Suggested Carb Jetting

[helebah]

Guy,

After collecting the items needed for my race engine over the last few years have finally taken them to be built. Specs as below:

1608
Head purchased from yourself last year, 105 BPF if I recall on inlet
Stage IIB cams from yourself, Inlet and Exhaust
Ross forged pistons, high dome
Forged rods
4 into 2 into 1 Exhaust header
Balanced etc
Megajolt Mapped Ignition
2 x Weber 40DCOE
Fuel 98 Octance Unleaded Pump, I am however going to investigate E85 Ethanol blend that is now available once I get a base point.

Engine is in a 125 and is used for Sprints and Track work and has a close ratio fourspeed if that makes any difference. I am about to rebuild spare Carbs and I am after a starting point for jetting. How about this?

Main venturi choke 34mm (Are these the biggest "standard" ones?)
Main jet 130
Air corrector jet 185
Auxillary venturi 45
Idle jet 45F8
Accl pump jet 35
Needle valve seat 175
Float level setting 8.5mm
Accl pump pickup jet 60
Emulsion tube F16

The car will be Dyno tuned when complete

With regard to choke size, a colleague who is building a similar engine is experimenting with larger choke sizes in DCOE 40, up to 38/39mm. He is fortunate in the fact he has access to fabricating and owns a Dyno so can measure change. Unfortunately he lives 5,000km away. What choke size do you recomend??

Lastly are you able to provide a ignition curve to suit the cams as a starting point?

regards

[Guy Croft]

Hi Graham

thanks for emailing me to 'jog' me into a reply.

Main venturi choke 34mm (Are these the biggest "standard" ones?) They are the biggest I would use in 40DCOE, yes.
Main jet 130 might be OK but try 140 also
Air corrector jet 185
Auxillary venturi 45
Idle jet 45F8
Accl pump jet 35 could be too small go 40
Needle valve seat 175
Float level setting 8.5mm
Accl pump pickup jet 60
Emulsion tube F16

Otherwise OK.

For simple non-mapped ignition you want 10 deg of static timing, with advance coming in from 1000 rom rising to 24 deg advance (34 total) at 3500.

I would avoid the ethanol experiment if I were you, use that gasoline while you can still get it!

Hope that helps in the 1st instance.

G

[timinator]

Hi Graham, glad to hear your project is progressing. Always seems to make the day a little brighter when things are getting done. I have been thinking about one thing that GC wrote that prompted me to post. You listed a Megajolt mapped ignition and asked for an ignition curve. His response was:

>For simple non-mapped ignition you want 10 deg of static timing, with advance coming in from 1000 rom rising to 24 deg advance (34 total) at 3500.

I got the impression that for a "new to you" engine combination he might be suggesting that a distributor could save you grief. I have seen the new distributor less ignition system put out by Mark Allison and read GC's favorable evaluation of it. Could be a good way to go.

That being said if you wish to continue with the Megajolt I have some thoughts I'd like to share. The physical representation of an ignition curve is a three dimensional graph. Most people do better understanding two dimensional graphs. For N/A engines full throttle acceleration approximates a two dimensional system since the manifold pressure depression stays somewhat uniform. Because of this many people think it would be a suitable ignition curve. When one creates his own mapped ignition matrix things get complicated in a hurry. In GC's book, in the chapter Ignition Systems, he shows sixteen different curves for the sixteen different manifold pressures of a mapped ignition system. Again, he shows this in a two dimension graph to help people understand.

When you begin to create your ignition timing matrix you will find that most driving occurs in only a certain region of the three dimensional graph. After you data log a few runs you will understand better. So what do you do with the rest of the open areas of the matrix? You have to guess that the trend that the timing was following for the data you have will continue. Only when you finally log a new rpm/vacuum region that you had not reached before the engine goes lean/rich. More fun.

One last thought is about the belief that timing curves should max at a certain number of degrees. In 1976 a well known drag racer named Bill Jenkins wrote a book about racing engines. He stated that in his opinion at a certain rpm, say 6000rpm, a timing curve should continue to increase 1.5deg for each additional 1000rpm. Also, that the lower gears can use 2-4deg. more timing than can safely be run in the top gear. These ideas made me go out and buy a distributor machine way back when so as to try and achieve what he was doing. It was very difficult. Today, with a vehicle speed sensor and a mapped ignition system, these kinds of adjustments are available to those that choose to experiment.

I blame GC for this long post because he put a thought in my head that would not leave. Good luck with your project Graham. Best regards, Tim

[helebah]

Thanks for the thoughts. It is pleasing to be making progress, I have been planning and collecting the parts I needed over the last few years, with a underlying mantra of "do it once and do it right....within budget", although a little easier if you are flexible with the budget. Looked at mapped ignition for a couple of reasons, engine will be for sprints and track work so hoping to get all the trorque I can out of it, and on a practical level the inlet manifold I want to use is straight, therefore the block mount distributor has to go and exhaust cambox ones do not fit on 125's without moving brake boosters etc. Also thinking of trying E85 in the future so the ability to switch between two tunes may be handy. I was looking for a "basic" curve to allow me to start and run without too much difficulty with the cams and head, and will then get the engined dynoed fairly quickly. The thoughts on increasing advance are interesting as I think there will be a few buckets left for tuning as the engine will either be idling or flat out, will mention this to the tuner.
Regards

[S&B125]

Hi Graham
Make sure you install an oxygen sensor in your exhaust system at the point where your pipes come into one. When choosing who is going to tune your carbs - make sure that this sensor location is what they insist on. Taking AFR/ Lamba readings from the tail pipe just doesn't cut it and neither does using narrow band oxygen sensors. I do quite a bit of jetting/tuning on Weber carbs (not professionally but for experience) and this is what's required to get the job done properly. I'd hate to see that lovely "T" under performing or that engine your building be damaged by bad mixtures.
Steve
The other "T" owner from the west ( in case you don't recognise the alias here)