How much does temperature affect motor?

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Projectile Dart

Projectile Dart

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I have a '68 Dart with a stock '76 360 out of a motorhome, edelbrock performer rpm intake, and unknown brand headers, a Holley 750 4bbl vacuum secondary and a 1/4" phenolic spacer. Car came with a 26" radiator and electric fan that I swapped out for a mechanical fan (from the motorhome that donated its engine). I feel like its important to state that the engine itself has never had any kind of overheating issues... It warms up and then the needle stays glued to its position on the gauge. I tuned the engine while warm originally, it idles in park at about 950-1000 rpm and about 800 rpm in gear, and the engine has seemed its happiest at about 14-15* initial timing. The carb has been jetted down 3 levels from its original 76 jets to 73.

The problem: I've noticed that when the engine is cold, I have to drive it really gingerly until it warms up (this is expected and does not surprise me). Once warm the engine will run GREAT for about 5-15 min but then after that is when the issues come up. It runs pretty similar to to how it runs while cold where if I put my foot in it then I get a noticeable stumble, and the engine shakes a lot while idling at a red light. My current band-aid fix is to pop it in neutral at red lights to raise the rpm and then it idles great but I'm not too happy to be doing that. I have noticed that the motor in general seems really happy on colder days (70*F or less) and becomes really unhappy on warmer days especially when outdoor temps are 90*+

Possible solutions: I'm thinking it has to be an issue with under hood temperatures and the open air filter sucking in that hot air and maybe the hot intake boiling the fuel in the bowls? I want to avoid getting a hood scoop for now if possible because I like the flat hood look (I'm weird I know). My possible fixes were going to be to either remove the intake and seal off the exhaust gas heat exchange section in the middle of the intake (would this make any difference?). Or to swap to an electric fuel pump putting out 10-15 PSI and then using a regulator with a return line to turn it down to 5.3-5.5 PSI to help keep the fuel circulating and cold.

Am I completely wrong or is there anything else that I am missing?
 
Before you start changing a bunch of stuff that you may or may not need, you should do a little diagnostics WHILE the problem is occurring. The ambient temp certainly can effect your tune up and the way the engine runs, but to know for sure you need to check. Drive the car on a hot day until it acts up. When it does, stop and take some temp readings of fuel bowls, fuel lines things like that so you have some info. Listen for fuel boiling in the bowls, check the level in the bowls, etc. If you diagnose it correctly you’ll save a bunch of time and money in the long run.
 
You can almost completely eliminate the under hood heat theory by simply taking the hood off on a hot day and going for a drive.
 
TT5.9mag said:
Before you start changing a bunch of stuff that you may or may not need, you should do a little diagnostics WHILE the problem is occurring. The ambient temp certainly can effect your tune up and the way the engine runs, but to know for sure you need to check. Drive the car on a hot day until it acts up. When it does, stop and take some temp readings of fuel bowls, fuel lines things like that so you have some info. Listen for fuel boiling in the bowls, check the level in the bowls, etc. If you diagnose it correctly you’ll save a bunch of time and money in the long run.
Click to expand...

I agree with you 100% about getting as much info as possible before changing/replacing parts, those are great suggestions. As far as getting some temp readings, do you think pointing the laser style temperature gun at those components will be accurate enough? If not then how do you suggest I get those readings?
My engine is LOUD, I can almost guarantee I wont be able to hear any fuel boiling even if I tried. Is there any other way for me to check if the fuel is boiling?
 
70 Buster 340 said:
Could be a heat induced vacuum leak... something warping? Maybe that spacer/gasket(s)
Click to expand...

Thats a good one, I will test this one when I get a chance, If I connect all the symptoms, it does sound almost like a vacuum leak... I have worked really hard to eliminate all types of leaks but didn't even consider that more can come when the engine is hot.
 
Is it a stock/off stock or built motor?
If it needs to idle that high,there are other issues that are contributing to your problems.
Vacuum leaks causing lean conditions will add to heat issues.
Check distributor function, timing advancing?
Full advance?
 
Did you block the heat riser when installing the manifold?
Have you re-torqued the manifold (and carb) after a few heat cycles ?
 
Projectile Dart said:
I agree with you 100% about getting as much info as possible before changing/replacing parts, those are great suggestions. As far as getting some temp readings, do you think pointing the laser style temperature gun at those components will be accurate enough? If not then how do you suggest I get those readings?
My engine is LOUD, I can almost guarantee I wont be able to hear any fuel boiling even if I tried. Is there any other way for me to check if the fuel is boiling?
Click to expand...
Yea my temp gun is pretty accurate and it’s a cheapo from NAPA. If you really are boiling the fuel then once you shut off a hot engine, things get hotter. And the fuel will be boiling away with the engine not running.
 
64mopar said:
Is it a stock/off stock or built motor?
If it needs to idle that high,there are other issues that are contributing to your problems.
Vacuum leaks causing lean conditions will add to heat issues.
Check distributor function, timing advancing?
Full advance?
Click to expand...

Engine is 100% stock other than the intake/headers. I will check again for vacuum leaks. While idling in park I was getting about 17-18* of vacuum if I remember correctly.
Distributor is stock style electronic ignition and also untouched/stock.
Timing is advancing as it should. I forgot the the exact number but it would reach the mid-30s* with the mechanical advance. vacuum canister in the dist. was tested and not leaking
 
70 Buster 340 said:
Did you block the heat riser when installing the manifold?
Have you re-torqued the manifold (and carb) after a few heat cycles ?
Click to expand...

I did not block the heat exchange section of the intake when installing it. When I put it all together, I didn't even know that it might cause any problems.
I have not gone back to re-torque the intake but I have removed the carb multiple times trying different spacers to try and eliminate these issues.
 
When it's acting up squirt water from a spray bottle to check for a vacuum leak. If idle drops you may have found something.
 
Could be carb related when cold but ignition related when hot? Coil getting heat soaked maybe ?
Its idling pretty high for a stock cam .
 
as suggested a vacuum leak or tuning..... or going through your carburetor setting the floats and cleaning
everything including filters
 
mbaird said:
Could be carb related when cold but ignition related when hot? Coil getting heat soaked maybe ?
Its idling pretty high for a stock cam .
Click to expand...

I can get it to idle lower in exchange for a little bit of an engine "wobble/shake" I can get it down to about 700-750 in park and itll drop to about 650-700 in drive but it shakes around more than I would like. At my current settings the engine idles really really smooth at start up.
I have also suspected maybe a faulty coil because I have no idea how old it might be but I want to exhaust all options before I start throwing parts and $$$ at the car.
Ill start with getting it real hot and then spraying around all the seals to see if the engine will react at all
 
70 Buster 340 said:
Was the manifold bought new ?
Were the heads ever resurfaced ?
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manifold was mounted on the 360 that came in the car. I thought something sounded weird about the engine so I had my mechanic take a look and he suspected a light knock on the bottom end. Instead of risking anything I decided to pull a free 360 I had available from a motorhome and just swapped the intake over along with all the accessories. That way I wouldn't risk hurting the previous 360 and I could have a good core to build up in the future. I did inspect the manifold and it hadn't been damaged or repaired in any way but was not new when I got it.
Also, the motorhome was gifted to me by the 2nd owner who bought it in the early 80's with less than 10k miles on it and he regularly drove it from California to Arkansas and back until he parked it in the early 2000's with 77k miles. I have no reason to believe that the engine was opened up before him and he said that he never had to do anything besides basic maintenance and add gas to the tank. The motor never once gave him an issue.
 
When tuned properly, a carburetor will be way too lean to keep a cold engine running. Which is why we have chokes. Or if you don’t have a choke you have to feather the throttle on/off for a while until the heat in the intake runner, plenum, chamber etc can vaporize the fuel. The same can be said for an engine that gets heat soaked. I suspect you have some carb tuning to do assuming your ignition timing is reasonable (12-18 initial and 32-36 all in) and stable. Take a look at the plugs and report back. My first instinct is to tell you to return the jetting back to what Holley sent with the carb and work on idle and transition fuel. After that you can focus on WOT.
 
What is the list number on your carb?
3310-???
 
TT5.9mag said:
What is the list number on your carb?
3310-???
Click to expand...
Sorry for the late reply, I got really busy at work the rest of the day yesterday and I just got a chance to respond
my carb is a LIST 3310-1
I forget if its either a 750 or 780 cfm. The carb belonged to my father who had it mounted on his '70 cuda 383 4 speed that he daily drove from '74 until '93. He said he never touched the carb once the entire time he had it and it never gave him any issues. My brother had the carb professionally rebuilt in the early 2000's in the hopes of getting the 'cuda back on the road but he never touched it after it came back from the shop.
I havent had a chance to pull a couple plugs yet to check their condition. When I do I will take a picture of them and upload to let you guys "read" them for yourselves
 
Projectile Dart said:
Sorry for the late reply, I got really busy at work the rest of the day yesterday and I just got a chance to respond
my carb is a LIST 3310-1
I forget if its either a 750 or 780 cfm. The carb belonged to my father who had it mounted on his '70 cuda 383 4 speed that he daily drove from '74 until '93. He said he never touched the carb once the entire time he had it and it never gave him any issues. My brother had the carb professionally rebuilt in the early 2000's in the hopes of getting the 'cuda back on the road but he never touched it after it came back from the shop.
I havent had a chance to pull a couple plugs yet to check their condition. When I do I will take a picture of them and upload to let you guys "read" them for yourselves
Click to expand...

3310-1 is a 750. Take a look here
https://documents.holley.com/techlibrary_carb_numerical_listing.pdf
And it will tell you everything about it. I would return that thing back to completely stock and start tuning from there. You have no idea what has or could have been done during all the years of rebuilds.
 
Let me also add under the heading of "general nonsense" that back in the day there were some carburetors that had what is called an "idle compensator". This is a thermostatic strip of metal (bends with change in temp) that had a wedge shaped valve at one end over a vacuum leak hole in the carb. When the carb heated up enough, the thing opened the "vacuum leak" and slightly leaned the thing out, as well as slightly increasing the idle speed

100_1029-1.jpg

(...........Shamelessly stolen from the www, misspellings and all.............)
Hot-idle Compensator Valve
High air temperature at carburettor inlet causes gasoline to evaporate rapidly, which intern-can produce highly rich idle mixture. In order to overcome this problem, many carburettors use
a hot idle compensator valve (Fig. 9.64), which is a thermostatic valve consisting of a bimetal spring, a bracket, and a small poppet. The compensator valve is usually located either in the carburettor barrel or in a chamber on the rear the of carburettor bowl. A dust cover is placed over the chamber. The hot-idle compensator valve is normally closed by spring tension and engine vacuum. As tempera-”~ ture rises, the bimetal strip bends. This uncovers an
auxiliary air passage, or air bleed, through which air enters the carburettor below the throttle plate. As this extra air mixes with excess fuel to lean out the idle mixture, it prevents stalling and rough idling. Once the carburettor temperature returns to normal, the compensator valve closes to shut off the extra air supply.
clip_image002_thumb.jpg

Fig. 9.64. Hot-idle compensator
 
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