Carb problem 4000RMP idle - help!

[spideyz]

Driving my 87 Brat to work today, as I let off the gas to slow down, the engine RPM increased to around 4000. I'm thinking "great, a stuck gas pedeal." I reached down and gave the gas pedal a yank and it seemed to be all the way up. I pulled over and played with the throttle lever. Seemed to be swinging back all the way and the cable was slack. I took off the air filter cover and looked into the carb...

 

Let me say now that I am a novice on carburetors.

 

So looking down on the carb, i noticed that the plate (throttle or choke plate??) that I think should be closed during idle was fully open. I tried to reach in there and wiggle it it wouldn't budge. When I put my glove over the carborator to restrict the airflow, the engine idled back down. The more air I restricted the lower the RPM.

 

So, I assume that my throttle (choke?) plate is stuck open.:-\ How does this gizmo work and how do I troubleshoot and fix this issue? It is an ECU controlled California car. Is the choke plate controlled mechanically, electrically or what? Thanks for any advice.

 

Keith

[GeneralDisorder]

The choke plate is only closed when the engine is cold, so what you saw is perfectly normal.

 

Choke is still mechanical on your car..... I would first see if the computer is giving you any codes. Note that the codes will be cleared every time you shut the car off, so you will have to drive it for 30 minutes or so, and then check the codes without shutting it off.

 

There is a fast idle cam inside the choke housing itself - if that were to bind up, then possibly it could set the idle high - but it would be more like 1500 - 2000, not 4000. Sounds like a problem with the feedback system - fualty sensor, or fualty control solenoids.

 

Best thing to do, is get a good running Hitachi without all that crap and put it on. Or a Weber, but not everyone can afford those.

 

GD

[spideyz]

Is the choke plate the same as the butterfly plate? Looks like only one of the holes on the top of the carb would be covered by the plate I am talking about. This plate is on the front half of the carb. Seems like the plate should be moving with the throttle. Since, when I put my hand over the entire carb circle within the air filter housing, it idles down, that plate closing should help lower the idle. Sorry if I sound confused... its because I am.

 

Keith

[GeneralDisorder]

Yes - that is the choke plate. It's controlled by a bi-metallic spring and an electric heater. It's designed to pull off after a specified interval (5-10 minutes I would say). And it should be open at all times after the engine's initial warm up. Under NO circumstances should this plate be closed after the engine has been running for a few minutes. It is used only to enrich the mixture durring startup and cold running. Cold metal in the engine tends to cause the gasoline to condense into droplets.... which don't burn.

 

To check the fast idle cam, you will have to open up the choke housing. Which means grinding the rivets off.... but I'm pretty confident you will find that it's working properly. Your problem is most likely in the feedback system.

 

GD

[Qman]

Check the ASV system. When they start to go bad or go bad for that matter they melt. The plastic innards of the bell(black plastic) melt and get sucked into the carb. This will cause the plate in the bottom of the carb to appear to be stuck open. I have had this happen twice on the Hitachi carb. You may have to remove the carb to get the garbage out.

 

 

Good luck.

[GeneralDisorder]

Yeah - I've had that happen as well. It's usually accompanied by a distinct change in the sound of the engine, and often you can smell the exhuast fumes from the melted silencer.

 

If you find this to be your problem, you can put a quarter in the pipe leading from the head to the ASV valve itself to block off the system. This will get rid of the noise and fumes, but you would still have to deal with whatever it damaged in the carb. Not a nice problem.

 

I was lucky in that the bits of silencer just jammed my choke plate open so the engine didn't want to start cold.

 

GD

[spideyz]

Does the ECU on the EA81 have the morse code LED flasher like the EA82s? I'll search this out and find the trouble codes.

 

I'm curious what is really controlling the idle? Does the feedback system (the O2 sensor and the ECU, right?) have a way of controlling the idle, so when something in that system is out of whack, the idle can climb to 4000rpm? I've read some threads about just unhooking the ECU altogether. Since I have to deal with CA smog checks, I'd like to fix it right.

 

Dumb question: What is AVS and what is the silencer?

 

Thanks for the help!

 

Keith

[GeneralDisorder]

Does the ECU on the EA81 have the morse code LED flasher like the EA82s? I'll search this out and find the trouble codes.

Yes - under the drivers side dash kick-panel.

 

I'm curious what is really controlling the idle? Does the feedback system (the O2 sensor and the ECU, right?) have a way of controlling the idle, so when something in that system is out of whack, the idle can climb to 4000rpm? I've read some threads about just unhooking the ECU altogether. Since I have to deal with CA smog checks, I'd like to fix it right.

Durring the year that I owned one, I never expereienced a climb in RPM's, but when my O2 sensor died, the car wouldn't even idle, so I would say the answer is yes - depending on the nature of the problem.

 

Dumb question: What is AVS and what is the silencer?

The ASV is the Air Suction Valve. You should have one on the drivers side head. The Silencer is a soft plastic muffler that is fitted to the intake of the ASV. When the ASV's reed valve fails, the hot exhaust will melt the silencer and the melted plastic will be sucked into the carb.

 

GD