Edlebrock 750 setup

[Dave-R]

The secondaries are weighted which means it is airflow (vacuum) that opens them.

A bog happens when throttles open too quickly. Manifold vacuum drops to near zero which means not enough fuel is delivered.

If your secondaries opened quicker it would make the bog worse. If anything you need them to open slower.

Have you got a single plane or dual plane manifold? Or an open spacer under the carb?

You want each side of the carb to only "see" 4 cylinders on a street car if possible.

[Dave-R]

Is running around 18/19 degrees.
Have tried either way with timing +_ 6 degrees, obviously either too advanced or too retarded.

Total in the 34-36 degree range at 3000rpm and half in at 2000rpm?

Have weakest springs in at the mo.

That is fine while tuning the timing and idle mixture but once that is done you need to swap in stronger springs that match whatever your manifold vacuum is. Refer to the handbook.

The idea is that as you open the throttle, vacuum will drop and the needles lift to allow extra fuel into the engine.

With the weak springs in, vacuum holds the needles closed in the jets. You will bog.
If the springs are too strong the needles open too fast or when you don't need them. That can cause a bog too and will make you run too rich all the time.

So the springs need to be just right so that as you open the throttle, the needles lift and dump the extra fuel needed into the engine.

Once the correct springs are in there you just need to go richer on the needles and jets until the bog goes.

If it does not go you have to have something wrong like a vacuum leak or something.

[GeneralWee]

Right then...too the bat cave.
You have talked me into having another proper go at it.
Let you know what happens - here's to better weather!
Thank for your time.

[GeneralWee]

Set timing at 20 degrees. was at 14!. Have plenty of vacuum. tried all springs, which make little or no difference.
Have changed rod for richer (power mode), but this seems to smooth out power, but also reduce performance.Bog still there.
Had accel pump in middle and high mode, but the bog always comes after the initial pedal stab ie when accel pump has done its stuff).
I have torker 2 single plane manifold. Have read these are not suited to 1407 edelbrock.
What do you reckon?
Leaner on the primarys?

[Dave-R]

Set timing at 20 degrees. was at 14!.

So was that with the vacuum disconnected? What is the total degrees @ 3000rpm now then? If you have more than 35-36 degrees it will detonate like crazy.

Have plenty of vacuum. tried all springs, which make little or no difference.

Again you do not give us the figures. How many inches of vacuum???

There is only ONE spring size to try and that is the springs that match the vacuum reading you have. It is pointless trying the other springs because only the correct spring will allow the carb to function correctly.

Have changed rod for richer (power mode), but this seems to smooth out power, but also reduce performance.Bog still there.
Had accel pump in middle and high mode, but the bog always comes after the initial pedal stab ie when accel pump has done its stuff).

Probably lean on the jets then. Probably your pump shot is not long enough either.

I have torker 2 single plane manifold. Have read these are not suited to 1407 edelbrock.
What do you reckon?
Leaner on the primarys?

I would say richer. You need to monitor your vacuum readings as you drive at a constant rpm. Tune for max vacuum at, say, 3000rpm. Use second gear if that rpm is a high road speed.

[Dave-R]

Try to keep this in mind. A bog is caused by the engine going lean.

You can get a bog from being too rich but you would have to be very rich indeed for that to happen. Like pouring the fuel in from a bucket.

[Dave-R]

Another point is the intake plenum design. Is it divided down the middle or an open type?

Open types half the vacuum signal to the carb so (if memory serves) you tend to need to go richer on the jets to avoid a bog.

[Dave-R]

Just looked the intake up. It has an open plenum on the manifold. It is a bad choice for a street car. Really intended for high rpm use. Rubbish at low rpms.

Whoever decided that was the right intake for the engine did not do their homework.

[GeneralWee]

5" of Vac, orange springs. Manual says if the springs don't make a difference then its air/fuel metering. obvious?
Read that these manual choke carbs come out of the box rich.
Unfortunately, the rod/jet kit doesn't allow me to go lean just on the power mode, so have had to go 1 stage lower on both power and cruise.
Ran out of light, fuel and money today, but bog nearly gone, certainly the best it has been so far - just a slight hesitation now, which feels like the accel pump has discharged, then before the secondary jets kick in.
Manifold was on the engine when I bought it S/H. Had a holley fitted - engine had been rebuilt not long before I got it. Had "homemade" adjustable push rods!!
Need to check timing setup/advance/mixture etc again tomorrow, but got MOT at 11am so had to stop and get the grease gun out.
Thanks guys for all your input - sorry if I'm not throwing loads of numbers at you, but I'm sure you realize, I don't live on a drag strip, so finding a hole between all the grannies in their Honda spazz's to give it the beans, on a wet A or B road (tricky at the best of times). Did get followed by the five-O last night whilest out on manouvers - they were hanging back, waiting for me to light it up, then I trundled onto the bypass and waved bye to them..lol. Don't think their diesel Corsa kept up...
If nothing else, I've done some miles in it this week.
Keep ya posted.

[GeneralWee]

OK, passed MOT
On way to MOT - bog was bad again. 20 minute run there - mixed motoring.Obviously they had it running there for a bit on the machine, then on way home, was still bad for 5 minutes, then bog pretty much dissappears???
Could it be heat in the inlet manifold??? Has big air gap underneath.
Have quite big alley radiator with flex fan. Thermostat is in. Engine never gets hot, hot even in summer.
Any ideas?
Have checked round manifold with carb cleaner, and doesn't appear to be any leaks.
Any thoughts?

[Dave-R]

Block off the heat passage through the intake. If you have cast exhaust manifolds make sure the exhaust by-pass valve is fixed open or removed first.

These carbs do not like high fuel pressure. Fit a good fuel pump along with a regulator and an in-line fuel pressure guage. Set the pressure to 5.5psi.

[GeneralWee]

So, still working on this one.
Timing. With vac plugged, got 13 degrees at idle and 34 degress at 3000 rpm. With vac on , I get 13 at idle and 50 degrees at 3000 rpm???? So I have 21 degrees mechanical in the dizzy - right.
Ditched the flexi fan - elecy fan due tomorrow.
I also see there is a plate for the 1407 Edelbrock, to be fitted between carb and certain manifolds, or "there will be a vacuum leak". Anyone used one?
Needless to say, I haven't got one.

[Dave-R]

You could probably do with more initial. Get an advance limiting plate from Turnip on here.

Warm the engine up.
Let it idle.
Turn the dizzy advanced to find the "sweet spot" is where the engine rpm goes up sharp and then just starts to level off.
Then re-set the idle speed on tha carb and see if the engine still starts OK.

If the engine kicks back or is hard on the starter then back off a touch and try again.

It the engine starts easy put your timing light on and see what the initial is now. The difference between what you have now and what you need is the number of degrees you need to remove from the dizzy. The plates Turnup sells in his role as FBO rep will have different positions to remove different amounts of timing.

[GeneralWee]

Well, the bog has gone - it seems engine temperature has made a massive difference. Not running perfect though - will invest in timing plate.
Flamin' P/S pump (11months old, done 500 miles) has started making a grinding noise when under load
Oh well, back in the workshop!

[Dave-R]

You problem at the moment though is tuning the car in these temps.

When the weather is cold like this you need to go richer. When the weather is hot you need to go leaner.
So tune the carb when the weather is mild.

I should have added that I sometimes have had a bog on the very rare occasions i used my car in cold winter months.