Chasing an idle
Thread Starter
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
Chasing an idle
Here is what I have:
383/10.5:1
NKB 68cc 200cc Aluminum Heads
Howards Cam PART # 180265-10 hyd. roller cam. Valve liftwith 1.5 Rockers: Intake .530/Exhaust .545 Duration @.050": Intake.233/Exhaust .245 Lobe sep. angle: 110º
Quick Fuel Slayer 750cfm (Part # SL-750-VS) withelectric choke.
Dual Plane air gap intake.
Ignition timing was set at 34 degrees.
No vacuum or intake leaks.
I want the engine to idle at about above 850 rpm and not more than 1000 rpm
When I set my idle, once the choke opens, to 1000 rpm it climbs to 1400 RPM when the engine is warmed up to 190* to 200*.
When the thermostat opens and brings temp down below 190* the idle drops.
If I set the idle at 1000 rpm when the temp is at 190* or below and the temp drops to below; the idle drops way below 850 and engine stalls unless fed.
Am I totally lost here or is the inherent to the large cam and I just need to live with it?
383/10.5:1
NKB 68cc 200cc Aluminum Heads
Howards Cam PART # 180265-10 hyd. roller cam. Valve liftwith 1.5 Rockers: Intake .530/Exhaust .545 Duration @.050": Intake.233/Exhaust .245 Lobe sep. angle: 110º
Quick Fuel Slayer 750cfm (Part # SL-750-VS) withelectric choke.
Dual Plane air gap intake.
Ignition timing was set at 34 degrees.
No vacuum or intake leaks.
I want the engine to idle at about above 850 rpm and not more than 1000 rpm
When I set my idle, once the choke opens, to 1000 rpm it climbs to 1400 RPM when the engine is warmed up to 190* to 200*.
When the thermostat opens and brings temp down below 190* the idle drops.
If I set the idle at 1000 rpm when the temp is at 190* or below and the temp drops to below; the idle drops way below 850 and engine stalls unless fed.
Am I totally lost here or is the inherent to the large cam and I just need to live with it?
Race Director
Joined: May 2006
Posts: 18,728
Likes: 2,579
when you say set the idle, does that mean you are adjusting the idle speed screw and the idle mixture screws once the engine is at 195 or above, assuming you have a 195 thermostat?
you have checked for vacuum leaks at the carburetor base?
you have checked for vacuum leaks at the carburetor base?
Last edited by MelWff; Dec 14, 2016 at 03:09 PM.
Thread Starter
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
Yes setting idle at throttle screw.
Mixtures being set with vac gage.
No vacuum leaks.
Yes 195 stat.
If it idles good at 1000 rpm at 190* or above is looses idle if it drops below 190*
Starts easy. Runs strong and idles smooth for the big cam.
Am I just asking too much for the big cam to maintain an idle at temp changes? Maybe be happy with the 1000 rpm after 190*?
With the Dewitt's radiator with oversized internal trans cooler the temp changes seem drastic when the stat opens and closes. It can be running at 190*-200* driving around and when the stat opens I seen it drop to what I think is about 160*-170*
Instructor
Joined: Dec 2004
Posts: 216
Likes: 1
From: Lapeer Michigan
I would say the choke & carb are not adjusted correctly, I run a cross ram with .625 lift and idles at 650 rpm. w/ full roller motor
what type of distributor are you running ?
if vacuum how is it hooked up port or manifold or mechanical advance
what type of distributor are you running ?
if vacuum how is it hooked up port or manifold or mechanical advance
Last edited by 1BAD80; Dec 14, 2016 at 06:22 PM. Reason: another question
Race Director
Joined: Apr 2009
Posts: 19,294
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From: Charlotte NC
I was thinking along the 'same lines'...that the springs in the distributor may be too weak....hard to say for sure due to not knowing what is going on.DUB
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
Read the carb's manual on setting the duration and idle speed of the choke assembly. The duration might be set too long and causing the RPMs to remain on the fast idle cam. With a cold engine, set the fast idle just high enough to keep the engine running, and then at operating temp, set the normal idle speed for a steady speed.
Instructor
Joined: Dec 2004
Posts: 216
Likes: 1
From: Lapeer Michigan
I was also going to say the choke is closed to much 
it has to be adjusted on a cold motor and the throttle held about 1/4 down and the choke should be open about 1/8"
when the motor turns over it sucks it shut and opens the crack upon starting
it has to be adjusted on a cold motor and the throttle held about 1/4 down and the choke should be open about 1/8"
when the motor turns over it sucks it shut and opens the crack upon starting
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Thread Starter
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
Thread Starter
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
Everything is new with 3k mile now.
I have a MSD HEI distributor that was set up by skip white on the dyno. Not a distributor or timing problem.
Most likely I just don't know how to adjust this choke. Prob going to take is to some one to adjust it.
Or just replace the carb with an FI system and let the computer adjust it LOL
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From: Corsicana, Tx
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So what is the initial timing at 1000 rpm and 850 rpm? Does it change in that RPM range? As vacuum drops the advance will go too...and often centrifugal can be bouncing around in that area.
First I would get initial to 20-25* at idle with no vacuum and limit total advance to get your 34-36*. Then limit vacuum to whatever it takes...but most MSD's add a ton of vacuum advance and unless they installed the largest bushing in the kit...the initial timing will be awful low with a 34* total.
After making sure that's correct....I'd look at idle mixture. If it's dropping off when it cools off...my gut tells me it's set to lean at idle. I never use vacuum gauge to set idle...almost always ends up lean at highest reading and a healthy cam just needs more fuel.
The wildly swinging temps leads me back to timing also.
JIM
First I would get initial to 20-25* at idle with no vacuum and limit total advance to get your 34-36*. Then limit vacuum to whatever it takes...but most MSD's add a ton of vacuum advance and unless they installed the largest bushing in the kit...the initial timing will be awful low with a 34* total.
After making sure that's correct....I'd look at idle mixture. If it's dropping off when it cools off...my gut tells me it's set to lean at idle. I never use vacuum gauge to set idle...almost always ends up lean at highest reading and a healthy cam just needs more fuel.
The wildly swinging temps leads me back to timing also.
JIM
Thread Starter
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
So what is the initial timing at 1000 rpm and 850 rpm? Does it change in that RPM range? As vacuum drops the advance will go too...and often centrifugal can be bouncing around in that area.
First I would get initial to 20-25* at idle with no vacuum and limit total advance to get your 34-36*. Then limit vacuum to whatever it takes...but most MSD's add a ton of vacuum advance and unless they installed the largest bushing in the kit...the initial timing will be awful low with a 34* total.
After making sure that's correct....I'd look at idle mixture. If it's dropping off when it cools off...my gut tells me it's set to lean at idle. I never use vacuum gauge to set idle...almost always ends up lean at highest reading and a healthy cam just needs more fuel.
The wildly swinging temps leads me back to timing also.
JIM
First I would get initial to 20-25* at idle with no vacuum and limit total advance to get your 34-36*. Then limit vacuum to whatever it takes...but most MSD's add a ton of vacuum advance and unless they installed the largest bushing in the kit...the initial timing will be awful low with a 34* total.
After making sure that's correct....I'd look at idle mixture. If it's dropping off when it cools off...my gut tells me it's set to lean at idle. I never use vacuum gauge to set idle...almost always ends up lean at highest reading and a healthy cam just needs more fuel.
The wildly swinging temps leads me back to timing also.
JIM
Otherwise this engine is full of power and fun to drive. I just want to be able to start it and not have mess with the peddle to keep it running when cold. Once it's warmed up it does stay running.
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
I helped a friend that was having similar idle problems- the advance springs in his distributor were so soft that ANY little change in idle speed or load changed the degrees of advance. His idle was all over the place,
Race Director
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From: Charlotte NC
DUB
Thread Starter
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
I recently went through this on '67 427 tri-power. The supposed rebuilt distributor with electronic ignition actually had the wrong springs in it and it would not get down below 1200 rpm's and would die. When the original distributor was installed...it ran fine and a curb idle could be set.
DUB
Race Director
Joined: Apr 2009
Posts: 19,294
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From: Charlotte NC
It often times is so hard to diagnose problems when 'we' are not there to see what is going on.
So...personally I would address the idle RPM issue and then see what goes on with the temp issue if that is even an issue after the idle has been corrected.
I am curious on how well it idles BEFORE the temp gets high enough to open the thermostat...because it should be able to achieve 'curb idle' BEFORE the temp reaches the point where the thermostat opens.
If the coolant temp is not to the point of boiling over (or overheating...which it appears it is not the case)...I am not taking any dynamics of the cooling system into consideration at this point.
DUB
Instructor
Joined: Dec 2004
Posts: 216
Likes: 1
From: Lapeer Michigan
depends on where the vacuum is hooked up carb port or manifold
if the auto trans is hooked to to the radiator it should be on a cooler of it's own
The temp sensor can be bad or loose wire
fan shroud on the motor for better flow ? are you using only the electric fans ?
there are 3 screws that hold the choke housing snug, just break them loose and turn the black housing, most have marks for lean/rich and you can tell which way the blade turns open or closed
if the auto trans is hooked to to the radiator it should be on a cooler of it's own
The temp sensor can be bad or loose wire
fan shroud on the motor for better flow ? are you using only the electric fans ?
there are 3 screws that hold the choke housing snug, just break them loose and turn the black housing, most have marks for lean/rich and you can tell which way the blade turns open or closed
Instructor
Joined: Dec 2004
Posts: 216
Likes: 1
From: Lapeer Michigan