This is the first year since I've owned the car I have been able to drive it in the winter now that the engine build was completed. Does engine run-on/dieseling occur during cold weather? Also, is it common to see the idle in park increase in cold weather by a small amount (100-200 rpm)?

Background details:
'70 Cutlass Supreme w/ 350 automatic
Engine crank balanced, block honed and milled
Heads rebuilt
Mild cam swap
MSD ready-to-run distributor
Edelbrok intake
Holley 750 DP
Mechanical fuel pump with single line (no return line)

During summer months, idle in park was around 800, in gear around 500-600 and car ran great. Carb was dialed in and not running rich or lean (based on plugs). Fast-idle works as normal and no other issues to note. Timing was dialed in to what the car liked after the cam swap and the advance weights and bushings were changed to accommodate a faster sweep (MSD).

Now that it's colder (30-35 degrees), I took that car out for a quick trip to the store and upon shutoff at the store I had a little bit of engine run-on. Everything else running great so not sure if this is to be expected or if maybe I should look at retarding the timing a bit for cold weather. Is that something that is common? Is this a common occurrence with setups with a single fuel line with no return line?

 

Check to make sure the choke is adjusted correctly.

 

OEM choke or upgraded to electrical?

 

Gasoline can be some of it. It may not happen everytime. I would make sure the choke is set correctly, as Oldcutlass says.

 

Yes. Upon switching to the Holley from the QJet, I’m using the electric choke on it. No issues with the choke to date.

 

Thanks. Are there things to look for on the electric choke? Check voltage to make sure it’s working and check to make sure it’s fully opening?

 

I know this is for a vac secondary carb, but the choke adjustment is basically the same.

 

Usually run on is caused by the throttle butterflies being open slightly in combination with a hot combustion chamber. Check your fast idle linkage and choke for proper adjustment.

 

A few questions. How cold ? The coldest I drove my car in was 28 degree days and i have a 750 HP series holley (no choke) the idle sits about 200 rpm higher but one thing to keep in mind that added rpm might be bringing in some timing so the dieseling could be a combination . try backing off the fast idle and fattening up the idle mixture. Just as a side note you are seeing a big drop in RPM between gear and park ideally you want 100 to 150 tops. How much of the transfer slots are exposed ?.

 

Avg temp is between 30-35 right now. I’m going to check the choke operation and back off the fast idle cam a bit as it takes the RPMs a little too high on warmup. I’m also noticed my MSD advance canister is on ported, I will swap that back to full manifold and back off the idle screw as it may have the throttle blades open a bit too much. Might not change much but worth a shot. I’ll have to look at the transfer slots to see how much is showing.

Plugs show not rich or lean and timing was dialed in to what the cam and the engine rebuild liked. I’ll have to check timing with my light again but with the bushings and springs on the MSD, total timing as between 34-36.

In the summer I though the dieseling was mainly due to how hot it was this year and the single fuel Lomé with no return line, but noticed it happen today with the cold so I’ll play with it and take a short drive tomorrow to see. Hopefully the switch to manifold vacuum instead of ported and changing the idle screw will make a difference.

 

It should be lean in the winter due to the thinner air . Once the choke is fully opened If your tune was not changed it should be lean in the winter vs the summer. I would look at your choke if that's the case.

 

As far as the timing goes im not talking about your total. You have to look at your initial. If you are running an msd distributor. Look at the graph compare your idle rpm rpm to where your advance comes in. If the idle rpm goes up you could be on the advance at that RPM.

 

No not when everything is in order and the car has correct gas, or close to it. I drive my car in the winter, once its well warmed up it has the same general engine characteristics that it has in the summer.

Not once its fully warmed up and engine is settled.

I'm guessing you have my same type 350 (310HP / HI Comp) but with a mild performance build, etc ?

575 RPM is factory setting for our factory setup drive idle, so that's in the ballpark.

My 69 was 675 park idle and 575 drive idle prior to getting my timing chain replaced, when fully warmed up / driven moderately. Cold or warm temps did not seem to alter that. The only times it clearly fluctuates are on humid rainy days and when i drive aggressively. Rainy and humid can lower idle 25-50 RPM. Getting on the car, IE driving aggressively and it can go up 25-50 RPM. Both of which are temporary... and range due to severity of situation.

This screams gas quality to me. Bad gas or too low octane rating. Modern gas is crap, modern high test is like low test when our cars where born... Unless your rebuild changed the need for high test I would put 93 octane minimum ?

For instance my car does not like WAWA high octane, which is 92.


PS Keep in mind my experience is relative to my 69s 350 which is stock with a quadrajet reman and points.

 

Wrong. If what you say is true, a choke wouldn't be needed. If anything, you richen a carb during colder weather. During cooler temps, such as evenings, cars make more power because the air density is higher. You richen things to take advantage of the cooler, denser air (oxygen).

Gasoline is seasonally "blended". During summer, gasoline vaporizes much easier due the ambient air temperatures. During colder weather gasoline doesn't vaporize as easily due to the colder temps. Generally during "winter/cold seasons" the humidity is lower, but air density (oxygen) is higher. If anything, "tweaking" the idle air adjustment screws a touch might be in order, after adding fresh fuel.
Now would be a good time for Joe P to weigh in and give us info from an engineering point of view.
.......Just my two cents worth.

 

Wrong . I didn't say density. I said thinner. Oxygen is composed other gasses which can compress. So.... When the molecules shrink you have more "air in one breath" I cold winter vs hot summer. . hence why...... Guys who run boost love cool weather. The rate at which you can compress the air into the engine is much higher as molecules are tighter the Same goes for carbs. I know this for a fact because my AFR gauge did not lie. I had a 13.7 /14.0 AFR in the winter on a 28 degree day . no changes to tune in 70 degrees was 13.0 12.8 ish. The air we breathe is gas temperature makes it expand and contract. More air in because of thinner more compressible air leans the fuel mixture.

 

Update:
After watching my Steelers lose to the NY Jokes, I needed some time alone with the Olds to cheer me up. I checked the choke to make sure it was working and I adjusted the fast-idle cam which was allowing RPM's to run a little too hight on warmup. I then disconnected vacuum advance to check initial timing and total timing based on the bushing and springs installed in my MSD and all checked out good. I then changed the distributor advance line from ported on the Holley to a full manifold port and then readjusted the curb idle. Took it for a nice ride and upon shutoff, the dieseling issue is gone so hopefully that combination of adjustments was the culprit. It smells a little rich and when I was adjusting under the hood but I'll be in stalling an AFR gauge and an O2 sensor in the spring to get accurate readings to see where I'm at.

Thanks for the input and advice from all and hopefully this took care of the main issue. Merry Christmas everybody.

Thanks
Tom

 

A step forward.

 

This is clearly the two of you talking past one another while saying the same thing.

He’s approaching this from the optimal tune changes that should be made to compensate for higher air density (adding fuel to maintain proper AF ratio) while you are approaching it from a no tune point of view (the more dense air will pull you leaner). You are both saying the same thing.

Also, “thin air” is typically used to describe low density air (i.e. high altitude, high temperature).

 

Glad you got it figured. I found as a general rule my cutlass ran great or even better (once started) in cold (-25C weather)