How much does temperature affect motor?

-
I vote we chip in & buy a new keyboard for Christmas for AJ.....'cause he must go through them pretty quickly...

I bet he does !
But if you follow those steps the tune should be spot on.
Its nice to have detailed steps instead of " jet your carb and adjust the timing"
There is a lot of interaction between the carb and the timing that can confuse the average guy . I have yet to find a book on performance that details the tuning procedure like A/J does.
Thank you A/J !
 
Why, why, and why?
Why does the stock engine idle so high.
Why does it need 14/15 * idle-timing. and
Why did you need to downjet?

But first;
>> Ok so your problem could be ; mechanical, or it could be ignition, or it could be fueling, or it could simply be the tune.
I always look at the mechanical first; Next up is ignition. Then fueling. And finally the tune.
Ok I see a lotta this has already been covered so back to the whys;
------------------
Last first: as for the jets;
73s are still too big on the primary side, but the secondary side should have a plate equivalent to about six/eight sizes bigger than whatever you run on the front. I would diagnose the Primary MJs by removing the PV and installing a plug, installing 68s, and then roadtesting in second gear, with a vacuum gauge, by rolling into the throttle at various rpms from 1800 to 2800, to various vacuum settings; to see at what manifold vacuum the primaries run out. You will feel it as the engine lays down.
You want this to be at about 10 inches. Change the MJs until you get it.
Now install a NEW PV, either a 12.5 or a 10.5 and roadtest for a smooth transition from 12 to 8 inches.

As for the high idle-rpm;
Make sure your secondaries are fully closed but not sticking.
Make sure that you have NO vacuum leaks, including into the Crankcase.
Make sure the PCV is plumbed to the FRONT of the carb.
Make sure the Brake Booster, if you have one, is plumbed to the back of the carb or to the Plenum, but NOT to just to one intake runner.
Make sure the Vacuum-advance can is plumbed to the spark-port.
Make sure ALL other vacuum ports are plugged.
Make sure the fuel-level is correct and stable. BTW, your carb does not care about the fuel pressure; it only cares about a stable design-correct wet fuel level. Pressure is what the system needs to get the fuel from the back and to maintain the wet fuel level.
For now; if you have an EGR, defeat it, so it doesn't confuse the tune.

Now back up the timing to 8 to 10 degrees, and reset the idle-speed to 650 in Neutral.
Now before you do anything else, check your PowerTiming. Unplug the Vcan, and just rev it up until it stops advancing. Could be 3000, or 3500, or even 4000. Wherever it stops, it should be less than 36*. Record the number. Rev it up and down gently a few times while watching the timing marks, to prove it is advancing smoothly and not sticking.
Next, with a Holley-type carb, set the mixture screws to between 1/2 and 3/4 turn.
Here's the thing;
The slow speed system of this carb consists of the Transfer Slots and the Idle enrichment screws. These two are synchronized to eachother by the Idle-Timing.
If the Idle-Timing is set too high, then the idle speed will be too high. If you then slow the engine down by using the speed screw, this automatically reduces the Transfer Slot fuel. Which makes the engine run rough; so you open up the mixture screws. And finally it idles. But, as soon as the engine comes OFF Idle, the AFR goes rich, because the mixture screws are out too far. Another thing that happens, in the above condition, is most times the engine will have a Tip-in; stumble, hesitation or sag. This is because the transfers are lean, they take a few milliseconds to come back up to speed. The cure is simple; just open the throttle some more, and reset the mixture screws. If the speed goes outta sight, you just retard the timing.
If the IdleTiming is not enough, your Idle speed will be too slow. So you will use the speed-screw to speed it up. This will open the transfer slots dumping in more fuel. So then you will shut off some of the fuel coming from the mixture screws. Now it idles. But as soon as the engine comes Off Idle, the AFR goes lean because of the dried up mixture screws.
You can play around with this for hours and hours searching for the perfect setting.
Or you can try the settings I gave you above.
Or you can just get it right the first time with
this method;
Remove the carb and drain it. Then with the throttles on the curb idle screw, keep it there and flip the carb upside down. Now go find the transfer slots, on the engine side of the blades and adjust them with the speed screw, to be square, to a little taller than wide but Not shorter than wide......... BOTH the same.
After that, make sure the secondaries are closed up tight but not sticking.... BOTH of them. Then reset the mixture screws to 1/2 turn from lightly seated.
These settings will get you daymn close.
NOW, after this, do NOT touch the speed screw. If you have to adjust the rpm, do it with timing advance in the range of 8 to 14 degrees, to get about 650 in gear or whatever does not BANG when shifting from Neutral, and is half-azzed smooth.
Now remember; the idle fuel is synchronized between the transfers and the mixture screws. So you can use the mixture screws as diagnostic tools. If the engine runs better with a lil more mixture screw opening, then you can adjust the transfers richer and put the mixture screws back to .5 to .75 turn. But if the engine wants less fuel, then tweak the speed screw the other way, and reset the screws again. Do not tweak the speed screw more than about 1/2 turn slower, nor more than 1 turn faster.
Now you are really close.
Reset the timing to recover your desired in-gear idle-speed.
Finally, road test it for a tip-in hesitation. see note-1
If you consistently get one, reset you accelerator pump per the factory instructions.
If you still get one, check/reset your float level.
If you still get one, add 2 degrees Idle-Timing but do not reset the speed screw setting.
If you still consistently get the stumble; ADD ONE FULL TURN to the speed screw fueling, reset the mixtures to .5 turn, take the two degrees timing back out, and try it again.

Note-1
I am talking about a Tip-in hesitation.
So Tip-in is what it implies. You get into the car, you put it into gear and let it idle, like you were sitting at a light behind another car. The light goes green, the guy in front of you starts moving, then it's your turn. So then you very gently tip in the throttle, and drive away.
In the next year, you will be doing this thousands of times, so if it stumbles, it will make your life miserable. So, IMO, you must not have a tip-in stumble. Take whatever time you need to get this set up right.

Ok so, that's my story and I'm sticking to it......


THANK YOU
This is a lot of information to take in but I like that, I love to learn. I am going to try and re-read this about 10 more times between now and Saturday and I will also print it out and tape it to my glovebox until I've worked my way through everything.
Just a couple questions:
1) You mentioned to plug the powervalve to isolate the primary jets, I like that idea but what would work for a proper plug? Would I use any old bolt or do you recommend I add an O-ring to make sure nothing sneaks by?
Or would I get an old powervalve that I know I will never use again and glue it completely shut?
2) You recommended that I plumb the PCV to the front of the carb but I don't think I have any ports on the front. I have the spark port (currently connected to dist) and one small port underneath the spark port which is plugged and is usually where I hook up my vacuum gauge. I have one large port in the base of the carb in the back currently hooked up to my brake booster, and then another port in the manifold just to the side of that which is where I have my PCV. Is this manifold connection okay for the PCV or do you still think I should move it elsewhere?
3) While testing the primary jets in the way you told me to above, should I put the heaviest spring possible in for my vacuum secondaries to make sure they don't come in at all during those tests?

I plan on digging into this as much as possible this Saturday but I am not sure how far I will get. I will do what I can and update accordingly

Thank you again to everyone trying to help me out, I really appreciate it
 
Oh Boy. Another train wreck coming.
Hate to see someone waste a bunch of time and possibly their engine.
So I'm going to put my .02 in

The reason the engine had to be jetted down is that there was more velocity under those conditions than the engine that carb was designed for. Both the primary and secondary blocks of that carb came with power valves. The primary power valve was 10.5 because the engine was a hottie and with somewhat poor part throttle combustion efficiency needed to go richer earlier.

You've got
'76 360 out of a motorhome, edelbrock performer rpm intake, and unknown brand headers,

@Wyrmrider hasnt been on for a while which is a shame, cause he'd probbaly know a fair bit about that engine.
It very likely has a fairly moderate cam, with lots of torque and power at the lower rpms. It may also have little passages under the exhaust ports. (When you put the headers on, you should have see them and plugged them. )

Assuming the fuel tank has summer fuel, the place to start solving the issue of dying when warmed up is what's going on at idle.
For that jets dont matter. For that the timing curve doesn't matter.
Base timing may matter, but more important to solving the problem is figuring out what is different after 15 minutes.
The possibility that it was running rich at idle cold, and then is too rich when warmed up, is worth pursuing.
> This could be too much pump pressure for the float position (you would see fuel dripping out the boosters)
> This could be a damaged power valve or PV gasket (either bowl).
> It could be because this carb is intended for an engine with lower vacuum at idle than your engine. However I'm leaning against that being enough of a difference in calibration to kill the engine.
> It could be gaskets have dried a bit (common with Holleys that sit depending on the gasket material) but usually there is some seepage visible externally.

Another possbility mentioned was on the fuel delivery side. If the fuel is getting too hot in the lines, this can push either excess vapor or sometimes alternating liquid fuel and vapor into the bowls. That generally results in low fuel (liquid) levels in the bowl, but can in some situations, usually after shut off and restart) cause flooding. So far the symptoms don't seem to match.

PS. I would not make any non-reversible changes to that carb because its relatively valuable especially if it has all its original pieces (other than vacuum pod cover). Changing jets, adjusting throttle position, secondary springs, pv, that wont hurt its value to collector.

Bottom line is to start with the carb. Make sure the gaskets and PV are good and the fuel level is correct and staying correct.
Something is changing between start and warmed up so keep on until you figure out what it is.
 
Last edited:
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.
When you say the engine is happiest at 14-15" initial do you mean it idles strong (high rpm) in neutral or in gear?
Because on this much I agree with AJ. Thats a big drop in rpm between neutral and drive for a stock engine.
A big drop usually happens when the idle is too lean for the conditions.
In which case the hotter the engine gets, the worse its going to be. There's cold, warming, and really heat soaked. Heat soaked is where its at after a couple hours on the interstate.

Throttle position at idle: It should show between .20 and .040 " of transfer slot.
see photo here: Holley idle mixture screws

The idle port supplies the rest of the fuel at idle. See illustration in the post linked above. This is the portion the idle mix screws control. If the mix screws can be turned in enough to kill the engine, the setup is at least in the ballpark.

My engine is LOUD, I can almost guarantee I wont be able to hear any fuel boiling even if I tried.
Might want to check for those holes under the exhaust ports... Dont ask me how I first learned about them...

Is there any other way for me to check if the fuel is boiling?
Check the fuel level in the bowls.


I'm no expert plug reader but these are examples of rich, amonst other things.
upload_2021-8-12_16-42-35.png

Champion (left) Autolite (right)
 
Last edited:
@Mattax Thanks for the reply. You left a lot of good info and I'll do my best to answer as much as I can. First off, you mentioned that the exhaust might have the extra ports under the headers, I know exactly which ports you are talking about because the 360 that came in my dart had those (plugged by the time I got the engine) and the one I put in did NOT have those.
The tank in my dart had rank fuel when I got it but was completely rust free and clean inside. it was emptied and has clean gas in it now that I put in about 1 month ago. I don't let the gas get older than 2-3 months tops before I burn through it all.
As stupid as it sounds, this carb holds sentimental value to me because it belonged to my father and he daily drove his car for so many years with no issues with it so while I did change out the cap to the vacuum secondaries to make tuning easier, I kept all the original parts and I would never do any irreversible changes to it even though I have no intention of selling it.
When I got the carb I did buy a trick kit and an assorted box of holley jet sizes. I replaced both primary and secondary power valves with the equivalent sizes to what was there when I opened it, and I replaced the jets with new ones to equivalent size (jets were 76 primary and 82 secondary) along with new gaskets on the fuel bowls and metering plates and then I set the fuel level while at idle (just barely weeping out of the sight hole and the engine was idling at about 850-900 rpm where its very stable) My mechanical fuel pump delivers fuel at about 5-5.5 psi at any given time according to the fuel pressure gauge mounted on fuel rail by the carb.
I drove around and I noticed that I was basically dumping raw fuel at all times so I played with my jetting gradually bringing it down to 70 primary jets and 76 secondary and then bringing it back up to 73 and 79 because that is where I noticed the *least* amount of stumble and the engine seemed happiest (and exhaust stopped stinking of fuel). My engine always had an issue of shaking around quite a bit at idle like something was off so then I played with the timing and idle speed and idle mixture screws until I got up to that 15ish* point of initial timing where the engine has seemed the happiest but something still just seems really off to me which is why I made this thread. I realize that I might have been tuning in the wrong direction and I have gotten a TON of help so far as I hope to apply it all soon to keep my dart happy and driving for many more miles.
As far as the loudness of the engine, I believe that a big factor is I suspect the headers aren't sealing too well at the flange with the rest of the exhaust system. I have new high quality header gaskets that I bought at a local performance shop so I know there aren't any leaks there. The noise is completely bearable but would easily drown out a lot of 'engine noises' such as fuel boiling in the carb. That will be a problem for another day, for now I will look at the carb and nothing else.
 
Well I sure aint Jesus but I'll try again to get you on the right path.
laugh2-gif.gif

So yes, I think the wrong path. Dont feel bad - you've got lots of company with those who have been there and done that.

I replaced both primary and secondary power valves with the equivalent sizes to what was there when I opened it, and I replaced the jets with new ones to equivalent size (jets were 76 primary and 82 secondary) along with new gaskets on the fuel bowls and metering plates and then I set the fuel level while at idle (just barely weeping out of the sight hole and the engine was idling at about 850-900 rpm where its very stable) My mechanical fuel pump delivers fuel at about 5-5.5 psi at any given time according to the fuel pressure gauge mounted on fuel rail by the carb.
Fuel level and pressure. Great!
Check if the fuel pressure is the same when hot and the engine is having issues. Next then check fuel level. You might have to shut it off if the engine is shaking alot. Then do a quick check and be prepared for spillage. First, look down the carb to see if its dry or wet. It ought to be dry. If its wet, then expect the fuel level to be high.

I drove around and I noticed that I was basically dumping raw fuel at all times so I played with my jetting gradually bringing it down to 70 primary jets and 76 secondary and then bringing it back up to 73 and 79 because that is where I noticed the *least* amount of stumble and the engine seemed happiest (and exhaust stopped stinking of fuel).
When you write dumping raw fuel that sounds like fuel dripping out the boosters leaking.
I'm guess you mean the exhaust smells rich with partially burned fuel; but possibly you mean you actually are smelling raw gas.

OK. So here's one of the places you went went off track, but as it happens I think you ended up OK.
Unless something is wrong, the primary jets should have virtually no effect on fuel metering at idle and low throttle.
The low speed circuit feeds off of the high speed circuit. The low speed circuit has its own fuel restriction that is much smaller than the jet. So unless the main jet is very small, the low speed restriction (usually called the idle feed restriction or IFR) controls how much fuel goes into the low speed circuit.
upload_2021-8-13_9-36-19.png


You can see the IFR on this 3310-2 primary metering block.
upload_2021-8-13_9-37-37.png



Changes to main jet should be very noticible cruising at interstate speeds and when you're got your foot deeper in the pedal.

I think you ended up OK because 73 primary main jet is closer to the original specs.
As I metioned last June, the 3310-1 came with 72 primary main jet (PMJ) and 76 secondary main jets. 10.5 Power Valve in the primary and 8.5 PV in the secondary.

Let me point you to some background on how these carbs work. That way you're not dependent on random voices on the internet all claiming they have the true path to tuning bliss.
IMO the best introduction to the principles is the first chapters of Urich and Fisher's Holley Carburetors and Manifolds. Any edition will do, no reason to pay for the latest edition with fuel injection, etc. There is also a smaller version just on the 4150/60 models which has much of the same explanation of the circuits.

Chrysler's Master Tech series covers the same material pretty well.
Here's a direct link Carburetion Fundamentals & Facts (Session 273) from the Master Technician's Service Conference
or if you prefer pdf format, go to mymopar.com and look under Master Technician's Conference 1970, Carburation Fundementals and Facts.

My engine always had an issue of shaking around quite a bit at idle like something was off so then I played with the timing and idle speed and idle mixture screws until I got up to that 15ish* point of initial timing where the engine has seemed the happiest but something still just seems really off to me which is why I made this thread.
We went through this last time but perhaps we weren't clear or you got distracted by one of the posts.
Let me see if I can clarify a few things.
If you set the timing to be 15* BTDC at 850-900 rpm, then the timing is likely not stable.
On most Chrysler distributors the timing advances very quickly above idle speed.
(a couple of examples here: Another Boggie One)
If the distributor was intended for an engine idling at 700 rpm, and you set the timing at 900 rpm, then your measurement is not the initial timing. Its the base timing plus the amount added by the centrifical advance.
So initial timing is something less than 15*. How much less depends on the distributor setup (internal).
A motorhome 360 ought to idle relatively low rpm.

The hesitation you mention is very possibly due to the throttle position being too far open or too far closed at idle.
At idle the primary transfer ports should be showing between .020 and .040" below the throttle plate.
If you have to adjust outside that range, increase the timing or reduce it to get the idle rpm to drop or increase.
It should idle below 800 rpm.
Urich explains why in this sidebar I posted here. Your engine is not radical and there is no reason the throttle plates should have to be modified.

I realize that I might have been tuning in the wrong direction and I have gotten a TON of help so far as I hope to apply it all soon to keep my dart happy and driving for many more miles.

Well hopefully now you'll head in the right direction.
Get the idle and off idle first.
After adjusting timing or throttle position, then check and readjust all the other things.
The goal is least drop in rpm or vacuum when going into gear at idle.
One big clue your in the ballpark with the throttle position at idle is whetehr turning the idle mix screws provides adjustment. if you can screw them all the way in with no effect, then throttle is way too far open.
If the carb is off the engine, you can measure and write down how many turns of the idle speed screw open the throttle from .020 to .040 showing on the transfer slot. Big time saver.

Good luck.
 
Last edited:
14-15 timing should be close enough to tune that engine as long as the cam isn't something crazy.

Be VERY careful with the needle and seat. Your carb has the thread killing jiffy kit N&S set up in it. The normal kit locks with a screw on top, not with the nut. Easy to over tighten that nut and pull the assembly up, tearing threads in bowl.

Make sure fuel isn't dripping into your venturis while the car is idling.
 
I finally had a chance to get the carb off the Dart and take a good look at it but I didn't have enough time to get it back onto the car to do any testing. I think I will have a chance at some point this week to dig into it again but as far as what I did on Saturday, I completely noticed my transfer slot were extremely long and I could not get them to close any more because the throttle arm wasn't seating on the idle speed screw
Long Transfer Slot.jpg

Idle speed screw.jpg

At this point I felt like an idiot because I have heard the "set transfer slot to square or just a hair long" a million times and had assumed I had already done it. I guess I just felt like I was close to the correct setting because the idle mixture screws did have a large effect on how the engine idled but I guess not.
I found an adjustment screw on the opposite side of the carb on the same arm that contacted the bottom of the choke assembly which was holding everything open. Turns out the little plastic arm on the back of the choke was broken anyway so I ended up removing the entire choke system since I basically had it set to remain open at all times.
Choke speed screw.jpg

At this point I gave the carb a good cleaning and made sure nothing was stuck/sticking and that everything that had a spring on it moved freely with good return pressure.
I noticed some damage to a couple of the threads on the side of the carb that hold the choke assembly and vacuum secondary canisters on the side so I want to repair those now that I have the carb on a table before dropping it back onto the car.
 
We have all been there .
There is a lot of variables involved in propper tuning and easy to gef lost .
That is why I appreciate A/Js detailed step by step instructions.
 
Glad you found something. @mbaird I totally agree. And this is why I always recommend, when using an unknown carb, return it to completely stock settings for a baseline to BEGIN tuning.
 
That screw is your fast idle “on choke” adjustment. It’s pretty clear that your choke was having some influence on how the engine was running. Like I said in the beginning, disable that choke completely and make sure it is disabled and having no effect until you get the carb set up and running good. Then if you’d like to use it, add it back on later.
 
-
Back
Top