C3 rpm drop
Thread Starter
Instructor
Joined: Aug 2015
Posts: 129
Likes: 0
C3 rpm drop
Sorry guys, I'm re editing my post for better info.....
......my c3 is no longer cfi, switched over to an edelbrock 600cfi carb, & computer is no longer hooked up.........
Have another question for you guys, my small block 350, carb, has a comp cam with a 520 lift, all I know about it, the guy who built the engine keeps giving me a part number that comes up for a 396, idk, anyway, the vette idles at about 1000rpm, but when you drop it into reverse or drive it drops to about 250-300rpm, kid was over tonight & said its cause I have a stock converter, but if I install a stall converter I'll tear up my 700r4 trans, is this true about the tearing up the trans & the reason why the rpm drops off so much, I have checked for vacuum leaks, none, I have tried vac advance hooked up both ways, vacuum, & it does the same, stumped again
......my c3 is no longer cfi, switched over to an edelbrock 600cfi carb, & computer is no longer hooked up.........
Have another question for you guys, my small block 350, carb, has a comp cam with a 520 lift, all I know about it, the guy who built the engine keeps giving me a part number that comes up for a 396, idk, anyway, the vette idles at about 1000rpm, but when you drop it into reverse or drive it drops to about 250-300rpm, kid was over tonight & said its cause I have a stock converter, but if I install a stall converter I'll tear up my 700r4 trans, is this true about the tearing up the trans & the reason why the rpm drops off so much, I have checked for vacuum leaks, none, I have tried vac advance hooked up both ways, vacuum, & it does the same, stumped again
Last edited by Dano1982; Jul 31, 2016 at 09:04 AM. Reason: Not enough info
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
yes, Big cam motors, not that .520 is lift s all that big,..do idle better with a higher stall converter... and if you still have the original 700r4 from 82 with the smaller input shaft, yes they are notoriously weak transmissions.. But, dropping from 1000 rpm down to 300rpm in gear is unusual . try increasing your initial timing to 12-14* and use ported vacuum source
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
Double check your coolant sensor at the front of the engine. This gizmo tells the ECM that your engine is cool/warm/hot so it adjusts the idle accordingly. Yours may be faulty or going bad. They are in stock and pretty cheap at most parts stores. It will be different than the original so you will have to splice the wires (careful not to cross).
Bump up the timing to (at least) 12. Your engine will like it.
Our engines are super vacuum quirky so your Cam creates a variable... do you have a vacuum gauge with readings you can share? You need to be between 17-20ish AT IDLE.
Thread Starter
Instructor
Joined: Aug 2015
Posts: 129
Likes: 0
I have a couple of things to consider as I had similar problems with my 82...
Double check your coolant sensor at the front of the engine. This gizmo tells the ECM that your engine is cool/warm/hot so it adjusts the idle accordingly. Yours may be faulty or going bad. They are in stock and pretty cheap at most parts stores. It will be different than the original so you will have to splice the wires (careful not to cross).
Bump up the timing to (at least) 12. Your engine will like it.
Our engines are super vacuum quirky so your Cam creates a variable... do you have a vacuum gauge with readings you can share? You need to be between 17-20ish AT IDLE.
Double check your coolant sensor at the front of the engine. This gizmo tells the ECM that your engine is cool/warm/hot so it adjusts the idle accordingly. Yours may be faulty or going bad. They are in stock and pretty cheap at most parts stores. It will be different than the original so you will have to splice the wires (careful not to cross).
Bump up the timing to (at least) 12. Your engine will like it.
Our engines are super vacuum quirky so your Cam creates a variable... do you have a vacuum gauge with readings you can share? You need to be between 17-20ish AT IDLE.
I'm no longer a cfi set up, I've converted over to carb setup, also pretty much I'm all mechical so no coolant sensor & I've disconnected my computer which also I'm wondering about the transmission converter lock up which I think the computer controlled, I don't understand the lockup part so if its locked up or not & if thats going to affect all this? Also the carb is edelbrock 600cfm electric choke, its new but that's no guarantee, if the secondary butterflies are not closing completely, as your noticing I'm not a mechanic, I know how to do somethings but don't understand a lot & why it works but I'm not a complete moron lol
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
Your torque converter lock-up has no effect on idle. It only locks in 4th gear , and your carb secondaries are not your issue....you have a ported connection on the carb, use it for vac advance, increase initial timing to 12*-14* ..
Thread Starter
Instructor
Joined: Aug 2015
Posts: 129
Likes: 0
Ok, I'll check it again but I'm pretty sure I've already got my timing at 14* & I've tried both vacuum ports on my carb for vac advance, 1 question about initial timing, what rpm do u want it at when u set ur timing, I can only find info for total timing
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
set idle to 900 rpm in PARK.. adjust initial timing to 14* @ 900RPM with vacuum advance disconnected .. Use Vacuum Gauge and adjust idle mixture screws on carb to get highest vacuum reading at idle .reconnect vac adv
Thread Starter
Instructor
Joined: Aug 2015
Posts: 129
Likes: 0
Thanks, I appreciate it, I was thinking 750 or so but I wasn't sure, thanks again
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
Thread Starter
Instructor
Joined: Aug 2015
Posts: 129
Likes: 0
Well no luck my man, got it timed & no changed, even got the Ole lady out to have her put it in gear & tried turning the distributor both ways, still doing it, pretty much only like 3 or 4 vacuum lines going to motor, plugged them off 1 by one to make sure I don't have a vacuum leak I'm not getting to, still no change, really stuck now ugh......
Thread Starter
Instructor
Joined: Aug 2015
Posts: 129
Likes: 0
Well no luck my man, got it timed & no changed, even got the Ole lady out to have her put it in gear & tried turning the distributor both ways, still doing it, pretty much only like 3 or 4 vacuum lines going to motor, plugged them off 1 by one to make sure I don't have a vacuum leak I'm not getting to, still no change, really stuck now ugh......