Carb tuning - RTFM!

[craggle]

Still no vacuum gauge by the way. I will try to get one and see what reading I can make of it. Don't think work will be willing to let me get one out the clean room and drive around with it in my car

I did sit down one night a few months ago and read the Edelbrock manual and I was surprised to find I understood it all and now know how the carburettor works.

It was the comment in the book that says if you have a flat spot or surge condition you probably have a lean mixture. Thats why I was going the wrong way.

Hopefully my fuel consumption will improve too.

Craig.

[wilf]

I think you will be surprised when you connect the vac guage.

The transition from cruise to power mode is not just affected by the amount of throttle opening - the vacuum builds up considerably as the revs climb, on anything short of WOT. And the secondary airflow is velocity controled - the butterfly might be open, but the air valve above it could still be shut - so it is possible to still be in cruise mode even with the secondaries cracked open.

Your flat spot could well be down to the vac advance, I still think. Had this problem before on my old car.

All other things being equal, I would go to position #23 next. Leave the power mode mixture alone, just alter one thing at a time.

[craggle]

Cheers Wilf

I need to nip out to get some fuel tomorrow night so will set up position 23 and move the accelerator pump to the minimum position and see what I get.

I will have a look for a vacuum gauge tomorrow.

Craig.

[craggle]

I'm still playing with the carb despite not having a vacuum gauge.

I setup position 23 in the carb today and moved the accelerator pump to the end position. This has made the car even more powerful and it runs even cooler.
Temperature didn't get above 80 except when stopped back at home.

Flat spot still slightly evident after pressing the accelerator but greatly reduced. May setup position #26 next and see if that makes it any better or worse.

I like this

Craig.

[robert]

Great post, sticky.

[craggle]

Thanks Rob........

But don't call me Sticky

Craig.

[wilf]

Originally Posted by craggle

I setup position 23 in the carb today and moved the accelerator pump to the end position. This has made the car even more powerful and it runs even cooler.

Craig.

OK - question for you.........when you say it is "more powerful", do you mean at Wide Open Throttle, or part throttle? I ask because leaning out the crusie mode alone should make no difference to the power mode mixture. What you could be seeing is much cleaner running and better throttle response because the original cruise mode mixture was far too rich.

I have to say that as I leaned out the cruise mixture on mine it ran a lot smoother. But WOT throttle response didn't change. (it's too bl**dy quick for my licence anyway!).

Position 26 is the next logical step. BUT! The WOT mixture still needs checking on a rolling road.

[craggle]

Hi Again

I should have worded that better.

It feels more powerful for the same amount of throttle usage. I don't think I have yet got the pedal to the floor as it seems to pick up so fast there is no need for it

MadJohnny has exactly the same engine as me and he is getting his setup on an engine dyno soon. I was going to simply copy his jet sizes into my carb and set the base timing at the same position or send him my carb and get it tuned to his engine.

Hopefully that will get us somewhere near the right figure.

Craig.

[wilf]

That sounds like a good plan.

Still want you to try a vac guage tho' - they are such a good diagnostic tool. Looks as tho' we are getting much better driveability in the cruise - I was suprised myself by the results. Should have done it 20k miles ago!

Once you have the vac gauge one thing we can look at is if your carb is too big for your engine (would also lead to the kind of hesitancy you report).

[craggle]

Still need to get a vac gauge but the other night I setup position 26 on the carb chart and tried again. This is the leanest setting for the cruise I can get with my carburettor and calibration kit.
It didn't feel as good as the last setting. It was less eager to pull under load and to accelerate. The flat spot seemed to have increased again.

Therefore the best setting I got for cruise was number 23 which is apparently 8% leaner than stock. I have now returned the carb to this position.
I will try to get a vacuum gauge and see where the hesitation is now occurring and decide weather to lean out the power mode as well or to change the step up springs.

Thanks for your help Wilf.

Craig.