Influence of mixture on engine temperature

[Ewald]

Hello

I am new to the forum having recently bought as TR3B.

Since I got the car, I suspected the engine was running on a rather rich mixture as the car would start from cold without choke. So when I got a chance I pulled the plugs and found them to be black and soothy. I did not have time to do a full tune of the carbs, so I just gave the adjusting nuts both about half a turn to make the mixture leaner.

Since then I have noticed two differences:

1. I need the choke for the first couple of minutes of driving (even though ambient temperatures here are 25-30 C here right now) and the engine is prone to stalling in the first 10 minutes of driving.

2. Engine temperature during driving has gone up significantly. Before it used to be at 185F, unless in stationary traffic. Now during driving it goes up to the next mark on the dial (half way between 185 and 230F). This causes the electric fan to kick in and I am assuming that then keeps it stable.

 

I am rather surprised by the large impact on the engine temp the (small) adjustment in mixture has made. I will spend some more time tuning the carbs better today, however I was surprised by the large effect. Anybody have any experience, or thoughts to share?

Thanks

 

Ewald

[peejay4A]

Half a turn is quite a large adjustment. A weak mixture can lead to high engine temperatures.

[Geko]

yes it has an impact on engine temperature. You may also play with the advance knurled knob by a few notches to see what happens...

[RogerH]

Hi Ewald,

welcome to the forum,

Remove the plugs and check the colour.

However on tickover they may well become black. Rev engine at 2000 rpm for 30 seconds then kill the ignition. Then check the plugs.

 

Retarded ignition will increase engine temp.

 

Roger

[Peter Cobbold]

Hi Ewald, welcome to the forum.

Going from a rich mixture to a lean mixture makes the flame spread slower.

Roughly speaking going from AFR 13 to 16 needs about 5 crank degrees of retard.

( slide 67

https://supertrarged.wordpress.com/2016/07/21/iwe-2016-technical-seminar/

 

https://supertrarged.files.wordpress.com/2016/07/combustion-talk-iwe-2016-v2-ppt.ppt

 

Running an engine with 5 crank degrees of retard will only kill about 1% of torque from maximum

( slide 60 ) That means only 1% more heat rejected to water jacket and exhaust gas. That's not going to make any detectable difference on the temp gauge.

 

If we take the temp gauge readings at face value, I suggest the engine is a long way away from correct tune. First step: check the spark timing curve as it may be well off delivering max torque. Correct curves in slide 104. My guess is your sparks are set badly retarded, and leaning he mixture has made combustion even slower, sufficient to heat up the water jacket.

 

And measure the mixture while driving with a wideband UEGO device.

 

Peter

Edited August 7, 2016 by Peter Cobbold

[Lebro]

Half a turn is quite a large adjustment. A weak mixture can lead to high engine temperatures.

I was also going to say ½ turn is quite a large change, I usually adjust 1 flat at a time.

Try going back1 or 2 flats, & see what happens.

Also, check you have oil in the dashpots - that will affect "stalling" or "gasping" when accelerating.

 

Bob.

Edited August 6, 2016 by Lebro

[boxofbits]

I would also add it might be worth setting the mixture back to its original setting, then driving the car again under similar circumstances to ensure that the temp rise is not coincidental, before coming to a conclusion.

 

Kevin

[TonyB]

Peter, to what does "slide no..." refer to? Is it a diagram or figure in the link referred to? If so how do you find it? I've looked and tried but have had no success.

 

Tony

 

Post script

Sorry, I looked again, I assume it referred to IWE2016. I can't get it to download.

Edited August 7, 2016 by TonyB

[ChrisR-4A]

Hi Ewald, by half a turn do you mean 180 deg. If the carbs were strombergs which have six flats I would suggest no more than one flat at a time between checking results.

[Peter Cobbold]

 

 

 

Peter, to what does "slide no..." refer to? Is it a diagram or figure in the link referred to? If so how do you find it? I've looked and tried but have had no success.

 

Tony

 

Post script

Sorry, I looked again, I assume it referred to IWE2016. I can't get it to download.

Tony, My apologies, the link to the powerpoint should be this:

https://supertrarged.files.wordpress.com/2016/07/combustion-talk-iwe-2016-v2-ppt.ppt

Peter

Edited August 7, 2016 by Peter Cobbold

[Ewald]

Gents, over the weekend I spend a bit more time setting up the carburetors properly. I indeed had made the mixture far too lean. I also filled up the dashpots. Combined that has made a massive impact! Car drives so much better now and temp has dropped back to normal levels!

thanks for the many tips.
Peter, thanks for the ppt. I will have a read of that next and have a look at timings...

 

Many thanks everyone!

Ewald

[TonyB]

Tony, My apologies, the link to the powerpoint should be this:

https://supertrarged.files.wordpress.com/2016/07/combustion-talk-iwe-2016-v2-ppt.ppt

Peter

Thanks Peter. I thought I might ft a supercharger to my 3A one day...... just a pipe dream really though. Like most things it's more complicated than I thought.

 

Tony

[RogerH]

Hi Tony,

'just a pipe dream' - well that's a start. Stick as blower on the pipe and a big SU on that and off you go.

 

Rogre

[TonyB]

WANTED supercharger, must work and be cheap. 12volt vacuum cleaner considered if it blows as well as sucks.

Also, pipe, fat and bendy with a smooth bore. Prefer one with low Reynolds number and good laminar flow.

 

Tony

[Geko]

WANTED supercharger, must work and be cheap. 12volt vacuum cleaner considered if it blows as well as sucks.

Also, pipe, fat and bendy with a smooth bore. Prefer one with low Reynolds number and good laminar flow.

 

Tony

check this post from Down Under http://www.tr-register.co.uk/forums/index.php?/topic/58349-should-i-be-tempted/?p=484725

[Peter Cobbold]

WANTED supercharger, must work and be cheap. 12volt vacuum cleaner considered if it blows as well as sucks.

Also, pipe, fat and bendy with a smooth bore. Prefer one with low Reynolds number and good laminar flow.

 

Tony

.....and high adiabatic effciency.

Peter

[TonyB]

.....and high adiabatic effciency.

Peter

Of course

Tony

[Peter Cobbold]

Of course

Tony

Tony,

'Reynolds number....laminar flow '.

I've been struggling to analyse the role of a venturi in SU carbs. You dont happen to know if Bernoulli's equation requires laminar flow....what if the air is turbulent?

Peter

https://supertrarged.wordpress.com/2013/08/24/tr6se-16-no-venturi-effect-in-an-su/

https://supertrarged.files.wordpress.com/2015/05/the-operating-limits-of-an-su-carburettor12.pdf

[Ian Vincent]

Peter,

 

If I recall my university thermodynamics there are two types of Bernoulli equation, one for compressible flow and one for non compressible flow. Because I was studying Civil Engineering, we focussed on the latter and I don't recall turbulence being an issue. It would make sense for turbulence to be an issue with compressible flow as the introduction of turbulence in aerodynamics has quite a marked effect on lift.

 

Rgds Ian

[Peter Cobbold]

Peter,

 

If I recall my university thermodynamics there are two types of Bernoulli equation, one for compressible flow and one for non compressible flow. Because I was studying Civil Engineering, we focussed on the latter and I don't recall turbulence being an issue. It would make sense for turbulence to be an issue with compressible flow as the introduction of turbulence in aerodynamics has quite a marked effect on lift.

 

Rgds Ian

Thanks Ian, I'm completely new to the subject, it never figured in biology. If there is a venturi-generated depression over the jet I think it will tend to suck the piston down onto the bridge !! So its a puzzle. It would be convenient if turbulence killed the effect.

 

Yesterday we walked by Telford's Horshoe Falls at Llangollen. He built a venturi flowmeter about 3-4 feet diameter to measure the flow out of the Dee into the Llangollen canal. Civil Eng 1800-style. Just 60 years after Bernoulli published the theory.

Peter

Edited August 11, 2016 by Peter Cobbold

[Ian Vincent]

In the 1800s there were really only two types of engineering, Military or Civil.

 

Rgds Ian

[TonyB]

Tony,

'Reynolds number....laminar flow '.

I've been struggling to analyse the role of a venturi in SU carbs. You dont happen to know if Bernoulli's equation requires laminar flow....what if the air is turbulent?

Peter

https://supertrarged.wordpress.com/2013/08/24/tr6se-16-no-venturi-effect-in-an-su/

https://supertrarged.files.wordpress.com/2015/05/the-operating-limits-of-an-su-carburettor12.pdf

Peter, you credit me with far more intelligence than I actually have. Either that or you are, in the nicest possible way of course, taking the mick. Sorry, I can't help with that one.

 

Tony

[Peter Cobbold]

Peter, you credit me with far more intelligence than I actually have. Either that or you are, in the nicest possible way of course, taking the mick. Sorry, I can't help with that one.

 

Tony

Tony, Not taking the Mick. Thought you might be an engineer after you mentioned Reynolds number etc. Peter