Rough idle keeps coming back - Page 3 - Chevelle Tech
Troubleshooting Diagnosing problems done here.

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post #31 of 43 (permalink) Old Oct 9th, 17, 4:04 PM Thread Starter
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Mike
 
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Re: Rough idle keeps coming back

I took a few minutes today and pulled/cleaned the idle mixture screws on the carb and sent some compressed air through their ports (which the ede video suggests doing), and then readjusted. While this really didn't do anything, I did realize that the fast idle when I started it up seems fine. Its just that once it warms up and RPMs drop, the idle becomes real rough. Could this mean that perhaps it is in fact carb related? My belief is that only a little fuel is needed to maintain the idle, so any minor obstruction could have a large impact. If so, how much would I have to dig into the carb to thoroughly clean it out?
Mike
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post #32 of 43 (permalink) Old Oct 10th, 17, 5:53 AM
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Geoff
 
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Re: Rough idle keeps coming back

I presume since you have fast idle, the choke is hooked up & working.

In fast idle mode, two things happen [ in comparison with how the carb operates once the engine is up to temp & off the choke ]. The carb idles on the idle ports plus more of the transfer slots are exposed to supply extra fuel. The closed/partially choke blade chokes off air supply & richens the mixture. So two separate carb circuits are at work to provide a richer mixture for a cold engine.

The fact that it idles ok when it is on warm [ fast idle ] but idles rough when warmed up [ lower idle speed ] suggests that it is likely too lean or two rich at operating temp. Though not the only possibility, more later.
Have you tried adjusting the idle mixtures screws at operating, choke off? As an initial adjustment, I would set the mixture screws at 2.5 turns off the seated position. Adjust both, in or out, a little at a time for best idle. Just listen & feel, forget about vac gauges & AFR meters, to do the idle setting.
This is not a fuel delivery problem if the fast idle is smooth, although it could be a float level problem; unlikely, but should be checked, @ 7/16".
Another possibility is cam duration. Depending on how fast the fast idle is, engine may have a 'cam' in it that idles smoothly at higher rpms, but idles roughly at reduced idle rpms. That could be normal behaviour for that cam with that combo. In this case, transfer slot position should be checked to ensure the no more than 0.040" of slot is below the pri blades at hot idle; requires carb removal to check.
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post #33 of 43 (permalink) Old Oct 10th, 17, 8:30 AM
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Gene
 
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Re: Rough idle keeps coming back

Quote:
Originally Posted by Snake Pliskin View Post
Replace the Edelbrock with a Holley. Sorry, I am just saying what others were thinking. Edelbrock is a glorified Rochester Quadrajet.

Ray
Hey clueless, the Qjet blows them all away in terms of metering. Please, a Carter ( EDEL ) is not a Rochester. Not even close. Is that what the "others" were thinking?> I bet it was.

Mike, Please replace your coil, especially if its in the cap. Also module. Impossible to test them properly, only way is to R&R. Heat is the trigger that causes me to say this.

Gene
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post #34 of 43 (permalink) Old Oct 11th, 17, 4:08 PM Thread Starter
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Mike
 
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Re: Rough idle keeps coming back

Thanks again for all of the great suggestions. I recently adjusted the idle mixture screwswith a vacuum gauge. However, yesterdayI warmed up the engine and, starting with the screws out about 2.5 turns, Itried to even out the idle. Even after severalmore turns out it still didnít fix the rough idle. Maybe that is where the problem lies?
Also, just wondering, if my fast idle is smooth can therestill be a problem with the coil. I donítmind replacing it, but I donít have a spare handy for diagnostics. So I just want to make sure before I order anew one.
Thanks again for all of the help!
Mike
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post #35 of 43 (permalink) Old Oct 12th, 17, 5:52 AM
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Geoff
 
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Re: Rough idle keeps coming back

Unlikely to be the coil or electrical if has smooth fast idle & rough idle.

Sounds like you are saying the idle mixture screws have no impact on idle quality?

Could be blocked idle jets/idle air bleeds, but most unlikely that both sides would be blocked.
Most usual cause of the above condition is t/blades open too far at idle exposing to much transfer which gives a rich & rough idle. The idle mixture screws lose control of idle mixture.
Have you checked for air leaks? Does carb base gasket seal all the passages?
Time to remove carb & check how much transfer slot is showing. Also check that sec blades close fully & are not binding.
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post #36 of 43 (permalink) Old Oct 12th, 17, 7:10 AM
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Gene
 
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Re: Rough idle keeps coming back

Geez man, I want to send you a Qjet! Or even God forbid , a Holley to try. Regardless of me bustin' pliskin for his ignorant prejudices, I run three Holleys atop my 427. Crude but effective.

Correct, it cannot be coil if it runs at fast idle. And a bad cam would shake all the time, and even give you a few "POPS" .

How far away from Hamden are you> WE know a guy there, who knows his musclecars....

Gene
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post #37 of 43 (permalink) Old Oct 12th, 17, 1:14 PM Thread Starter
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Mike
 
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Re: Rough idle keeps coming back

Everything ran fine together for 5+ years, then I startedgetting this as an intermittent problem. And the last time it started while I was sitting at a light. Geoff, does that make anything you mentionmore/less likely? I plan to pull thecarb next. Gene, I am not too far fromHamden. I am going to try and troubleshoot this myself for only a little while longer, and then will enlist the pros(any contact info you can provide would be much appreciated).
Mike
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post #38 of 43 (permalink) Old Oct 13th, 17, 6:34 AM
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Geoff
 
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Re: Rough idle keeps coming back

Mike, carb could have a build up of sediment/dirt/corrosion inside.
I would remove the carb top & check it out. Unlike brand H [ & clones ], you do not need any gaskets if you are careful. These carbs are simple in design, sophisticated in operation.
I will walk you through it. Take some pics first of the linkages etc so that you can re-install properly.
- remove fast idle/choke linkage circlips
- remove air cleaner stud
- remove acc pump linkage circlip, note which hole is used in the pump arm.
- loosen metering rod piston cover screws 2 turns. Swivel covers to one side & remove met rod assemblies and the springs.
- if using rubber fuel line to the carb, you can probably leave the line connected to the carb.
- remove screws around carb top [ airhorn ] .
- lift airhorn straight up, may need a light tap to loosen it, make sure gasket comes away with the airhorn. If the gasket sticks to the body anywhere, pry it away gently with a knife.
- turn airhorn over & lay it on the engine.

Things to check:
- are the float bowls clean, check jets for blockages. Idle jets are fed from primary main jets.
- check that floats are set at 7/16". With needle & seat wear, this can reduce & give a rich mixture.
- remove the float pivot pins, one at a time, then remove the float needle, check the rubber tip for wear/dirt. If they are worn, they should be replaced, can be bought separate from a repair kit.
- remove two screws holding acc pump nozzle; underneath in the pump passage, you should see the top of a brass needle. It should be DOWN.
- check that all the booster mounting screws are tight.

If all of the above checks out, it gets to really obscure reasons for the problem. Has the carb base gasket deteriorated; is there an air leak there?
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post #39 of 43 (permalink) Old Oct 13th, 17, 10:42 AM Thread Starter
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Mike
 
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Re: Rough idle keeps coming back

Geoff thanks so much for the helpful steps. I will pull the carb over the weekend and doas you describe. I will let you knowwhat I find out. Yesterday, after it waswarmed up and idling rough, I sprayed some carb cleaner around all sides of thebase and got no change. I tried the idlemix screws again. If you dial them all theway down, the engine will barely run (which I would expect to happen). Backing them out helps, but I am not gettingthe same range of adjustment like it has in the past. Given the junk I found in the tank, perhapsthat might turn out to be the problem.
Mike
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post #40 of 43 (permalink) Old Oct 16th, 17, 11:43 PM
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jeremy
 
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Re: Rough idle keeps coming back

A longshot, but being a '66, when it was converted to HEI was the resistor wire to the coil eliminated with a non resistor wire? Low voltage could be causing this issue especially when hot. Just a thought
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post #41 of 43 (permalink) Old Oct 20th, 17, 5:06 PM Thread Starter
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Mike
 
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Re: Rough idle keeps coming back

Hi Jeremy, thanks for the reply, the engine harness I havewas bought new specifically for an HEI set up.
Geoff, I pulled the carb off completely and followed thesteps you described. Now I know what theinside of an edelbrock carb looks like! I found a few small particles here and there which I cleaned out. More notably, I found some metal shavings inthe float bowls and even in a couple of the metering jets. These shavings are non-magnetic and as far asI can tell probably came from when the threads were cut in the carb housing(?). I cleaned it all outcarefully. After getting everything backtogether and back on the engine, I just started it up. Fast idle ran fine (like before). After it got up to temp, idle is muchsmoother now Ė 100% better (I just need to spend some time getting my idle mixscrews back in the right place). Couldit be these particles that I cleaned out of the carburetor were causing all ofmy problems?
Mike
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post #42 of 43 (permalink) Old Oct 21st, 17, 5:55 AM
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Geoff
 
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Re: Rough idle keeps coming back

Mike,
It certainly sounds like the metal particles were some/all of the problem.
The Idle Feed Restrictions, which are fed by the pri main jets are generally 0.031- 0.034" diam, so wouldn't take much to them.
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post #43 of 43 (permalink) Old Oct 21st, 17, 1:46 PM Thread Starter
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Mike
 
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Re: Rough idle keeps coming back

Thanks Geoff. I have to run it a little longer this weekend just to make sure the problem is resolved. Hopefully it is and I can get in a few rides before the winter.
Mike
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