses.

[Mike Self]

Since you don't have a vacuum advance distributor and presumably the other emission stuff has been removed, have you checked all the vacuum ports--on the manifold as well as the carb--to make sure they are securely plugged?  If one of those ports has become uncovered, it'll totally upset the fuel-air mixture, especially at low rpm.

 

mike