Help diagnose idle dip.
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
Help diagnose idle dip.
Interesting symptoms: When the car is still warming up (but off choke), the idle will dip from 750 to 500 on initial deceleration timed with throttle lift. Throttle lift or deceleration by themselves don’t appear to have any effect. Once the car is fully warmed up the problem more or less disappears or at least becomes much less pronounced. I ran the float level at the middle of the sight glass (BG 750) for the summer without these symptoms. After a jet and power valve change was when the symptoms first started. They were made better by running the float level on the bottom 1/4 of the sight glass but they still haven’t gone away. Of note the power valve was changed from a 6.5 BG high flow PV to an 8.5 Holley PV with 14 inches of vacuum at idle. 4 speed car.
Race Director
Joined: Oct 2008
Posts: 19,183
Likes: 1,669
From: Long Island
I would guess - without an Air/ Fuel meter that the engine is going rich / lean when it decelerates.
Again I would guess when cold it cannot handle the extra fuel. I would try going down a jet size. (or so)
Normally with 14" of vacuum at idle I would choose a 6.5 or 7.5 PV.
Just my thoughts
Again I would guess when cold it cannot handle the extra fuel. I would try going down a jet size. (or so)
Normally with 14" of vacuum at idle I would choose a 6.5 or 7.5 PV.
Just my thoughts
Last edited by BLUE1972; Dec 18, 2018 at 09:22 PM. Reason: hate auto spell
Tech Contributor
Joined: Aug 1999
Posts: 14,376
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
If your're running your vacuum advance off "ported" vacuum, the ported signal can "partially" bleed off at throttle lift and eventually settle down to full retard at idle, thus changing idle speed. Put the vacuum advance on full manifold vacuum and see if the idle speed stabilizes (you may need to re-set idle speed and idle mixture once you do this). Power valve has no effect on idle unless it's ruptured. See my article here:
https://www.corvetteforum.com/forums...gine-idle.html
Lars
https://www.corvetteforum.com/forums...gine-idle.html
Lars
Last edited by lars; Dec 18, 2018 at 09:55 PM.
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
Thanks guys, I owe you more details on the setup. I will endeavor to check timing tomorrow (kicking myself for not writing it all down somewhere), but I know the vacuum advance is hooked up to manifold and it idles terrible with the vacuum advance plugged (I think the no-vacuum initial advance is 10 deg). I did up the idle RPM to around 775 or so for the drive today and while it still dipped while warming up coming to a stop it wasn't as detrimental to drivability.
After it was warmed up I did notice one additional thing: After lifting the throttle and coming to a stop the idle would sometimes (more often than not) only fall to ~850-900 for about 8-10 second before finally dropping down to ~750-775. I did not blip the throttle to bring it down - it just settled down on it own after that period of time.
After it was warmed up I did notice one additional thing: After lifting the throttle and coming to a stop the idle would sometimes (more often than not) only fall to ~850-900 for about 8-10 second before finally dropping down to ~750-775. I did not blip the throttle to bring it down - it just settled down on it own after that period of time.
Tech Contributor
Joined: Aug 1999
Posts: 14,376
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
See if you can induce the condition at no-load idle by working the throttle lever on the carb with your hand: Using your hand on the carb throttle lever, bring the rpm up to 2500 rpm-or-so. Then, let it go and see where the idle ends up at. If it's high, grab the throttle lever and wiggle it. If the idle immediately returns to the low rpm by wiggling the throttle lever, you have worn out primary throttle shaft bushings.
Lars
Lars
Thread Starter
Racer
Joined: Jan 2008
Posts: 291
Likes: 15
From: Virginia Beach VA
Lars and all,
Sorry for the long delay. I finally thing I have one of my issues sorted. It was a sticky throttle cable - I ended up putting on a Lokar Cable and that seems to have solved my idle issue. I didn’t get a chance to drive it yet so I’ll report back tomorrow. More data:
Idle Vaccum: 11 in. hg
MSD Distributor PN 8572
Initial Advance: 12 Deg.
Vacuum Can (Manifold): B28
MSD Advance Bushing: Blue (+21 Deg)
MSD Springs: 2 x Light Blue (all in by ~2900 RPM)
Total Advance: 33 Deg.
Thanks for all the suggestions!
Sorry for the long delay. I finally thing I have one of my issues sorted. It was a sticky throttle cable - I ended up putting on a Lokar Cable and that seems to have solved my idle issue. I didn’t get a chance to drive it yet so I’ll report back tomorrow. More data:
Idle Vaccum: 11 in. hg
MSD Distributor PN 8572
Initial Advance: 12 Deg.
Vacuum Can (Manifold): B28
MSD Advance Bushing: Blue (+21 Deg)
MSD Springs: 2 x Light Blue (all in by ~2900 RPM)
Total Advance: 33 Deg.
Thanks for all the suggestions!
