Carb size for my setup

[erik70rt]

Lack of vacuum will cause a lot of problems.  Good cams that make good vacuum can not be stressed enough.  That's why I run the Mr. Six Pack cams.  They make great vacuum and that in turns makes it easy to tune and great running engines.  The 340 engine we just tuned on the dyno last Friday made 18 pounds of vacuum at 850 idle.

What is your vacuum at idle?

I'll have to recheck that since I don't remember off the top of my head.

[erik70rt]

Lack of quench will cause detonation well below safe range

It would also backfire through the exhaust when you let off the gas at highway speeds. 

I have not romped on it much since it's only got about 700 miles in it since the build. 

[HP_Cuda]

I would agree with Neil on the Proform carb but if you want to improve gas mileage you could move towards a vacuum secondary setup. But as he said before you only improve your gas mileage as long as you can restrain from overuse of the fun pedal.

[Shane Kelley]

In my opinion a manual trans car works better with a mechanical secondary carb. With a vacuum secondary it will close the secondaries between shifts. Then pull them back open. Automatic trans I like the vacuum secondary carb. I run 11-1 compression with 38 degrees total timing with no spark knock on 93. I run one of my 340's with the timing locked at 38 degrees. Instant throttle response.  I do have aluminum heads which does help with detonation. I have had some 340's that really run great with 650 cfm. The smaller the carb the snappier the throttle response. My preferred carb is a 750 mechanical secondary with a 4 speed. My favorite carb of choice is a Quick Fuel Q750. Sweet right out of the box. 

[HP_Cuda]

Funny I don't have any problems with a vacuum secondary carb and a manual car. I have one on my Cuda right now.

The secondaries don't close that fast between shifts. If you take off the vacuum secondary unit and pull the diaphragm closed you will see it closes pretty slowly.

I have had plenty of double pumpers in my past and they definitely have their place but it wouldn't be on a car where you wanted better gas mileage IMHO.

[erik70rt]

My list of things to do tonight:

1.  Install the lightest set of springs I have in the carb.
2.  Check on/reset the advance on the distributer
3.  Set timing with more advance
4.  Recheck vacuum
5.  Get cam specs posted
6.  Have cold one 

[erik70rt]

I changed out the springs in the carb to the lightest set I have - 3 in hg.  I reset the timing to approx. 18 degrees initial and 36 at 2500 rpm.  The vacuum at idle is 5 in lbs and 18 at 2500 rpm and steady. 

My cam is Comp grind 292H-10 part # 20-243-4 - part of the Magnum series (not Xtreme like I posted earlier).  It has .501 lift intake and exhaust.  Duration @.050 is 244 intake and exhaust.  Lobe lift .3340 for both and lobe separation is 110.0

I also rechecked my engine specs and it's 10:1, not 9:1.  So I may need a higher octane gas, too.

[Cuda Cody]

Maybe I'm to cautious, but I don't like being under 10 to 12 on vacuum at idle.  And if you have 10.1 to 1 with iron heads you might have to run some race fuel to get it to not ping.  I try not to go over 9.5 to 1 with iron heads.

With that little vacuum does it idle okay?  Do you run power brakes on that low of vacuum? 

Plus you have 3.55's which is a great rear, but not as friendly on fuel as maybe something like 3.23's.  If it's running good and you like everything else, you might have to be okay using fuel.  But 6 MPG sounds really low for what you have.  I personally think 800 is a lot of carb.  Do you have a buddy or someone you can borrow a 750 and see how you like it?

The vacuum at idle is 5 in lbs

[erik70rt]

It runs well at idle.  I have it set around 850 rpm. 

I knew I wouldn't have enough vacuum for power brakes, so I installed manual discs in front and drums in back. 

I'll see if I can scrounge a 750 from someone.  If the Eddy carbs are that fickle about vacuum (I was not aware they were), then I might switch to the Proform carb that others recommend.

[Cuda Cody]

The carb might be fine with that little vacuum.  If it runs and idles great, the only change it to see if you can get some better fuel mileage.   

It runs well at idle.  I have it set around 850 rpm. 

I knew I wouldn't have enough vacuum for power brakes, so I installed manual discs in front and drums in back. 

I'll see if I can scrounge a 750 from someone.  If the Eddy carbs are that fickle about vacuum (I was not aware they were), then I might switch to the Proform carb that others recommend.

[Chryco Psycho]

I changed out the springs in the carb to the lightest set I have - 3 in hg.  I reset the timing to approx. 18 degrees initial and 36 at 2500 rpm.  The vacuum at idle is 5 in lbs and 18 at 2500 rpm and steady. 

My cam is Comp grind 292H-10 part # 20-243-4 - part of the Magnum series (not Xtreme like I posted earlier).  It has .501 lift intake and exhaust.  Duration @.050 is 244 intake and exhaust.  Lobe lift .3340 for both and lobe separation is 110.0

I also rechecked my engine specs and it's 10:1, not 9:1.  So I may need a higher octane gas, too.

OK now we are getting to the real info !
5" at idle makes sense , cam has a lot of duration with very little lift , far better grinds are available if you are into a cam swap .
There is no reason why you cannot get around 10+ MPG but a better carb will make a lot of difference , you want a carb with adjustable air bleeds with low vacuum like that .
You may have to back the timing down again as your heads have no quench , or add better fuel .
what are you running for T stat temp ?

[erik70rt]

Thermostat is 160.

I can't get race gas around here.  Best I can do is the 93 octane.  I'll have to find that post about octane boosters and read up on which ones are good.

[Chryco Psycho]

Change to 180*

[Cuda Cody]

Why?    Wasn't the goal originally to get the car up to operating temp as fast as possible, so they used a stat to stop the coolant until it was hot?  So in a daily driver it makes sense to use a 180 and let the car warm up.  If the car is like most E-Bodies these days, it's not a daily driver and usually only driven on nice warm days.  So getting the car up to temp fast isn't as big of a deal as it was back in the day.  I want my engine to stay as cool as possible as I manly go cruising and sometimes sit stuck in traffic.  I have even removed the stat all together and instead use the wide 3 core (or 4 core) radiators with a custom dimple core to slow the water down where it's needed.... in the radiator.  I don't want the bottle neck of the coolant flow to be in the stat, I want the coolant to get to the radiator, then the dimples slow it down and get the coolant as cold as possible before it goes back to the engine.  I never had over heating trouble even when I'm sitting in a parade idling in 100 temps.   

Change to 180*

[Chryco Psycho]

The engine can run too cool accelerating wear inside the engine & cause incomplete combustion ,ie back fire out of the pipes