What are the causes for a change in idle speeds on a carbureted, stock 305? I'm seeing a lower idle speed when my engine is cold than I do when it's warm, and I was wanting to know what I could look at to try to correct this.

I've checked/replaced some of the vacuum lines and while I've found a leak or two, fixing them didn't seem to help. I have noticed a VERY slight miss at higher speeds (~2000-2500)rpm with a occasional mild 'spit' thru the carb if I romp on the pedal while in this range- can a vacuum leak cause this or should I be looking for something else?

Thanks!


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Original owner - '84 T/A w/stock 305HO 5-speed

 

DT,

If anything, the idle should be higher when the engine is cold. Make sure the high idle cam is operating freely, and the choke thermnostat is adjusted correctly. The steps on the high idle cam should have a throttle stop screw resting on them when the carb is cold, and this needs to be adjusted for a specific idle speed. Certainly, a vacuum leak does not make the engine run any better, but typically will make idle speed higher than if the leak didn't exist.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Vader- that brings up another point I've noticed- the fast idle cam had been sticking on me the past few days. It first happened Friday and I found there was gasolene varnish on the pivot shaft, making it stick in the topmost fast idle position (just in that position- a nudge would be enough to free it to drop down). That got cleaned off with Chemtool until it wouldn't stick. But it tried to stick again yesterday- this time I added graphite to the pivot shaft. It's behaved so far, but then again the weather's been warm enough where this morning there was no choke (i.e. the fast idle cam didn't engage) so it's hard to tell right now how effective the fix has been....

But actually I'm more concerned with the spitting I experienced - other than a vacuum leak, the only other explanation I can come up with is either bad plug wires (the wires are ~9 years old, but I haven't noticed any arcing when viewing the engine at night) or a jumped timing chain (though I would think for that the engine would be running pretty bad). But I'm not positive on either of these possibilities, and I'd like to know more about the symptoms, or what else to check to narrow down the possibilities.

 

DT,

Running very lean can cause backfire. The valves tend to get quite warm and can ignite fuel before the intake gets fully closed. Obviously, a vacuum leak would tend to make you run lean. Since you're dealing wiht a Rochester E4ME carburetor, the mixture control solenoid is very significant in this RPM range under light load/cruise conditions. If the oxygen sensor is reporting incorrectly and/or the MC solenoid is not adjusted correctly, you might also be running on the lean side. Additionally (since EGR seems to be the topic-of-the-week) the EGR valve is supposed to be at a fairly large opening under these same conditions. If it is not, you could have even hotter chambers than from lean operation alone. If you have a dwell meter or duty-cycle meter, you can check the MC solenoid operation fairly easily to eliminate that possibility. Adjustment of the solenoid is fairly simple as well, if necessary.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

I'm not familiar with the carb in question, but the fact that cold idle is lower than hot idle, & the engine spits (when only cold??) suggests the following, assuming the carb has these components. Thermostat choke coil & vaccum pull off diaphram. The biggest mistake most people make is trying to adjust themselves outside the factory choke settings to correct a problem. Instead, they really should replace the components I mention, as they can deteriorate or fail over time. Another thing to consider, if the carb has one, is the electric coil throttle stop. I know on other non-GM products, the plungers get sluggish, giving unpredictable idle speed.
Regards, FJK

 

Vader-

Yes I do have a dwell meter so I will go ahead and check the MC cycling to make sure it's not running lean/rich. On the adjusting of the solenoid tho, don't you need special tools to do that?

Oh and by the way this morning the choke came on- choked for a few minutes (maintained around 1400rpm) then came off, and when it did it was idling a bit high, but correctly ('correctly' meaning it was at the RPM that it runs when hot. I need to adjust the idle down a little, tho- fyi it's idling at around 950rpm and I need it down to 750)



[This message has been edited by David Trimble (edited September 20, 2000).]

 

FJK- well the spitting is occuring (when it does) when the engine is warm. Haven't tried WOT when it's cold.

The carb does have the choke coil and pull-off diaphram - the diaphram was replaced a year or two ago, and the choke coil has never been touched (it's riveted in place). However I've checked with the engine hot and the choke is off at that point.

Electric coil throttle stop... do you mean the idle solenoid? There is one but it's only activated for the air-conditioning. Otherwise there's a idle stop screw that's adjusted to set hot idle.

 

DT,

It's been a while since I've toyed with an E4ME, but as I recall there are no special tools required. The adjustment screw may still have the factory plug installed above it, but the screw is just plain-Jane. Maybe this file would help: E4ME.pdf.

You'll need Adobe Acrobat Reader to open the file (see link below if necessary).


------------------
Later,
Vader
------------------
"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Vader- thanks, this file will help a lot! But you're right, there's factory plugs that I'll have to remove - but I think I can just drill a small hole and then pop them out by inserting a short screw.

Thanks again!