Influence of low surrounding temperature on AFR

Hi,

I drive a mild street tuned 200 six with OZ250 head, headers, 2V intake, Holley 2300 etc.

At moderate or high surrounding temperature in the summer (> 70 degrees F.) the engine accelerates great under all circumstances.
If the temperature is low (lets say 50), I feel a slight hesitation when accelerating from cruise drive and low revs (from about 1200 rpm).

Do I potentially have a to-lean or a to-rich situation when accelerating, which only is noticeable with lower surrounding temperatures?

Any hint is appreciated.

Cheers
Joerg

My engine: https://www.dropbox.com/s/i370ihcw1g88b ... e.JPG?dl=0

Hi, try to read a spark plug or get someone to help with air / fuel ratio. Your engine can use more fuel (bigger jet) under cooler temps. You may be a tiny bit on the lean side right now. Good luck.

65Fastback":36ulbeow said:

Hi,

I drive a mild street tuned 200 six with OZ250 head, headers, 2V intake, Holley 2300 etc.

At moderate or high surrounding temperature in the summer (> 70 degrees F.) the engine accelerates great under all circumstances.
If the temperature is low (lets say 50), I feel a slight hesitation when accelerating from cruise drive and low revs (from about 1200 rpm).

Do I potentially have a to-lean or a to-rich situation when accelerating, which only is noticeable with lower surrounding temperatures?

Any hint is appreciated.

Cheers
Joerg

My engine: https://www.dropbox.com/s/i370ihcw1g88b ... e.JPG?dl=0

Click to expand...

@65Fastback,

I expect that your engine is warmed to operating temperature (water temperature at least 160 or so) when you have this hesitation? If not, that could be your issue - before your engine has warmed to operating temperature, the intake and cylinder head walls are cool enough that some fuel drops out of suspension in the air/fuel mixture, leaning the mixture out. Chokes are the traditional solution to this issue, but tricky to get just the right adjustment on.

Otherwise, when the outside temperature is cold, outside air is denser and you get a denser charge of air into the engine, also leaning the mixture out.

Reading plugs is the traditional way to troubleshooting this, but unless you're very careful and go through a very specific sequence of operations, your plugs are going through many different engine conditions - idle, part throttle, steady-throttle cruise, and wide open throttle - and it can be difficult to tell what the air/fuel mixture is like under specific conditions. IMO the best way to diagnose this type of issue is with a wideband air/fuel ratio gauge. You've made quite an investment in your OZ head, headers, 2V intake and Holley 2300 - the wideband would leverage your investment by allowing you to tune for all engine conditions.

Forum member wsa111 can give you expert information about tuning your Holley 2300, which is the same carb I am running (on a 250 with the CI aluminum head and CI intake). He knows much more than I do about carb tuning, but it could be that you need a power valve with a different vacuum rating, or different size power valve channel restrictors (PVCRs). Your cam choice affects A/F ratio because the cam choice can change how much vacuum you have under different engine conditions.

Thanks
Bob

Thanks, Bronco and Bob!

I mean under operating engine temperature. Choke adjustment or cold run behavior is not an issue.
Plugs look normal with tendency to the lean side.

Looks like I am operating slightly on the lean side.
As Bob said, an AFR gauge would be the final solution for diagnoses. Just need to get the sleeve welded in for the probe.

I will ask wsa111 for details of his carb setup.
Cam is a 260H from ComCams, 0.058 main jets, 8.5 power valve.

Great forum!

Cheers
Joerg

Hello,

I just want to give the feedback that all hesitation problems disappeared after some of the recommended changes. Thanks guys!
E-Choke tuning is the only thing left for stone-cold-engine-first-3-minutes improvements.

First I connected the heater hoses to the OZ250 intake, which eliminated the stumble when accelerating with warm engine from low rpm (<1800) in low surrounding temperature (<60F) thing. I thought there must be a reason why the water connections are there. Seems that vaporization was an issue with the big intake and German temperatures in spring and autumn.

Next I went up with the PV and down with the main jets to get a crisp kick when stepping on the pedal from 2000.
There were 61 mains in, not 58 as I posted above. Now I am on 9,5 PV and 59 mains. AccPump cam and nozzle finally unchanged (orange/ 1st/31), no improvements observed in this area with different setups.

Next week I am able to loan an afr gauge to see the readings for verification and further improvements. I think I can go down a bit with the mains. I will post the afr results here, maybe combined with a couple of further questions. I think there are some people here with a similar setup than mine (see footer).

Any comments or hints highly appreciated.

Again, great forum!

Cheers
Joerg

65 Mustang Fastback, 68 Inline 200, OZ 250 Head, Holley 350 cfm, Comp 260 cam, Hooker Headers, 2” dual exhaust, DSII, MSD 6A. 4 speed toploader, 3.2 rear end.

Joerg, one thing you can do is stagger the main jets. Run a 60 on the outer & 59 on the inner. Are you setting the accerator pump correctly. When you open the throttle all the way, take a pair of pliers & open the pump arm till it bottoms out, there should be a .015" gap between the end of the lever to the spring loaded adjustment screw.
Make sure at idle there is no delay on the movement of the accerator pump arm when the throttle is opened.
Give me what you have as far as distributor specs. Total centrifugal, initial advance & how do you have the vacuum advance hooked up, manifold or ported.
At a warm curb idle the throttle blade should be in the lower 1/3rd of the idle transfer slot.
What is your static compression ratio & what is your cranking compression taken with the throttle wide open??
Give me this information & then we will go from there. Your carb is a 7448-350 correct.

Hello Bill,

thanks a lot for your hints for further optimizations. Yes, it is a 7448 carb.
The acc pump is adjusted correctly on both end of the levers travel.
Regarding the mains I will first have a look on the afr values above cruise, once I get the gauge.
The tip with mixed mains right and left is good, never thought about this.

My distributor advance comes in too late for my feeling.
Initial is 16, centrifugal starts at 1000 rpm (4 degr.) and does increase even above 3000 rpm. At 3000 rpm it is 30.
So in total a 46+ which is a lot. But no pinging on uphill acceleration.
Vacuum advance is on ported. Full manifold vacuum resulted in hesitation. Vacuum reading at idle is 12 and 15 at cruise.

Regarding the transfer slots, I cannot say where they are at idle. I see a significant part of them from the top, so I guess they are open in the lower area. If this check is crucial, I can unseal and put off the carb.

The same with compression, I don't know, sorry. As the head/block were not milled or zero decked, static compression should be in a moderate area. Never did a crank compression test but I can do so of course.

I've got a question about my spark plugs: I have in the ACDELCO R85TS which were chosen by the former owner of the engine. Good choice? 0.5 gap. (MSD ignition) Ok?

Cheers
Joerg

You need a recurved distributor for your engine. You only need 34-36 total for your combination.
With the DS11 distributors the MSD-6al box with a MSD 8869 adapter harness is a direct plug in. Gap plugs @ .045"
I do ship my distributors to Europe if interested. See my listings in the small six for sale section. PM me if interested Bill

Hello,

I just want to give feedback after replacing my old DSII with Bills optimized one.
Well, this was the best upgrade ever since I got the engine. Thanks Bill!
All bad spots in engine behavior disappeared. Smoother idle, better cold run (stays running now), better off the line drivability, excellent off-cruise acceleration. I need to take the car out of the town next to look on WOT performance.

By the way, is there a trick to get the oil pump drive in easy, when dropping in the distributor? It took me 1.5 hours.

The 20 degr. initial advance is what this engine seems to need to achieve modern-car-like drivability at lower revs.
The vacuum advance, adjusted by Bill, appears to work much better than on the stock distributor.

Being on ported vac now I like to test full manifold instead.
When ignition setup is finished, I can utilize my Innovate AFR gauge for further carb fine-tuning.

Cheers
Joerg

65 Mustang Fastback, 68 Inline 200, OZ 250 Head, Holley 350 cfm, Comp 260 cam, Hooker Headers, 2” dual exhaust, DSII, MSD 6A. 4 speed toploader, 3.2 rear end.