Carb tuning - RTFM!

[wilf]

I have an Edlebrock 1412 (800cfm) carb on my 408 stroker.

When I put my car on the road back in 2001, I naturally got the WOT mixture dialled in properly on a rolling road. Two jet sizes up on the secondaries and it was fine. I never really gave much thought to the cruise mixture, except that a year or so later I richened it in an attempt to cure a slight stumble. It didn't improve but my attention wandered onto other things at the time, and I never went back to it - after all it was running pretty fine. I fogot all about it.

I started to read the manual properly(!) a month or so ago. One of the things it says is to keep leaning out the cruise mixture until you get a problem, then go back a stage. I never did that before, so I decided to do it now. With these carbs, you can lean out the cruise mixture without affecting the power mode. A little concentration, and the graphical representation of this in the manual made perfect sense.

A couple of weekends later, I am 16% leaner in the cruise (with the edlebrock it is dead easy to change rods and jets) and she is running better than ever. What this will translate to in terms of mpg I don't know yet, but it will probably be very +ve. Oh - and the stumble I referred to earlier is gone (was getting far too rich with the accelerator pump shot - which I have also reduced now).
I really don't know why I did not do this earlier, wonder how much extra fuel I have paid for in the meantime?

So.............RTFM, do what it says, and save some money. At this rate I will qualify to join the Green party (shudder). Cruise mixture can be very much leaner than you think, without any issues.

[tonym]

For all you non-American types
RTFM means "read the fuc**ng manual"

[craggle]

Good advice Wilf

I have the calibration kit for my 750 carb and have been playing about with it, but didn't think of going leaner. I have been going up stages to make it richer.

Maybe I will come back a stage on the cruise and see what happens.

I too get a flat spot in the middle of the rev range and accelerating doesn't really help until a few seconds after you press the pedal. Maybe my accelerator shot is too much as well.
Did you change the step up springs or stick to the stock items?

I feel some road testing coming on this weekend!

Cheers

Craig.

[wilf]

Craig - I hooked up a vacuum guage to see when the step up springs were coming in - that let me judge if they were the issue or not. Not, in my case.

My advice would be to get a vacuum gauge , and go out and see what the guage is doing when you get your flat spot - you can easily see if the rods will be up or down at that precise time if you do that.

What kind of rpm/throttle opening does your flat spot occur at?

[craggle]

Seems to be anywhere from 1500 rpm until the secondaries open.
Cruises all right but doesn't like gradual pick up. If you floor it to get the secondaries open though it seems OK.

I guess you can plug a vacuum gauge in the spare carb port for the distributor advance (manifold vacuum port)

Craig.

[wilf]

There are two vac take offs - one is "full time", the other "ported", and I suspect that the second one is also "venturi effect" - i.e. vacuum related to airflow, not manifold pressure. Haven't checked properly to see tho'. You want to use the lh one as you look from the front of the car. Personally I would disconnect the vac advance totally whilst trouble shooting the carb.

You need to find out first if it is cruise or power mode that is giving you problem. I would go back to the standard step up springs if you have changed away from them.

What point on the graph in the manual are you at right now?

[craggle]

Hi Wilf

Carb I have is the 1407 750CFM version. Engine is a 350 chevy with Edelbrock RPM heads and cam.

At the moment I am at number 9 on the calibration chart which is one step richer in the cruise than stock.

I tried number 10 but, although it drove quite well it was down on power and sooted the plugs.

I am currently on the strongest (silver or natural) step up springs.

My next move was to go back to the stock needle rod but keep the stronger springs.

Maybe I should go to 19 on the chart and back to the stock springs?

Craig.

[craggle]

Just been out for a quick blast of the local country lanes.

I decided to lean the mixture by going to position 19 on the chart. This is one step leaner than stock on both the power mode and the cruise. I also returned to the stock orange step up springs and moved the accelerator pump to the middle hole.

Boy did it go!!
also seems to run a bit cooler.

Feels far more powerful than before but there was still a hesitation when accelerating. Could still be too much fuel on the accelerator pump so will try the hole furthest from the pivot next time.

I will try again tomorrow night.

Craig.

[wilf]

Ah - you got to play whilst I was still at work! Great.

I suspect you may still be rich in both modes - going to the stock step up springs has delayed the transition to a lower vacuum level - i.e. wider throttle opening.

The stumble on opening the throttle - if it is the accelerator pump giving too much fuel, it should be a very transitory event, so going to the smallest shot should give you an instant feel for whether this is down to the accelerator pump, or an issue with the overall mixture. Are you talking about going to full throttle suddenly or just rolling the throttle open to about 1/2 to 2/3rds fully open?

I would also repeat my advice to get a vacuum guage hooked up - without that you just can't troubleshoot an Eddy carb properly, since it tells you when the move to power mode takes place (at the inches WG of the step up spring rating). You will be surprised at how much throtle it takes to do this, more so with any revs on. You should also see about 1" WG at WOT and max revs - any less and the carb is too big (could contribute to your current stumble as well) any more and it is too small. If any combination of throttle opening and revs drops the vacuum below 1"WG, no amount of fiddling with the mixtures will help - basicallly the carb cannot meter properly in that condition - but will resume working as the revs (and vacuum) build up.

I believe you should get WOT mixture set up on a rolling road before you start in on the cruise mixture adjustments. That is the most critical (from an engine safety perspective) setting to get right first. Once that is done you can just keep on leaning out the cruise mixture untill you get a flat spot, then back a stage and it's done!

Lastly - if you disconnect your vac advance, does that affect your flat spot?

[craggle]

Hi Wilf

Flat spot seems to be after pressing the pedal at any revs but not flooring it. I was definitely still in the primary throttles as I can feel the pedal get slightly harder to press when the secondaries open.
After the initial press of the pedal and the flat spot it then clears itself and go's. Kind of feels like it is choking on the accelerator pump shot but then once that clears it is OK again

I haven't tried disconnecting the vacuum advance on the distributor yet. May try that as well tomorrow if I get a chance.

Seems to be quite a step now in the calibration chart to go leaner on both the power and cruise. Looks like the next step for both is position 22.

Craig.