California summer gas? Atmospheric temp/pressure/density?

[Ottmundr]

'68 318
performer intake and 600cfm carb
675 heads
Advertised Duration (Int/Exh): 256/262
Duration @ .050 (Int/Exh): 213/220
Gross Valve Lift (Int/Exh): .454/.475
LSA/ICL: 112/108
cam was installed at factory specified timing, not advanced or retarded
stock exhaust manifolds

Car runs great in the winter, but not in summer.
During the winter, for the last two years;
the car warms up just fine
idles just fine
runs great.

During the summer for those same two years;
the car warms up just fine
Idles for crap, I need to constantly have to cover the gas pedal and watch the tach
runs not so great.
When under load, either the valvetrain is rattling like a can of rocks bouncing down a mountain, or it's detonating, especially going up hill and that's with the 91 fuel we have here.

When I adjust the idle screw on the carb in the summer, it either wants to race at around 1300+ or stall. I can set it just fine in the winter. Timing was good in January.

Things I know I need to do;
new points or the new dizzy sitting next to my keyboard here.
check plugs (haven't checked since replaced last summer, but car was running just fine until about a month or so ago).
compression check
390 or 470 stroker (for those who say to ditch the teener)

After the to do list above, any thoughts or recommendations? Do I need to add anything else to the to-do list to help diagnose this issue and resolve it?

 

[mopowers]

Are you sure it's the gas and not the warmer ambient temperature? Is there vapor in the fuel line?

 

[nm9stheham]

Does this start acting up after the first of April (if you are in So Cal)? That is when the summer reformulated fuels some out in SoCal; I resume it is a bit later in northern Cal.

Does this crappy running occur only when the engine is warmed up? What carb brand is this?

A narrow points gap (low dwell) will cause crappy idling. The idle be gin unstable could be:
- mechanical advance trying to kick in at way too low an RPM
- secondary stop screws being set wrong

 

[MOA]

What is the compression Ratio? What pistons e are in it? Do you have a spacer between carb and manifold?

 

[Ottmundr]

Are you sure it's the gas and not the warmer ambient temperature? Is there vapor in the fuel line?

I am not sure it's the gas, but suspect it is because this has occurred every year around May.

I do not think it is the temperature, but I am not 100% sure. We do have some warm days in the winter but the re pretty constant in the SF bay area.

I cannot be certain about vapor in the fuel line, but if it were, wouldn't I get it all year, instead of regularly in May?

 

[Ottmundr]

Does this start acting up after the first of April (if you are in So Cal)? That is when the summer reformulated fuels some out in SoCal; I resume it is a bit later in northern Cal.

Does this crappy running occur only when the engine is warmed up? What carb brand is this?

A narrow points gap (low dwell) will cause crappy idling. The idle be gin unstable could be:
- mechanical advance trying to kick in at way too low an RPM
- secondary stop screws being set wrong

Northern CA here. Not sure when they switch to the other formulation.

Yes, it only occurs when the engine is warmed up. The choke has it idling fast enough in the morning. If I blip the throttle, and the choke comes off, then idle goes to hell and the engine wants to die.

The carb is the Summit MO8600VS.

I will probably put in the points and set dwell next week. This weekend is out unfortunately.

 

[yellow rose]

If your tune up is on the edge for winter blend fuels (they lower the Reid vapor pressure so they will start better when cold and a few other things that I can't think of) and you switch it over to summer blend you could be out of your tune up.
You may be able to go one heat range colder on plug in the summer to help. Or work on your timing curve a bit. Do you use a vacuum advance?

 

[Ottmundr]

What is the compression Ratio? What pistons e are in it? Do you have a spacer between carb and manifold?

Compression ratio should be the stock ratio for a 68 318, which if I remember right from my service manual is 9.1:1 correct me if I am wrong. Also, shouldn't the bigger than stock cam bring the dynamic compression ratio down?

Stock pistons, they did not show any evidence of detonation when heads were off last summer for cam swap. I do not have pics of them on this phone. If I remember right from my measurements, they were 0.080" down in the hole at tdc.

1/4" divided spacer between carb and intake manifold. The car seems to like it better than no spacer.

 

[Ottmundr]

If your tune up is on the edge for winter blend fuels (they lower the Reid vapor pressure so they will start better when cold and a few other things that I can't think of) and you switch it over to summer blend you could be out of your tune up.
You may be able to go one heat range colder on plug in the summer to help. Or work on your timing curve a bit. Do you use a vacuum advance?

Yes, vacuum advance.

 

[yellow rose]

Yes, vacuum advance.

Is it adjustable? You may be able to pull a few degrees out of the can, if you can can.

Get it?

 

[DFX 340 Duster]

Ottmundr, have you tried different brands of gas?
I had to stop using ARCO because their summer formulation change caused pinging.
Keep in mind that the methanol content can be UP TO 10% but it is not guaranteed to be that amount.

 

[Ottmundr]

Is it adjustable? You may be able to pull a few degrees out of the can, if you can can.

Get it?

It is the stock dizzy. I have a Mallory unilite electronic with vacuum advance on my desk at home, but can't check the model number until tomorrow. Gotta run to chemo now

 

[nm9stheham]

Ottmundr, have you tried different brands of gas?
I had to stop using ARCO because their summer formulation change caused pinging.
Keep in mind that the methanol content can be UP TO 10% but it is not guaranteed to be that amount.

That is a good suggestion.... we have found a big difference between Shell premium and the locally branded non-ethanol premium. But, that is a true 10:1 CR engine. The OP's engine is a lower compression one; its real static CR is around 8:1 (despite the factory rating of 9:1). So I would not expect the fuel itself to act that much differently, aside from the summer changes mandated in places like CA.

 

[nm9stheham]

Northern CA here. Not sure when they switch to the other formulation.

Yes, it only occurs when the engine is warmed up. The choke has it idling fast enough in the morning. If I blip the throttle, and the choke comes off, then idle goes to hell and the engine wants to die.

The carb is the Summit MO8600VS.

I will probably put in the points and set dwell next week. This weekend is out unfortunately.

Well, you do realize that when the temp is warmer, the choke can close less tightly when cold and will come off sooner. Does this crappy idle continue when the engine is fully warmed up? If so, then the choke setting is a side issue.

The summer blend mandated in some areas is less volatile, and will adversely effect the ease of combustion.

One thing I would be looking at is the ignition system. If it was weaker, it would have a harder time firing the lower volatility summer fuels. This may not be the issues, but it sure is a possible cause of your symptoms.

I know this sounds dumb, but the ballast resistance is CRITICAL for the stock system (points or non-points). If you don't have the OEM ballast, then all of the aftermarket ballasts will weaken spark to some degree or another. The closest is the MSD 0.8 ohm (cold) ballast. But I have a replaced a wrong ballast in a stock points Mopar system and had a biiiig improvement in cold warm up and idling quality. Someone had put in a wrong replacement ballast and when I put in an OEM one, it was all better. The OEM one is 0.5-0.6 ohms cold; PN is 2095501.

Also, a weak coil could cause poor running on low volatility gas. The Pertronix Flamethrower 40011 has the right primary resistance and is a bit hotter than the stock coil, and works with the stock points system.

 

[72bluNblu]

If your compression is stock there's no way you should be detonating with 91 octane. You should be able to run 87 all year long. That's what I did with the mostly stock 318 in my Challenger. Your earlier engine probably has higher compression than that one did. But you're not even in the ballpark for my 340, it's a true 9.8:1 (not the factory rating, actual measurements) and I run it on California 91 octane all year. I fill up pretty much every week and I've never noticed a difference that made me think "yup, summer blend!". Or winter blend, or any of the changes in blend. And I run the cheapest gas I can buy. I never noticed any remarkable change with the challenger at all, fuel blend, temperature, pretty much anything. That stocker would run on darn near anything I bet.

With the 340 in my Duster I do adjust my idle speed and idle mixture a tiny bit with the warmer temperatures. I have in the past changed the primary jets, I have an A/F gauge and notice the mixture changes that come from the temp change in the air the carb's pulling. But I stopped messing with the jets, it's not a big enough difference on a daily driver to warrant messing with the carb that often.

But yeah, I think your issue is somewhere else. Your ignition, timing, or fuel mixture must be borderline enough that the temperature change is enough to give you grief.