1970 Idle Speed - "Decel Valve"
Thread Starter
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
1970 Idle Speed - "Decel Valve"
Hey there. My 1970 is running very well these days, but I'm always chasing the little things (aren't we all?). I've set my idle speed, timing, mixture, and dwell recently and the car runs great - except it wants to idle at around 900 instead of the 750 I set it at (L-46 4-speed).
The car will purr at 750 when warm no problem if I close the throttle by hand - but as soon as I flick the throttle again it will want to idle at around 900. That 900 is constant, it doesn't change - I do not believe this to be a loose throttle shaft problem. It's also not likely to be timing, etc., either.
Has anyone else with a 1970 had problems with idle because of the "deceleration valve" that these cars had rather than the standard speed screw used before and after? The plunger seems to hold the throttle out a little - to about 900 rpm - and if I apply a little pressure to depress the plunger, the speed will go down to the sweet sounding 750. Have a look at the vid and pics.
Brian
First video I've tried to post here, so hopefully it works:
The car will purr at 750 when warm no problem if I close the throttle by hand - but as soon as I flick the throttle again it will want to idle at around 900. That 900 is constant, it doesn't change - I do not believe this to be a loose throttle shaft problem. It's also not likely to be timing, etc., either.
Has anyone else with a 1970 had problems with idle because of the "deceleration valve" that these cars had rather than the standard speed screw used before and after? The plunger seems to hold the throttle out a little - to about 900 rpm - and if I apply a little pressure to depress the plunger, the speed will go down to the sweet sounding 750. Have a look at the vid and pics.
Brian
First video I've tried to post here, so hopefully it works:
Last edited by Brcmpbl; Sep 4, 2015 at 09:37 PM.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Looks like that dashpot is sticking alright.
Me, I would install a screw/spring and be done with it.
If you want it "original", try some carb cleaner and see if it frees up.
You "might" find a NOS unit at Doc Rebuild.
Me, I would install a screw/spring and be done with it.
If you want it "original", try some carb cleaner and see if it frees up.
You "might" find a NOS unit at Doc Rebuild.
Drifting
Joined: Jul 2006
Posts: 1,259
Likes: 154
From: Deplorableville Nebraska
I have 2 original '70 L46 cars. Your de-cel valve looks and operates like the 4 valves I have, including an NOS one I've been saving. Both of my cars came with 2 accelerator springs, one fore and one aft. I do not know exactly what the factory spring set up was, Bubba was there before I acquired them. I have tried using only one spring to the rear like yours, and had the same problems. I've also backed the valve out a little bit after stretching the little spring along the valve shaft.
Looks like you have a '69 dated carb, what is your build date?
Looks like you have a '69 dated carb, what is your build date?
Thread Starter
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
Hi Dave. It's very early - January of 1970, ...00523.
Maybe I'll add a second spring and see what happens. The spring that is on there is a new one - I thought at one point that maybe the original spring had just gotten a little soft.
Where was the second spring mounted by Bubba?
Maybe I'll add a second spring and see what happens. The spring that is on there is a new one - I thought at one point that maybe the original spring had just gotten a little soft.
Where was the second spring mounted by Bubba?
Thread Starter
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
Hi - no hole on my inlet for a spring. Are both of your cars like that?
When I bought the car the spring was set up through one of the linkage holes (I've done a lot of work since this picture was taken!):
When I bought the car the spring was set up through one of the linkage holes (I've done a lot of work since this picture was taken!):
Drifting
Joined: Jul 2006
Posts: 1,259
Likes: 154
From: Deplorableville Nebraska
no hole on my inlet for a spring. Are both of your cars like that?
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Thread Starter
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
Honestly I can't say I've seen the front mounted spring like that before on a C3. I bet it's effective though.
My car is early enough to have a few 1969 parts - it takes a 1969 style choke rod for instance, not the one that GM lists as the 1970 one.
Before I try anything else I think I will try to connect the spring to the linkage hole it was in before I rebuilt the carb - I don't remember having the idle increase issue back then, it just ran badly generally instead. Now it's great otherwise.
A lot of our cars are missing the valve or even the whole original carb. Maybe your second springs were common fixes for the problem I'm having back in the day, or maybe they're factory and mine is missing. The AIM etc doesn't show the second spring, but it doesn't really show the valve either from what I remember.
Many little mysteries with the 1970 cars. I like them.
My car is early enough to have a few 1969 parts - it takes a 1969 style choke rod for instance, not the one that GM lists as the 1970 one.
Before I try anything else I think I will try to connect the spring to the linkage hole it was in before I rebuilt the carb - I don't remember having the idle increase issue back then, it just ran badly generally instead. Now it's great otherwise.
A lot of our cars are missing the valve or even the whole original carb. Maybe your second springs were common fixes for the problem I'm having back in the day, or maybe they're factory and mine is missing. The AIM etc doesn't show the second spring, but it doesn't really show the valve either from what I remember.
Many little mysteries with the 1970 cars. I like them.
Drifting
Joined: Oct 2005
Posts: 1,615
Likes: 150
From: Forth Worth TX
Is your TCS system hooked up and functional?
I've found with my 70 L-46 that with the TCS system functional (no vacuum to distributor) the decel valve will be screwed in enough to function properly.
With the TCS system inoperative (manifold vacuum going to the distributor at idle), you can't back out the decal valve far enough to get a 750 rpm idle. It hangs at 900 rpm like yours.
FYI - Just one return spring is used, as you have it.
I've found with my 70 L-46 that with the TCS system functional (no vacuum to distributor) the decel valve will be screwed in enough to function properly.
With the TCS system inoperative (manifold vacuum going to the distributor at idle), you can't back out the decal valve far enough to get a 750 rpm idle. It hangs at 900 rpm like yours.
FYI - Just one return spring is used, as you have it.
Thread Starter
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
Hi Revi - my TCS is a goner, unfortunately. It's about the only thing on the car that's really missing mechanically. The wires are still there on the firewall, but that's it.
It would be nice to get it down to 750. It sounds so good there, and at 900 it will also want to diesel on shutdown (I drag it down with the clutch a little to prevent this).
Good info here guys, thanks.
It would be nice to get it down to 750. It sounds so good there, and at 900 it will also want to diesel on shutdown (I drag it down with the clutch a little to prevent this).
Good info here guys, thanks.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Hi Revi - my TCS is a goner, unfortunately. It's about the only thing on the car that's really missing mechanically. The wires are still there on the firewall, but that's it.
It would be nice to get it down to 750. It sounds so good there, and at 900 it will also want to diesel on shutdown (I drag it down with the clutch a little to prevent this).
Good info here guys, thanks.
It would be nice to get it down to 750. It sounds so good there, and at 900 it will also want to diesel on shutdown (I drag it down with the clutch a little to prevent this).
Good info here guys, thanks.
Just try a screw. If it doesn't stall, your golden. If it does, you know why.
Thread Starter
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
I put a second stock spring on the throttle linkage, and this does in fact close the throttle so that the car will idle at around 750 - this is at a cost in drivability though - the pedal is now very heavy of course.
I'm going to put a regular screw on the carb in place of the valve and see how the car behaves with that. I could always store the valve for safekeeping (those things are rather hard to come by).
I'm going to put a regular screw on the carb in place of the valve and see how the car behaves with that. I could always store the valve for safekeeping (those things are rather hard to come by).
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
I put a second stock spring on the throttle linkage, and this does in fact close the throttle so that the car will idle at around 750 - this is at a cost in drivability though - the pedal is now very heavy of course.
I'm going to put a regular screw on the carb in place of the valve and see how the car behaves with that. I could always store the valve for safekeeping (those things are rather hard to come by).
I'm going to put a regular screw on the carb in place of the valve and see how the car behaves with that. I could always store the valve for safekeeping (those things are rather hard to come by).
Melting Slicks
Joined: Feb 2004
Posts: 2,111
Likes: 32
From: Damascus MD
Idle Speed
My 70 L46 has the same problem with the TCS disconnected. TCS Hooked up and working properly it idles perfectly.
I can run full time vacuum and it will still idle fine. What I found interesting is that everything would start up and kick down fine until it was fully warmed up and I had run through the gears. Then it too would idle at 900 RPMs.
I also got an electronic clicking in my factory radio until I hooked up the TCS.
Bill
I can run full time vacuum and it will still idle fine. What I found interesting is that everything would start up and kick down fine until it was fully warmed up and I had run through the gears. Then it too would idle at 900 RPMs.
I also got an electronic clicking in my factory radio until I hooked up the TCS.
Bill
Thread Starter
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
The TCS will likely get restored to the car at some point, but not this year.
Good point about throttle shaft wear - the pedal was too heavy as well - so the extra spring is off. I've replaced the decel valve with a standard idle speed screw and it seems to be working well. I'll take it out tomorrow morning and see how it behaves after a run.
Good point about throttle shaft wear - the pedal was too heavy as well - so the extra spring is off. I've replaced the decel valve with a standard idle speed screw and it seems to be working well. I'll take it out tomorrow morning and see how it behaves after a run.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
The original return spring(s) were nested- a smaller one inside the outer. And they were not super stout either. Just enough. And pulled to the rear. I still have the original springs on my 69.
If you're not 100% stuck on original, look for the vent hole in that dashpot. Find a drill bit that fits the existing hole and ream it out a little bit.
If you're not 100% stuck on original, look for the vent hole in that dashpot. Find a drill bit that fits the existing hole and ream it out a little bit.
Last edited by TimAT; Sep 5, 2015 at 09:30 PM.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
U need a "dual" return spring. Most parts stores have them on the shelf in the restoration section.
Heres one from NAPA #NOE 7321113 ($6): http://www.napaonline.com/Catalog/Ca...113_0435569883
Heres one from NAPA #NOE 7321113 ($6): http://www.napaonline.com/Catalog/Ca...113_0435569883