Strange Idle Issue
Thread Starter
Drifting
Joined: Oct 2002
Posts: 1,748
Likes: 0
From: Titusville FL
Strange Idle Issue
Well I got the 72 back on the road to break in the newly rebuilt motor (Small Block 350 bored .040 over with Edlebrock E-street 202 heads, performer intake, and Holley 650 double pumper with mechanical secondaries). After driving about 20 minutes or so as I pulled into my neighborhood, the idle dropped very low (around 350 rpm in gear and 500 rpm in park/neutral). I was able to feather it back into my garage and shut it down. I restarted it a few times and the idle acted the same. If I got down on the gas it would rev with no issue and felt as strong as ever. Now here is the odd part.. After letting it sit for about 10 minutes, I ended up just trying it again and the idle seemed to be back to normal 
And right now idle seems to be fine again.
I couldn't find any leaks around the intake and carb. Was possibly thinking fuel filter as this seemed to happen after filling the tank (it was on just about empty before so thinking there could have been some sediment). But if it was the filter I doubt it would go back to normal. So wanted to see where I should start.
And right now idle seems to be fine again.I couldn't find any leaks around the intake and carb. Was possibly thinking fuel filter as this seemed to happen after filling the tank (it was on just about empty before so thinking there could have been some sediment). But if it was the filter I doubt it would go back to normal. So wanted to see where I should start.
Melting Slicks
Joined: May 2006
Posts: 2,590
Likes: 403
From: Nashville TN
St. Jude Donor '09
Did you set idle when engine was hot and choke was off? Is choke operating correctly? Maybe disconnect all vacuum lines and plug manifold/carb vacuum ports to see if problem still exist, then reconnect one at a time until problem reappears to isolate.
Burning Brakes
Joined: Oct 2001
Posts: 1,241
Likes: 4
From: Okinawa Okinawa
[QUOTE=redman76;1582758849]Did you set idle when engine was hot and choke was off? Is choke operating correctly? QUOTE]
Make sure choke is good and idle is set when the engine is fully warmed up.
Make sure choke is good and idle is set when the engine is fully warmed up.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
I've seen this issue caused by timing variations: Either a sticky centrifugal advance, or issues with the vacuum advance. Check your actual timing (with everything hooked up and running normally) when the car is idling normally, and then check it again when the problem occurs: I'll bet you'll see that the timing has retarded when the idle speed drops...
The other issue I've seen is failure of the secondary throttle blades to fully seat and close consistently. Check the secondary idle speed, and verify the secondaries are closing fully every time.
Lars
The other issue I've seen is failure of the secondary throttle blades to fully seat and close consistently. Check the secondary idle speed, and verify the secondaries are closing fully every time.
Lars
Last edited by lars; Jan 7, 2013 at 02:02 PM.
Drifting
Joined: Jan 2002
Posts: 1,645
Likes: 11
From: Schaumburg IL
As redman76 said - disconnect the vacuum lines or pinch them off with clamps. A serious vacuum leak will react the way you describe. Engine cold, choke on and ok, engine warm at idle and dies. Have been there and seen that.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Some thoughts:
* Likely not the fuel filter.
* Did you set the floats after the engine achieved operating temp (as recommended)?
* As stated above, make sure your mechanical advance weights aren't binding. Also, make sure you have enough spring tension to return the weights to the 'closed' (no advance) position. I've run springs that we're not strong enough for the job. Try the next stiffer pair.
* Vacuum advance? Ported or manifold?
* If manifold (recommended) do you know what the vacuum rating of the can is? If it's rated higher than your actual engine idle vacuum, you'll see inconsistent idle speeds. A quick fix is to switch to a ported vacuum source, but a better fix is to find a vacuum can that's "all in" just below your idle vacuum reading.
* When the car comes to a stop, does the engine quickly return to idle speed or does it take a while to finally settle into the correct idle RPM?
* Does the engine run fine otherwise? Any surging, etc?
* Likely not the fuel filter.
* Did you set the floats after the engine achieved operating temp (as recommended)?
* As stated above, make sure your mechanical advance weights aren't binding. Also, make sure you have enough spring tension to return the weights to the 'closed' (no advance) position. I've run springs that we're not strong enough for the job. Try the next stiffer pair.
* Vacuum advance? Ported or manifold?
* If manifold (recommended) do you know what the vacuum rating of the can is? If it's rated higher than your actual engine idle vacuum, you'll see inconsistent idle speeds. A quick fix is to switch to a ported vacuum source, but a better fix is to find a vacuum can that's "all in" just below your idle vacuum reading.
* When the car comes to a stop, does the engine quickly return to idle speed or does it take a while to finally settle into the correct idle RPM?
* Does the engine run fine otherwise? Any surging, etc?
Thread Starter
Drifting
Joined: Oct 2002
Posts: 1,748
Likes: 0
From: Titusville FL
Once again, thanks for all of the input.
Floats were set at temp, but will double check again.
Vacuum advance is from the manifold. Don't know what the current can rating is, but this got me thinking as I have read the paper that Lars wrote on vacuum advance. I will check the current vacuum coming from the manifold as my new cam is definitely producing less vacuum, which could be making my current advance can to not function.
Will keep digging and hopefully get this sorted out so I can continue with my engine break in.
And of course to add to the fun, I noticed a small leak on my drivers front caliper
If it is not one thing it is another.
Floats were set at temp, but will double check again.
Vacuum advance is from the manifold. Don't know what the current can rating is, but this got me thinking as I have read the paper that Lars wrote on vacuum advance. I will check the current vacuum coming from the manifold as my new cam is definitely producing less vacuum, which could be making my current advance can to not function.
Will keep digging and hopefully get this sorted out so I can continue with my engine break in.
And of course to add to the fun, I noticed a small leak on my drivers front caliper
If it is not one thing it is another.
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Melting Slicks
Joined: Aug 2012
Posts: 3,487
Likes: 83
From: Titusville Florida
I just had a similar issue with my Vette, cold it would run and idle just fine but once it got warm while sitting at a light I had to be concerned that it would die, the idle would bounce, idle fine for a few seconds then stumble to the point I thought it would die. I finally figured out that my mechanical advance was not working, put a new distributor in and now it's as smooth as can be...
Last edited by AirborneSilva; Jan 8, 2013 at 09:27 AM. Reason: spelling